Patent Number: 059498374
Section: summary

TECHNICAL FIELD The present invention relates in general to light water nuclear reactor designs which employ thorium as a fuel. The reactors can burn with the thorium, nonproliferative enriched uranium, weapons grade plutonium or reactor grade plutonium. BACKGROUND ART Nuclear power remains an important energy resource throughout the world today. Many countries without sufficient indigenous fossil fuel resources rely heavily on nuclear power for the production of electricity. For many other countries, nuclear energy is used as a competitive electricity producer that also diversifies their energy mix. Further, nuclear power also makes a very important contribution to the goals of controlling fossil fuel pollution (e.g., acid rain, global warming), and conservation of fossil fuels for future generations. In terms of numbers, nuclear power provides approximately 11% of the world's electricity. At the end of 1994, there were 424 nuclear power plants in 37 countries. Plants under construction will bring this number to approximately 500 plants by the end of the decade. Although safety is certainly a major concern in the design and operation of nuclear reactors, another major concern is the threat of proliferation of materials which could be used in nuclear weapons. This is of particular concern in countries with unstable governments whose possession of nuclear weapons could pose a significant threat to world security. Nuclear power must therefore be designed and used in a manner which does not cause proliferation of nuclear weapons, and the resulting risk of their use. Unfortunately, all present nuclear power reactors create large amounts of what is known as reactor grade plutonium. For example, a typical 1,000 MWe reactor creates on the order of 200-300 kg per year of reactor grade plutonium. It is not difficult to reprocess this discharged reactor grade plutonium into weapons grade plutonium, and only approximately 7.5 kg of reactor grade plutonium is required to manufacture a single nuclear weapon. Accordingly, the fuel discharged from the cores of conventional reactors is highly proliferative, and safeguards are required to insure that the discharged fuel is not acquired by unauthorized individuals. A similar security problem exists with the vast stockpiles of weapons grade plutonium which have been created as the U.S. and the countries of the former U.S.S.R. have dismantled their nuclear weapons. Other problems involved with the operation of conventional nuclear reactors concern permanent disposal of long term radioactive waste products, as well as the quickly diminishing worldwide supply of natural uranium ore. Regarding the former, government owned repository spaces are virtually nonexistent and the Yucca Flats project located in the United States has now been delayed by Congress. As to the latter, significant problems with supplies of natural uranium ore are foreseen within the next 50 years. As a result of the foregoing problems, attempts have been made in the past to construct nuclear reactors which operate on relatively small amounts of nonproliferative enriched uranium (enriched uranium having a U-235 content of 20% or less), and do not generate substantial amounts of proliferative materials, such as plutonium. Examples of such reactors are disclosed in my two previous international applications, Nos. PCT/US84/01670, published on 25 Apr. 1985 under International Publication No. WO 85/01826, and PCT/US93/01037, published on 19 Aug. 1993 under International Publication No. WO 93/06477. The '826 and '477 applications both disclose seed-blanket reactors which derive a substantial percentage of their power from thorium fueled blankets. The blankets surround an annular seed section which contains fuel rods of nonproliferative enriched uranium. The uranium in the seed fuel rods releases neutrons which are captured by the thorium in the blankets, thereby creating fissionable U-233 which burns in place, and generates heat for powering the reactor. The use of thorium as a nuclear reactor fuel in the foregoing manner is attractive because thorium is considerably more abundant in the world than is uranium. In addition, both of the reactors disclosed in the '826 and '477 applications claimed to be nonproliferative in the sense that neither the initial fuel loading, nor the fuel discharged at the end of each fuel cycle, is suitable for use in the manufacture of nuclear weapons. This is accomplished by employing only nonproliferative enriched uranium as the seed fuel, selecting moderator/fuel volume ratios which minimize plutonium production and adding a small amount of nonproliferative enriched uranium to the blanket whose U-238 component uniformly mixes with the residual U-233 at the end of the blanket cycle, and "denatures" the U-233, thereby rendering it useless for manufacture of nuclear weapons. Unfortunately, Applicant has discovered through continued research that neither of the reactor designs disclosed in the aforementioned international applications is truly nonproliferative. In particular, it has now been discovered that both of these designs result in a higher than minimum production of proliferative plutonium in the seed due to the annular seed arrangement. The use of the annular seed with both an inner, central blanket section and an outer, surrounding blanket section cannot be made nonproliferative because the thin, annular seed has a correspondingly small "optical thickness" which causes the seed spectrum to be dominated by the much harder spectrum of the inner and outer blanket sections. This results in a greater fraction of epithermal neutrons and a higher than minimum production of proliferative plutonium in the seed. Both of these previous reactor designs are also not optimized from an operational parameter standpoint. For example, moderator/fuel volume ratios in the seed and blanket regions are particularly crucial to minimize plutonium production in the seed, permit adequate heat removal from the seed fuel rods and insure optimum conversion of thorium to U-233 in the blanket. Further research indicates that the preferred moderator/fuel ratios disclosed in these international applications were too high in the seed regions and too low in the blanket regions. The previous reactor core designs were also not particularly efficient at consuming the nonproliferative enriched uranium in the seed fuel elements. As a result, the fuel rods discharged at the end of each seed fuel cycle contained so much residual uranium that they needed to be reprocessed for reuse in another reactor core. The reactor disclosed in the '477 application also requires a complex mechanical reactor control arrangement which makes it unsuitable for retrofitting into a conventional reactor core. Similarly, the reactor disclosed in the '826 application cannot be easily retrofitted into a conventional core either because its design parameters are not compatible with the parameters of a conventional core. Finally, both of the previous reactor designs were designed specifically to burn nonproliferative enriched uranium with the thorium, and are not suitable for consuming large amounts of plutonium. Thus, neither of these designs provides a solution to the stockpiled plutonium problem. DISCLOSURE OF INVENTION In view of the foregoing, it is an object of the present invention to provide improved seed-blanket reactors which provide optimum operation from both an economic and a nonproliferative standpoint. It is a further object of the present invention to provide seed-blanket reactors which can be easily retrofitted into conventional reactor cores. It is another object of the present invention to provide a seed-blanket reactor which can be utilized to consume large quantities of plutonium with thorium, without generating proliferative waste products. A still further object of the present invention is to provide seed-blanket reactors which produce substantially reduced amounts of high level radioactive wastes, thereby resulting in a significant reduction in long term waste storage space requirements. The foregoing and other objects of the invention are achieved through provision of improved seed-blanket reactors which utilize thorium fuel in combination with either uranium or plutonium fuel. The first preferred embodiment of the present invention comprises an improved version of the nonproliferative reactor disclosed in the '477 application. Through the use of specific moderator to fuel ratios and a novel refueling scheme, this embodiment of the invention achieves a fuel burn up efficiency which has heretofore been impossible to achieve in any known reactors, and generates only nuclear wastes that are incapable of being used for formation of nuclear weapons. A second preferred embodiment of the invention is designed specifically for consuming large quantities of both reactor grade discharge plutonium and weapons grade plutonium in a fast, efficient manner. Again, the waste material generated thereby cannot be employed for forming nuclear weapons. The first embodiment of the invention is known as the nonproliferative light water thorium reactor, and is so named because neither its fuel nor its waste products can be employed for forming nuclear weapons. The nonproliferative reactor's core is comprised of a plurality of seed-blanket units (SBUs), each of which includes a centrally located seed region and a surrounding, annular blanket region. The SBUs are specifically designed to be easily retrofitted in place of fuel assemblies of a conventional reactor core. The seed regions in the SBUs have a multiplication factor greater than 1, and contain seed fuel elements of enriched uranium with a ratio of U-235 to U-238 equal to or less than 20% U-235 to 80% U-238, this being the maximum ratio which is considered to be nonproliferative. The enriched uranium is preferably in the form of rods and/or plates consisting of uranium-zirconium alloy (uranium-zircalloy) or cermet fuel (uranium oxide particles embedded in a zirconium alloy matrix). The blanket regions have a multiplication factor less than 1, and contain blanket fuel elements essentially comprising Th-232 with a small percentage of enriched uranium (again enriched as high as 20% U-235) to assist the seed in providing reactor power during the initial stages of operation when the thorium is incapable of providing power on its own. By adding enriched uranium to the blanket, the blanket can generate approximately the same fraction of power at start up that it does later when a large number of neutrons released by the seed fuel elements have been absorbed by the thorium fuel elements in the blanket. This absorption generates fissionable U-233 which is burned in place, and provides power from the blanket once the reactor is up and running. The 20% enriched uranium oxide in the blanket also serves to denature any residual U-233 left in the blanket at the end of its lifetime by uniformly mixing the U-233 with nonfissionable uranium isotopes including U-232, U-234, U-236 and U-238. This denaturing is important because it is nearly impossible to separate the residual U-233 from the nonfissile isotopes thus making the residual U-233 unsuitable for use in the formation of nuclear weapons. Light water moderator is employed in both the seed and blanket regions of each SBU to control reactivity. Unlike in conventional uranium cores, boron is not dissolved in the water moderator during power operation because this would unacceptably lower the multiplication factor of the blanket, thus resulting in a drastically lower blanket power fraction. The volume ratios of the water moderator to fuel in each region are crucial. In the seed region, to insure that the reactor will not generate sufficient amounts of plutonium waste to be considered proliferative, the moderator/fuel ratio must be as high as practicable to slow down the neutrons in the seed, and decrease the likelihood that they will be absorbed by the uranium-238 in the seed, thereby generating plutonium. Unfortunately, to increase the moderator volume in the seed naturally implies that the fuel volume must be correspondingly decreased, and this increases the power density which, if increased too far, will generate too much heat. Both of these factors must therefore be taken into consideration in order to determine the optimum moderator/fuel ratio in the seed region. Use of uranium/zirconium alloy for the seed fuel permits a higher moderator/fuel ratio because of its higher thermal conductivity compared to that of oxide fuel. Using these types of fuel elements, the moderator/fuel ratio in the seed region should be between 2.5 and 5.0, and preferably between 3.0 and 3.5. Another benefit of the use of the high moderator/fuel ratio in the seed is that it results in a substantial reduction in the generation of high level radioactive wastes, particularly transuranic actinides. This, combined with the fact that the blanket fuel rods remain in the core for approximately 10 years, results in a substantial reduction in long term waste storage space requirements. The moderator/fuel volume ratio in the blanket region should be considerably lower than that in the seed region because it is desirable that the thorium fuel in the blanket absorb as many neutrons as possible. These are necessary to convert the thorium into fissionable U-233 which is burned in place, and supplies a substantial portion of the reactor power. Research has established that the optimum moderator/fuel volume ratio in the blanket region should be in the range of approximately 1.5-2.0, and preferably approximately 1.7. If the ratio is higher than 2.0, too many thermal neutrons will be absorbed by the water, while if the ratio is below 1.5, too much protactinium will be formed in the blanket region which will also interfere with the formation of U-233. A once-through fuel cycle is employed with the first preferred embodiment which eliminates the need for reprocessing spent fuel assemblies for future use. In addition, a novel refueling scheme is employed which maximizes fuel consumption in both the seed and blanket regions, and further reduces the likelihood that any of the fuel remaining in the spent fuel elements can be reprocessed and employed in the manufacture of nuclear weapons. In this refueling scheme, the seed fuel elements are replaced in a staggered manner in which a portion, preferably 1/3, of the total seed fuel elements is replaced at the end of each fuel cycle, and each seed fuel element remains in the core for more than one, preferably three, fuel cycles. Each fuel cycle is approximately 13 months in length. The blanket fuel elements, because they are comprised predominantly of thorium, can remain in the core for up to nine fuel cycles, or approximately 10 years. However, shuffling of the SBUs in the core is performed at the end of each fuel cycle to improve power distribution throughout the core. This refueling scheme enables the enriched uranium seed fuel rods to be depleted down to less than 20% of their original U-235 content. In addition, the long residency time in the core of the seed fuel elements increases the generation of Pu-238 to the point where it denatures the relatively small amount of Pu-239 which is generated by the seed fuel elements. As a result, the spent seed fuel elements are effectively rendered useless for the formation of nuclear weapons. The second preferred embodiment of the present invention uses the same basic seed-blanket core arrangement as the first preferred embodiment with a plurality of SBUs that can be retrofitted into a conventional reactor core. However, this embodiment of the invention is designed specifically for consuming very large amounts of plutonium, either weapons grade or reactor discharge grade, with the thorium in the blanket. Thus, the thorium oxide is mixed with plutonium in the blanket fuel rods, while the seed fuel rods are formed predominantly of plutonium-zirconium alloy. Unlike the first embodiment whose goal is to maximize the amount of power generated by the thorium in the blanket, the goal of the second embodiment is to maximize the consumption of plutonium without generating large amounts of new plutonium as typically occurs in a conventional reactor. The plutonium incinerator embodiment also employs a high water moderator/fuel volume ratio, preferably between approximately 2.5 and 3.5. However, the reason for the high ratio is different than that for the first embodiment. In particular, the high water to fuel volume ratio provides a very thermal spectrum in the seed regions. This simplifies core control since all control is concentrated in the seed regions, and control can thereby be effected without boron chemical control or increased use of control rods. In the blanket region, the only notable difference in the plutonium incinerator embodiment is that the thorium oxide in the blanket fuel rods is mixed with a small percentage of plutonium oxide to assist during initial reactor operation. In addition, it is very important that approximately 2-5% by volume uranium tailings (natural uranium with its U-235 content reduced to approximately 0.2%) are added to the blanket fuel rods. These tailings serve to denature (render useless for use in the manufacture of nuclear weapons) the U-233 which is formed in the blanket during reactor operation. The moderator/fuel ratio in the blanket region is preferably between approximately 1.5 and 2.0 to satisfy neutronic and thermal hydraulic constraints.