Putting the Genie Back in the Bottle
Uranium and Nuclear Weapons Proliferation
Alexander Glaser 
The emphasis of proliferation analyses, in recent years, has been on weapons plutonium disposition, namely, what to do with the plutonium released from dismantled U.S. and Russian nuclear weapons to make it difficult to steal or re-use for weapons. However, the proliferation risks associated with excess stocks of highly enriched uranium (HEU) and those associated with the continued use of HEU in the nuclear fuel cycle pose additional challenges for the international non-proliferation regime. More recently, the potential theft, diversion, or loss of HEU has also raised particular concerns due to its usability in crude nuclear explosive devices and its potential role in nuclear terrorism scenarios.
This article summarizes some general aspects of existing HEU stocks and the current use of this material in the nuclear fuel cycle. Options, perspectives, and challenges of ending the use of HEU for non-weapons purposes are discussed. The overview is based on the implicit assumption that at least excess HEU stocks will indeed be eliminated by their owners once the material can no longer be used for military or civilian purposes.
 | |||||||||||||||||||||||||||||||||||||||||||
|
 Figure 1: Critical mass of a beryllium-reflected uranium sphere as a function of the uranium-235 enrichment (—). Enrichment is given in weight percent (wt%). MCNP 4B simulations at 300 K. Reflector thickness is 10 cm.
Table 1: Military stocks of fissile material, end of 1999[10] | ||||||||||||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||||||||||||||||
Characteristics of HEU
Diluting HEU with natural or depleted uranium and reducing the uranium–235 fraction to less than 20% (low-enriched uranium, LEU),[1] essentially eliminates the proliferation risks associated with the material. In particular, and most importantly, this is due to the critical mass of uranium, which increases quickly with lower enrichment (close to 20% and below, cf. Figure 1). The material essentially becomes unusable as fissile material in a nuclear explosive device.
In practice, however, due to the outstanding performance of HEU as fuel for certain reactor types and in spite of international efforts to avoid and to discourage the use of this material for non-weapons purposes, HEU is still used in some civilian research reactors and in naval propulsion reactors.
Production, Role, and Use of HEU in the Past
The production of enriched uranium began during World War II as part of the Manhattan Project. Although, HEU was available at an early stage, and used in the nuclear weapon that destroyed Hiroshima, the production capacity was low at that time. The large enrichment facilities under construction were completed only after the war. From 1945 to 1947, the HEU production capacity in the U.S. was eight times higher than the production capacity of weapons plutonium in dedicated production reactors.
In Table 1, the military stocks of fissile material are summarized. The mass ratio of the world inventory of HEU compared to the inventory of weapons plutonium is currently greater than six.[2] All major nuclear weapon states obtained significant quantities of HEU that exceed the corresponding weapons plutonium inventory in every case.[3]
The high HEU to plutonium ratio is remarkable since the nuclear weapons programs in some countries were initially focused on plutonium and the HEU production capacity was added at a later time only. While HEU can be used in implosion type primaries, the high inventory of military HEU in the nuclear weapon states suggest additional explanations:
Gun-type weapons: Only HEU (and not plutonium) can be used in the simple, but inefficient, gun-type design. Nevertheless, even the gun-type method apparently allows weapon designs that are much more compact and lighter than the first gun-type device (Little Boy, Mk–I), which contained 62 kg of HEU and weighed approx. 4,000 kg. For instance, the W33 warhead, an artillery shell developed in the 1950's, had a total weight of approx. 100 kg only. Especially the U.S. army was interested in these robust small diameter warheads and promoted the production and use of HEU for gun-type weapons. As a consequence, several gun-type weapons were designed early in the nuclear weapons age and kept in the active U.S. stockpile until the 1980's.
High-yield fission weapons: Pure fission weapons, designed to have a very high yield of up to 500 kt (TNT), required unusually high quantities of fissile material. Apparently, HEU was preferred for this purpose because the pre-detonation probability of corresponding plutonium quantities was high even when advanced implosion technologies were used. The interest in high-yield fission weapons decreased only when the feasibility of thermonuclear weapons had been confirmed in October 1952.[4]
Thermonuclear weapons: In the thermonuclear stage (i.e. the secondary) of a nuclear weapon, significant quantities of uranium are placed next to the fusion fuel. This component is usually called the "pusher." When high energy neutrons emerge from the deuterium-tritium fusion reactions, the uranium is fissioned and contributes significantly to the total yield of the weapon. Even though natural uranium can be used for this purpose, HEU is the preferred material due to its higher fission probability.[5] Apparently, weapons designers shifted from natural uranium to HEU when the latter became available in sufficient quantities in the 1980's.[6]
In addition to these weapons applications, HEU is used to fuel military naval and civilian research reactors. Around 4 metric tonnes of HEU are currently used per year to fuel naval (mostly U.S. and Russian) reactors.[7] The inventory of HEU in the civilian sector is small compared to the current military stockpiles: it has been estimated at approx. 20 metric tonnes,[8] which is still enough material for some 1,000 nuclear weapons. Although the number of HEU-fueled research reactors in the world is decreasing, the remaining facilities, still operated in more than 20 different countries, require a total of approximately one metric tonne of fresh HEU per year.[9]
Estimation of de facto Excess HEU Quantities
In addition to the total amount of HEU listed in Table 1, it is of particular interest to look at the quantities that are effectively surplus to current military uses (i.e. as long as nuclear weapons arsenals are reduced rather than increased). Russia and the U.S. provided corresponding numbers. Russia declared 500 metric tonnes of HEU (assumedly weapons-grade) excess. This material is being blended down to LEU and purchased by the U.S.[11] Similarly, in March 1995, the U.S. declared 174 metric tonnes of HEU surplus to its military needs, of which only 33 metric tonnes are enriched to at least 90%.[12] Due to the lower enrichment level, the 174 tonnes correspond to a much lower amount of HEU wg-eq. Vice versa, these numbers of declared excess material, however, cannot be correlated to the quantities absorbed by operational nuclear weapons. By way of an example, the situation in the U.S. is discussed below.
Discussion of the U.S. Case
In Table 2, the operational U.S. nuclear weapons are listed by type and estimated number deployed in the years 2000 and 2002.[13] The amount of HEU absorbed in these weapons can be estimated very roughly by two simple approximations.[14]
Number and mass: Based on the assumption that the maximum number of nuclear weapons ever deployed by the U.S. (approx. 32,000 in 1967) absorbed the main portion of the HEU stockpile available for military purposes in the U.S. (500-600 metric tonnes of HEU wg-eq), the average amount of HEU per warhead would be 15–20 kg.[15] Using this average value for the roughly 7,600 nuclear weapons operational today, an active HEU inventory of 120-150 metric tonnes is deduced.
Yield: Based on the maximum cumulative yield of the operational U.S. nuclear weapons (max. 1,790 Mt (TNT) according to Table 2) and using the fact that the fission of one kilogram of uranium-235 is equivalent to the energy of at least 18 kt(TNT), the total HEU inventory can be estimated making the following additional assumptions: 50% of the total explosive yield stems from HEU fission, which is certainly a very conservative number (i.e. a high percentage), and a fraction of not more than 50% of the HEU in the weapon is fissioned. These assumptions lead to an estimated HEU inventory in use of approx. 100 metric tonnes.
These simple approximations suggest that the active HEU inventory in the U.S. should be 100—150 t, probably even less. Consequently, more than 400 metric tonnes of U.S. HEU are potentially surplus. The situation is similar in Russia, although a significant portion of excess HEU has been addressed in the 1993 HEU-agreement with the U.S.
As a consequence, and especially if the number of operational nuclear weapons reaches the currently envisioned levels in the mid-term future, the de-facto excess HEU quantities in the world may reach values close to 1,000 metric tonnes.[16] This quantity would be the result of a successful disarmament process. However, it will have to be carefully analyzed how the corresponding disposition process should be organized, bi- or later possibly multilaterally, and what influence the possible non-weapons uses of HEU might have on the disposition process.
| |
|
 Figure 2: Flow of direct-use fissile material: highly enriched uranium |
| |
The HEU Life Cycle
In order to assess potential proliferation risks associated with HEU, the broader context in which the material is embedded has to be taken into account. In Figure 2 a highly simplified "flow chart" for the HEU life-cycle is given. As long as no new HEU is produced, proliferation risks are associated with the use or storage of existing materials.[17]
A major step to reduce the proliferation risks associated with HEU is to phase out its use as soon as technically and politically feasible. As will be discussed below, in the past, the main focus of corresponding efforts was directed at HEU fuels for a number of research reactors globally. These efforts led to the conversion of many facilities, provided funding for alternative LEU fuels with very high uranium densities, and prevented the construction of new HEU-fueled reactors.
The use of HEU in the military sector, where the material is used to fuel naval reactors, attracted much less attention so far. In the U.S., significant quantities of HEU are reportedly placed in reserve for future use in naval reactors on submarines and military surface vessels.[18]
Comparing these facts with the corresponding situation in the plutonium context, it is important to note that no military non-weapons applications exist for surplus weapons plutonium. Due to this structural difference, once an amount of plutonium is declared surplus to weapons needs, nothing hinders designation of the plutonium for disposition. In this sense, triggering the disposition of HEU is more complicated.
The Civilian Context of Reactor Conversion
The development of new research reactor fuels was initiated in the late 1970s in the wake of the International Nuclear Fuel Cycle Evaluation (INFCE) conference. In particular, the Reduced Enrichment for Research and Test Reactors (RERTR) program has provided the principal impetus to these efforts by coordinating the international activities, encouraging reactor operators to abandon HEU, and preparing feasibility studies for the conversion of existing reactors.[19]
The actual conversion of older reactors built prior to 1980 is an immediate success of the activities put forward by the RERTR program. In general, the conditions for conversion of reactors that are still fueled with HEU have since been elaborated in detailed feasibility studies. In a few cases, suitable LEU fuels are currently unavailable and only the next generations of research reactor fuel (uraniummolybdenum and monolithic fuels) will allow the conversion of those facilities. In some important cases, agreements exist between fuel providers and reactor operators, which guarantee that conversion will actually take place as soon as the specified fuel is available.
Research and development of advanced LEU research reactor fuels have increased the effective uranium density from initially 1.5 g/cc to 4.8 g/cc today. The so-called monolithic fuels, which could become available within the next five years, promise effective uranium densities of up 16 g/cc.
With the exception of the German FRM-II, discussed below, all reactors currently planned or under construction will generally use LEU fuel. This includes projects in Australia, Canada, China, France, Morocco, Thailand, and Taiwan. In particular, the Chinese and the French project are remarkable since they reflect the political re-orientation of former "classical" HEU users. All official nuclear weapon states support the emerging non-proliferation norm not to build HEU fueled reactors and to abandon, at least gradually, the use of HEU in the civilian sector. The U.S. recently announced that it will convert all its remaining domestic research reactors as soon as suitable fuels are available. By 2015, the FRM-II might be the last civilian facility that requires the supply and use of HEU.
A Special Case: The New German Research Reactor FRM-II
In 1996, construction of a new research reactor, the FRM-II (Forschungsreaktor München II), started in Garching near Munich in Bavaria (Germany). The facility will be operated by Munich University of Technology (TUM), which received the last partial license required to start operation in April 2003. The fact that the operator designed the reactor with HEU fuel has been strongly criticized nationally and internationally. Nevertheless, due to the support of the Bavarian and the former German Federal Government, construction of the reactor commenced without seriously contemplating the use of LEU.
Different alternative LEU designs and calculations have been proposed in the past, the first ones even before construction of the facility had actually begun. The most comprehensive alternative designs were elaborated by Argonne National Laboratory and studied further by other independent analysts.[20] The results of these calculations show that with a (slightly) different core design the FRM-II could be converted to LEU fuel with a moderately reduced thermal neutron flux. Near the so-called cold neutron source, where most of the neutrons are extracted, the loss is 10–15%, and the quality of the neutrons, i.e. the available energy spectrum, almost identical to the original HEU spectrum.
The operator would have been able to evaluate these performance parameters at an early stage and was fully aware of the negative consequences the use of HEU would have: besides problems associated with fuel supply and disposition, criticism and opposition on a national level (from the public and the licensing authorities) as well as on an international level were foreseeable.
| |||||||||||||||||||||||
|
Table 3: Incentives for an operator of a research reactor (top) and of a naval propulsion reactor (bottom) to convert to low-enriched uranium. | ||||||||||||||||||||||
| |||||||||||||||||||||||
Ending the Use of HEU: Incentives and Disincentives
From a purely technical point of view, HEU is always superior to LEU due to the higher percentage of the fissile isotope U–235 it contains. Consequently, in the absence of other criteria, there is no a priori incentive for a (military or civilian) reactor operator not to use HEU.
This may be evident in the military case, but it also holds true, in general, for research reactor operators. Their primary task is to operate their facility reliably, economically, and at an optimum performance level. Hence, for a conversion process to take place, additional elements have to be introduced. Interests of operators and possible measures that may trigger or support conversion are summarized in Table 3. The RERTR program, supported by several other programs, is the most important driving factor for the research reactor conversion process. These programs use push and pull mechanisms. On the one side, restricting the supply of HEU is an effective—opponents would argue: unfair—measure to make the choice of LEU more attractive. In the U.S., this policy is substantiated in the Schumer Amendment.[21] Unfortunately, as some operators (who converted their facilities) have pointed out, the use of LEU is economically not necessarily more attractive than HEU fuel, which indicates a serious flaw in pricing policies.
Additional incentives for conversion of research reactors can be introduced at the back-end of the fuel cycle. Since the number of countries that operate research reactors is much higher than the number of countries that operate commercial power reactors, it is evident that most of them were and will be unable to implement domestic fuel disposition programs for research reactor fuels. In the past, it was common practice that suppliers took the irradiated fuel back in order to re-use the remaining enriched uranium after re-processing.
In this respect, the current U.S. Foreign Research Reactor Spent Nuclear Fuel Acceptance Program, which was initiated in 1996 for a duration of 10 years,[22] is a new key element. For the first time ever, research reactor fuels are taken back by the country of origin without the intention to reprocess the fuel for further use. The U.S. might develop and apply the socalled Melt & Dilute process to prepare the spent fuel for direct dispsoal.
The future policy of Russia in this field could be crucial in this context. With reference to fuel disposition of the relevant German research reactors, operators have outlined the effect a Russian offer could have: "If Russia will take the waste this can be an excellent option, which will be taken by nearly all research reactors".[23] It is evident that without any special requirements or obligations for the HEU recipients, such a 'simplified' return program could jeopardize international conversion activities—especially, if combined with an HEU supply option.
Conclusion and Outlook
Due to the outstanding performance of HEU as fuel for certain reactor types and in spite of international efforts to avoid and discourage the use of this material for non-weapons purposes, HEU is still used in some civilian research reactors and in naval propulsion reactors. The fact that these applications continue to exist perpetuates the risk of theft by non-state actors and the risk of HEU diversion by states for weapons-purposes. The technical means are now at hand (or in reach) to abandon the use of HEU in research and to some extent also in naval propulsion reactors. Advanced LEU high-density fuels reach or exceed the performance of HEU fuels still in use. Therefore, regarding the latest fuel generation, it has to be guaranteed that adequate funding is made available to develop and qualify these fuels without delay.
As long as non-weapons applications of HEU exist, there is no driving momentum to blend the entire stocks of excess military material down to LEU. The material remains either in the military sector for the intended future use as fuel in naval reactors, or it is offered for use in civilian research reactors. If excess HEU stocks were not available, there would be no incentive to supply HEU for existing or planned research reactors that could be fueled with LEU. Similarly, a clear HEU 'shortage' could prevent operators from designing new research reactors with HEU in the first place. The elimination of excess HEU stocks would also strongly support the irreversibility of the disarmament process.
In the civilian context, the new German research reactor FRM–II represents a dramatic exception of the emerging non-proliferation norm to fuel new research reactors with LEU only. For the first time in more than ten years, an operator deliberately challenges the measures and instruments provided, by RERTR and other support programs, to make the use of LEU fuel more attractive. In the near-term future, the activities related to conversion, operation and management of research reactors have to be carefully observed. The related programs must eventually be equipped with new and more effective tools in order to reach the goal of phasing out the use of HEU in the mid-term future.
A "Global Cleanout and Secure" effort for HEU can only be successful if the use of this material is terminated without narrowing the focus on specific countries where the near-term proliferation risk is considered especially high. Since a major fraction of HEU is currently used in the nuclear weapons states, it is crucial that HEU users worldwide, even those with unrestricted access to the material, participate in this important effort to maintain and actively support global security.
An earlier version of this article was presented at the 13th International Summer Symposium on Science and World Affairs, July 21–30, 2001, European Academy, Berlin (Germany).
|
|
||||
