Burning Waste Minor Actinides and Plutonium

The word actinides refers to a family of heavy metals, some of which are fissionable, while others are fertile. Of the actinides only uranium and thorium occur in large amounts in nature. Both Uranium and Thorium undergo nuclear decay. U-238 and Th-232 decay slowly, and are present in the Earth's crust in fairly significant amounts. Neutron absorption by U-238 produces Plutonium 239 a fissionable isotope. Neutron absorption by Th-232 produces U-233 a fissionable isotope of Uranium.

Pu-239 is viewed as a desirable weapon constituent, while the military value of U-233 is problematic due to hard radiation from a very contaminant product associated with U-233 production in reactors. Only one U-233 weapons test was ever conducted by the United States, and that was not considered successful. In addition any deliverable U-233 weapon would require heavy shielding, making it extremely awkward for use by terrorists or by a military organization. Nations wishing to acquire nuclear weapons for military purposes have inevitably preferred Pu-239 or U-235 over U-233 although U-233 is quite easy to manufacture in low cost and technically unchallenging to build reactors. Pu-239 can be manufactured in such reactors and has always been preferred by weapons developers.

In the "Military Effects Test" of April 15, 1955 Weapons designers reportedly found that by substituting U-233 for U-235 in a standard test weapon, they lowered the weapon yield from 31 kt to 22 kt, a yield reduction of nearly a third.

A secret 1963 document, titled U-233 and prepared by Hanford weapons designer, A.E. Smith, set out Smith's concerns about U-233 weaponization. Smith indicated that in order to produce U-233 with low levels of U-232 contamination, a weapons material productions reactor at Hanford or Sevannah River would require three years. A test using high U-232 contaminated U-233 could be conducted more quickly, but it would involve problems that were not known, but which would probably involve "real sacrifices and risks to overcome." People assigned the task of purifying the U-233 would "take high radiation exposures." The U-233 would have to be purified to remove U-232 daughters, followed by rapid processing and assembly. A U-233 bomb test itself would cost between $7.5 and $15 million in 2011. Expected shelf life of a U-233 weapon would be 4 years, but disassemble and refurbishment would be more complicated and difficult than with a plutonium weapon.

Nations wishing to produce U-233 based nuclear weapons, would already have the resources to produce Pu-239 weapons, or would be capable of developing technology to produce U-235 weapons. U-235 or Pu-239 weapons would have superior military characteristics relative to U-233 based weapons. The potential militarization of U-233 seems unlikely to increase the risk of nuclear proliferation by nations, while the expenses, risks, costs and unknown challenges of militarizing U-233 would prove daunting to a terrorist organization.

U-235 was developed as a weapons material at the same time Pu-239 was. U-235 is extracted from Raw uranium which contains 99.3% U-238 and 0.7% U-235. It is possible to design quite simple weapon with U-235, and if a nation can design a low cost U-235 separation technology, U-235 is the royal road to nuclear power. South Africa proved, during the 1960's, 70's and 8Os's that developing a low cost U-235 separation technology was a relatively insignificant challenge for even a small country. Pakistan, a larger country, but one with even fewer industrial resources than South Africa, also developed U-235 separation technology at about the same time. Both nations used information that was readily available in other countries, as well as parts clandestinely obtained from other countries. Libya also undertook to obtain U-235 separation technology, and Iran has had developed a successful Uranium enrichment program prior to the appearance of the Stuxnet virus.

It should be assumed then that if a nation possesses a low cost, easily mastered route to nuclear weapons, the acquisition of a second, higher cost, less easily mastered route to nuclear weapons, will not increase the likelihood of nuclear proliferation. And given the choices of two routes, on difficult and one easy, nations can be expected to chose the easy cheap route over the more difficult and expensive path.

In addition to other fissionable materials, fairly small amounts of neptunium and americium can at least in theory can be used to power nuclear explosive devices according to the DoD. Reference to nuclear explosive devices rather than nuclear weapons suggest that the path to use of these materials in nuclear weapons.

Np-237 is produced by a double neutron capture conversion process which converts U-235 into Np-237. About 19% of U-235 fails to fission after a slow neutron capture, thus conversion of U-235 to NP-237 is fairly common. 19% of U-235 slow neutron capture leads to conversion rather than fission. In addition Np-237 is also produced via an (n,2n) reaction with U-238 after it encounters a high energy neutron. Np-237 is the product of alpha decay of Am-241.

In theory about 10 times as much Np-237 will be produced in a Uranium fuel cycle reactor than in a Thorium fuel cycle reactor. However, when NP-237 remains in a reactor for a significant period of time, it undergoes conversion to Pu-238 by the further capture of another neutron.

All uranium cycle reactors are capable of producing large amounts of plutonium will also produce Np-237. The ratio of plutonium to neptunium will be very high, over 20 to one, but enough Np-237 will be left in the nuclear waste to build a nuclear device every couple of years, if the nation who possess the nuclear waste. However, no nation is known to have built an Np-237 weapon, although the French reportedly may have done some research in that direction. However, no data has been published on Np-237 explosive devices or weapons designs, leaving the would be weapons designers to guess. Different statements on how much Np-237 is required to produce a nuclear explosion, with 60 kg asserted for a gun type device, and perhaps 30 kg for an implosion type device.

David Albright and Kimberly Kramer state,

Thus, large quantities of neptunium 237 are found in spent nuclear fuel. Each year, a typical 1,000-megawatt-electric light-water reactor produces about 25 tonnes of spent fuel containing about 10 kilograms of neptunium 237. The same spent fuel contains about 230 kilograms of plutonium. By weight, neptunium 237 discharges are about five percent of plutonium discharges and about 0.05 percent of spent-fuel discharges.

It should be noted that both uranium fuel cycle and thorium fuel cycle MSRs produce some Np-237, with LFTRs can be expected to produce less than 10% of the Np-237 produced by conventional LWRs.

David Albright and Lauren Barbour note that as a nuclear explosive, americium would be considerably more problematic than Np-237.

The three most important isotopes are americium 241, americium 242m, and americium 243. All three have bare-sphere critical masses, but they vary widely and are uncertain. Americium 241 has a bare-sphere critical mass between about 60 to 100 kilograms. Americium 242m has the lowest bare-sphere critical mass, about 9 to 18 kilograms. The bare-sphere critical mass of americium 243 is somewhere between about 50 and 150 kilograms, indicating that public estimates of its critical mass vary enormously.

Despite its explosive capacity. Americium has civilian uses, including in smoke detectors, as medical diagnostic tracers, and in neutron sources. In all of these uses very small amounts of americium are used. It would appear that would be proliferators who might chose americium weapons would have strong motives for preferring U-235 or Pu-239 weapons. Any state which possess the capacity to manufacture americium would also possess the capacity to produce weapons grade Pu-239. Weapons grade Pu-239 is far more predictable that americium, and thus far more desirable for weapons purposes.

Finally, in addition to Np-237 and americium, Protactinium-231 might also be mentioned as a further fissionable minor actinides. Pa-231 is very rare and can occur naturally as a U238 or U-235 decay product. Pa.231 is the longest lived Protactinium isotope, having a half life of 32,000 years. Small amounts of Pa-231 as well as Pa-233 would be produced in thorium cycle reactors. Even less is known about the explosive and military value of Pa-231, than is known about americium, and thus any weapons designer would be left to guess amount important details such as critical mass, and some authorities appear to deny that any amount of Pa-231 can explode. Once again, any state capable of building a Pa-231 weapon would also be capable of producing a Pu-239 weapon, and any sane weapons designer would prefer to use Pu-239.

It would be highly desirable to dispose of actinides in fast neutron reactors, and liquid sodium cooled reactors, either burners or breeders, would be very serviceable for this task, although fast MSRs would be equally serviceable, and might offer some advantages. Even thermal MSRs can be used for actinide disposal, although less energy would be extracted in the process than would be the case with fast reactors and other aspects of the process might turn out to be less efficient.

The Molten Salt Reactor Family: Fuel

I intend to offer a series of posts designed to explain the sometimes bewildering complexity of Molten Salt Reactor Technology. This first post explains two nuclear fuel breeding cycles.

Rather than offering a single potential reactor design, the Molten Salt Reactor (MSR) idea offers a large number of design options, each of which would require a significant amount of research, before a prototype reactor could be built. The Molten Salt Reactor designer is faced with a bewildering number of elective choices, each offering a set of advantages and disadvantages. Each choice that the designer makes will dictate a number of design features some of which require further choices.

Lets start with nuclear fuel. My father first demonstrated that not only U-235 but also Pu-239 could be used as a reactor fuel in MSRs. During the ORNL Molten Salt Reactor experiment Oak Ridge scientists tested the use of the three fissionable materials that can be used as nuclear fuels, Plutonium-239 (Pu-239), Uranium-235 (U-235) and Uranium-233 (U-233). Once during the operation of the Molten Salt Reactor Experiment (MSRE) they used all three potential fuels in the reactor at the same time.

Of the three potential fuels, U-233 had some significant advantages. Neither U-235 nor Pu-239 produced enough neutrons per neutron hit, to support breeding more nuclear fuel at a slow (thermal) neutron speed range. U-233, produced by breeding thorium did produce enough neutrons to breed thorium at a slow temperature range. We will see that this offers a very large advantage. U-235 is not efficiently produced by breeding, while Pu-239 can only be produced in the breeding range with fast neutrons.

Breeding means that for every fuel atom used in the nuclear process, at least one new fuel atom is produced. Thus in a plutonium fast breeder, if a neutron strikes a plutonium atom, it is very likely to fission into two smaller atoms, almost always with three neutrons left over. Those neutrons will be moving fast and will contain a lot of energy. Fast neutrons are more likely to produce fission in plutonium atoms than slow neutrons. Neither U-235 nor Pu-239 produce enough neutrons to maintain breeding if they encounter a slow (also called thermal) neutron. Thus Plutonium can only be produced as a nuclear fuel in so called fast reactors. There are, as we shall see, some major disadvantages to fast reactors.

Fast reactors are often thought of as having liquid sodium as their coolant, although liquid lead, and a liquid lead-bismuth mixture have also been used as a coolant in fast reactors. In addition it is possible to build fast Molten Salt Reactors. The stability of Molten Salt Reactor operations in enhanced by Xenon-135 removal. Xenon-135 is a radioactive gas that is a byproduct of nuclear fission and has a very large neutron cross section. Because it is very likely to capture neutrons, Xenon-135 can adversely effect a chain reactor in a reactor. Thus it would be highly desirable to get Xenon-135 out of a reactor core quickly after it is produced. That is impossible in a solid core reactor, but it is not difficult to do in a Molten Salt Reactor. The presence of Xenon-135 adversely effects to the ability of reactors to breed nuclear fuel, so any MSR that is designed as a thorium breeder would have a system for moving Xenon-135 out of its core.

There are decided advantages for fuel reprocessing with MSRs. Compare the fuel reprocessing technique for a Molten Salt Reactor with the fuel reprocessing technique proposed for the Integral Fast Reactor (IFR) a LMFBR. In two fluid MSR, the blanket salt flows out of the blanket, and protactinium and U-233 are withdrawn from it by chemical processes. Once they are processed out of the carrier salt, the U-233 is re-fluoridated and returned to the core. The protactinium is set aside until it undergoes a nuclear transformation to U-233, and then that U-233 is returned to the core. In a IFR, the spent fuel is fished out of the reactor core, and once recovered, dumped into a molten salt bath, in which it dissolves. Then by use of electroplating, various material from the old fuel, for example plutonium, are separated out of the bath, and deposited on electrodes. Eventually the separated metal, is recovered, melted and mixed into an alloy, which is then cooled enough to serve as fuel elements, and then returned into the reactor. The MSR fuel reprocessing technology is much simpler than the fuel reprocessing technology designed for the IFR.

In addition fast reactors require 10 times as much nuclear fuel to produce a chain reaction as thermal breeder reactors. It does not really matter if the fast reactor is cooled by liquid metal of liquid salts, a fast breeder reactor just needs a who lot more fuel in order to operate than a thermal breeder reactor does. This makes fast reactors poor candidates to replace fossil fuels like coal with nuclear power, because many reactors will have to be built quickly, and fueling enough fast reactors quickly will be a big challenge.

There are two breeding cycles, the Uranium 238 breeding cucle, and the thorium 232 breeding cycle. Both cycles have some advantage. Plutonium-239 produces more neutrons per fission event than thorium, but fewer fission events per neutron in the thermal spectrum. In fact Pu-239 produces so many fewer fission events in the thermal spectrum than in the fast spectrum, that it is impossible to achieve a positive breeding ratio for the U-238/Pu-239 breeding cycle in a thermal reactor. On the other nand U-233 produces about as many neutrons per fission event in the thermal range as in the fast range, and about as many fission events. That means that the Th-232/U-233 breeding cycle is as effective in the thermal range asin the fast range, and because thorium breeding only requires about 10% of the nuclear fuel in the thermal range as U-238 breeding requires in the fast range, thorium breeding cycle reactors can be deployed far faster.

In addition Liquid fuel reactors have advantages over solid fuel reactors. Once a sollid fuel is inserted into a reactor it almost always stayes there for a year or more, while fission products build up in the fuel. We have already seen that Xenon-135 becomes a reactor control problem, although Xenon-135 eventually reaches an equalibrium because of its short half-life. The presence of Xenon-135 in a nuclear core, can interfear with a reactor's capacity to bread, especially in the thermal breeding range. Thus Thorium fuel cycle breeder reactor are better candidates for rapid deployment than U-238 fuel cycle breeders, and liquid fuel thorium breeding reactors have advantages as solid fuel thorium breeder. Liquid fueled thorium breeders, as we have already noted, have advantages over solid fuel U-238 breeders. Thus the Thorium fuel cycle Molten Salt Reactor (often called the LFTR) would seem to offer several advantages over U-238 fuel cycle liquid metal fast reactor.

In the next post of this series I intend to explain the difference between single fluid and twoi fluid Molten Salt Reactors.

Understanding Molten Salt Reactors: 1. How are MSRs Different from LFTRs?

The name Molten Salt Reactor (MSR) is more inclusive than the name Liquid Fluoride Thorium Reactor (LFTR). The LFTR is a type of Molten Salt Reactor that features the use fluoride salts and a thorium fuel cycle. Strictly speaking a LFTR need not be designed to produce fuel in a breeding range, but breeding is an important justification for the use of the thorium rather than the uranium fuel cycle.

In chemistry a salt is an ionic compound that is produced when acid undergoes a neutralizing reaction with a base. Some salts are compounds of metallic and non-metallic elements. Salts become molten (or liquid) when heated. Different salts have very different melting temperatures. Some salts may melt at relatively low temperatures, while other salts require several hundred degrees centigrade of heat before they melt. Mixed combinations of salts may melt at a lower temperature than individual salts will melt.

Some molten salts are very good conductors of heat, and some molten salts may not boil until they reach 1400 degrees centigrade, or even higher. These qualities make molten salts potentially excellent coolants for high temperature reactors. Research on high temperature fluoride salt cooled reactors continues at Oak Ridge National Laboratory (ORNL).

Two families of salts have been identified as potentially excellent reactor coolants. They are Fluoride and Chloride salts. Of these two salt families, ORNL scientists quickly chose the former, as presenting fewer developmental challenges while offering greater opportunities for commercial reactor use. Liquid chloride salts were identified as presenting more problems for reactor developers. Chlorine was, in particular, less useful as a neutron moderator than fluorine. However, Chloride salts were suitable for fast reactors, and a Molten Chloride Fast Breeder Reactor was both possible, and probably technologically less challenging than the Liquid Metal Fast Breeder Reactor, as a uranium fuel cycle breeder. In addition, the MCFBR would have offered far fewer safety problems than the LMFBR, while offering a route to technologically superior and lower cost fuel reprocessing.

ORNL Scientists however, also noted that liquid fluoride salt offered a superior performance precisely because of their neutron moderating performance. Moderation removes energy from neutrons. Heavily moderated neutrons are called thermal neutrons, and reactors that are built with large amounts of neutrons moderating materials such as graphite or heavy water are called thermal reactors. Neutron moderation decreases the amount of fissionable material required to sustain a chain reaction, and graphite and heavy water moderated reactors can produce chain reactions with natural uranium. While graphite and heavy water moderated reactors are useful tools for the production of plutonium, they are not particularly efficient tools for burning Pu-239 or even U-235 as nuclear fuels. However, it is possible to breed thorium in thermal reactors. Thermal reactor breeding of thorium offers significant advantages over fast reactor breeding of U-238. Thermal Reactors can operate with 10% perhaps as little as 3% of the fissionable material required to operate fast reactors. Thus thermal reactors can potentially be started at a far more rapid rate than fast reactors.

Fast reactor advocates claim that it is possible for fast reactors to breed at a much more rapid rate, but there appear to be safety problems involved in more rapid fast reactor breeding, and current documented advanced fast reactor designs appear to breed at a similar rate to thermal thorium breeding molten salt reactors.

The term breeding refers to the production more nuclear fuel than is used in a nuclear reactor. If a fertile material, either uranium 238 or thorium 232, is included with the fissionable material placed inside a reactor core along, the result will almost inevitably be the conversion of some of that fertile material into a fissionable material. In the case of thorium, a thorium 232 atom inside the reactor can absorb a neutron, and then becomes a thorium 233 atom. The nucleus, that is the center, of a thorium 233 atom is unstable. A neutron in the unstable Th 233 atom will eventually emit an electron, changing the neutron to a proton, the new proton in turn converts the atom to protactinium 233, and Pa 233 is also unstable. After a few days, a Pa neutron emits an electron, and as a consequence converts to a proton. The added proton makes the atom uranium 233. U 233 is fissionable, and can be used as reactor fuel.

The uranium fuel cycle is similar. If an U-238 atom absorbs a neutron a process that is similar to the process we find with thorium 233 occurs, and the U-239 atom is converted into plutonium 239.

Most reactors produce added nuclear fuel by converting U-238 into Pu-239. The amount of plutonium produced usually equals somewhere in the neighborhood of 60% of the amount of U-235 burned in a conventional reactor. Neither U-235 nor Pu-239 are ideal nuclear fuels at conventional neutron speed. WASH-1097 states

From a nuclear standpoint, the use of U-233 in a thermal reactor makes it possible to achieve higher fuel conversion ratios and longer fuel burnups than is practical with either U-235 or Pu-239. . .

The higher conversion ratios which can be obtained in thermal-spectrum reactors when using U- 233 instead of Pu-239 can result in a significantly better utilization of natural uranium fuel resources with thorium-fueled reactors than with the low-enrichment, light-water cooled uranium-fueled reactors . . .

WASH-1097 defines the fuel conversion ratio,

The fuel conversion ratio (CR) is the ratio of the amount of fissile fuel produced per unit of fissile fuel destroyed. . .

Breeding takes place when the conversion ration is greater than 1 to 1. While plutonium theoretically produces more neutrons and therefore faster breeding in fast reactors,

A higher breeding ratio can be obtained with Pu-239 than with U-233 in a very high-energy, fast- neutron spectrum reactor. On the other hand, in a degraded (10 to 100 keV) fast spectrum, U-233 would probably be as good as, or better than, Pu-239. Also, the variation of U-233 and Pu-239 cross sections with energy are such that improved reactivity coefficients would be obtained with the use of U-233 in a large sodium-cooled FBR. This leads to improved nuclear safety characteristics. . . .

The energy dependence of the fast-fission cross sections of Th-232 and U-238 is such that the use of Th-232 would produce an improved reactivity coefficient in a liquid-metal-cooled FBR. The fast fission cross-section of Th-232 is much lower than that of U-238 so that use of the latter leads to much larger conversion ratios in fast-spectrum reactors.

Thus not only is thorium breeding attractive in molten salt reactors, thorium breeding in fast reactors enhances their safety, and increases their conversion ratios. This point has not been lost on Indian reactor scientists who plan large scale production of thorium-uranium fast reactor breeding hybrids. Breeding thorium as a nuclear fuel in fast reactors would produce a large amount of fissionable U-233 that can be used as nuclear fuel in conventional reactors. This point has not been lost on Indian nuclear scientists, who plan to use the extra U-233 in Advanced Heavy Water Reactors. U-233 fueled non-breeder or converter Molten Salt Reactors offer attractive, safer and lower cost alternatives to conventional water cooled reactors, while reducing but not eliminating the nuclear waste problem. Thus, a modified Indian system could be developed, that would feature thorium breeding in IFRs, with U-233 burning MSRs. The rub for such a system would be that LFTRs would be cheaper to develop, cheaper to build, safer and cheaper to operate, and would virtually eliminate the nuclear waste problem.

The LFTR then is a thorium breeding MSR, that offers what may well be the simplest and best solution to the problem of producing sustainable nuclear power.

Two, Four Six, Eight, We Don't Want to Proliferate

Members of the British House of Commons have the right to submit questions to members of the Government . In 1992, Labor MP Bob Cryer ask the following question:
"To ask the Secretary of State for Foreign and Commonwealth Affairs what information he has about how North Korea was able to build a military grade plutonium production reactor outside international safeguards based on blueprints of the United Kingdom military magnox plants at Calder hall, Sellafield."

The British Foreign Secretary Mr. Douglas Hogg responded : "Technical information about the Magnox reactors at Calder hall, Sellafield has been in the public domain for over 25 years. We continue to urge the Democratic People's Republic of Korea to fulfil her obligations under the non-proliferation treaty and sign an agreement with the International Atomic Energy Agency which would place all of her nuclear facilities under safeguards".

The British government, in its great wisdom, allowed the blueprints of the Calder Hall Magnox Reactors to be declassified. The Magnox reactor was, after all, a primitive power generating reactor, that had been offered for sale to other countries. By the mid 1960's its design was dated.

But the Calder Hall Magnox Reactors were not just innocent power reactor, they were graphite reactors capable of burning natural uranium as fuel. Natural uranium burning required frequent fuel changes, but military planners knew that it the fuel change cycle was speeded up, the post reactor fuel would be an excellent source of reactor grade plutonium. The primary function of the Calder Hall Magnox Reactors was weapons grade plutonium production. Power production was an after thought.

During the 1950's and 1960's the militant British Coal Miners Union, was a thorn in the flesh of the British Government, the British Economy, and the British public. Electrical power generation in the UK was dependent of Coal mined by Union Miners, and thus the possibility existed that the British ability to generate electric power would be lost in the event of a long coal miners strike. Thus the British government proposed to give the Calder Hall Magnox Reactors the ability to generate electricity. The catch was that electricity produced by Calder Hall Magnox Reactors would be far more expensive than electricity produced by coal fired plants, but the British government had an answer to that. The real function of the Calder Hall reactors was the production of weapons grade plutonium. Thus the British built a series of Magnox reactors for dual purpose use.

By the mid 1960's Magnox reactors were technologically obsolete, and the British saw no reason to withhold the blueprints from the public. Thus anyone who could cough up the considerable blueprint copying fee, could obtain a copy of the Calder Hall blueprints for his or her personal use and pleasure. There was one tiny question about this British government decision. The Calder Hall Magnox Reactors were built for decidedly military purposes. They were relatively simple, and easily to duplicate, by nations which possessed a far less substantial knowledge and industrial base than the United Kingdom. The plans for the Calder Hall Magnox Reactors, were open to be stolen by soviet spies, and probably were. The Soviets in turn would have been in a position to pass on the Magnox plans to what they regarded as friendly countries. The Soviets might well have regarded the technologically unsophisticated Magnox as a perfect "nuclear research starter kit", for North Korea, exactly because building one was within North Korea's industrial and technological reach. It would, however, not been beyond the capacity of North Korea to obtain the Magnox plans without Soviet help.

The Magnox reactor is the ideal type reactor for the anti nuclear types, and indeed the Greenpeace script sounds as if every reactor is a Magnox . This is not true, and the North Koreans knew exactly what they were doing, when they decided to focus their nuclear weapons program on plutonium production from the Magnox type reactor.

Only two nuclear moderators have been ever been used for Weapons grade plutonium production. They are Graphite, and Heavy Water. All of the major nuclear powers appeared to have exclusively relied on graphite reactors for military Plutonium production. Israel and India have used heavy water reactors for the same purpose. Which ever type of reactor is used, the presence of the moderator allows natural Uranium to be used as a nuclear fuel. The fuel is burned for a relative short period of time in order to maximize the production of Pu-239. The longer the fuel stays in the reactor, the more other types of Plutonium - Pu-238, Pu-240, Pu-241, and Pu-242 - build up. From the stand point of the weapons designer, the creation of other forms of plutonium in a nuclear reactor is an highly undesirable development.

Pu-238 is just plain hot, in addition to being radioactive. Managing the heat and radiation from from Pu-238 would be a significant challenge for weapons designers, because in a weapon incased with a outer shell of high explosives, the heat will build up until the layer of explosives melt, rendering the weapon useless. PU-240 poses an even bigger problem. Although it is not not fissionable under reactor conditions, it fissions spontaneously at an alarming rate. No less than 415,000 Pu-249 atoms fission every second, for every 2.2 pounds (1 kg) of Pu-240. Pu-241 is fissionable in a reactor, but it is quite radioactive. Furthermore, Pu-241 emits beta particles, and transmutes itself into even more highly radioactive Americium-241. When withdrawn from LWRs, Reactor grade plutonium is 53% Pu 239, 25% Pu-240, 15% Pu-241, 5% Pu-242 and 2% of Pu-238.

According to Carson Marks. "Reactor-grade material would generate more than 10.5 watts per kilogram. As Gerhard Locke has recently emphasized, a crude nuclear explosive containing perhaps eight kilograms of reactor-grade plutonium would put out nearly 100 watts of heat-much more
than the eight watts emitted from the approximately three kilograms of weapons-grade plutonium he suggests would be in a modern nuclear warhead. Since the high-explosive (HE) around the plutonium core would have insulating properties only a few times poorer than wood (about 0.4 watts m-oC-1 ) only 10 centimeters of HE could result in an equilibrium temperature of the
core of about 190°C.5 Apparently, the breakdown rate of many types of HE begins to become significant above about 100°C.

Johan Swahn of the Technical Peace Research Group of Chalmers University in Goteborg, Sweden has developed data indieating that the surface dose exposure rate of material such as the reactor- grade plutonium is about six times larger (and MOX-grade over eight times larger) than that from the weapons-grade material which, again, is handled routinely.Marks speaks of nuclear explosions with reactor grand plutonium, but not nuclear weapons.

Two nuclear weapons designers, Carson Marks and Alexander DeVolpe have debated the usefulness of reactor grade plutonium in weapons. DeVolpe pointed to two tests:
1. The two UK "Totem" 1953 experiments in Australia, which were designed to evaluate the yield reduction resulting from plutonium of less than weapons grade".
2. The 1962 United States nuclear test which appeared to involve an intermediate grade plutonium.

DeVolpe argues that "[a]lthough the Totem explosive yield was highly destructive, they evidently confirmed that it was not good enough for military-quality weapons. Because these results would have been shared with the US, we can guess that the [1962] Nevada test might have been conducted with plutonium closer to the low end (81%) of the definition". He adds, that the British were quite disappointed with their 1950's test results.

DeVolpe argued that information about the 1962 test was probably distorted for political reasons. Most of what we know about the test comes from a series of terse statements:

"The 1962 detonation involved plutonium of a quality below that of weapons grade. To reinforce its 1967 announcements that "high-irradiation level reactor-grade plutonium can be used to make nuclear weapons," the US government added in 1977 that "a nuclear test was conducted using reactor grade plutonium" and "it successfully produced a nuclear yield." As a result of the Openness Initiative formulated by Secretary O'Leary, DOE announced in 1994 that the plutonium was "provided" by the UK and the upper limit of explosive yield was 20 kt."

DeVolpe also pointed out, "[c]ompared to data released about other nuclear detonations, the information disclosed about the 1962 test has little substance". According to DeVolpe the French "scorned the US government affirmation that it successfully exploded a weapon made with 'reactor-grade' plutonium."

DeVolpe argues that the quality of the plutonium in the 1962 device would have then been considered "reactor grade", but in the 1990's it would have been classified as fuel grade. He argued that public accounts of the 1962 Nevada test was inconsistent with published data on the 1950's British "Totem" experiments.

DeVolpe complains about the continued classification of information the 1962 test, long after far more information of conventional plutonium devices had been declassified. This would include general information on device design, plutonium quality, plutonium metallurgical chemical form, and Explosive yield. DeVolpe argues, "A lower yield would be suggestive of greater resistance to proliferant use. No nation with other options would choose such material as the basis for a nuclear-weapons program, and none are known to have done so".

Los Alamos chief weapons designer Carson Marks, together with Marvin Miller and Frank von Hippel, responded to DeVolpe's argument. Many of their arguments would seem very ambiguous. For example, they state, "The information disclosed about this test in 1977 represented a compromise between policy makers in the Carter Administration who wished to high-light the proliferation risks of of civilian plutonium use and those responsible for protecting classified weapons-design information".

This statement highlights the political influence on the 1977 statement, but does not respond to DeVolpe's argument that more was classified on the 1962 tests, than was classified on nuclear weapons test conducted at that time. If the goal of the Carter Administration was to high-light the proliferation risks of of civilian plutonium use, maintaining the classification of information that was inconsistent with that goal would have certainly been possible. DeVolpe argued that the classification of the 1962 was idiosyncratic, the Marks et al statement does not conflict with DeVolpe's argument.

Marks et al, made a further statement: "To our knowledge, all U.S. nuclear weapons use weapon-grade plutonium, i.e. plutonium with an isotopic fraction of at least 93.5 percent Pu-239. The same is probably true of the weapons in the arsenals of the other weapon states. There are several reasons for this. One of these is that the natural-uranium-metal fuel used in early production reactors had to be discharged after low U-235 burnup because of both reactivity and metallurgical fuel constraints. Such reactor operation naturally produces plutonium with a high Pu-239 fraction. It was also recognized that radiation exposure to workers fabricating plutonium weapons components in glove boxes would be minimized if the plutonium had a low fraction of the higher plutonium isotopes".

They added,

"However, the most important factor in motivating the high Pu-239 content plutonium in early nuclear weapons was the problem of pre-initiation of the chain reaction. In nuclear designs such as the Nagasaki weapon, where the chain-reaction was designed to be initiated at the point of maximum core compression, neutrons from the spontaneous fission of the even plutonium isotopes (primarily Pu-240) could pre-initiate the chain-reaction leading to significant reduction of the yield of the device.

Even with very high Pu-239 plutonium used in the Nagasaki bomb, there was an estimated 12 percent of reduced yield from this cause. For weapon-grade plutonium as defined today, this probability would have been considerably higher -- on the order of 50 percent -- unacceptably high to the U.S. military. This provided one of the many motivations for going to more sophisticated designs of fission weapons which incorporated faster assemblies and smaller quantities of fissile material. The introduction of "boosting," i.e. having a low-yield fission explosion ignite deuterium-tritium fusion in the primary releasing neutrons which increase the fission yield by an order of magnitude, further reduced the sensitivity to pre-initiation".

They conclude, "we are not arguing that a proliferator would not prefer weapon-grade plutonium or highly-enriched uranium to reactor-grade uranium. However, the possible use of reactor-grade plutonium cannot be discounted".

It is also important to note that Marks et al, did not challenge DeVolpe's discussion of to the British "Totum" tests.

A 1998 Indian test might be shine a little more light on the subject. In May, 1998, India tested a series of nuclear devices. At least one of those devices was believed to use reactor grade plutonium. The yield was reported to be between 0.2 and 0.6 kilotons, but some Indian scientists speculated that it was much smaller.

We ought to take a further note of the "Totum" experiments. These experiments used plutonium from the Calder Hall Magnox reactors and were conducted because plutonium from the Calder Hall reactors had a relatively high Pu-240 content, although probably >10%. The first test is reported to have produced an explosion on around 10 kilotons. But the exact sized of the second explosion is something of a mystery, with estimates running as high as 7 kilotons, and as low as 0.25 kts.

We ought to look at the 2006 North Korean nuclear test, which probably used "fuel grade plutonium from their Magnox type reactor. Prior to the test, the North Koreans told the chinese that they intended to set off a 4 kt nuclear device. Estimates of the actual size of the North Korean device varied widely with estimates running as low as 0.1 kts and as high 1.0 kt. Since the seismic reading fell into to the range of a large conventional explosion, some experts suggested that the North Koreans had not used a nuclear device at all. The Wall Street Journal suggested that the blast was equivalent to the explosive force of about $100,000 worth of ammonium nitrate. It was not clear if the North Korean test had been a success, a failure, or not even a test.

We should also to observe that all but one tests of nuclear devices with more than weapons grade levels of Pu-240 were conducted with Plutonium from Magnox reactors. The British 1953 Totum tests were conducted with a >10% level of Pu-240 and were deemed less than satisfactory. Given the published statement that the Plutonium for the 1962 Nevada test came from a UK source, it would appear that the source was a Magnox reactor. The Pu-240 content ran up to 15%. Finally the plutonium for the Indian sub lt test came from a heavy water reactor. The exact Pu-240 content was unclear.

Thus it would appear that no nuclear test has ever been conducted using plutonium extracted from LWR "spent fuel". Such Plutonium can include as much as 25% Pu-240. With so much Pu-240, the dangers of a spontaneous and premature explosion would be enormous. The amount of neutron radiation from such a device would be significant enough to require more than glovebox containment.

Finally it should be noted, that as Carson Marks has suggested, no nation has chosen to build nuclear weapons from using "reactor grade plutonium". Even North Korea chose to use plutonium that was near weapons grade, and their test of the explosive properties of that material may well have ended in failure.

Considering that no nation has ever even attempted to build a nuclear device using plutonium from spent light water reactor fuel, the case that reactor grade plutonium containing as much as 25% Pu-240 can be weaponized is open to question. In fact the history of nuclear proliferation efforts i9s not consistent with the notion that reactor grade plutonium woule ever be used as a nuclear proliferation tool. Even if such a reactor grade plutonium weapon were possible, easier routes to nuclear proliferation seem likely. The North Koreans seem willing to sell their possibly defective technology to the highest bidder. The many members of the A.Q. Kahn nuclear proliferation gang have never been arrested, and Kahn appears to be on the loose again. Kahn was able to procure of Pakistan the technology to build nuclear weapons, and the Kahn gang was known to have sold nuclear weapons technology to North Korea, Iran and Lybya. The South Africans developed an uranium separation technology that allowed them to enrich about 80 kgs of uranium every year to the 80% U-235 level, that is required for weaponized use.

The evidence is thus that even small nations like South Africa, given a determined government, may posses sufficient industrial, scientific and technical resources, may be able to develop the technology to produce a small number of nuclear weapons, without using reactor grade plutonium. The evidence is further that the cost of producing conventional nuclear weapons by conventional routes is not highly expensive, and the weapons produced are far more likely to produce predictable results. The probable continued survival of the A.Q Kahn criminal proliferation organization, and recent reports of North Korean involvements in building a reactor in Syria suggest that conventional proliferation resources are available to rogue states which wish to produce nuclear weapons. Thus alleged the civilian power reactor-proliferation link appears to be utterly without merit. Reactor grade plutonium produced by civilian power reactors, would appear to be a very undesirable proliferation tool, while inexpensive and far superior proliferation tools are available through criminal organizations and rogue states, to even small countries which wish to acquire nuclear weapon.

Ganted the availability of superior, low cost proliferation options, arguments that reactor grade plutonium from light water reactors constitute an added proliferation risk appear irrational. Thus the so called danger of nuclear proliferation risk ought to be taken off the table in discussions of the advantages and disadvantages of power production from reactors. building power reactors effectively adds no added risk of proliferation avove the super means already available to the would be proliferator.