The Use of Weapons Plutonium as Reactor Fuel

At the end of the Cold War, the United States and Russia face an unprecedented and unexpected problem: surpluses of plutonium and highly-enriched uranium (HEU), the two key materials used to make nuclear weapons. In principle, the uranium poses the lesser problem of the two, because it can be blended down into the low-enriched uranium fuel that is widely used in nuclear reactors. In 1993, the United States and Russia signed a deal according to which the United States agreed to purchase, over a period of 20 years, 500 metric tons of Russian HEU that is being blended down into reactor fuel in Russia. Although implementation of this agreement was initially slow, it is now going forward at the agreed rate.
More difficult is the issue of converting the surplus plutonium into forms not usable for making nuclear weapons. The United States has declared about 50 metric tons (out of a total stock of about 100 metric tons) to be surplus,1 while Russia has not yet made any formal declaration of surplus. Total Russian plutonium in the military sector is thought to be about 130 metric tons, perhaps more.

There is disagreement between the United States and Russia about the best way to handle surplus weapons plutonium. The Russian Ministry of Atomic Energy (Minatom) regards plutonium as a valuable energy resource, but the prevailing US view (notwithstanding some disagreements that continue) is that it is a security and economic liability. Despite their conceptual differences, the United States and Russia have been working together since 1994 on methods for disposition of this surplus weapons plutonium. The Joint United States/Russian Plutonium Disposition Study, prepared by teams of scientists and officials from both countries and published in September 1996, is one result of this joint work.

The joint study outlines a number of options, and reflects agreements as well as disagreements between the governments of the two countries. Both governments agree that it is very important to put surplus military plutonium into non-weapons-usable forms in a timely manner. In the report, the US and Russia present four options jointly, while Russia presented two options in addition on its own. The four jointly presented options are:

use as MOX in light water or heavy water reactors

use as MOX in fast reactors

immobilization in glass or ceramics

direct geologic disposal of plutonium
The two options presented by the Russian side alone are: (i) high-temperature gas reactors, and (ii) accelerator-based systems.

The first two reactor options involve using plutonium in reactor fuel. The plutonium would be converted into an oxide chemical form, mixed with uranium oxide, and fabricated into ceramic fuel pellets (called MOX fuel for short). The isotope of uranium used in MOX fuel is uranium-238, which is not fissile. MOX fuel would be put into fuel rods and loaded into reactors as a complete or partial substitute for the uranium fuel currently used, which is enriched in the fissile isotope, uranium-235. Of the options considered, MOX fuel (in LWRs and fast reactors) and immobilization (the mixing of plutonium with glass or ceramics), are the two technologies under serious consideration for implementation in the near-term.

The study concludes that the most mature of the technologies considered are those involving "reactor options involving known and demonstrated reactors and MOX fabrication technologies." Immobilization technologies are deemed the next most mature. This judgment is based primarily on the European experience of using MOX in LWRs, and Russian experience in the development of MOX fuel for fast reactors. However, a number of differences between civilian plutonium (used in Europe) and military plutonium make this judgment less certain. Further, the decades of European experience in vitrification (the most developed method of immobilization) of high level radioactive waste appears not to have been factored into the overall judgment of relative technological maturity.

Recognizing the differences that exist between the two governments, the report states that "the United States and Russia need not use the same plutonium disposition technology. Indeed, given the very different economic circumstances, nuclear infrastructures, and fuel cycle policies in the two countries, it is likely that the best approaches will be different in the two countries."2 Furthermore, each country may use more than one option.

MOX has not been fabricated from weapons-grade plutonium on an industrial scale. Current industrial MOX facilities use plutonium dioxide derived from facilities that reprocess spent power reactor fuel (called reactor-grade plutonium). There are some important differences. Commercial reprocessing plants currently use aqueous technology (that is, acids and other liquid solvents) to separate plutonium and uranium in spent fuel from fission products and from each other. The final product is a plutonium dioxide powder that can be directly used in MOX fuel production. In contrast, most military plutonium is in the form of "pits" which consist of plutonium metal with small quantities of other materials. Further, in the United States and Russia (and probably in other nuclear weapons states as well) weapons plutonium is alloyed with up to one percent gallium. Gallium complicates the MOX fuel fabrication process and therefore it must be almost completely removed from weapons grade plutonium prior to fuel fabrication. Hence, weapons plutonium metal must both be purified and converted into oxide form (not necessarily in that order) before it can be used. Thus, MOX fuel fabrication from weapons-grade plutonium involves steps and processes that are not needed for reprocessed plutonium from power reactor fuel.4

The current processes for making weapons plutonium into suitable feed for a MOX fuel fabrication plant use aqueous technology similar to reprocessing, which involves huge liquid waste discharges (for more information on reprocessing, see E&S #2 and Science for Democratic Action Vol. 5 No. 1). Dry processes that could be used to make plutonium oxide and remove gallium have not yet been developed beyond the laboratory scale. They will take four to five years more to reach the industrial scale needed for plutonium disposition using MOX. The U.S. has declared its intent to use the dry ARIES process to remove gallium from plutonium pits, while Russia is primarily considering aqueous and molten salt technologies (it is cooperating in this work with France).

In the United States, MOX fuel was used in tests in LWRs during the 1960s and 1970s. MOX has been made in the U.S. only in small-scale glove-box facilities. If the U.S. decides to pursue a MOX option, it would have to construct a new fuel fabrication plant or complete the partially-finished Fuel Materials Examination Facility at the Hanford site in Washington state, built in the 1970s to produce fast breeder reactor fuel.

Russia has a long history of development of MOX fuel for breeder reactors, but Minatom had apparently not considered using MOX in LWRs until the U.S. plutonium disposition program created greater incentives to look at this option. If the fast reactor option were pursued, MOX fuel fabrication would take place at Mayak (near Chelyabinsk), where the partially-built Complex 300 facility is located. If the water reactor option is pursued, plutonium conversion and MOX fuel fabrication facilities would be built at the RT-2 plant in Zheleznogorsk (Krasnoyarsk-26). (See article on Russian MOX fuel fabrication.)

The joint study cites a number of safety precautions necessary in the fabrication of MOX fuel relative to uranium fuel. MOX fuel emits higher gamma radiation and much higher neutron radiation than uranium fuel. Therefore, a separate fresh fuel storage facility designed for MOX only fuel containers for on-site use, and transport equipment for fresh fuel may be necessary. Dust resulting from MOX fabrication is also a concern for worker safety because of the dangers of inhaling plutonium (see article on health effects of plutonium).

Reactor Options Under Consideration

The time it would take to convert plutonium into non-weapons-usable irradiated fuel in reactors depends on a number of factors:

the number, size, and type of reactors used

the average reactor power output

the percentage of plutonium in the MOX fuel

the percentage of the reactor core that is loaded with MOX fuel
It should be noted that all of the reactor options are widely expected to take considerably longer than some vitrification options for meeting the goal of putting surplus plutonium into a non-weapons-usable form. In addition, the initial timeframe estimates for reactor options are likely underestimates. The options involving reactor construction are likely to take the longest.

Russia is considering using MOX fuel (a mixture of the oxides of plutonium and uranium) in both fast reactors (also known as fast breeder reactors) and light water reactors (LWRs) for disposition, while the United States declared in December 1996 that it would pursue a "dual-track" strategy of studying the use of MOX in light water reactors as well as immobilization options that do not involve the use of plutonium as a fuel at all.5 Although the U.S. contributed to the section of the joint report which discusses MOX use in fast reactors, it will not pursue this option. The following sections look at the main options for using LWRs and at Russian possible plans to use MOX in fast reactors.

Existing thermal reactors

The U.S. has a large number of operating reactors which could potentially be loaded with MOX. The Department of Energy has obtained expression of interest at one time or another from 18 utilities offering 38 reactors for burning plutonium as MOX. Not all are currently interested, but the situation is fluid. A formal process for utilities to develop proposals and for the Nuclear Regulatory Commission (NRC) to license them to use MOX (if it believes the license applications to be appropriate) is underway.

Russia's options for plutonium disposition using existing thermal reactors are more limited. For safety reasons, the graphite-moderated RBMK reactors and small light water VVER-440 reactors have been excluded from consideration. Only the larger LWR design, the VVER-1000, could be loaded with MOX, and only with a one-third MOX core (in other words, two-thirds of fuel rods in the reactor would be conventional uranium fuel, and the remaining one-third would be MOX). However, a 1995 report by the United States National Academy of Sciences (NAS) notes that even VVER-1000s "do not currently meet international safety standards,"6 and therefore must be upgraded prior to MOX use. A further complication is that Russia's seven operating VVER-1000 reactors would not be able to consume 50 metric tons of surplus plutonium within the timeline of 20 to 40 years set by the joint panels. In order to pursue a water reactor option, three partially-built VVER-1000 reactors in Kalinin and Rostov would need to be completed. Another proposal has been to load eleven VVER-1000 reactors in Ukraine with MOX fuel in addition to the Russian reactors. Other possible measures to shorten the time needed for disposition such as extending the reactors' operating lives beyond the currently foreseen 30 years, loading more than a one-third MOX core, increasing the plutonium content of the MOX (beyond the 3.9% current envisioned) would pose additional safety risks that have not been adequately addressed.

Even with a one-third MOX core, modifications will probably be necessary before VVER-1000s can be loaded with plutonium fuel. The joint report mentions several possible measures, most of which are connected with maintaining reactor control (see below for further discussion of safety issues). The timeline given in the joint report assumes the first VVER-1000 reactor would accept MOX in 2001, and disposition (using 10 reactors with one-third MOX cores and a plutonium content in the MOX of 3.9%) would be completed in 2028.

"Evolutionary reactors"

Both the U.S. and Russia are considering plans to use newer reactor designs that would be able to take a 100% MOX core because appropriate provisions have been made for additional control. In the U.S. three existing System-80 reactors of the Arizona Public Services Company located at Palo Verde could be used. Russia is also considering construction of up to five VVER-640 (NP-500) reactors (with instrumentation and control systems provided by Siemens). However, even if 100 percent MOX cores were allowed in these reactors, the percentage of plutonium in the MOX would likely be relatively low, so that a larger amount of MOX fuel would have to be fabricated. Hence the advantages from the point of view of speed of disposition of such an approach may be relatively small. The joint report says that "it is believed" that the VVER-640s would be able to take a full MOX core, with 3.7%.7

A third option considered by both the U.S. and Russia is the Canadian heavy water reactors (called "CANDU" reactors, which use natural uranium as fuel and heavy water as a moderator and coolant). Unlike LWRs, which are shut down periodically for refueling, these reactors are continually fueled.

CANDU reactors would use 100 percent MOX cores. According to the Atomic Energy of Canada Limited (AECL), CANDU reactors can use 100 percent MOX cores containing from 0.5 to 3 percent plutonium without physical modification,8 but new licensing would be required because no CANDU reactors are currently licensed to use MOX fuel. CANDU reactors could accommodate 100 percent MOX cores because they have adequate space for any additional control blades (similar to control rods) that may be needed.

CANDU reactors appear to have a number of significant advantages in the use of MOX fuel in terms of controllability. The power production per unit of fuel would be higher with MOX fuel than with natural uranium fuel. With higher power production, the volume of high-level radioactive waste produced by these reactors would be smaller than that now produced by CANDU reactors.

Yet CANDU reactors also possess many disadvantages, such as the need for international transport of MOX fuel, which can be chemically separated into uranium and weapons-usable plutonium in a relatively straightforward manner. Because CANDUs use small fuel bundles and have the potential for on-line removal of fuel bundles (because they are continuously refueled), greater security against theft and diversion of plutonium is necessary. Use of CANDU reactors may also require production of a greater volume of MOX fuel than use of LWRs, since the fuel would contain between 1.5 percent and 2.7 percent plutonium,9 rather than the 2.5 to 6.8 percent range possible in light water reactors (depending on the specific reactor).

Fast neutron reactors

The U.S. discontinued its fast reactor program (also called "breeder" reactors) due to their high cost and concerns over proliferation. However Minatom, because it views plutonium as an energy treasure, has continued extensive research into breeder reactors. Currently, Minatom is operating one fast neutron reactor, the BN-600 at Beloyarsk, loaded with highly-enriched uranium fuel. Four additional fast neutron reactors have been planned, three at Mayak and one at Beloyarsk. Construction was started on two of these (one at each site) in the 1980s, but was halted in the early 1990s because of lack of funds and local environmental opposition. Minatom has recently declared its intention to resume construction and the projects are now undergoing licensing review, but funding is still very uncertain.

Disposition of plutonium can be accomplished in a fast neutron reactor by removing the breeding blankets around the radius of the reactor core, thus turning the reactor from a plutonium producer, to a net burner (note that this does not mean that all of the plutonium is consumed, just that there may be somewhat less in the spent fuel than in fresh fuel). Of course, one problem with breeder reactors from a proliferation standpoint is that the uranium blanket can be inserted and used to make more plutonium, including weapons-grade and super-grade plutonium.

Minatom proposes to build one BN-800 at Mayak for plutonium disposition. BN-800 reactors are designed to take 100% MOX cores, and joint report states that a BN-800 reactor could use 1.6 metric tons of plutonium per year, thus completing disposition of 50 metric tons of plutonium in 30 years. BN-800s are designed to take MOX with reactor-grade plutonium, but, based on calculations that are two decades old, the report states that use of weapons-grade plutonium would not significantly change reactor performance. A more recent and independent evaluation would appear to be needed in view of the seriousness of the issue.

Minatom also plans to complete construction of a second BN-800 at Beloyarsk which could be fueled with MOX containing the approximately 30 metric tons of commercial plutonium which have already been separated at the RT-1 plant at Mayak. This second reactor could serve as a backup for plutonium disposition as well. The timeline given in the report foresees construction on the first BN-800 to be completed by 2005, contingent on adequate financing, which has not yet been arranged.

The joint report states that the existing BN-600 could be used as a demonstration reactor for MOX use as early as the year 2000, assuming early funding for conversion and fuel fabrication facilities. However, the BN-600 is only able to handle a partial MOX core, and the report states that additional research would need to be conducted on the safety of using MOX fuel in this reactor with no radial breeding blanket. This reactor could consume about 0.5 metric tons per year, or about 5 metric tons before the end of its operating life in 2010.

Disposition in breeder reactors poses a number of additional safety and proliferation risks. MOX fuel for fast reactors has a significantly higher plutonium content than fuel for LWRs. Because of the higher plutonium content of the fuel, there would be additional plutonium in the spent fuel as well: breeder MOX spent fuel would have approximately 20% according to the report. Although Minatom declares the safety and environmental record of the BN-600 to be '"excellent," the report also notes that about 30 sodium leaks have occurred in its first 14 years of operation. In addition, the international experience with fast breeder reactors has not been very positive. Safety and technical operating problems or accidents have resulted in the temporary or permanent shut downs of this type of reactor in the United States, Japan, and France.

Light Water Reactor Safety and Licensing Issues related to MOX

The vast majority of LWRs were not designed to use plutonium as a fuel. While both plutonium-239 and uranium-235 are fissile materials that generate similar amounts of energy per unit weight, there are a number of differences between them as reactor fuels that affect reactor safety. The basic set of concerns relates to control of the reactor. The chain reaction in a reactor must be maintained with a great deal of precision. This control is achieved using control rods usually made of boron and (in pressurized water reactors) by adding boron to the water. Control rods allow for increases and decreases in the levels of reactor power and for orderly reactor shut-down. They prevent runaway nuclear reactions that would result in catastrophic accidents.

It should be noted that while all commercial LWRs have some amount of plutonium in them which is made during the course of reactor operation from uranium-238 in the fuel, the total amount of plutonium is about one percent or less when low enriched uranium fuel is used. When MOX fuel is used, the total amount of plutonium would at all times be considerably higher. It is this difference that creates most reactor control issues.10

Changing the fuel can affect the ability of the control rods to provide the needed amount of reactor control and modifications to the reactor may be required before the new fuel can be used. Therefore, changing the fuel in any significant way also requires re-licensing of the reactor.

Several differences between the use of MOX fuel and uranium fuel affect safety:

The rate of fission of plutonium tends to increase with temperature. This can adversely affect reactor control and require compensating measures. This problem is greater with MOX made with weapons-grade plutonium than that made with reactor-grade plutonium.

Reactor control depends on the small fraction of neutrons (called delayed neutrons) emitted seconds to minutes after fission of uranium or plutonium. Uranium-235 fission yields about 0.65 percent delayed neutrons, but plutonium yields only about 0.2 percent delayed neutrons. This means that provisions must be made for increased control if plutonium fuel is used, if present control levels and speeds are deemed inadequate.

Neutrons in reactors using plutonium fuel have a higher average energy than those in reactors using uranium fuel. This increases radiation damage to reactor parts.
Plutonium captures neutrons with a higher probability than uranium. As a result, a greater amount of neutron absorbers are required to control the reactor.

The higher proportion of plutonium in the fuel would increase the release of plutonium and other transuranic elements to the environment in case of a severe accident.

Irradiated MOX fuel is thermally hotter than uranium fuel because larger quantities of transuranic elements are produced during reactor operation when MOX fuel is used.

Overall, the issues related to reactor control, both during normal operation and emergencies, are the most crucial. Most independent authorities have suggested that only about one third of the fuel in an LWR can be MOX, unless the reactor is specifically designed to use MOX fuel. However, there are some operational problems associated with using partial-MOX cores since MOX fuel is interspersed with uranium fuel. Their differing characteristics regarding control, radiation and thermal energy mean that there are non-uniform conditions in the reactor that can render operation and control more complicated. Some reactor operators claim they can use 100 percent MOX cores without needing to make physical changes to the reactor or control rods. The safety implications of such claims need to be independently verified.

The details of licensing procedure in the United States are well known. It is an elaborate, public and expensive process that will almost certainly be contentious, as the joint report acknowledges. However, the role of Gosatomnadzor, the Russian nuclear regulatory agency, is not yet clear; nor is the issue of whether it will have sufficient resources to assure a thorough licensing process. The joint report acknowledges that Gosatomnadzor has not yet begun considering MOX licensing issues, and public participation in the licensing process is also a question mark. The report gives no details about the Russian licensing process but says only that "all facilities are assumed to be licensed by appropriate national authorities."

Plutonium is both used up and produced when MOX fuel is used in reactors. MOX spent fuel contains more plutonium than conventional spent fuel (that is, spent fuel resulting from loading an LWR with low enriched uranium fuel). Conventional spent fuel from LWRs typically contains about one percent plutonium when it is withdrawn from the reactor. The amount of residual plutonium in MOX spent fuel would depend on the initial plutonium loading (percent of plutonium in the fuel), the burn-up of the fuel, and the configuration in which the fuel is used.

For light water reactors using MOX fuel, the NAS calculates that residual plutonium in the spent fuel would range from 1.6 percent (for a 33% MOX core with 4% plutonium loading) to 4.9 percent (for a 100% MOX core with 6.8% plutonium loading). Ranges of 2.5 percent to 6.8 percent plutonium loading have been suggested. In the case of a CANDU reactor using a 100% MOX core, the percentage of plutonium in MOX spent fuel would be between 0.8 and 1.4 percent for MOX fuel containing 1.2 percent and 2.1 percent plutonium, respectively.12

Repository disposal of MOX spent fuel is complicated not only by the higher plutonium content in MOX, but by the larger quantities of transuranic elements in the spent fuel as well. This results in MOX spent fuel being thermally hotter than conventional spent fuel. The presence of greater amounts of transuranic radionuclides like americium-241 also cause persistent higher spent fuel temperatures, and cause the decay of thermal power level to be slower. MOX spent fuel use may therefore require that a host of issues be revisited, such as design of transportation and disposal canisters, and design of on-site spent fuel storage casks. For instance, the higher temperatures may cause storage problems at reactors that have limited storage room in their spent fuel pools. The higher temperature may also result in a need for more repository space, unless a repository is designed to take hotter fuel and withstand higher temperatures. Greater repository space would result in proportionally higher repository disposal costs. In addition, if the amount of residual gallium in MOX spent fuel is too high, it may result in deterioration of the spent fuel cladding, create new issues in evaluating the suitability of a repository, and pose greater risk of groundwater contamination. There are some uncertainties as to the concentration of gallium that might adversely affect spent fuel integrity. The differences between spent MOX fuel and spent uranium fuel pose many complications for reprocessing as well.

Non-proliferation concerns

While much of the official discussion about MOX is that it would "burn" the plutonium, in reality plutonium is both consumed ("burned") and produced in nuclear reactors, as noted above.13 The main function of plutonium disposition is not to get rid of all the plutonium. Rather it is to:

mix plutonium with other materials, usually very radioactive fission products, so that it would be very difficult to re-extract for use in weapons; and

prevent diversion of plutonium by putting it into highly radioactive storage forms that would be lethal to anyone wanting to steal it.

The joint report judges each plutonium disposition option on non-proliferation criteria, according to its timeliness, resistance to theft or diversion, and resistance to retrieval, extraction, or reuse. It was agreed that in order to meet the timeliness goal, the options should provide for disposition of 50 metric tons of plutonium within 20-40 years. The commonly-used yardstick to measure the resistance to theft and diversion of the final form of plutonium after disposition is the so-called "spent fuel standard." This criterion was identified by the NAS in their 1994 report, and means that the plutonium should be as inaccessible to theft, diversion, and re-extraction as plutonium in stored commercial low-enriched spent fuel.
However, there is a major flaw in this standard when judging the long-term security of plutonium. The "spent fuel standard" inherently assumes that the plutonium will remain in spent fuel (or whatever form it has been placed into)--that is, that it be slated for geologic disposal. However, the joint report states that Russian policy does not allow for final burial of "plutonium-bearing materials" (which would include spent fuel), but rather the reextraction of plutonium through reprocessing. Minatom has stated very clearly on numerous occasions that it intends to reprocess spent MOX fuel, rendering the "spent fuel standard" effectively meaningless over the long-term. The U.S. appears to ready to allow Minatom to reprocess spent MOX fuel from the plutonium disposition program. The joint report notes that ". . .Russia will ultimately recycle any plutonium left in the [MOX] fuel. The U.S. objective of plutonium disposition is satisfied when the isotopics of the weapons-grade plutonium have been altered by irradiation, the fuel attains a significant radiation barrier, and the fuel is stored for several decades before reprocessing."14

Even though plutonium will be used to generate electricity in nuclear reactors, the use of MOX fuel will involve net costs. This is because it is more expensive to fabricate MOX fuel even when the plutonium is free than it is to purchase low-enriched uranium fuel, taking all costs, including raw material costs, into account (for further discussion of costs see E&S #1). Using NAS estimates, MOX fuel costs for 50 metric tons of plutonium will be about $2 billion. If the plutonium content of the MOX is 5 percent, the excess costs for disposition of 50 metric tons of plutonium would be about $500 million for MOX fuel fabrication alone, compared to uranium fuel costs. The actual U.S. costs are likely to be far higher because utilities want subsidies to carry out the disposition mission and because many other uncertainties and delays are likely to raise costs.

Overall cost estimates in the U.S. and Russia differ because of differences in the structure of reactor ownership and operation, and because of differing spent fuel policies. In addition to the fuel costs themselves, there would be licensing costs for reactors, transportation and safeguard costs, and reactor construction and modification costs (if required). In general, Russian cost estimates are less certain because of the rapidly-changing economic situation. Because of the policy to reprocess spent fuel, Russian cost estimates include only 50-year storage costs rather than those of final disposal.

Selected IEER plutonium publications:
Plutonium: Deadly Gold of the Nuclear Age
by IPPNW and IEER International Physicians Press, 1992 Price: $17. Also available in French, German, and Japanese.

Fissile Materials in a Glass, Darkly
by Arjun Makhijani and Annie Makhijani IEER Press, 1995 Out of print: photocopy price: $5. Also available in Russian.

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ENDNOTES Almost 12 metric tons of this is non-weapons-grade plutonium produced in military plants.
Joint United States/Russian Plutonium Disposition Study, September 1996, p. ExSum 2.
Unless otherwise mentioned, technical aspects of the use of MOX fuel in reactors are from: Panel on Reactor-Related Options for the Disposition of Excess Weapons Plutonium, Committee on International Security and Arms Control, Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options, National Academy Press, Washington, DC, 1995.
Fabrication of lead test assemblies in Europe has been considered in order to allow MOX to be tested in reactors before new fabrication facilities are built, but seems increasingly unlikely.
Unless otherwise mentioned, the facts regarding DOE's options are from: Storage and Disposition of Weapons-Usable Fissile Materials Final Programmatic Environmental Impact Statement: Summary, Office of Fissile Materials Disposition, U.S. Department of Energy, December 1996. Information on Russia's options is taken from the Joint United States/Russian Plutonium Disposition Study, September 1996. Unfortunately, the report is available only in English. The summary was published in Russian in mid-1997.
NAS 1995, p. 137.
Joint Report, p. WR-27 - WR-29.
By comparison, MOX fuel in an LWR core would range from one third to 100% of the core with a plutonium content of 2.5 to 6.8 percent.
See NAS 1995, pp. 146-151, for a discussion of advantages and disadvantages of the use of CANDU reactors relative to U.S. LWRs. The 1.5 to 2.7 percent range of MOX has been suggested by the reactor manufacturer.
For more information on reactor control, see Science for Democratic Action, Vol. 5, No. 4, February 1997.
NAS 1995, pp. 121-122.
NAS 1995, p. 252, Table 6-1.
Plutonium is formed in commercial reactors from the transmutation of uranium-238 under bombardment by neutrons.
Joint study, p. WR-36-37.

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