Plutonium Burning for Disposal of Pure Plutonium


Chapter For Encyclopedia of Environmental Remediation

Richard Wilson

Mallinckrodt Professor of Physics

Harvard University


Contents


I Historical introduction

A The Clear and Present Danger

II The nuclear physics of plutonium

A Properties of each isotope

(i) Decays

(a) alpha emission

(b) beta emission

(c) electron capture

(d) spontaneous fission

(ii) gamma ray emissions

(iii) x-ray emissions

(iv) The fission process

(v) energy released in fission

(vi) number of neutrons in fission

(vii) fraction of delayed neutrons

III The production of plutonium isotopes

A The ordinary chain U238 -> Np239 -> Pu239

B The production of Pu240 and higher actinides

IV Mixed Oxide (MOX) Fuel

A Plutonium equivalency

B Fuel Fabrication

C Experience with MOX fuel

D Differences between Weapons Grade and Reactor Grade fuel

(i) favorable to use of weapons grade plutonium

(ii) unfavorable to use of weapons grade plutonium

E The optimum reactor for plutonium burning

F Use of plutonium metal fuel

G Use of thorium fuel

V Total technical availability of reactors

VI Effects of fuel handling on health

VII Environmental Effects

VIII The political discussions

A Critical mass of plutonium

B The Pre-ignition hope

C Plutonium heat generation

D Present accepted explosion hazard

E Proliferation hazards

(i) Third world country actions

F The setting of an example

(i) Atoms for Peace

(ii) the reprocessing ban

(iii) the present conflict

(iv) the international non-proliferation demands

G Energy Scenarios

(i) Maintaining the nuclear Option

(ii) timing of a breeder reactor need

(iii) Final destruction of plutonium



I. HISTORICAL INTRODUCTION

Plutonium was discovered by Seaborg, McMillan, Segre, Kennedy and Wahl in 1941. In 1951 McMillan and Seaborg received the Nobel prize in Chemistry for their work on the chemistry of the transuranic elements that included this discovery. It is a useful material that can bring great advantages to mankind but is dangerous because it can be misused and be a source of war, suffering and perhaps the end of civilization. Although it is not alone in this, it is regarded by many in the US public as the embodiment of evil, and the use of plutonium whether in a breeder reactor or recycled into a thermal neutron reactor, has become a symbol for public concern about nuclear power. Plutonium is useful because it is a way to unlock the energy available in uranium 238 and put it to the service of man. Dangerous because a small quantity - merely 5 kg - suffices to make a nuclear fission bomb of considerable explosive power.

The world has for a long time been concerned about the possibility of nuclear war; for 45 years the main attention was on the rivalry between the two superpowers, the USSR and the USA, with their huge arsenal of nuclear weapons. Since 1990 this concern has been replaced by another, the spread of nuclear weapons, and weapons technology among smaller countries and terrorist groups. President Eisenhower recognized this problem 40 years ago; he realized that the main thrust of anti-proliferation measures must be political, although technical matters can assist. He therefore proposed the landmark Atoms for Peace conference in 1955, at which it was made clear that the superpowers would share their knowledge of nuclear fission for peaceful purposes with other countries. In exchange they would demand openness so that it could be less likely that nuclear weapons would be made in secret. This policy was followed by the nuclear NonProliferation Treaty, NPT, signed by more most of the nations in the world and with an indefinite renewal voted in 1995.

Before 1975 there were plans for a nuclear fuel cycle in the world which, for brevity, I will call Fermi's dream. One starts with uranium ore, processing it, enriching the natural uranium in the fissile isotope uranium 235, burning the uranium 235 in an electric power producing reactor, reprocessing the fuel to separate the uranium 235 and plutonium 239 for use in subsequent reactors. If the reactor is a fast neutron reactor, other transuranic elements can also be broken up and destroyed by fission. In addition a fast neutron reactor is capable of producing more plutonium fuel than the uranium fuel it burns, leading to a breeder reactor. All that would be left for subsequent waste disposal would be fission products themselves, almost all with half lives of 30 years or less. The fast neutron reactor of preference was cooled with liquid sodium. With this it was envisaged that all the energy in uranium, including both isotopes could be unlocked.

In the early 1970s problems appeared in the Fermi dream. It was realized that the existence of many tons of chemically separated plutonium might lead to a possibility of theft, or "diversion", of enough material to make a nuclear bomb. The presence in the hands of a small "rogue" country, or of a terrorist group, is unacceptable and would be a nightmare. This led a study sponsored by the Ford Foundation (1) and the decision of President Carter on April 7th 1977 to abandon the plans in the U.S. to reprocess spent nuclear fuel, and slow the development of the breeder reactor. Fossil fuel supplies had and have been more plentiful and cheaper than anticipated, uranium ore had and has been more plentiful, and the cost of the experimental breeder reactors has been greater than expected. Nonetheless other countries have not followed the US lead and have continued to reprocess nuclear fuel. Since a breeder reactor is not yet necessary to ensure adequate electricity supplies for the world, the separated plutonium has begun to be burnt in thermal reactors to produce electricity. Although the US experience of burning plutonium in "ordinary" reactors is limited to a few test fuel assemblies, the world experience is considerable.

A. The Clear and Present Danger

The whole issue of plutonium disposal was reopened at the end of the cold war when nuclear fuel from weapons was declared surplus from military stockpiles in the United States and the USSR and discussions began on how to dispose of it. Since 1994 several committee reports in America have discussed the question of how to prevent the plutonium from getting into "unauthorized" hands. The first, a committee of the National Academy of Sciences (NAS) chaired by W.K.H. Panofsky (2), addressed how to dispose of "weapons grade" plutonium declared surplus to the excessively large arsenals of Russia and America. They stated a belief that the existence of this plutonium (especially in the hands of the unpredictable Russian government) presents "a clear and present danger" to the United States, mainly because they were uncertain of the reliability of the Russian security. President Yeltsin's proposal for an internationally monitored plutonium storage facility in the Urals might help in this regard. These carefully chosen words have been often repeated and imply that the President of the United states should act. Other committees while not being so dramatic, agreed (3,4). A second committee of the National Academy of Sciences (5) expanded upon the reactor disposition options, and a third was a joint committee of the US National Academy of Sciences and the Russian Academy of Sciences (6). Other committees of the Department of Energy, the Amarillo group (4) and a special committee of the DOE chaired by Dr Allen Sessoms (7) have also discussed the matter.

The restriction of the discussion of the NAS committee to excess weapons plutonium was deliberate, and was intended to address the steps necessary to safeguard or dispose of weapons plutonium additional to those necessary for the 1000 tons already present in "spent" nuclear fuel discharged from reactors at electricity generating stations. They coined the phrase "spent fuel standard". If the fuel can be diluted with other material so that the safety is at least that of spent fuel, then their (limited) task would be done. The committee of the American Nuclear Society (ANS), chaired by Glenn Seaborg, Richard Kennedy and Myron Kratzer (3) addressed the management of all plutonium including that from nuclear power. The ANS committee was primarily composed of persons who had positions of responsibility in the US, but also in Europe and even in Russia.

In this article I will discuss the physics and chemistry that underlie any discussions of the problem. While acknowledging the need to address the issue of disposal of weapons plutonium, I will also address the way in which plutonium might be handled in the longer term in a civilian nuclear power program.

II THE NUCLEAR PHYSICS OF PLUTONIUM

There are fifteen isotopes of plutonium known and listed in the Handbook of Physics and Chemistry and in the "Table of Isotopes (8). Their half lives and types of emission are listed in Table I.

Table I

Plutonium Isotopes
Isotopic Mass Half Life Decay Mode (% of each) Specific Activity

109 Bq/g

Spontan-eous Fission Neutrons neutrons/

gm/sec

Heat Genera-tion mW/gm Principal Daughter Product Half Life of Daughter Product
233 20.9 m E.C.(99.9),

(0.1)

Np-233 36.2 m
234 8.8 hr E.C. (94),

(6)

Np-234 4.41 d
235 25.6 m E.C. (99+),

(0.003)

Np-235 396 d
236 2.852 yr 1.9 x 104 37 x 103 - U-232 71.8 yr
237 45.4 d E.C. (99+),

(0.003)

Np-237 2.14 x 106 yr
238 87.74 yr 6 x 102 2.6 x 103 560 U-234 2.447 x 105 yr
239 2.413 x 104 yr 2 0.03 1.9 U-235 7.038 x 108 yr
240 6.571 x 103 yr 8 1.0 x 103 6.8 U-236 2.342 x 107 yr
241 14.356 yr ,

(0.003)

3.7 x 103 - 4.2 Am-241 432 yr
242 3.764 x 105 yr 0.1 1.7 x 103 0.1 U-238 4.468 x 109 yr
243 4.955 h Am-243 7.37 x 103 yr
244 8.05 x 107 yr 1.7 x 103 U-240 14.1 hr
245 10.49 hr Am-245 2.05 hr
246 10.85 d Am-246 25.0 m


Values mostly from Lederer et al., 1977 (8), Table of Isotopes, Seventh Edition.

In addition to the radioactive emissions from the isotopes themselves there are emissions from the radioactive daughter products when these have half lives less than or comparable to the half lives of the parent. For example Plutonium 239 has a half life of 24,390 years and decays to uranium 235. But uranium 235 has a long half life of 7 x 108 years so that the emissions from this daughter are small. But plutonium 241 has a half life of 14 years. The daughter nucleus is Americium 241 with a half life of only 458 years. Americium will therefore build up over a time of 14 years with a rate of decay 14/458 that of the initial plutonium 241. The next daughter however is Neptunium 237 with a very long lifetime of 2 million years. The decays of Neptunium 237 after equilibrium will be at a rate 15/2,000,000 of that of the plutonium 241. For the purposes of this discussion therefore, neptunium 237 and subsequent daughters can be neglected.

A. Properties of Each Isotope.

The name plutonium is given to all materials with atomic number 94, regardless of the their mass. The atomic number (Z) of 94 is the number of atomic electrons attached to the nucleus to form a neutral atom, and these electrons control the chemistry of the material.

Each "isotope" of plutonium has a different mass. The ratio of this mass to the mass of a proton is called the atomic mass (sometimes called atomic weight). More recently the atomic masses are scaled to the mass of the carbon nucleus 12C =12.000000. The mass number (or atomic number) (A) is this mass rounded off to the nearest integer and is used to list these isotopes as shown in Table 1. The isotope of most interest here is that with mass number 239 and the designation of the plutonium is 94Pu239.

(i) Decays

Each of the plutonium isotopes is unstable and decays into other materials. The observed decays are:

(a) emission of an alpha particle 2He4 which results in a daughter nucleus with Z value reduced by 2 and A value reduced by 4. For the 239 isotope the result is 92U235. Most of the relevant plutonium isotopes decay by alpha emission to an isotope of uranium.

(b) decay by the weak interaction with emission of a positive (positron) denoted beta + in the table, or negative electron, denoted beta - accompanied by a neutrino. This increases (decreases) the atomic number by 1 for an electron (positron). The heavier plutonium isotopes decay in this way. The important isotope is Pu241 which decays by beta - emission to Am241 .

(c) capture of an atomic electron which also decreases the atomic number by one. This end product is similar to that from positron emission. This decay occurs when there is insufficient energy to produce the masses of a positron electron pair. Otherwise positron decay predominates. Sometimes decay can occur either by alpha decay or by electron capture (to different daughter products). The percentage of each is shown in Table 1.

(d) Spontaneous fission in which the nucleus splits into two almost equal halves.

The probabilities of all of these can be added to produce a decay constant of the isotope. The reciprocal of this decay constant divided by the natural logarithm of 2 is the half life of the isotope.

94Pu239 decays by (a) only. The lighter isotopes tend to decay by positron emission and the heavier ones can decay by electron emission.

(ii) the alpha or beta decay usually leads to an excited state of the daughter nucleus. This excited state then decays to the ground state by gamma ray emission.

(iii) x ray emission occurs after beta or alpha emission as the electron cloud around the daughter nucleus rearranges itself.

(iv) The fission process.

The mass of a heavy nucleus is greater than the sum of the masses of two parts into which it can split. It is held together by the forces between nucleons, but when excited the nucleus can split by a process called fission. The nuclear fission leads to a pair of fission products, with slightly unequal atomic weights. The distribution in atomic mass of these fission products is shown in figure 1. The distribution is not exactly the same for plutonium 239 fission as for uranium 235 fission. This leads to a large difference in a vital quantity - the number of delayed neutrons that will be discussed later.

FIGURE 1 HERE

Most of the heavy elements with atomic weight of uranium and higher undergo fission upon capture of a neutron. However the cross section for fission by a neutron (probability of fission) depends upon the neutron energy as well as the element under neutron bombardment. This is shown in figure 2 (averaged over individual narrow resonances). The symmetries in the forces between individual nucleons result in nuclei of even atomic weight as having a higher rate of spontaneous fission than others, and especially those with an atomic weight that is a multiple of 4. A nucleus which has an odd atomic weight, and especially 4N-1 where N is an integer, will readily undergo fission when they capture a neutron and produce the fissionable nucleus in an excited state.

FIGURE 2 HERE

Of the isotopes of interest therefore, only those of odd atomic weight, uranium 233, uranium 235, plutonium 239 and plutonium 241 will undergo fission with thermal neutrons. These are called fissile. However, as the neutron energy is increased above 1 Mev all the other isotopes of plutonium, and americium 241 undergo fission readily. Uranium 238 does also, but not as readily. There is a distribution of energy of the neutrons from fission (the fission neutron spectrum) with a peak between 1 and 2 Mev. However, Pu240 and Pu242 are fertile: so called because on capture of a neutron they produce the fissile Pu241 or Pu243

As noted in Table 1, the plutonium isotopes of odd atomic weight all have a small probability of spontaneous fission.

In a thermal reactor, the neutrons are slowed down before capture, and only the fissile elements will undergo fission. But in a bomb, or in a fast neutron reactor, the other isotopes can undergo fission also.

(v) The energy released in fission

That energy can be released in fission of a heavy nucleus was clear already in the 1920s when atomic masses were measured. The binding energy of the nucleons (neutrons and protons) in a nucleus is the mass of all the individual nucleons minus the mass of the nucleus. The binding energy of a heavy nucleus of atomic number 239 is about 7.6 Mev per particle; that for the fission products with masses about 150 and 90 about 8.5 Mev per particle. The energy released in fission can then be derived approximately as 238 (nucleons) x [8.5 - 7.6] = 215 Mev. This is well over a million times the typical energy release per unit weight in chemical processes. It is this large energy per unit mass that enables large efficient bombs to be made and also enables electricity to be produced with comparatively small amounts of mining and other environmental consequences.

(vi) The number of neutrons in fission

The heavy nuclei have a larger number of neutrons than protons in their nuclei as the electrostatic repulsion of the protons drives them apart. Lighter nuclei have a more equal number of protons and neutrons. Therefore on fissioning of a heavy nucleus there is an excess of neutrons. A part of this excess goes to producing unstable fission products that are "neutron rich" compared with their stable isotopes. But a part goes to instantaneous production of fission neutrons. The number of neutrons in fission is an important quantity since it is this number that if greater than unity, can lead to a chain reaction. It is different for different materials and varies with energy. For thermal energy the fission of U235 yields 2.4 neutrons per fission, Pu239 yields 2.9 neutrons per fission and U233 yields 2.5 neutrons per fission. This increases with energy and at 1.5 Mev becomes 2.6, 3.1 and 2.6 respectively.

(vii) The fraction of delayed neutrons in plutonium fission

Fermi once said that "without delayed neutrons we could not have a nuclear power program." A fraction of all neutrons produced in fission is delayed. Without them the time constant for power changes in a nuclear reactor would be short enough that effective control would be impossible. The delayed neutrons arise from beta decay of a fission product nucleus to a state of a neighboring nucleus sufficiently highly excited that it can decay by neutron emission. An example is shown in figure 3. The fission product Br87 decays by beta - emission with a half life of 55.6 seconds to an excited state of Kr87 which can either decay by beta - emission to Rb87 or by neutron emission to the Kr87 ground state.

FIGURE 3 HERE.

There are six such groups of delayed neutrons as shown in Table 2. These come from different fission products or groups thereof. Because there is a difference in the fission product yield distribution for the different fissionable elements, there results a difference in the number of the decayed neutron. When this is divided by the average number of neutrons in fission (Table 3) the fraction of fission neutrons that is delayed is different (smaller) for plutonium than for uranium (0.3% vs 0.65%) making safe reactor control slightly more difficult. This might necessitate a change in the control and shutdown systems.



Table 2. Delayed Neutrons from Thermal Fission

Fuel Isotopic Purity ( %) Group No. (i) Half Life i 2, (sec) Decay Constant (i, sec-1) Relative Yield (i/) Yield (Neutrons per Fission)
U235 99.9 1 55.72 0.0124 0.033 0.00052
2 22.72 0.0305 0.219 0.00346
3 6.22 0.111 0.196 0.00310
4 2.30 0.301 0.395 0.00624
5 0.610 1.14 0.115 0.00182
6 0.230 3.01 0.042 0.00066
Total 1.000 0.0158
Pu 239 99.8 1 54.28 0.0128 0.035 0.00021
2 23.04 0.0301 0.298 0.00182
3 5.60 0.124 0.211 0.00129
4 2.13 0.325 0.326 0.00199
5 0.618 1.12 0.086 0.00052
6 0.257 2.69 0.044 0.00027
Total 1.000 0.0061
U233 100 1 55.0 0.0126 0.086 0.00057
2 20.57 0.0337 0.299 0.00197
3 5.0 0.139 0.252 0.00166
4 2.13 0.325 0.278 0.00184
5 0.615 1.13 0.051 0.00034
6 0.277 2.50 0.034 0.0022
Total 1.000 0.0066




Table 3. Delayed-neutron Fractions and Weighted Mean Decay Constants

Effective Neutron Energy Fuel Yield (Delayed Neutrons per Fission) (Neutrons per Fission) (Delayed- neutron Fraction)
Thermal U235 0.0158 2.432 0.00650 0.0767 0.405
Pu239 0.0061 2.874 0.00212 0.0648 0.356
U233 0.0066 2.482 0.00266 0.054 0.279
1.45 Mev U235 0.0165 2.57 0.00642 0.0784 0.435
1.58 Mev Pu239 0.0063 3.09 0.00204 0.0683 0.389
1.45 Mev U233 0.0070 2.62 0.00267 0.0559 0.300


III PRODUCTION OF PLUTONIUM ISOTOPES

A. The Ordinary Chain U238 -> Np239 -> Pu239

In a nuclear reactor fueled by uranium, the uranium 235 is the fissionable fuel, whereas the uranium 238 is a neutron capturing impurity. But it is called fertile because of the chain of events after the capture is of especial importance.

The capture of the neutron forms a compound nucleus of uranium 239 in an excited state. The compound nucleus promptly (within a hundredth of a microsecond) emits gamma rays and de excites to become uranium 239 in the ground state

n + U238 -> U239*

U239* -> U239 + gammas

In turn the Uranium 239 is radioactive and decays by emission of an electron (beta -) with a half life of 23.5 minutes to become neptunium 239 (Np239). In turn (Np239) decays to become plutonium 239 (Pu239) which has a half life of 2.36 days. The plutonium with the half life of 2.41 x 104 years stays in the reactor.

B. The Production of Pu240 and Higher Actinides.

Higher isotopes of plutonium are formed by neutron capture when the plutonium is left in the reactor with a large flux of neutrons. For example Plutonium 240 is formed by the reactions:

n + Pu239 -> Pu240*

Pu240* -> Pu240 + gammas

Plutonium isotopes up to Pu242 are produced in such a way. However, successive addition of neutrons can produce Pu243 but as shown in table 1 this rapidly decays to Am243, and is not available in the reactor to capture further neutrons. In a mixture of isotopes in a slow neutron reactor, the induced fission of the (fissile) odd isotopes leads to a their depletion relative to the (fertile) even isotopes and the mixture becomes progressively harder to use in a thermal neutron reactor. However in a fast neutron reactor all the isotopes can be burned up.

In addition very small amounts of Pu238 and other lighter plutonium isotopes are produced by fast neutron reactions. Plutonium 238 is of practical interest because of its heat generation which is larger than that of the other isotopes due to the short half life. Combined with thermoelectric cells it is a source of electricity for pacemakers and space probing satellites. It also makes it harder to make a bomb, as discussed later.

A simplified schematic of the important nuclear reactions of interest is shown in figure 4. it will be evident that we are primarily interested in those isotopes that are made plentifully in reactors, those with atomic weights 239 to 242. Since Pu241 decays intoAmericium 241 with a half life of 12 years, we are also concerned with Americium 241 in "aged" spent fuel which has been around for a time.

Figure 4 here "Decay Modes of Transuranic Elements"

The relative amounts of each isotope will depend upon the detail of operation of the reactor. If the plutonium is kept in the reactor a relatively short time, it has little time to be burned up by fission as it is produced, and little chance to capture a neutron and form higher isotopes. This, therefore, is the mode of operation of a reactor producing plutonium for bombs. In contrast, a power reactor making electricity will normally keep the fuel in the reactor for a long period, and the higher isotopes will be increased. The period over which the fuel has been used is measured in units of energy produced divided by the mass of the fuel - usually MW days per ton, or in a fraction or percentage of the maximum burn up when all the fissile material is consumed. The distribution of plutonium isotopes for various fuel "burn-ups" is shown in Table 4 for various reactor types.

Table 4. Average Isotopic Composition of Plutonium

Produced in Uranium-Fueled Thermal Reactors (from Ref. 9)

Reactor Type Meanfuel burn-up (Mwd/t) Percentage of Pu isotopes at Discharge
Pu-238 Pu-239 Pu-240 Pu-241 Pu-242
Magnox 3 000 0.1 80.0 16.9 2.7 0.3
5 000 * 68.5 25.0 5.3 1.2
CANDU 7 500 * 66.6 26.6 5.3 1.5
AGR 18 000 0.6 53.7 30.8 9.9 5.0
BWR 27 500 2.6 59.8 23.7 10.6 3.3
30 400 * 56.8 23.8 14.3 5.1
PWR 33 000 1.3 56.6 23.2 13.9 4.7
43 000 2.0 52.5 24.1 14.7 6.2
53 000 2.7 50.4 24.1 15.2 7.1
* Information not available.

This table from OECD (9) shows the distribution of isotopes at discharge from typical thermal neutron reactors, showing the variation with the type of reactor used and the "burn-up" of the fuel given in the amount of electricity produced per ton of fuel. The high burn up of the fuel in a modern power reactor produces fuel with large content of Pu240.

Of the plutonium isotopes, plutonium 239 and plutonium 241 undergo fission by thermal neutrons (are fissile materials) and are therefore suitable fuels for thermal neutron reactors. The other isotopes are mild poisons, as they capture neutrons. All the produced plutonium isotopes can undergo fission by fast neutrons, leading to a more complete destruction of the plutonium in such a reactor.

IV. PLUTONIUM AS A REACTOR FUEL (MIXED OXIDE (MOX) FUEL)

The most important fact to recognize about the use of plutonium as a reactor fuel is that the problems it poses are not different in character from those that a reactor designer faces with any fuel. When any reactor is loaded with fuel, extensive calculations are made of the optimum distribution of fuel rods in the core, the proper location of poisons and other details to ensure reliable operation even as the fissile fuel is burned up. Adding plutonium is no exception. In the context of destruction of plutonium it is necessary to use high power reactors to enable enough plutonium to be burned up and destroyed in a reasonably short time. The world has had considerable experience of "mixed-oxide" fuel in power reactors. Although there is experience with plutonium metal fuels, this has mostly been with smaller experimental reactors. It must also be recognized that even a power reactor fueled with uranium burns plutonium for the later part of the fuel burn up, since plutonium is continuously produced from neutron capture on uranium 238.

239Pu is as fissile as 235U, and fuel made with approximately the same mixture 239Pu/238U as the normal mixture 235U/238U will behave in a similar way in thermal reactors to the pure uranium oxide fuel . Such a fuel is called mixed oxide fuel or MOX.

A. Plutonium Equivalency

If plutonium with more than one isotope is to be used, we must be careful to define the amount of plutonium correctly. Three different amounts are discussed.

Total plutonium by weight (PuT)

Total fissile plutonium (Puf)

Plutonium equivalent for reactivity (Pueq)

The non fissile materials add weight but no reactivity. Thus a mixed oxide fuel with 6% plutonium oxide but 50% non fissile plutonium will behave similarly to a fuel with 3% plutonium oxide with only fissile isotopes, This is expressed more precisely by defining a quantity called "equivalent plutonium" or Peq to be defined as producing the same energy as the same weight of Pu239. This will be about the same for all fissile isotopes, U235, and Pu241 but will be negative for the non fissile isotopes. But if a fast neutron reactor is used, even the even isotopes which are not fissile for thermal neutrons can undergo fission leading to a small equivalency. For the practical purposes of this report, the equivalency factor for a mixture is given by

Peq = fi Peqi

where the equivalency factors for each isotope are given in Table 5.

Table 5. Plutonium Equivalent Worths Peqi in LWRs and FBRs (modified from Ref. 9)

Pu-239 = 1.00

ISOTOPE

LWR FAST NEUTRON REACTOR

PHENIX (250 MWe)

U-235 + 0.8 + 0.71
U-238 0.0 0.0
Pu-238 - 1.0 + 0.49
Pu-239 +1.0 +1.0
Pu-240 - 0.4 + 0.20
Pu-241 + 1.3 +1.40
Pu-242 - 1.4 + 0.086
Am-241 - 2.2 - 0.23
This table provides an illustration of the energetic value of the main uranium and plutonium isotopes (and americium-241) in comparison to the plutonium-239 isotope (atom per atom) in LWRs and a fast neutron reactor.

B. Fuel Fabrication (Steady State Burning of Plutonium)

In a typical light-water reactor environment, the replacement ratio between fissile plutonium (Pu239 + Pu241) is approximately 1.3 to 1 on an atom to atom basis. This results from the need to increase the fissile loading of the plutonium fuel to compensate for a large absorption of lower energy neutrons by Pu240 at the resonance of 1.05 eV. For weapons grade plutonium, with less Pu240, the amount of plutonium needed for replacement of the U235 would be less. One third of the fissions at end of fuel life in a thermal reactor are from plutonium, so it seems reasonable to allow one third of the fissile fuel in a new fuel load to be plutonium without appreciable change.

For a burning of plutonium that is produced in a civilian electricity producing reactors, a simple replacement of a uranium oxide core by a 30% MOX core is approximately enough for a steady state. Studies have been made on the effect of MOX cores on safety. A European study (10) concluded:

The recycling of about 30% of mixed oxide fuel in large LWR cores should not induce special problems, provided precautions are taken in core design to minimize the differences with UO2 cores, taxing account a limited margin of uncertainty.

The influence on core behavior during the hypothetical accidents investigated in the studies was not very important and dose not necessitate restrictions for at least a 30% Pu fraction in the core. Obviously, the possible case of non-optimized (core) loading patterns and the uncertainty about some parameters and event sequences should not be forgotten.

The big "political" discussion is whether a reactor can be allowed to have a "full MOX core" safely; that is a core with all the uranium 235 replaced by Pu239. It is desirable to begin burning up excess weapons plutonium as soon as possible. If there were a limited number of reactors, use of a full MOX core would be of help. The technical limitation is availability of oxide preparation and fuel fabrication facilities, although there may be a political limitation on the number of reactors permitted to burn MOX.

But the use of plutonium equivalency will not account for all effects. If plutonium is recycled for several passes through a reactor, the non-fissile even atomic number isotopes are not destroyed and the fraction builds up. Eventually it becomes impracticable to burn them in a thermal neutron reactor. The disposition of such plutonium is discussed in a later section.

C. MOX Fuel Fabrication

The basic requirement for reactor fuel is that it stay intact as the fission process proceeds, as some of the material changes its chemical form. This can clearly be achieved for a short time with any fuel, but after a time, there are changes in fuel integrity - it falls apart. The fuel is usually encased in a metal tube about 1 cm in diameter (the fuel rod) which contains the fission products. It is of course particularly hard to contain the gases. Early reactors used metal fuels, but only achieved a burn up of about 1% before the fuel rods leaked badly. Most electricity producing reactors today (the large power reactors) use fuel with sintered oxides. The uranium (or plutonium) oxide is sintered and pressed into a solid with somewhat less than the maximum density of the oxide. The pellets of sintered oxide are then placed into the metal (usually zirconium) tube to form a fuel rod.

The chemical processes involved with fabrication of Mixed oxide fuel (MOX) have received much attention (11-14). The latest procedures in France, Belgium and England are described in Golinelli et al (15), Vanderbrock et al (16), and McLeod and Yates (17). Starting with either a plutonium nitrate solution (from a reprocessing plant) or pure plutonium metal from a dismantled bomb, all plutonium is converted to plutonium oxide by a calcine process. If necessary the plutonium is dissolved and purified by ion exchange or solvent extraction. Gallium (present in weapons plutonium see below) may have to be removed by a thermal process. The resultant oxide is mixed with uranium oxide (or maybe thorium oxide) for making into the usual sintered pellets.

The main potential problem in using weapons plutonium is the presence of gallium; when preparing plutonium oxide from the metal for use in MOX fuel it can evaporate and may, if present in large concentrations, prevent proper operation of the chemical process.

There are fewer delayed neutrons from plutonium as compared to uranium fission. This demands more care in ensuring that no rapid insertions of reactivity can take place. It is this latter reason that makes some people hesitate to use full MOX cores in some reactors.

The cross section for neutron fission as a function of neutron energy is different for plutonium and for uranium as shown earlier in Figure 1. As a result the mean free path for neutron fission is shorter for plutonium than for uranium. This also makes reactor control a little different.

These are detailed differences. In any reactor start up a full accounting must be taken of all materials in the core with their cross sections. There exist computer programs for this, such as ORIGEN (18) and there seems to be no difficulty in handling MOX cores rather than pure uranium cores.

The first MOX assembly to have been recycled in a reactor in the world was fabricated by Belogonucleaire (MN) in Belgium and loaded in a small pressurized water reactor (PWR) named BR3, in 1963. In 1970 a full core load of MOX fuel was loaded into the Garagliano reactor (also a BWR) in Italy. Tests in the USA included installation in commercial power reactors of test MOX assemblies in Big Rock Point Reactor in Michigan, and in Quad Cities (a BWR) (19).

In 1996 there were 32 reactors in Europe authorized to use MOX fuel, and on October 1 1996 MOX was loaded into 19 of these reactors (Table 6). In the USA the Combustion Engineering reactors were explicitly designed with a capability of handling a full core load of MOX fuel.(1) Both GE and Westinghouse maintain that their reactors can handle a full MOX core also, although many analysts believe that supplementary control rods and burnable poisons should be added if more than 30% MOX loading is desired. This addition is not particularly difficult. Japan plans to use MOX fuel for 30% of the core in two GE reactors shortly, and plans are being made in Japan for full MOX core load in five years.

Table 6

Reactors in operation MOX authorized reactors "Moxified" reactors Cumulated Tons as of 1/10/96 First MOX loading
France 54 16 9 279 1987
Germany 20 10 6 200 1972
Belgium 7 2 2 20 1995
Switzerland 5 4 2 20 1988
Data provided by Seimens, Inc.


It has been said that the proof of the pudding is in the eating. The statements and experience of the reactor operators has therefore some significance. The operators of Electricite de France find no appreciable difference between MOX loading and ordinary loading. The EDF experience is that they can achieve as great a "burn up" with MOX fuel as with recent other fuels with none of the early problems, and fuel disintegration (Figure 5).

D. Differences Between Weapons Grade Fuel and Reactor Grade Fuel in MOX.

Most of the experience of burning plutonium (as MOX) in the world has been with reactor grade plutonium with only 50% to 60% of Pu239, and no gallium. We need to know how applicable this experience is to weapons grade plutonium where the plutonium has a much greater fraction of the isotope 239Pu and some gallium is present. The differences are shown in Table 6.

(i) Favorable to use of weapons plutonium

In reactor grade plutonium there are more non fissile isotopes than in weapons grade plutonium. These do not contribute to nuclear fission but do add to neutron absorption and therefore act as mild poisons. To compensate for their presence or absence, the reactor operator must adjust control rod absorbers, or boron liquid absorber.

Weapons plutonium has less 238Pu with its high thermal power

Weapons plutonium has less gamma and neutron radioactivity (from 241Am)

The alpha radioactivity is contained in the fuel rods and stopped by the cladding. As a result weapons plutonium is easier to handle. This is a minus from a non-proliferation standpoint, but a plus from ease of preparing fuel rods.

Pu 239 has fewer gamma rays than the other Pu isotopes and is therefore easier to handle.

(ii) Unfavorable to use of weapons plutonium.

Weapons plutonium contains a small amount of gallium which is deliberately placed therein to enable the plutonium to pass phase transitions easily when the material is compressed during the bomb explosion. As a reactor fuel in thermal reactors gallium is a mild poison since it absorbs neutrons. The cross section for capture of thermal neutrons averaged over (gallium) isotopes is 3 x 10-24cm2 (3 barns) which is moderately high. Most experts feel that while this is important, it is easy to make compensation by adjusting the boron poison control and the reactor control rod adjustment.

However the gallium will interact with the fuel cladding and create pinholes if present in too large a concentration. This puts a limit on the acceptable amount of gallium which is presently under discussion. One guaranteed solution is to remove the gallium at the stage of conversion of the plutonium metal "pits". This can, however cause delay.

Care must be taken to avoid a critical assembly during fuel fabrication. This is more difficult for the weapons plutonium.

E The Optimum Reactor

Most of the experience with MOX fuel has been with its' use in pressurized water reactors (PWRs). It is this experience that is described in the OECD report (9). Detailed calculations for a PWR including a wider lattice, are shown in Breeders et al (20). These authors show that an equilibrium "steady state" is achieved after 80 years with a ratio of UOX to MOX loadings of 5/3. Much of the experience is applicable to any reactor using thermal neutrons. In particular the Advanced Boiling Water Reactor (ABWR) manufactured by General Electric Company and being built extensively in Japan, has been proposed as an excellent candidate for similar reasons.

The Advanced Gas Cooled Reactor (AGR) used in Great Britain can be used but is regarded as unfavorable economically. Most attractive, perhaps, is the possibility of using plutonium fuel in the Canadian heavy water reactors (CANDU) (21). These reactors are designed to operate with natural uranium fuel, and because they use heavy water rather than the neutron absorbing light water as moderator, are more efficient in the use of neutrons. The economics discussed in the paper by Veeder and Didsbury suggest that the use of plutonium can extend the fuel burn up considerably and therefore lead to a very favorable cost advantage compared to the use of PWRs. This possibility has been specifically studied by Greene et al (22) in their volume 2. Greene et al note that the economic optimization is different from that of Veeder and Didsbury (21). Because the aim is destruction of plutonium, rather than sustainable production of electricity, It makes sense to use more plutonium in the fuel than considered optimum by Veeder and Didsbury (21), both to destroy the plutonium more rapidly, and also to reduce the overall cost of fuel fabrication.

Although no CANDUs are operating in the weapons states (USA and Russia) that are planning to dispose of excess weapons plutonium, the fact that Canada has shunned plutonium recycle and the breeder reactor might make it politically acceptable to ask their help in the disposition.

F. Plutonium Metal Fuel

Although the large fast neutron reactors such as Phoenix and Superphoenix in France used fuel of plutonium oxide, there has been extensive experience with plutonium metal fuel (and usually fuel of weapons grade) in smaller experimental reactors in the United States and elsewhere. Until about 1985 not much of the plutonium in a metal fuel had ever been successfully burned. It is necessary for the fuel to remain intact as the drastic chemical changes of nuclear fission are taking place. In early days, the fuel integrity failed when 1% of the fissile material was burned up. This made these fuels less suited for burning plutonium, or indeed less suited for any reactor operation, than oxide fuels.

Since 1985 in work at Argonne National Laboratory metal fuels, mixed with some zirconium or other material (corium), have been fabricated. The fuel is fabricated to less than the full density and the fission product gases are produced they can fill the spaces rather than break the fuel apart. With such a fuel 20 of the plutonium in the fuel has been successfully burned. Two advantages accrue from the ability to use fuel made from plutonium metal. The thermal conductivity is greater than that of oxide fuel, enabling reactor safety to be achieved more readily. Secondly it is possible to use pyroprocessing and electrorefining rather than the solvent extraction to remove the plutonium. This forces the plutonium to be extracted along with higher actinides with the same electropotentials, so that the resultant fuel, though still suitable for a fast neutron reactor is unsuited for bomb making without further chemical processing. This opens the possibility of a fuel cycle that is not easily diverted to military uses.

The large operating fast neutron reactors in the world that could be used for plutonium burning in the immediate future include Phoenix and Superphoenix in France (with thermal powers of 563 and 3000 respectively) and at Beloyarsk in Russia. Serious considerations are being given to the use of the French reactors for this purpose (23, 24).

G. Use of Thorium Fuel

When plutonium oxide is mixed with uranium oxide to form MOX fuel, and this fuel is burned in a reactor, more plutonium is inevitably produced by neutron capture in the fertile uranium 238. This replacement is less if the MOX has a higher fraction of plutonium. Some critics of such plutonium burning have argued that this burning plutonium would not be helpful in reducing the total plutonium stocks because of this replacement. But if the plutonium is used to replace uranium 235 in a reactor that would operate anyway, the reduction of plutonium relative to what would otherwise occur is more important. The boundary of the problem should include the electricity generation. More important, perhaps is the replacement plutonium automatically will meet the "spent fuel standard".

Nonetheless one may consider the alternate of using a mixed oxide of thorium oxide and plutonium oxide. Such a cycle has been considered by Veeder and Didsbury (21) for the CANDU reactor (their Table 11). The aim at that time was to find a cycle that is self sustaining or almost so. The non-fissile thorium 232 captures a neutron to become fissile uranium 233, enough to be used as the fuel in subsequent cycles with only 0.5% plutonium make up. Of course it would be possible to use the thorium on a once through system and throw away the uranium 233. If plutonium is free (as would be the case for disposing of weapons plutonium) the fuel plutonium/thorium cycle has an advantage over a uranium 235/thorium cycle. No problems were anticipated with build up of the plutonium isotopes of even atomic weight.

Although it is possible to make a nuclear bomb with uranium 233, it is harder than with plutonium, and it is easy to dilute it with natural uranium before disposal to make it as secure as natural uranium.

There are, however, disadvantages with use of thorium. Although the initial fuel fabrication will not have this disadvantage, uranium 232 (radiothorium) will build up in the reactor by fast neutron (n,2n) reactions on Th232 and U233. This has a 70 year half life and gives rise in turn to a progeny of gamma active daughters, the most noxious being Tl208 (Thorium C") with a penetrating gamma ray of 2.615 Mev. Any recycling would therefore require fuel fabrication a remote facility to avoid the strong gamma ray doses. If the desire is only to address the "clear and present danger" subsequent recycling need not be considered.

There seem to have been no systematic studies of the use of plutonium oxide/thorium oxide mixtures in light water reactors, and this option does not seem to be considered seriously by the political committees discussed below. However it may be attractive both for satisfying the purist who wants to avoid making more plutonium, and also as a first systematic commercial study of a thorium cycle which might eventually point to a nuclear power future for the world with little or no plutonium, and one which might therefore be more publicly acceptable.

V. TOTAL TECHNICAL AVAILABILITY OF REACTORS

If the world were sufficiently concerned about burning up plutonium to cooperate to some extent in the endeavor, perhaps half of the light water reactors in the world would be available to burn MOX fuel, and one quarter available without modification to burn a full core load. This corresponds to 100 Gigawatt electric with partial MOX and 50 Gigawatt electric with 100% core availability. The destruction of the fissile plutonium 239 (Pu239) in a full MOX core is roughly the same as the consumption of U235 in an ordinary reactor. This is about 1 tonne per GW yr. Thus 75 tonnes of weapons Pu could be burned up every year if the problem were faced as a world wide problem. This statement contrasts with the technical reports written for the US Department of Energy (7). DOE have suggested a long time schedule of over 10 years for burning weapons grade plutonium in US light water reactors. The limitation is NOT the existence of enough reactors, but their availability for political reasons and the availability of the facilities for converting the metal in the fuel pits to plutonium oxide and the MOX fabrication facilities.

The problem of fabrication facilities could be technically solved if the USA and USA and Russia were to pay France and Great Britain to stop reprocessing spent nuclear fuel and to fabricate the fuel rods for the weapons fuel disposition. But the USA and Russia have been reluctant to ask for help from the rest of the world in this way. This possibility is implicitly ruled out in the reports of all committees noted above (2-6), indicating that these reports are more political than they are technical. The word "political" here should not be taken in a pejorative context, but is used to describe the fact that many considerations other than the technical possibilities must be considered by the body politic and these other considerations dominate, and perhaps should dominate, the discussion.

VI EFFECTS OF FUEL HANDLING UPON HEALTH

Pu 239 is an alpha emitter and the energy of the alpha particles (about 4 Mev) is similar to a number of other well known and more common alpha emitters such as radon gas. However it decays into U235 which has a long half life, so that, unlike radon gas, it has no daughters in equilibrium. The other isotopes of plutonium in bomb fuel and spent reactor fuel are also alpha emitters with alpha particles of similar energy, so from this point of view the hazards of different isotopes will be similar. But the neutrons and gamma rays from Pu240 and higher isotopes add to the hazard. This is illustrated in table 7 which shows the dose at the surface of a 1 kg sphere of various isotopes.

Table 7. Surface dose rates for 1 kg spheres of pure nuclides (milliSv per hr.)
Isotope X-Rays -Rays Spontaneous Fission Neutrons* % of Isotope in 33 000 Mwd/t PWR Fuel
Pu-238 5.7 x 103 2.4 x 102 640 1.4
Pu-239 8.9 3.2 < 0.01 57.1
Pu-240 72 0.8 300 22.0
Pu-241 - 120 - 13.7
Pu-242 1.3 - 310 5.5
Am-241 4 x 103 2.7 x 104 0.15
* Excludes alpha-neutron reactions which would add to doses from oxide fuels.


There have been extensive studies on the absorption and metabolism of plutonium by humans (25, 26), and by animals, with the latest authoritative review by Clarke, et al. (27). These enable us to use data on cancer induction in animals with some confidence in our prediction in humans. There have been a number of studies on experimental animals, rodents, dogs, and non-human primates. The animal data, for example, show that the incidence of lung tumors is no grater when the plutonium is deposited as discrete particles compared with the situation when plutonium is dispersed uniformly. This is contrary to a speculation (28) often called the "hot particle" theory. The organs of primary interest are the lung, but secondarily the liver, bone, red bone marrow and lymph nodes. For discussions of plutonium burning we are mostly concerned with the occupational hazard, and the routes of administration are inhalation of plutonium dust (from machining for example) or external radiation.

The data on high levels of exposure to plutonium in humans is very sparse, Although many hundreds of tons of plutonium have been processed and even machined, the average intake by workers in western countries seem to have been consistently low. However some large exposures were recorded in the early days of the atomic bomb project (at Mayak) in the USSR (29, 30). At the highest exposures several microCuries of Pu were found in the bones on autopsy. The lung cancer rate seems elevated by 50%. This increase is consistent with what was expected from other data. Other cancers, such as leukemia, were not increased. One leukemia could well have been caused by the concomitant high gamma ray exposure.

If the plutonium is vaporized and breathed in, the alpha emissions can be damaging to the lungs in the same way as alpha emissions of naturally occurring radon gas. But in bulk material the alpha emissions in all but a thousands of an inch of surface can be neglected. If there is a thin covering of a thousandth of an inch, all that remains will be a few easily absorbed X-rays. Thus plutonium 239 itself is not very hazardous in many applications. Indeed, one of the first members of the US Atomic Energy Commission, Professor Robert Backer, personally described to this author how, in 1945, when most of the Los Alamos scientists had returned to their home universities, he undertook the task of deciding which of the pieces of metal in the store room were uranium and which were plutonium. He handled each one personally; those which were warm (due to the radioactive heating) were plutonium. Professor Bacher lived to a ripe old age with no ill effects. Thus the handling of plutonium in fuel fabrication needs few special safety precautions over and above those needed for ordinary industrial procedures and for uranium oxides.

Although plutonium should be handled both on the laboratory and industrially with all the care appropriate to its toxicity, the present knowledge and excellent experience, suggest no reason to consider health effects to be an important consideration in any policy discussion.

VII THE ENVIRONMENTAL ISSUES

In this section are discussed the impact upon the natural environment. separated from this discussion, because of its extreme importance is the discussion of nuclear weapons proliferation and other explosion hazards, and the important but easily handled and historically well handled, matter of health hazards.

Plutonium in monitored storage, particularly as metal, but also as oxide or even as nitrates have relatively little impact upon the environment. The explosion at Kyshtym in the Ural mountains in 1958 was a chemical explosion of an inadequately monitored facility.

Plutonium burning in a reactor will inherently produce a stream of wastes; but this waste stream is similar to the waste stream from burning of uranium fuel. Therefore if existing plutonium replaces uranium in a reactor which would in any case operate to produce electricity there is no net change. But there is a significant reduction in the environmental effects from uranium mining, the isotope separation and the processing to form uranium metal.

VIII THE POLITICAL DISCUSSIONS

Since 1945 there has been no public concern that has been more continuous and more threatening to mankind, than the threat of a major war using nuclear weapons. It is not surprising that there have been disagreements on how to handle the many problems that arise. These disagreements have tended to polarize into two opposite views, neither of which can be proven correct in a rigorous sense, and probably neither is completely correct. On the one hand, there are those who would hanker after a simpler world when mankind had not heard of nuclear fission. To return they would abolish all traces of a nuclear industry. On the other there are those who say that "the genie is out of the bottle" and this cannot be done. They argue further that the benefits of nuclear technology, including nuclear electric power, can help mankind to overcome the poverty, disease and despair that leads to wars and the concomitant desire to make nuclear weapons. It is in the balancing of these views that disagreements, often heated, arise. This section will attempt to put some of the technical facts about plutonium burning into this political perspective.

A. Critical Mass of Plutonium

It is useful to discuss the bare critical mass of a material (31) as the least amount of the pure material which if assembled, will immediately explode. These are also shown in Table 8.

Table 8. Critical Masses of Plutonium and Americium-241
Isotope Bare Critical Mass

kg, -phase

Pu-238 10
Pu-239 10
Pu-240 40
Pu-241 10
Pu-242 100
Am-241 100


It is easily seen that the critical mass is concerned is no barrier to making a bomb with any combination of plutonium isotopes. It is possible to make a bomb with less material than a critical mass, about half, by judicious use of neutron reflectors and by compression. The physical principle is that of the several neutrons produced in fission, at least one must be available to produce another fission. The others may capture without fission on impurities or on isotopes of plutonium. The rapid assembly of the material is achieved either by a gun type system with an assembly velocity of about 30,000 cm/sec (the "thin man" or an explosive compression arrangement with an assembly velocity of about 500,000 cm/sec (the "fat boy").

B. The Preignition Hope

The spontaneous fission of plutonium isotopes especially of Pu240 produces a background of neutrons that can cause preignition if the assembly is not rapid enough. For many years it was therefore a hope, particularly in the civilian sector, that it would not be possible to make a bomb with plutonium with the mixture of isotopes from a power reactor - because of the preignition possibility. This was not a completely unfounded idea. Some early bomb designers insisted upon it as recently as 20 years ago (32) Until about 1990 requirements of military secrecy prevented those who had more recently designed bombs and understood the requirements of bomb making to convincingly explain otherwise. There have since been a number of unclassified discussions that make this point abundantly clear (33, 34). Although the salient facts have been in the public domain a long time and are derivable from the early lectures of Serber (31) which were publicly released many years before the recent publication in book form. It is now pointed out that even the "fizzle yield" that might result from preignition is still a few percent of the maximum yield, and may well be enough for a small country or a terrorist (first three rows of Table 9).

It is widely stated that so long as the fraction of plutonium 239 is greater than 50% it is possible for an expert to make a bomb provided the expert has an adequate staff of up to 4,000 persons!) Table 4 shows that plutonium fuel from a commercial reactor satisfies this requirement. Even if the expertise necessary is greater for reactor grade plutonium as time passes, the expertise is likely to become less rare.

However, bombs in the military arsenals of the bomb countries have relatively small amounts of non fissile plutonium isotopes that for bomb making count as impurities. The prototype was the "fat boy", the first of which was exploded at Alamagordo and the second dropped over Nagasaki. In this a hollow plutonium sphere was compressed rapidly by implosion devices and the critical assembly made.

Table 9. Potential risks for theft, diversion, and retrieval

Plutonium conversion Transit MOX fuel fabrica-tion Transit Reactor Transit Reposi-

tory

Threat

Covert threat (domestic) High Medium High/

Medium

Medium Medium/

low

Low Low
Overt threat (domestic) Medium high Medium Medium high/

medium

Medium high Medium/low Low Low
Diversion (internat'l) High Medium High/

medium

Medium high Medium/

low

Low Low

Criteria 1

Material form High High High/

medium

high

Medium Medium

high/low

Low Low
Environ-ment High Medium High/

medium

Medium

high

Medium

high/

medium

Med-ium Medium/low
Safeguards and security High Medium High/

medium

Medium Low/low Low Low

Criteria 2

Detectabil-ity High High High/

medium

Medium Medium/

low

Low Low
Irreversibil-ity High Medium High/

medium

Medium Medium/

low

Low Low


National Academy of Sciences, Management and Disposition of Excess Weapons Plutonium, National Academy Press, 1994 (2).

C. Heat Generation

An examination of Table 1 shows that Pu238, because of its relativity short half life, produces 540 watts per kilogram of heat. Two percent of Pu238 in a plutonium assembly of 10 kg critical mass would then produce 108 watts.

Table 4 shows that spent fuel from a PWR at 43,000 Mwd/t burns up or from a BWR at less than 30,000 Mwd/t, contains 2% of Pu238. Such a high burn up in a commercial reactor is no longer unusual (see Figure 5), which shows the distribution of burn up from French reactors.

This heat generation is not important for fuel handling but makes it harder to make a bomb, although how much harder is a matter of dispute. It has been said that none of the designs in the arsenals of the weapons countries would work with this much Pu238; either the assembly itself would melt or the high explosive trigger would self destruct. On the other hand, one scientist has said, "It would not be my preference, of course, but I would have no difficulty in making a highly reliable weapon with 100 watts of decay heat." Even if it is only somewhat harder, this leads to a suggestion that further protection against misuse of plutonium for making bombs can be provided by increasing the Pu238 content of the spent fuel. A simple method might be to add a small amount (0.03% of total uranium content) of Am241 to the fuel (either original uranium fuel or MOX). The Am241 would become Pu238 by the chain Am241 + n Am242 ( decay 16h) Cm242 ( decay 163 days) Pu238 and increase the Pu238 content of the fuel by about 1%. Although Am241 is not available in weapons grade plutonium, it is present in reprocessed aged reactor fuel.

FIGURE 5 HERE

D. The Present Accepted Explosive Hazards

Mark (33) states:

(1) Reactor-grade plutonium with any level of radiation is a potentially explosive material.

(2) The difficulties of developing an effective design of the most straightforward type are not appreciably greater with reactor-grade plutonium than those that have to be met for the use of weapons-grade plutonium.

(3) The hazards of handling reactor grade plutonium though somewhat greater than those associated with weapons-grade plutonium, are of the same type and can be met by applying the same precautions. This, at least would be the case when fabricating only a modest number of devices. For a project requiring an assembly line type of operation, more provisions for remote handling procedures for some stages of the work might be required than would be necessary for handling weapons-grade material or for handling a limited number of items.

(4) The need for safeguards to protect against the diversion and misuse of (chemically) separated plutonium applies essentially equally to all grades of plutonium.

Mark's conclusions (33) have been accepted by all the committees on plutonium disposition refereed to earlier (2-6). Unless there is convincing evidence to the contrary, the authority of the author who used to be head of the theoretical physics division at the bomb designing laboratory, Los Alamos Scientific Laboratory, suggests that this should be the basis for future policy about plutonium (33). The committees argue the technical distinction between security (against theft) of reactor grade chemically pure plutonium and bomb grade chemically pure plutonium, is a small one compared with the security that dilution with radioactive isotopes provides. This changed perspective and realization has profound implications for the nuclear industry which are not fully appreciated by all countries and by all people in the industry in the United States. However, Jones (35) among others have emphasized that a bomb is not easy to make.

(A) For each transuranic element a criterion for its proliferation potential cannot be established just by calculating its properties as a fissile core; it must be based on the likelihood of the applied sciences achieving the complete explosive system.

(B) Open discussion of the complexities of design and applied science which achieve such systems is prohibitively proliferative and rarely appreciated by the predominantly non-technical policy makers; criteria which are more apparent and widely understood are needed.

(C) The achievement to be sought by a threshold National effort and that by a terrorist organization are different and must be addressed distinctively.

The distinctions are important for decisions involving countries (35):

(a.1) None of the 5 major NWS, despite anything the later ones learned from the earlier ones, built either their initial or follow-on capabilities on anything other than high purity U235 or Pu239.

(a.2) This is also true for threshold states where we know their design intent.

E. Proliferation Hazards - Terrorist or other Theft

In this technical discussion it is not sensible or possible to make definitive statements about security of facilities against diversion to undesired or unauthorized uses or military purposes (bomb making). However, it is useful to outline the parts of the fuel chain which are most susceptible to such diversion. It is also useful to distinguish diversion of material by a sovereign country with presumed approval of its own government but in disregard of the wishes of other nations, and diversion of material by a terrorist group which is also in disregard of the wishes also of its own country. The political actions and the technical aids to these actions are somewhat different. The terrorist is searching for a short term advantage by holding society at ransom; the rogue country is usually searching for a longer term advantage.

The locations of the fuel cycle where chemically pure plutonium is available and therefor where theft would be most effective are:

Storage of weapons

Storage of weapons pits including those not used

Transportation of pure plutonium to fuel fabrication site

Fuel fabrication plant

Transportation of fuel assemblies

Storage and loading of assemblies at the reactor site

It seems clear that safeguarding the material in the burning process is simpler and can be more effectively realized than is the case with the existing handling of plutonium for bombs. For example, although fuel assemblies can be broken open and the MOX chemically treated with greater ease than spent fuel, it is more difficult than with the pure metal of the weapons pits.

Once the fuel assemblies are loaded into a reactor, they are hard to remove, and after even 24 hours of operation, their radioactivity makes them even harder to handle. Therefore the reactor would only need increased security, (increased over the security needed for ordinary uranium operation) if at all, during fuel loading. This, however, does not apply to CANDU or RBMK reactors that are continuously fuelled.

The transport of the mixed oxide fuel, and its safeguarding before insertion into the reactor are of secondary importance. Once within a reactor the fuel is hard to remove, and when it has been irradiated it will be mixed with fission products and hard for any clandestine operation to handle. As one expert put it, "Inside a reactor is the safest place for the plutonium" (36).

Then we are left with the sensitive location of the fuel fabrication plant that needs continuous security, but no more than existing military complexes for weapons manufacture, and the initial transport which is similar to what has been handled routinely by the military for many years.

A layout of the various security concerns is shown in Figure 5, taken from Greene et al. volume 2 (22) for the disposal of weapons plutonium in a CANDU reactor.

figure 6 here

(i) Third world country actions

It is obvious that a country can muster more effort to create nuclear weapons than can a terrorist group. It is assumed here that a country will not normally steal plutonium from another country, and other countries are sufficiently concerned that they will not sell weapons usable plutonium. But a country could take steps to acquire the material, and the expertise to use it, themselves, although if they do so the protection against terrorism discussed in the previous section would apply. If they do resort to terrorist activity it has in the past been by sponsoring small terrorist groups, rather than utilizing the full available resources of the country.

The United States built a facility for making plutonium and produced enough for the first two bombs in about 2 years without any previous knowledge and experience. Now that the knowledge and experience is in the public domain, it is clear that any industrial country could make plutonium in this short a time unaided from outside. A less industrialized country might take longer. Thus it has been widely assumed, following President Eisenhower's Atoms For Peace declaration, that the primary attempt to control and limit nuclear weapons must be political. The discussion here is about the technical factors that can influence this political discussion one way or another. It is readily apparent that the main effect of administrative deterrents, such as export controls or embargoes, is delay. While stating this, the role of delay in human affairs must be fully recognized. It allows time for hot heads to cool; for negotiators to act; for scientists, engineers, technicians, generals and politicians to ponder and reconsider their actions. The disagreements arise because of different perceptions on the way an action by one country is perceived by others.

F. The Setting of an Example

(i) Atoms for Peace

The choice of technology by a country can be used to set an example to another country. The declared aim of the Atoms for Peace declaration of President Eisenhower, was to set the example that the United States proposed to use nuclear technology for peaceful purposes and wished to encourage others to do so.

As noted in the introduction, the nuclear industry in the USA in 1975 was pursuing a dream of unlimited nuclear energy for mankind, fueled by conversion of the non-fissile uranium 238 to the fissile plutonium 239. This was being done with urgency, to head off what was perceived to be an impending world energy crisis with its possible outcome of a world (nuclear) war. President Kennedy added to the Eisenhower policy with a statement about the need for cheap energy. The unintended example that was being set was that every country should separate plutonium (reprocess the spent nuclear fuel) and have a breeder reactor. Contracts were signed for Germany to sell a reprocessing plant to Brazil (which only had one operating reactor) and for France to sell a reprocessing plant to Pakistan (which also had only one operating reactor).

(ii) The reprocessing ban

Unlimited expansion of reprocessing was realized to be "incredibly dangerous" as it would put the technology for nuclear weapons material in the hands of many countries (37). The Carter administration decision to stop reprocessing was to set an example to the rest of the world that the US did not believe this to be necessary. This decision was continued by the administrations subsequent to the Carter administration, and has been reemphasized by the Clinton administration. Indeed the increased availability of oil and gas, and the ability of countries to use energy more efficiently in response to price increases, partially justify this decision. However, other countries did not accept the Carter example, as shown by the International Fuel Cycle Evaluation begun on his initiative (38). An interesting historical perspective on the decision to ban reprocessing has been made by Rossin (19). The important issue for the present day is whether it should be a permanent decision or a whether a more appropriate modification might be to study intensely ways in which the advantages of nuclear power can be achieved without proliferation of nuclear weapons, and to modify reprocessing appropriately.

(iii) The present conflict

The conflict between these two examples is at the root of much of the debate today including that on whether to burn plutonium or to bury it. While the debate rages, a thoughtful approach is to use time gained by delay and by the increased availability of fuels to study carefully the technical and political considerations that can reduce the dangers. Alas, the most usual resolution of such issues is to do nothing. The burning of weapons plutonium in a reactor is strongly opposed in some quarters just because it implies (sets an example) that burning of plutonium is always "acceptable". Following such an example would be a justification for further "reprocessing" and separation of chemically pure plutonium. Using the European or Japanese reactors for burning plutonium, while the quickest means of accomplishing the task of burning weapons plutonium, would send a signal (undesired by this group of people) that the European and Japanese approach (which more closely resembles the Eisenhower approach than the Carter one) has merit. But the converse could also be true. A refusal to ask the rest of the world to help in burning plutonium can be, and in some quarters is taken as a signal that the United States is not serious about destroying weapons stocks.

(iv) International non-proliferation goals.

It is important, even vital, to realize that the motives of the United States and Russia (and to a lesser extent the other nuclear weapons states) are not necessarily considered benign by the third world countries who have not (yet) made nuclear weapons and hopefully never will. The committees above (2-7) were committees dominated by the United States although that of Seaborg et al. (3) had 50% representation from overseas. It is important to ask for and consider the opinions of those in the third world countries. In this there may well be an important distinction between weapons grade and reactor grade plutonium that is not usually discussed although it was implied in Seaborg et al., (3). The weapons states have only used weapons grade plutonium in their arsenals, shunning the reactor grade material. Indeed, it has been suggested that North Korea was also proceeding on this route. It is likely that any country would proceed in this way. The technical reasons include:

greater assurance of no preignition,

less heat from other isotopes, particularly Pu238,

fewer gamma rays and consequent greater ease of handling

It is unlikely that Russia and USA would ever use reactor grade plutonium for their nuclear arsenals in the future. Therefore the conversion of weapons grade material to reactor garde would accomplish a task that is politically significant to the rest of the world: it would represent a clear open public demonstration that the fuel would never again be used for weapons, and that Russia and the USA are truly beginning to meet their responsibilities under article VI of the Nuclear Non-Proliferation Treaty (NPT).

G. Energy Scenarios

It is dangerous to make predictions - especially for the future. Nevertheless before 1975 it was common to make long term energy forecasts. Traditionally (over the last two centuries) they forecast an "imminent" energy crisis for the world. The world wide scare of oil shortage in 1973 emphasized this. But the increased availability of oil and natural gas since 1980 has made this unpopular. It is instructive to compare a typical energy forecast of the 1960s with a typical forecast of the 1990s. In 1960 (with President Kennedy's policy of cheap energy) analysts projected an expansion of energy use per capita that would be 40 Kw per capita - four times what actually occurred. This was to be met by a rapid expansion of nuclear energy. Attempts were to be made by the developed world to raise the living standards of the developing (third) world to closer to US standards with an even greater increase in fuel use. Today it is politically correct to project a declining fuel use per capita, although there is no clear political path to achieve it since incentives to save (such as a carbon tax) are politically rejected. Projections that China for example will burn more coal in the next century than all OECD countries are met with alarm rather than as a world challenge on how to help.

Since the formation of the United Nations in 1945 the world has taken seriously the task of reducing disease in the whole world and of feeding the whole world. In that task energy projections have their place, and neither of the two extremes of the preceding paragraph is acceptable. The twin environmental issues of the decade: the effects of air pollution upon health (39) and of the potential for global warming (40, 41) might change this. Moreover it is still important for mankind to prepare options for the future. Is Fermi's dream for nuclear power, probably considerably modified, still a reasonable option? Does such an option include the use of plutonium?

A. Energy Scenarios for the Future

(i) Maintaining the nuclear option

Before 1939 there were three known sources of uranium ore in the world. The Joachimstal mine in Czechoslovakia, the mines of Union Miniere de Haute Katanga in the Congo (now Zaire) and the Eldorado mine in Canada. Although military needs stimulated world wide exploration, only ore with 1% uranium was used. Now ores with 0.03% uranium are economically mined and this economically available fraction may well increase. Moreover world wide resources have increased. Therefore there seems less immediate need to unlock the energy in uranium 238, and the desire for a breeder reactor, with a dependence upon plutonium, has receded.

There has been little public discussion, but many public declarations on this subject. In the United States there seems a general consensus that although 22% of all electricity is from nuclear power, nuclear power is on the decline and there is no political will to change this. The considerations that involve plutonium include the following:

The present use of nuclear power using uranium fuels inevitably produces plutonium. Therefore the issue of plutonium management must be faced. What is the best way to manage the material, which now comprises 1000 tons world wide? Burn it? Bury it? Dilute it? Can the procedure for doing any of these be made acceptable? Can one change to a fuel cycle using thorium and will this remove the technical and political obstacles?

(ii) Timing of a breeder reactor need

As noted in the Introduction, energy projections before 1975 suggested that a breeder reactor would be needed very soon to satisfy energy demands. Energy demand has flattened off considerably so that the need for an operating breeder reactor program is not needed, if it is needed at all, till the middle of the next century. This implies that the world need do nothing at this time. But a countervailing argument is that the technical and political problems of a fast neutron reactor have been shown to be more difficult than previously thought, so that more research and development is needed. This suggests that research and development should continue with a modified aim of resolving these problems. This has been particularly argued by Davis (42).

(iii) Ultimate plutonium destruction

One might, (hopefully) imagine a world in which plentiful and affordable (sustainable) non nuclear fuels have been found, and where all countries have decided to destroy all nuclear weapons. It might then be thought desirable to actually destroy all appreciable concentrations of plutonium to ensure the stability of such a world. This would be for psychological reasons. In this endeavor there exists no other way of destroying plutonium than fissioning in a reactor or particle accelerator.

Of course man could always make more. It took only 3 years to make the plutonium for the first atomic bomb starting from scratch, with little information on how to proceed. Now the knowledge exists, this time could be shorter in the future. Knowledge, once acquired by mankind has rarely been lost. Therefore If mankind continues to develop, knowledge, once gained, will not be lost. Therefore a policy of complete destruction of plutonium, to be effective (i.e. to achieve more than a superficial psychological gain) would have to be accompanied by a ban on making new plutonium and a detailed watch on any attempts to make new plutonium. Many people believe that if a nuclear power industry ceases to exist, this watchfulness might well be harder than with an industry with an understanding and an economic incentive to prevent misuse.

But this ultimate burning could not be done in a thermal neutron reactor. As noted above, when fuel is recycled through the reactor several times, the plutonium 239 and plutonium 241 produced will undergo fission. The plutonium 240 and other even isotopes are non fissile. Therefore the isotopic ratio will change and consist more and more of the non fissile isotopes as recycling proceeds several times. Eventually the buildup of the non fissile isotopes will poison the reactor (by introducing neutron absorbing but non fissile material).

An examination of Figure 1 and Table 3 shows that this destruction could be done in a fast neutron reactor since for these even isotopes are fissile with fast neutrons. In the commercial nuclear industry, for example, a long term option has always been to use a fast neutron reactor after 20 years or so of recycling in a thermal reactor. Such a program would allow for the ultimate destruction of plutonium. Alternatively, if costs are unimportant or the cost estimates for such a procedure come down, this could be arranged in an accelerator driven subcritical assembly. Indeed an accelerator driven assembly has an advantage that some non fissile elements are reduced by spallation with neutrons of 20 Mev or more energy than is available from fission neutrons. While accelerator driven assemblies were originally discussed as a means of breeding fissile material from non fissile thorium or uranium 238 (43), more recent studies have focussed on alternate fuel cycles and the use of an accelerator as a plutonium and actinide burner (44-47).

A complete plan for either for use of nuclear energy or for abandonment thereof that is valid over the centuries and millenia must therefore include some policy and procedures for controlling or elimination of plutonium and msot important, it must include procedures for preventing or controlling facilities for producing plutonium.

XI ACKNOWLEDGMENTS

The author acknowledges many helpful comments on the manuscript from Drs. J. Landis, C. Till, and Ambassador R. Kennedy, and general discussions with Dr. R. Garwin and Ambassador Imai.

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FIGURE CAPTIONS

Figure 1. Fission yield versus mass number for (a) 233U and 239Pu and (b) 14 MeV and thermal 235U.

Figure 2. The neutron cross-section for fission of the principal plutonium and uranium isotopes (and americium-241, a decay product of Pu-241) against neutron energy.

Figure 3. Origin of delayed neutrons from Br87.

Figure 4. Decay modes of transuranic elements.

Figure 5. The distribution of "burn up" of 900 MWe fuel assemblies (from Electricite de France).

Figure 6.









1.