On May 11 and 13, 1998, India shocked the world by conducting five underground nuclear test explosions. India was known to have nuclear weapons capability (see previous FAQ), but both the boldness of its 1998 tests (in a climate of global reduction in nuclear arsenals and tests), and the sophistication of the tests (including thermonuclear technology), took international observers by surprise. Pakistan followed shortly afterwards with nuclear test explosions of its own.

The Indian tests have two possible links to Canadian-supplied technology: (1) some of the plutonium used may have been generated in the CIRUS research reactor supplied jointly by Canada and the USA about thirty years earlier (see previous FAQ); and (2) some of the tritium used in the development and implementation of India's thermonuclear devices may have been extracted as a by-product from the heavy-water moderator of India's pressurized-heavy-water power reactors based on Canadian CANDU technology [1]. It is also possible that India used some of its unsafeguarded "CANDU-derivative" reactors (the copies of its two CANDU reactors, but unlike the CANDU units, not covered by UN-based safeguards) to generate weapons-grade plutonium [2]. However, much of this information remains unconfirmed, and subject to speculation by outside observers.

There is little doubt that India pursued a nuclear weapons development programme steadily and indigenously since the 1970's [3,4]. India developed its own plutonium and tritium production technology despite international sanctions on nuclear technological development since 1974. Although India's nuclear weapons program has undeniable links to Canadian-supplied technology from thirty years ago, international experts place more emphasis on the technology developed indigenously since that time [2,4,5]; for example:

[1] T.S. Gopi Rethinaraj, "Tritium Breakthrough Brings India Closer to an H-Bomb Arsenal", Jane's Intelligence Review, January 1998.

[2] M. Hibbs, "India Made About 25 Cores for Nuclear Weapons Stockpile", Nucleonics Week, June 11, 1998, p.15.

[3] J. Stackhouse, "How the nuclear ban bent for India", The Globe and Mail, June 15, 1998.

[4] D. Albright, "The shots heard 'round the world", The Bulletin of the Atomic Scientists, July/August 1998.

[5] R. Silver and M. Hibbs, "Opponents Say South Asian Tests Show All CANDU Exports Should End", Nucleonics Week, June 25, 1998.

[6] For current tritium-production plans in the US, see the US DOE's "Commercial Light Water Project" homepage at http://www.dp.doe.gov/dp-62/default.htm

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This is a question that cannot be answered simply. First, however, two related misconceptions must be put to rest:

Nuclear fuel (spent or otherwise) inside or outside of a CANDU reactor (or any power reactor) cannot explode like an atomic bomb (see related FAQ); and, An atomic bomb has never been manufactured using spent fuel from a CANDU reactor (see the two previous FAQs for more on the misconception that India may have done exactly this).

The issue of nuclear weapons proliferation, involving any form and purity of fissile material, is an important concern. It is important that we understand it, and neither underestimate nor overestimate the risks. To commit the former invites obvious consequences; to commit the latter can be equally catastrophic: anti-proliferation resources can be diverted to less worthy (i.e. lower risk) targets, and related infrastructures (e.g. civilian nuclear power) can be degraded or disassembled for illegitimate reasons.

"WEAPONS-GRADE" VS. "REACTOR-GRADE" PLUTONIUM

First, what makes the plutonium in spent fuel different from the plutonium in atomic bombs? The answer relates to the isotopic mixture of the plutonium.

Weapons-grade plutonium, the kind used by all nuclear-weapons states in their fission and fusion plutonium bombs, contains at least 93% pure Pu-239, with the remainder a mix of other (mostly heavier) plutonium isotopes such as Pu-238, Pu-240, Pu-241, and Pu-242. Some of these other isotopes, while able to sustain a fission chain reaction, are spontaneous neutron emitters. Their presence in the warhead of an atomic bomb, therefore, raises the probability of pre-initiation or predetonation. This is what happens if the chain reaction initiates before it is supposed to (by even one stray neutron), leading to a much reduced explosive yield.

In a nuclear weapon, the nominal yield is achieved by waiting until maximum reactivity is inserted, when neutron-multiplication is most efficient (for example, by applying maximum compression to a plutonium core), and then introducing a free neutron to initiate a divergent chain reaction. With plutonium of any isotopic mixture, there is a certain probability that a stray neutron will appear on the scene prior to this crucial point, starting the chain reaction prematurely and leading to a less-than-nominal yield. The minimum yield (excluding the possibility of a complete "dud") is that achieved by introducing a neutron at the earliest effective time (the point of criticality). The minimum yield is also referred to as the fizzle yield, possibly a few percent of the nominal yield [3].

In reactor-grade plutonium, the kind produced as a by-product of fission in power reactors, the fraction of Pu-239 is typically 60-70% 1 , and therefore the chance of a predetonation would be greater if it were to be used in a nuclear weapon. In any reactor core, plutonium is manufactured when U-238, which constitutes over 99% of natural uranium, absorbs a neutron, and eventually decays into Pu-239. The amount of Pu-239 builds up as the fuel remains in the reactor, but at the same time more and more undesirable plutonium isotopes are created through subsequent neutron absorption and decay. In a weapons-grade plutonium-production reactor, therefore, the "trick" is to leave the fuel in long enough to create reasonable amounts of Pu-239, but not long enough to exceed the 7% limit on other plutonium isotopes. These reactors typically shuffle fuel so fast through their core that economical electricity generation is simply out of the question.

In power reactors the fuel normally remains in the core for over a year, leading to a 30-40% fraction of higher plutonium isotopes. Not only does this crucial difference significantly increase the probability of "pre-initiation" in a would-be bomb, but two more effects must also be considered:

the neutron and gamma dose from this material (estimated at about 17 rem/hr at the surface, for 1 kg of plutonium metal separated from spent LWR fuel [2]) is significant; and

the heat generated by radioactive decay (about 11 Watts/kg for spent LWR fuel [2] – almost 100 Watts for a critical mass) would melt the high-explosive material needed to compress the critical mass prior to initiation.

It is popular knowledge that the scientists in the Manhattan Project, arguably the largest collection of the most intelligent people in the world at the time, spent vast amounts of time and money (the total budget was roughly $2 billion) trying to produce high-purity plutonium. In the end the explosions of both the Trinity test bomb, and the Nagasaki delivered bomb, were testimonials to their success. (The Hiroshima bomb, on the other hand, used highly-enriched uranium – another, relatively simpler, route to the same result, which the Manhattan Project followed in parallel with the plutonium effort.)

PROLIFERATION RISK

Since the potential for making an explosive from reactor-grade plutonium is not zero, however, spent fuel is a safeguarded item. This means that utilities must comply with international safeguard protocols when handling, transporting, and storing spent fuel, subject to periodic inspection by the United Nations. (see related FAQ, as well as the CNA fact sheet, PDF file, 130 kb: "Does Canada contribute to nuclear weapons proliferation?")

Safeguards are one aspect of a general consideration of the proliferation risk of reactor-grade plutonium. One must also be concerned with the details of acquisition, manufacturing, tactical and strategic usefulness on both a national and subnational level, as well as technical viability. Although this document will deal mainly with the latter issue – technical viability – it will touch on the "grander" issue of proliferation risk as well.

EXPLOSIVE POTENTIAL OF REACTOR-GRADE PLUTONIUM

Suppose for the moment, however, that someone were able to steal or divert enough spent fuel to make a critical mass with the enclosed reactor-grade plutonium. At approximately 80 g of plutonium per discharged CANDU fuel bundle [19], and assuming a reactor-grade critical mass (with effective neutron reflection) of 8 kg [3], this would require 100 spent fuel bundles, weighing two tonnes without shielding. Not only would the theft be extremely difficult, but since it would also be easily and quickly detected, it would be followed by the necessary evasion of a top-priority manhunt employing most likely the full resources of the country's security infrastructure.

Suppose further that this person were then able to separate the plutonium from the rest of the spent fuel, requiring remote tooling because of the radioactivity of the fuel. What then, would the explosive yield from such plutonium be?

Understandably, this topic sits squarely at the boundary of classified and declassified military knowledge. A public analysis must necessarily rely upon declassified information, including declassified statements made by nuclear weapons experts. As one such expert [30] reassures, "that is not to say, however, that relevant information that is considered classified cannot be derived from unclassified sources by competent people."

In the last twenty-five years many authors have addressed the issue of reactor-grade bomb potential in the public literature, and while they usually assume a typical discharge plutonium assay from LWR reactors, the assay from CANDU reactors will not be different enough to change the general results, which are approximate anyway. Although most authors generally agree on the probable upper limit of explosive yield from reactor-grade plutonium, there is wide variation in the predictions of the average yield.

The reason for this variation is that the calculation is highly non-trivial. Only one of the many relevant inputs to the analysis is the yield distribution of a predetonation, which several authors [2,3,4] have attempted to characterize (and some degree of "public" validation can be achieved by comparison with declassified statements made by Oppenheimer and Groves after the 1945 Trinity test [3,4]). There are many other parameters to take into account [4,5], including the critical mass required, the metallic phase (and change of phase) of the plutonium, the degree of radial compression achievable, the effect of subcritical multiplication (sometimes overlooked by weapons experts because it plays no significant role in "weapons-grade" bomb design), the maximum reactivity insertion achieved, and the surface leakage of neutrons. The easiest approach is to assume the "worst-case" scenario, i.e. that maximum success is achieved in all areas in the above list; this approach is non-informative, however, beyond indicating what the maximum achievable yield might be.

The fact remains that making a nuclear bomb is not an easy task, despite the popular perception that the essential knowledge and capability is widespread in the post-Hiroshima world. While the fundamental concepts are widely appreciated, the task remains an enormously complex and expensive undertaking. Most authors agree that the complexity is increased significantly by adding the ingredient of reactor-grade plutonium.

The estimates of the probable yield vary, and the only way to make sense of the information is to view it from the point of view of "sophistication" – are the perpetrators terrorists, or nuclear weapons states, or something in between? The sophistication manifests itself in many aspects of design, but most importantly in the degree of reactivity insertion and speed of compression. Of the lower range of sophistication, often called the "crude" approach, Taylor, Mark, et al write [6]:

"If the assembly velocities ... are quite low, the earliest possible preinitiation could lead to an energy release (equivalent weight of high explosive) not many times larger than the weight of the device. If the velocities are quite high ... the lowest preinitiation yield may still be in the 100 tons range, even in a crude design."

Mark expands upon this point elsewhere [3], describing the fizzle-yield as "a few percent" of the nominal yield, which, for a Trinity/Nagasaki style device with a nominal yield in the tens of kilotons, puts the fizzle-yield in the hundreds of tons. Mark's work is particularly indicative of the interesting classified/declassified nature of this topic: he was Director of the Theoretical Division at Los Alamos from 1947 to 1972, and yet bases all of his crucial probability calculations – necessarily – on recently-declassified comments made by Oppenheimer and Groves in letters written in 1945.

There are those that predict lower values: Sahin [5] puts the upper limit at 100 tons for typical reactor-grade plutonium; Miettinen [7] reports predictions of attainable yields less than one kiloton. There are also those that predict higher values: a study [8] by the US National Academy of Sciences (NAS) refers to an "assured" yield for a "relatively simple" device of the order of "one or a few kilotons". This claim is noteworthy because it implies guaranteed kiloton-range yields; however, some clarification is supplied by Dr. William Sutcliffe [9], one of the authors of the restricted LLNL report [10] referenced by the NAS authors. Dr. Sutcliffe is necessarily general in his response, but makes it clear that yields of all ranges are possible (even zero), and he doesn't believe that the NAS authors were making a general statement about "all" forms of reactor-grade weapons.

Meyer's work [2] is useful in sorting out the ambiguity because he reports a range of results depending on the target reactivity insertion (related to the level of sophistication) of the bomb design. The higher the target reactivity insertion, the greater the nominal yield of the device, but also the greater the probability of predetonation that must be overcome. As a sort of "median" result, it may be inferred from Meyer's analysis that a device with 50% chance of predetonating, would produce a nominal yield of around one kiloton. Note that this value corresponds to that used in the NAS statement, but it is far from "assured" according to Meyer.

A thorough treatment is found in DeVolpi's 1979 seminal book on the subject [4], elements of which appear elsewhere [18,23]. DeVolpi attempts a quantitative assessment based upon the many factors listed earlier (predetonation probability, critical mass, metallic phase, radial compression, subcritical multiplication, reactivity insertion, and surface leakage), pulls all the contributing probabilities together, and sorts them according to three levels of "sophistication". He predicts average yields, from reactor-grade bombs, over a range from 10 tons to 10 kilotons, in order from lowest sophistication to highest. DeVolpi's predicted average yield does not go above 1 kT until a sophistication level appropriate for a national program is reached, and the highest level corresponds to a nuclear-weapons state development program. He also points out that many "conventional" weapons designers tend to overlook the issue of subcritical multiplication because it is irrelevant to designs with weapons-grade plutonium. This one factor alone can amplify the predetonation neutron field by factor of ten.

DeVolpi's work was supported by the late Hans Bethe, Director of the Theoretical Division at Los Alamos during the war and a notable nuclear weapons expert. It also has its critics, notably Amory Lovins [4,22], who claims that achievable reactivity insertions are sufficient to reduce the distinction between denatured forms of plutonium, in terms of probable yield. DeVolpi counters that Lovins' insertion rates are 10 to 100 times greater than rates derived from the unclassified literature, and that, if true, Lovins' claims imply that militarily useful weapons can also be made with low fissile fractions of U-235 as well. This contradicts experience, and the opinion of experts like Bethe. In a more recent article [17], Lovins repeats the claims, but also points out that the unreliability of "unsophisticated designs" is not a deterrent, since it matters not "whether the designer can accurately predict the yield, but rather that the intended victims cannot."

USEFULNESS OF REACTOR-GRADE PLUTONIUM BOMB DEVELOPMENT

All this may appear academic in light of the fact that a one-kiloton bomb, or even a one-hundred-ton bomb, is a potentially destructive weapon in its own right [3]. This is especially true when you consider that a bomb of several hundred tons yield may be used to ignite a thermonuclear "booster" [11] and achieve much higher yields (although the most "crude" of the predicted reactor-grade devices are probably too weak to be used this way [12]).

It is important, therefore, to consider mitigating factors such as complexity, expense, acquisition, and consequently the level of sophistication required, which are not translated in simple statements addressing only the issue of possible yield. In all likelihood, it is impossible to evaluate the true proliferation risk of reactor-grade plutonium without addressing these concomitant factors.

A 1995 report [31] from Los Alamos, the nuclear weapons centre of expertise in the U.S., makes a clear statement on the relative proliferation risk of reactor-grade and weapons-grade plutonium:

"The isotopic mix of typical LWR spent fuel is generally considered to be proliferation resistant due to its low concentration of Pu-239 (~56 wt%) and higher concentration of Pu-240 (~23 wt%), Pu-241 (~14 wt%), and Pu-238 (~1 wt%) – all of which increase the level of difficulty required to design and fabricate and effective weapon. The level of difficulty increases due to the internal heating caused by alpha decay from Pu-238; the additional neutron source rate due to spontaneous fissions from Pu-240, which increases the likelihood for a weapon to pre-initiate; and the highly penetrating gamma rays from Am-241, which arise from the decay of Pu-241 increasing the handling and fabrication difficulties of LWR plutonium. [...] Thus, for these reasons, LWR-like isotopic mixes of plutonium do incrementally increase the proliferation resistance of the material by increasing he level of difficulty in designing and fabricating the weapon."

The Los Alamos report then stresses that this increased proliferation resistance of reactor-grade vs. weapons-grade plutonium should not be used as a discriminating factor in the definition of conservative weapons-proliferation-resistance policy, since reactor-grade plutonium can still be used in a nuclear weapon (with a "likely fizzle yield" estimated as "several kilo-tonnes").

Consider, for the sake of illustrative argument, representatives of the two ends of the scale of "sophistication", each with a motive to pursue reactor-grade weapons: at one end is a terrorist group; at the other is a national defence program.

The terrorist group, without a national-level development program, is probably limited to the kiloton range as the average result of their efforts, with a certain probability of achieving less than this. If they desire higher yields, or more reliability of kiloton yields, they need to develop methods to shorten the compression times, increase the degree of compression, or accelerate the fission chain reaction, all of which necessarily adds to the complexity, expense, and time-scale of their undertaking. At the same time, they will have to deal with the problems of radiation exposure and self-heating in the plutonium core. The latter is especially important in the application of the device since the heat generated by reactor-grade plutonium is enough to melt the high explosive normally packed around the core as a detonator. Both the radiation and heating problems, like that of average yield, are in theory solvable, but certainly daunting in comparison with more conventional weapons that can deliver equivalent results. Even if the group must simultaneously detonate several "Oklahoma City"-style bombs (an example of what are colloquially referred to as "blockbusters") to achieve the same mass destruction as one reactor-grade plutonium weapon, this is more than compensated by the ease of acquiring the ingredients (fertilizer and diesel fuel), the conventional and reliable nature of the detonation, and the easily masked process of manufacturing and delivery.

The national-level defence program, on the other hand, can assign its best minds to the task and solve the problems of unreliable low yield, radiation field, and heat generation. In the end they might attain a more reliable, multi-kiloton weapon, or even a primary to a multi-stage fusion weapon. How much incentive is there to pursue this route, however, when (a) the inherent complexity remains greater than that of the weapons-plutonium or weapons-uranium route, and (b) the size and weight of the device would still compromise its military usefulness? Why waste defence resources solving problems of heat generation and predetonation probability, with higher critical mass and higher radiation fields, when you can avoid the issues in the same manner they did in 1945? Even if you possess spent power reactor fuel in storage, is the expense and complexity less than that associated with a weapons-plutonium or uranium-enrichment route? Furthermore, if you are a signatory to international safeguards, would you risk detection in so obvious a manner when other options can be pursued in smaller, cheaper, even indigenously-designed (and therefore unsafeguarded) facilities [13]?

The answers to these questions are not readily apparent, but surely there is validity enough in the above two scenarios to render the proliferation risk of reactor-grade plutonium lower than that based upon technical viability alone. Certainly it is known that nuclear-weapons states themselves have little use for reactor grade plutonium; for example, the US weapons program has considered a program to isotopically enrich reactor-grade plutonium in spent power reactor fuel, which indicates its lack of faith in the military utility of the un-enriched, reactor-grade product. The U.S. government has also offered to help pay for the conversion of Russia's "dual-use" plutonium production reactors to solely civilian use, a move based on the recognition of civilian plutonium's low military utility [28].

John Holdren, chairman of the Committee on International Security and Arms Control (CISAC) of the US National Academy of Sciences (NAS), a body that recommended the "burning" of surplus weapons-grade plutonium in power reactors as one strategy for reducing its proliferation risk (see related FAQ), points to the difficulty that even nuclear-weapons states would encounter in trying to make a weapon with the resulting reactor-grade plutonium, as one rationale behind the Committee's conclusions [21].

In late 2000 CISAC published an update [32] on its surplus weapons plutonium recommendations, declaring plutonium isotopic content to be of "moderate" importance as a proliferation barrier for both "host" and "proliferant" states. For "subnational groups" the report considers the matter to be of "low" importance, since groups in this category (e.g. terrorist organisations) would be less concerned about final bomb yield.

The relative nature of this assessment is informative: According to this NAS CISAC report, plutonium isotopic content is the most important inherent barrier to the re-use of surplus weapons plutonium by a "host" state such as the U.S. or Russia (supporting the comments by CISAC chairman John Holdren quoted above). Furthermore, in terms of providing an inherent barrier to proliferation by "proliferant states" such as Iraq, isotopic content was found to be equal in importance to the so-called "radiation barrier" – itself a central tenet of the weapons-plutonium disposition programme. [32]

Although finding the overall significance of plutonium isotopic content to be of low to moderate importance in the context of the disposition of surplus weapons plutonium, CISAC concludes that it does form the final inherent barrier to proliferation, and thus increases in importance for scenarios involving the separation of plutonium metal from the final dispositioned forms:

"If it is assumed that proliferators in all categories will ultimately be capable of obtaining reasonably pure plutonium metal starting from the dispositioned forms -- as we believe to be the case -- then the main intrinsic barriers in this category [final utilization] are those associated with deviation of the plutonium's isotopic composition from 'weapons grade'. " [32]

It is apparent that predetonation is enough of an issue even with weapons-grade plutonium. Despite recent statements by Ted Taylor [20] that "ways to avoid this problem [of predetonation] were developed and demonstrated before the end of the 1950's", predetonation remained a serious issue in the US weapons program decades later [4]. Taylor also writes:

"Contrary to opinions expressed by many nuclear engineers that are not familiar with the still secret intimate details of nuclear weapon design and operation, plutonium extracted from all types of spent fuel removed from nuclear power plants or research reactors can be used for making modern fission or thermonuclear weapons that are reliably predictable in performance, over a very wide range of yields, from fractions of a kiloton to megatons of high explosive equivalent. This has been true for decades, and confirmed by numerous nuclear weapons tests."

This is a broad statement, and given that Taylor only specifies a range of yield, and not the corresponding levels of required sophistication, and also that his range includes "fractions of a kiloton", his statement is in agreement with other observers. However, Taylor, who specialized in lightweight, highly-sophisticated nuclear weapons during his time at Los Alamos in the 1950s, is presumably not referring to reactor-grade material in his last sentence. The only publicly known US test of a reactor-grade device was a 1962 explosion, partially declassified in 1977. However, in 1962 the term "reactor-grade" included any purity less than 93% Pu-239 [14]. The plutonium for the 1962 test came from a British MAGNOX reactor (a dual-purpose electricity/plutonium-production design), and is suspected of being in the range 80-90% Pu-239, although this fact remains classified [14,15,16,33].

THE NEED FOR SAFEGUARDS

It must be emphasized that the existence of safeguards is implicit in the assumptions of proliferation risk presented here. It can be argued that people will build nuclear weapons whether nuclear power fuel is available or not, and people will generate nuclear power whether nuclear weapons are available or not. The overriding need is for international scrutiny and cooperation, in the form of an enforceable and effective safeguards regime.

PERSPECTIVE

After assessing the realistic proliferation risk of nuclear power reactors, the next step is to balance this risk against the benefit of nuclear power reactors. There are environmental and economic arguments to be made, but an anti-proliferation argument also exists that deserves attention. Nuclear power has the promise to bring abundant energy supply to developing and less privileged nations. Since energy is a driving force behind economic development, it stands to reason that an increase in energy availability can be linked to an increase in standard of living. Surely this is a stabilizing sociopolitical influence with anti-proliferation consequences, and therefore this aspect must not be overlooked in rational assessments of the overall issue.

Furthermore, through the current Non-Proliferation Treaty (NPT) regime, nuclear power is a "carrot" used to entice non-nuclear-weapons states to remain that way. If the proliferation risk of nuclear power were deemed sufficiently intolerable for it to be taken out of the equation, then such treaties, while perhaps not perfect, become moot. Nuclear reactors will continue to be built, but in an unsafeguarded environment.

Perhaps the necessity for proper perspective is best illustrated by analogy with other industrial links to weapons of mass death and destruction. Most man-made destruction of life and property over the past century was inflicted with chemical explosives and chemical poisons, and yet global commerce in the chemical industry is allowed to flourish, despite a continuing (and, in the case of chemical poisons, escalating [24]) threat. Similarly, the biotechnology industry is not generally opposed on the grounds that its direct products are used as weapons of war and terrorism [25,26,29] (and even as a component of nuclear weapons proliferation [27]). Accelerators and lasers can be used to generate and enrich fissile material for use in nuclear weapons, while simultaneously contributing enormously to applications in engineering and health care. Finally, virtually all modern design and "testing" of nuclear weapons is performed in the digital world of supercomputers. India, in bringing itself to the self-declared point of thermonuclear statehood, relied at least as much on digital technology as it did on tritium and plutonium production technology. There is no inherent aspect to any of the technologies listed here that restricts it from being applied maliciously - the decision is sociopolitical.

NOTES

This range of plutonium isotopic variation in spent fuel is remarkably small, given the diverse reactor designs to which it applies. The total specific energy ("burnup") produced by CANDU fuel, for instance, can be four to six times less than that of LWR fuel, but due to CANDU's use of natural uranium and highly thermalized neutron spectrum, the relative abundance of higher plutonium isotopes ends up similar.

REFERENCES

[1] A. DeVolpi, "The Physics and Policy of Plutonium Disposition", Physics and Society, Vol. 23, No. 3, http://www.aps.org/units/fps/newsletters/1994/july/ajul94.cfm#al, July 1994.

[2] W. Meyer et al, "The Homemade Nuclear Bomb Syndrome", Nuclear Safety, Vol. 18, No. 4, July-Aug. 1977.

[3] J. Carson Mark, "Explosive Properties of Reactor-Grade Plutonium", Science and Global Security, Vol. 4, 1993.

[4] A. DeVolpi, Proliferation, Plutonium and Policy – Institutional and Technological Impediments to Nuclear Weapons Propagation, Pergamon Press, 1979.

[5] S. Sahin, "Non-proliferation", letter to Nature, Vol. 287, 1980 October 16.

[6] J. Carson Mark, T. Taylor, et al., "Can Terrorists Build Nuclear Weapons?", in P. Leventhal and Y. Alexander, eds., Preventing Nuclear Terrorism, Lexington Books, 1987.

[7] J.K. Miettinen, "Nuclear Miniweapons and Low-Yield Nuclear Weapons Which Use Reactor-Grade Plutonium: Their Effect on the Durability of the NPT", in Nuclear Proliferation Problems, Stockholm International Peace Research Institute (SIPRI), MIT Press, 1974.

[8] Management and Disposition of Excess Weapons Plutonium, Vol. 1 and 2, National Academy of Sciences (NAS), Committee on International Security and Arms Control, National Academy Press, Washington DC, 1994.

[9] W.G. Sutcliffe, personal communication, Lawrence Livermore National Laboratory (LLNL), June 1998.

[10] W. G. Sutcliffe and T.J. Trapp, eds., "Extraction and Utility of Reactor-Grade Plutonium for Weapons", Lawrence Livermore National Laboratory, UCRL-LR-115542, 1994 (S/RD).

[11] M. Miller and F. von Hippel, "Usability of Reactor-grade Plutonium in Nuclear Weapons: A Reply to A. DeVolpi", comments in Physics and Society, Vol. 26, No. 3, http://www.aps.org/units/fps/newsletters/1997/july/cjul97.cfm#a1, July 1997. (see also A. DeVolpi's reply.)

[12] See discussion of small (tactical) fission weapons in "The High Energy Weapons Archive – A Guide to Nuclear Weapons", at the URL http://nuclearweaponarchive.org.

[13] R.W. Morrison, "Is Canada Peddling Nuclear Bombs World-Wide in the Guise of Nuclear Reactors?", Science Forum, Vol. 10, No. 6, 1977.

[14] DOE Facts, "Additional Information Concerning Underground Nuclear Weapon Test of Reactor-grade Plutonium", Washington, D.C., http://apollo.osti.gov/html/osti/opennet/document/press/pc29.html, June 1994.

[15] A. DeVolpi, "A Cover-up of Nuclear Test Information?", Physics and Society, Vol. 25, No. 4, http://www.aps.org/units/fps/newsletters/1996/october/aoct96.cfm#a2, October 1996.

[16] Nuclear Electricity, Seventh Edition, Uranium Information Centre, Melbourne, Australia, http://www.uic.com.au/ne7.htm, 2003.

[17] A. Lovins, "Plutonium Disposition", letters to Physics and Society, Vol. 23, No. 4, October 1994.

[18] A. DeVolpi, "Denaturing Fissile Materials", Progress in Nuclear Energy, Vol. 10, No. 2, 1982.

[19] Author's calculation.

[20] T. Taylor, "Utility of Reactor Grade Plutonium in Nuclear Weapons", comment submitted to a debate on the internet listserver of the Canadian Network to Abolish Nuclear Weapons (CNANW), June 10, 1998.

[21] J. Holdren, "Work With Russia", Bulletin of the Atomic Scientists, March/April 1997, p. 40.

[22] A. Lovins, "Nuclear Weapons and Power-Reactor Plutonium", Nature, Vol. 283, 28 February 1980.

[23] A. DeVolpi, "Fissile Materials and Nuclear Weapons Proliferation", Annual Review of Nuclear and Particle Science, Vol. 36, 1986.

[24] F.J. Gaffney, Jr., "Making the World Safe for VX", Commentary Magazine, October 1998.

[25] E. Masood, "Bioweapons scientist says microbes can be made safer", Nature, Vol. 394, 23 July 1998.

[26] G. Achcar, "The Spectre of Bioterrorism", Le Monde Diplomatique, September 1998.

[27] A. Sen, "The Nuclear Bug", New Scientist, 11 July 1998.

[28] T. Perry, "Stemming Russia's Plutonium Tide: Cooperative Efforts to Convert Military Reactors", The Nonproliferation Review, Monterey Institute of International Studies, Center for Nonproliferation Studies, 4, 2, Winter 1997.

[29] R. Taylor, "All Fall Down (Bioterrorism Special Report)", New Scientist, 11 May 1996.

[30] T. Taylor, "Nuclear Safeguards", Ann. Rev. of Nucl. Sci., 25, 406, 1975.

[31] D.A. Close, B.L. Fearey, J.t. Markin, D.A. Rutherford, R.A. Duggan, C.D. Jaeger, D.L. Mangan, R.W. Moya, L.R. Moore, R.S. Strait, "Proliferation Resistance Criteria for Fissile Material Disposition", Los Alamos National Laboratory report, LA-12935-MS, April 1995.

[32] Committee on International Security and Arms Control, National Academy of Sciences, "The Spent-Fuel Standard for Disposition of Excess Weapon Plutonium: Application to Current DOE Options", National Academy Press, Washington, D.C., 2000.

[33] B. Pellaud, "Proliferation Aspects of Plutonium Recycling", Journal of Nuclear Materials Management, 31, 1, Fall 2002.