Managing Military Uranium and Plutonium in the United States and the Former Soviet Union: Technical Background

TECHNICAL BACKGROUND

Bunn, Matthew, and John P. Holdren. "Managing Military Uranium and Plutonium in the United States and the Former Soviet Union." Annual Review of Energy and the Environment 22 (1997): 403-486.

The materials that make nuclear bombs possible are those few isotopes,4 among the hundreds found in nature or producible by technology, capable of sustaining an explosively growing chain reaction. Two isotopes of uranium fit this description (U-233 and U-235), as do all isotopes of plutonium (most importantly Pu-239, Pu-240, Pu-241, and Pu-242).

Uranium-235 is the only potential nuclear-explosive isotope that occurs naturally in significant quantities; it constitutes 0.7% of natural uranium, but its nuclear-explosive properties emerge only if the proportion of U-235 atoms in the uranium is much higher than in the natural element. Nuclear explosives can in principle be made with material containing somewhat less than 20% U-235, but the amount of material required at that level of enrichment is very large; in international practice, all uranium with a U-235 concentration of 20% or more is referred to as highly enriched uranium (HEU) and is subject to increased safeguard measures. For fission explosives, nuclear-weapon designers prefer a U-235 fraction of more than 90%, and HEU in this concentration range is called "weapon-grade."5 Increasing the U-235 concentration above its level in natural uranium— uranium enrichment— is a technologically demanding and costly enterprise.

Plutonium is virtually nonexistent in nature but can be produced by bombarding uranium-238 with neutrons in a nuclear reactor or an accelerator. (U-238 is the most abundant uranium isotope, constituting 99.3% of natural uranium.) Reactors have proven to be more practical than accelerators for producing plutonium in large quantities. In order to use the plutonium produced in a nuclear reactor in a nuclear weapon, it must be chemically separated from the fission products produced along with it, and from the residual U-238, by reprocessing the nuclear fuel. Reprocessing is also a technically demanding and costly operation; because of the intense gamma-radioactivity of the fission products, and the health risks posed by the alpha-activity of plutonium if inhaled or otherwise taken into the body, reprocessing also requires more stringent measures to mitigate its health and safety hazards than does enrichment.6

Although virtually all combinations of plutonium isotopes can be used to manufacture nuclear explosives,7 nuclear-weapon designers prefer to work with plutonium containing more than 90% Pu-239 (called "weapon-grade" plutonium). This high Pu-239 concentration is commonly achieved by removing the plutonium from the reactor before the higher isotopes (which result from successive neutron absorptions) have a chance to build up. The longer refueling intervals typical of civilian nuclear electricity generation result in plutonium that contains only 60-70% Pu-239, which is called "reactor-grade" plutonium.

This term notwithstanding, reactor-grade plutonium can also be used to produce nuclear weapons at all levels of technical sophistication. This point is crucial to how a regime for protecting and safeguarding plutonium should be structured, and since it has been the source of considerable confusion over the years, we want to be as clear about it as classification boundaries permit.

Three aspects of reactor-grade plutonium pose additional difficulties for weapons design and manufacture: (a) the increased neutron background (which increases the chance of "pre-initiation" of the nuclear chain reaction at a moment before the weapon reaches the optimum configuration for maximum yield); (b) the increased heat (which may affect the stability and performance of the weapon's components); and (c) the increased radiation (which results in greater dangers to those fabricating and handling weapons produced from reactor-grade plutonium). The more sophisticated the designer, the greater the degree to which these difficulties can be overcome. Unsophisticated weaponeers could make crude but highly destructive nuclear bombs from reactor-grade plutonium, using technology no more sophisticated than that required for making similar bombs from weapon-grade plutonium, and sophisticated weaponeers could use reactor-grade plutonium to make very effective nuclear bombs quite suitable for the arsenals of major nation-states (8, 9, 33, 40).

A significant nuclear-explosive yield results even if the weapon goes off prematurely at the worst possible moment; in a design identical to the Nagasaki design, for example, this "fizzle yield" would be in the range of a kiloton— that is, the equivalent of 1000 tons of conventional high explosive— which would still have a destructive radius between a third and a half that of the Hiroshima bomb (40). Regardless of how high the concentration of troublesome isotopes was in the reactor-grade plutonium used, the yield would not be less than this figure.

The US government recently declassified a particularly explicit statement on the weapon-usability of reactor-grade plutonium (36, pp. 38-39):

The degree to which these obstacles [to using reactor-grade plutonium in weapons] can be overcome depends on the sophistication of the state or group attempting to produce a nuclear weapon. At the lowest level of sophistication, a potential proliferating state or subnational group using designs and technologies no more sophisticated than those used in first-generation nuclear weapons could build a nuclear weapon from reactor-grade plutonium that would have an assured, reliable yield of one or a few kilotons (and a probable yield significantly higher than that). At the other end of the spectrum, advanced nuclear weapon states such as the United States and Russia, using modern designs, could produce weapons from reactor-grade plutonium having reliable explosive yields, weight, and other characteristics generally comparable to those of weapons made from weapon-grade plutonium....Proliferating states using designs of intermediate sophistication could produce weapons with assured yields substantially higher than the kiloton range possible with a simple, first-generation nuclear device.

Obfuscation on this point, which continues in some debates concerning recycle of plutonium for civilian nuclear energy generation, is irresponsible and dangerous. Thus, in this article, we refer to separated plutonium of any grade, and all uranium enriched to 20% or more U-235, as weapon-usable material, as distinct from weapon-grade material.

Limited access to the principal weapon-usable materials has been for many years the primary technical barrier against the spread of nuclear-weapons capabilities to additional nations and to subnational groups,8 for the following reasons. 1. As already noted, the technologies for producing separated plutonium and HEU are demanding and costly. 2. Plutonium and highly enriched uranium that have been produced have mostly been well guarded or have resided in forms awkward to steal and difficult to use in weapons (such as plutonium in spent fuel, not separated from accompanying uranium and fission products). 3. In contrast to the relative difficulty of acquiring weapon-usable materials, the knowledge and expertise needed to use these materials to make (at least) crude nuclear weapons is very widely available, that is, available to virtually any country and potentially to some subnational groups. The ability to buy such materials on a nuclear black market could shorten a third-world bomb program from a decade to months or less.

The quantities of weapon-usable material needed to make a nuclear weapon are not large. Although the amounts used in specific nuclear-weapon designs are classified, numbers in the range of 4-6 kg of plutonium metal are widely cited in the unclassified literature as typical (and the figure would not be very different if reactor-grade rather than weapon-grade plutonium were used); a comparison of critical masses suggests that obtaining a comparable explosive yield from weapon-grade HEU would require a mass of uranium metal approximately three times greater.9 The necessary amounts of material are easily carried by one person and easily concealed. These materials themselves are not radioactive enough to deter theft and handling of them; because of the very long half-lives of Pu-239 (24,000 years) and U-235 (0.7 billion years), the radiological dose rates from these materials are orders of magnitude lower than those that arise, for example, from spent fuel when it is unloaded from a nuclear reactor, which contains intensely radioactive fission products such as Cs-137 and Sr-90.10 Unless proper security and accounting systems are in place, therefore, a worker at a nuclear facility could simply put enough material for a bomb in his briefcase or under his overcoat and walk out.

For comparison to the 4-6 kg figure mentioned above, a large civilian nuclear-power reactor of the most widely used type produces, in the course of a year of operation, about 250 kg of reactor-grade plutonium embedded, along with roughly a ton of intensely radioactive fission products, in the reactor's low-enriched uranium fuel. (In the prevalent "once-through" fuel cycle, the spent fuel is not reprocessed, and the plutonium therefore remains intimately mixed with fission products and the residual uranium and thus is not directly usable in nuclear explosives.)

4 An element is uniquely characterized by the number of protons in its nucleus; an isotope is uniquely characterized by the combined number of protons and neutrons. For example, each uranium nucleus contains 92 protons; uranium-235 is the isotope of uranium in which each nucleus contains 92 protons and 143 neutrons (the 235 designation being the sum of these two numbers).
5 In the most basic nuclear weapon designs, the entire nuclear-energy release comes from fission. In the more complicated thermonuclear weapons that predominate in the arsenals of the declared nuclear-weapon states, a "primary" nuclear-explosive component, in which the nuclear-energy release comes mainly from the fission of plutonium or highly enriched uranium, ignites a "secondary" nuclear-explosive component that derives its energy from a combination of fusion and additional fission. Such weapons generally contain both plutonium and HEU (see e.g. 8, 9). Uranium used in such secondaries covers a range of enrichments, some not within the range typically referred to as "weapon-grade."
6 Other potential nuclear-explosive isotopes, such as U-233, are also produced by neutron absorption (in thorium, in the case of U-233) and separated by chemical reprocessing but have not played any significant role in nuclear arsenals to date.
7 The exception is plutonium containing substantial quantities of Pu-238, which generates such intense heat that it is not practical to make nuclear explosives from it; plutonium containing 80% or more Pu-238 is hence exempted from international safeguards.
8 There are, of course, important political barriers to such proliferation, above all the international norm against proliferation embodied in the Non-Proliferation Treaty— extended indefinitely in 1995 with overwhelming support from its now185 parties— and the International Atomic Energy Agency safeguards that monitor compliance with this and related agreements. For a useful examination of why states forgo nuclear weapons programs see (41).
9 These figures apply to relatively simple pure-fission weapons or to the primary and not the secondary components of thermonuclear weapons. For an interesting discussion, based on unclassified sources, of the minimum amount of material from which bombs might be made, see (42).
10 An intact sphere containing 4 kg of weapon-grade plutonium, for example, would have a gamma dose rate at 1 m of 0.005 roentgen-equivalent-man per hour (rem/h), whereas a 6-kg sphere of reactor-grade plutonium would have a dose rate of 0.03 rem/h at the same distance. By contrast, the equivalent dose rate for a spent fuel assembly irradiated to a typical burnup, 10 years after leaving the reactor, would be 2200 rem/h (see 9, p. 270).