KR100313964B1 - Seed-blanket reactor - Google Patents

Abstract

The seed-blanket reactor core 10,100 is employed to combust thorium fuel with conventional reactor fuel, including non-diffuse enriched uranium, and inorganic or reactor grade plutonium. In the first embodiment, the core 10 is completely non-proliferative in that the reactor fuel and the resulting waste cannot be used to produce nuclear weapons. In a second embodiment of the present invention, the core 100 is employed to combust a large amount of inorganic grade plutonium with thorium and employs a conventional mechanism in which stored inorganic grade plutonium can collapse and be converted into electrical energy. to provide. The cores of both embodiments consist of a plurality of seed-blanket units 12, 102 having seed regions 18, 104 located centrally and surrounded by annular blanket regions 20,106. The seed region includes uranium or plutonium fuel rods 22, 110, while the blanket region includes thorium fuel rods 26, 118. The moderator / fuel volume ratio and the relative size of the seed and blanket areas are optimized so that neither of the two embodiments produces waste that can be used for nuclear weapons manufacturing. In addition, the new fuel supply scheme maximizes seed fuel utilization and also It is employed with the first embodiment to ensure that the spent fuel cannot be used for nuclear weapon production.

Description

Seed-Blank Reactor {SEED-BLANKET REACTOR}

Nuclear power is an important energy resource worldwide today. Most countries without sufficient fossil fuel resources depend on nuclear power to produce electricity. In most countries, nuclear energy is used to diversify their energy ratios and as a competitive power producer. Moreover, nuclear power contributes greatly to achieving the goals of controlling fossil fuel pollution and to preserving fossil fuels for future generations. Numerically, nuclear power supplies about 11% of the world's electricity. As of the end of 1994, there are 424 nuclear power plants in 37 countries. There will be about 500 power plants under construction at the end of the century.

Although safety is a major concern in the design and operation of nuclear reactors, another major concern is the threat of the proliferation of materials that can be used for nuclear weapons. This is of particular concern in countries with unstable governments where having nuclear weapons can pose a huge threat to global security. Therefore, nuclear power must be designed and used in such a way that nuclear weapons will not proliferate and cause the dangerous consequences of using them.

Unfortunately, all current nuclear reactors produce large amounts of plutonium known as reactor grade plutonium. As an example, a typical 1,000 MWe reactor produces about 200-300 Kg of reactor grade plutonium per year. Reprocessing these discharged reactor grade plutonium into inorganic grade plutonium is not difficult, and only about 7.5 kg of reactor grade plutonium is needed for the production of a single nuclear weapon. Therefore, the fuel discharged from the core of the conventional reactor is highly diffusive, and safety measures are required to ensure that the discharged fuel is not obtained by unauthorized individuals. The same safety concerns exist with the massive weapons grade plutonium that has arisen as the United States and the former Soviet Union dismantled their nuclear weapons.

The problems associated with conventional reactor operation are related to the drastic reduction in the global supply of natural uranium minerals and the permanent disposal of long-term radioactive waste. On the latter, government-owned storage is virtually nonexistent and the Yucca Flats project required in the United States is currently being delayed by Congress. For the former, serious problems with the supply of natural uranium minerals are expected in the next 50 years.

As a result of this problem, attempts have been made to build a reactor that is operated with a relatively small amount of non-diffusive enriched uranium (enriched uranium with a U-235 content of 20% or less) and does not produce significant amounts of diffusible materials such as plutonium. It has been done in the past. Examples of such reactors are International Patent Application PCT / US84 / 01670, published internationally by Applicant International Patent Application WO 85/01826 in 1985, and International Publication No. WO 93/06477, 1993. Patent application PCT / US93 / 01037 is disclosed. '826 and' 477 disclose seed-blanket reactors that derive significant proportions of seed-blanket reactor power from a thorium-fueled blanket. The blanket surrounds the annular seed portion comprising the fuel rods of non-diffusive enriched uranium. The uranium in the seed fuel rods emits neutrons that are captured by the thorium in the blanket, producing fissile U-233 that burns properly and generates heat to power the reactor.

The use of thorium as the reactor fuel in this method is attractive because thorium is much richer than uranium. In addition, the reactors disclosed in '826 and' 477 are required to be non-proliferative in that none of the fuel discharged at the initial fuel loading and at the end of each fuel cycle is suitable for use in the manufacture of nuclear weapons. It employs only non-diffuse enriched uranium as the seed fuel, selects moderator / fuel volume fraction that minimizes plutonium yield, and a small amount of non-diffuse in the blanket where the U-238 component mixes uniformly with the remaining U-233 at the end of the blanket cycle. By adding enriched uranium and by "denaturing" U-233, thus making it useless in the manufacture of nuclear weapons.

Unfortunately, the Applicant has learned through continued research that none of the reactor designs disclosed in the above international patent application is truly non-proliferative. In particular, these designs have all been found to result in more than the minimum yield of diffusive plutonium in the seeds due to the cyclic seed arrangement. The use of the annular seed in combination with the inner central blanket portion and the outer envelope blanket portion is non-proliferative because the thin annular seed correspondingly has a small “optical thickness” which causes the seed spectrum to be governed by the stronger spectrum of the inner and outer blanket portions. It can not be prepared. This results in more diffuse plutonium in the seed than the minimum yield and a larger portion of the epithermal neuron.

These conventional reactor designs are also not optimal in terms of operating parameters. As an example, the moderator / fuel volume ratios of the seed and blanket regions yield a minimum plutonium in the seed region, allow for proper heat removal from the seed fuel rods and in particular to ensure optimal conversion of thorium to U-233 in the blanket. It is important. Subsequent studies indicate that the preferred moderator / fuel ratio disclosed in the above international patent application is too high in the seed region and too low in the blanket region.

Conventional reactor core designs are not particularly efficient in terms of non-diffuse enriched uranium consumption in the seed fuel element. As a result, fuel rods are discharged at the end of each seed fuel cycle containing a number of remaining uranium that must be reprocessed for reuse in other reactors.

Reactors disclosed in '477 also require complex mechanical reactor control devices that are not suitable for retrofitting into conventional reactor cores. Similarly, the reactor disclosed in '822 cannot be easily retrofitted into a conventional core since the design parameters of this core are incompatible with those of the conventional core.

Finally, conventional reactor structures are all designed to burn non-diffusive enriched uranium with thorium, which is not suitable for large plutonium consumption. Thus, none of these designs provide a solution to the stored plutonium problem.

The present invention generally relates to light water reactor structures using thorium as fuel. The reactor may be nuclear, with thorium, non-diffuse enriched uranium, inorganic grade plutonium or reactor grade plutonium.

1 is a schematic cross-sectional view of a reactor core constructed in accordance with a first preferred embodiment of the present invention, known as a non-diffusive light thorium reactor.

FIG. 2 is a detailed cross-sectional view of seed-blanket assembly units (SBUs) employed in the first embodiment. FIG.

3 is a partial cross-sectional view of an SBU modified to include a combustible poison rod for reactor control.

FIG. 4 is a graph illustrating the reactivity level as a function of the maximum power up day for the first seed fuel cycle of various variants for the modified SBU illustrated in FIG. 3.

5A-5I illustrate a fuel loading map corresponding to each of nine different seed fuel cycles employed during operation of the reactor core illustrated in FIG. 1.

6 is a schematic cross-sectional view of a reactor core constructed in accordance with a second preferred embodiment known as plutonium incinerator.

7 is a detailed cross-sectional view of an SBU employed in accordance with a second preferred embodiment.

8 is a core map showing the accumulated fuel consumption and reloading configuration for the second preferred embodiment.

In view of the above, it is an object of the present invention to provide an improved seed-blanket reactor that provides optimum operation in terms of economics and non-proliferation.

Another object of the present invention is to provide a seed-blanket reactor that can be easily retrofitted into a conventional reactor core.

It is a further object of the present invention to provide a seed-blanket reactor that can be used to consume large amounts of plutonium with thorium without producing diffuse waste by-products.

It is a still further object of the present invention to provide a seed-blanket reactor that significantly reduces long-term waste storage requirements by yielding significantly reduced amounts of high level radioactive waste.

The above and other objects of the present invention are achieved through the provision of an improved seed-blanket reactor utilizing thorium fuel in combination with uranium or plutonium fuel.

A first preferred embodiment of the present invention includes a reactor that improves on the non-diffuse reactor disclosed in '477. With the use of new moderators and specific moderators for fuel ratios, this embodiment achieves fuel consumption efficiency that could not be achieved in any known reactors, and produces only nuclear waste that cannot be used for nuclear weapons production. A second preferred embodiment of the present invention is designed to consume large quantities of reactor grade exhaust plutonium and inorganic grade plutonium in a high speed and efficient manner. In other words, the waste thus produced cannot be used for nuclear weapon production.

The first embodiment of the present invention is named as a non-diffusive light-water thorium reactor because the fuel and waste by-products of this reactor cannot be used for the production of nuclear weapons. The core of the non-diffuse reactor consists of a plurality of seed-blanket units (SBUs), each of which includes a centrally located seed region and an annular blanket region surrounding it. SBUs are designed to be easily retrofitted instead of fuel assemblies in conventional reactor cores.

The seed region of the SBUs has a multiplication factor greater than 1 and contains a seed fuel element of enriched uranium with a U-235 to U-238 ratio that is less than or equal to 20% U-235 to 80% U-238 ratio, This is the maximum rate that is considered to be non-diffusion. This enriched uranium is preferably in the form of a plate and / or rod consisting of a uranium-zirconium alloy (uranium-zircaloy) or cermet fuel (uranium oxide particles embedded in a zirconium alloy matrix).

The blanket area has a multiplication factor of less than 1, and a small percentage of enriched uranium (concentrated to 20% U-235) to assist seeding in nuclear supply during the initial start-up stage when thorium fails to power itself. And a blanket fuel element necessarily including Th-232. By adding enriched uranium to the blanket, the blanket can generate at start up a power ratio equal to the power ratio that would occur later when multiple neutrons emitted by the seed fuel element are absorbed by the thorium fuel element of the blanket. . This absorption results in a properly burned fissable U-233, which is powered from the blanket once the reactor is started and started up.

The 20% enriched uranium oxide in the blanket also remains in the blanket at the end of its lifetime by uniformly mixing U-233 with non-fissile uranium isotopes such as U-232, U-234, U-236, and U-238. Denatures any remaining U-233. This metamorphosis is important because it is nearly impossible to isolate the remaining U-233 from non-fissile uranium isotopes, making it inappropriate to use the remaining U-233 in the manufacture of nuclear weapons.

Hard water moderators are used in the seed and blanket areas of each SBU to control reactivity. Unlike conventional uranium cores, boron does not dissolve in hard water moderators during power operation because the multiplication ratio of the blanket is unacceptably small, thus resulting in a sharp decrease in the blanket power ratio.

The volume ratio of hard water moderator to fuel in each area is very important. In the seed zone, in order to ensure that the reactor does not produce a significant amount of plutonium waste that is considered to be diffusive, the moderator / fuel volume ratio should be high enough to be useful for slowing down the neutrons in the seed, and these neutrons are separated by uranium-238 in the seed. By reducing the likelihood of absorption, it produces plutonium. Unfortunately, increasing the moderator volume in the seed naturally means that the fuel volume must be correspondingly reduced, which increases the power density which will produce too much heat if excessively increased. Therefore, these two factors must be considered to determine the optimum moderator / fuel ratio in the seed region. The use of uranium / zirconium alloys for seed fuels allows for a high moderator / fuel ratio because the alloy's thermal conductivity is higher than that of oxide fuels. Use of this type of fuel element should have a moderator / fuel ratio in the seed region between 2.5 and 5.0, preferably between 3.0 and 3.5. The advantage of using a high moderator / fuel ratio in the seed region results in a significant reduction in the generation of high levels of radioactive waste, particularly ultrauranium actinium. This, combined with the fact that the blanket fuel rods remain in the core for about 10 years, results in a significant reduction in long term waste storage space requirements.

The moderator / fuel volume ratio in the blanket region should be significantly lower than the moderator / fuel volume ratio in the seed region because it is desirable for the thorium fuel of the blanket to absorb as much neutrons as possible. It is necessary to convert thorium into properly burned nuclear fissable U-233, providing a significant portion of the reactor power. Research has been established that the optimum moderator / fuel volume ratio in the blanket area should be about 1.5-2.0, preferably about 1.7. If this volume ratio is greater than 2.0, excess thermal neutrons will be absorbed by the water, while if this volume ratio is less than 1.5, it will also interfere with the formation of U-233 and excessive formation of protactinium in the blanket region.

Once-through fuel cycles are used in conjunction with the first preferred embodiment, which does not require reprocessing spent fuel assemblies for future use. In addition, new fueling schemes are employed that maximize fuel consumption in the seed and blanket areas, and further reduce the likelihood that any fuel remaining in the spent fuel element will be reprocessed and used for nuclear weapons production. In such a fueling scheme, the seed fuel element is replaced in a non-smooth way with a portion, preferably 1/3, of the entire seed fuel element replaced at the end of each fuel cycle and the core for one fuel cycle preferably at least 3 fuel cycles. Remains on. Each fuel cycle is about 13 months in duration. Because blanket fuel is mostly made of thorium, it remains in the core for up to nine fuel cycles or about 10 years. However, shuffling of SBUs in the core is performed at the end of each fuel cycle to improve power distribution across the core.

This fueling system allows enriched uranium seed fuel rods to be reduced to less than 20% of their original U-235 content. The long term residence time at the core of the seed fuel element also increases the production of Pu-238 to the point of denaturing a relatively small amount of Pu-239 produced by the seed fuel element. As a result, the spent seed fuel element becomes virtually useless for the formation of nuclear weapons.

The second preferred embodiment of the present invention uses the same basic seed-blanket core device as in the first preferred embodiment having multiple SBUs that can be retrofitted into a conventional reactor core. However, the second embodiment is designed for consumption of large amounts of plutonium, either inorganic or reactor grade, with thorium in the blanket. Thus, thorium oxide is mixed with the plutonium of the blanket fuel rods, while the seed fuel rods are formed predominantly of plutonium-zirconium alloys. Unlike the first embodiment, which aims to maximize the amount of power generated by the thorium of the blanket, the goal of the second embodiment is to reduce the consumption of plutonium without generating large amounts of new plutonium, as is typically the case in conventional reactors. To maximize.

The plutonium incinerator embodiment also utilizes a volume ratio between 2.5 and 3.5, preferably as a high water moderator / fuel volume ratio. However, the reason for using the high volume ratio is different from that in the first embodiment. In particular, the high water to fuel volume ratio provides the ultra-high temperature spectrum in the seed region. This simplifies core control because all controls are concentrated in the seed region, so that control can be made effective without increasing boron chemical control or rod control.

In the blanket region, a significant difference in the plutonium incinerator embodiment is that thorium oxide in the blanket fuel rods is mixed with a small proportion of plutonium oxide to aid during initial reactor operation. In addition, about 2-5 volume% uranium debris (natural uranium with a U-235 content reduced to about 0.2%) is added to the blanket fuel rods. These debris serve to denature U-233 formed in the blanket during reactor operation, becoming obsolete for use in the manufacture of nuclear weapons. The moderator / fuel ratio in the blanket region is preferably between about 1.5 and 2.0 to satisfy neutron constraints and thermal hydraulic constraints.

The features and advantages of the present invention will become apparent from the following detailed description of numerous embodiments with reference to the accompanying drawings.

A. Non-Diffuse Hard Water Thorium Reactor

Referring in detail to the first preferred embodiment of the present invention, known as a non-diffusive light-water thorium reactor, FIG. 1 includes a plurality of fuel assemblies 12 arranged in a hexagonal configuration and hexagonal in cross section, known as a seed-blanket unit. The reactor core 10 is shown. The core 10 is of the same geometry and size as the VVER-1000 and consists of 163 SBU fuel assemblies 12 so that it can be easily retrofitted into a conventional Russian light-water reactor known as the VVER-1000. The difference between the core 10 and the VVER-1000 reactor core is in the configuration of the SBUs 12 as described in detail below. It will be appreciated that the shape and arrangement of the core 10 and the SBUs 12 may be modified as needed to facilitate retrofitting with any type of conventional light pressurized water reactor (PWR). For example, conventional PWRs in the United States and other countries use fuel assemblies with square cross sections, and SBUs 12 may have square cross sections if designed to be retrofitted with such PWRs.

As shown in FIGS. 1 and 5A-5I, the reflector 14, which preferably comprises a plurality of reflector assemblies 16, surrounds the core 10. Each of the reflector assemblies 16 includes a mixture of water and core barrel / pressure vessel metal. Alternatively, each of the reflector assemblies 16 may also be formed primarily of thorium oxide.

2 shows each configuration of the SBU fuel assembly 12. Each of the SBU fuel assemblies 12 includes a centrally located seed region 18 and an annular blanket region 20 surrounding the seed region 18. Seed region 18 is an uranium-zirconium alloy initially containing U-235 / U-238, initially enriched with 20% U-235, which is initially non-proliferative, ie, the maximum concentration that should be considered to be unavailable for nuclear weapon production. It comprises a plurality of seed fuel rods 22 formed. While it is not necessary to maximize the initial U-235 enrichment by 20%, it is desirable to use this enrichment level to minimize plutonium output from the seed during reactor operation. Alternatively, fuel rod 22 may be made of cermet fuel with uranium oxide particles embedded in a zirconium alloy matrix. The use of zirconium alloys (zircaloys) in the seed fuel rods 22 is preferred for oxide type fuels because the zirconium alloys have high thermal conductivity. As will be described in detail below, it is important because the size of space required for the SBU 12 for heat removal can be reduced, thereby increasing the space size useful for the water moderator. Seed region 18 may also be provided with a plurality of water tubes for receiving water moderators (or conventional combustible poison rods and / or control rods as detailed below) to control reactivity in seed region 18. It includes.

The blanket region 20 preferably comprises a plurality of blanket fuel rods 26 formed of mixed thorium-uranium oxide. The uranium volume of the initial thorium-uranium mixture is preferably in the range of about 2-10%, to help fuel the blanket region 20 at start-up before thorium has a chance to absorb neutrons from the seed region. And U-233, a blanket fissionable fuel. As with the seed fuel rods 22, the uranium oxide contained in the blanket fuel rods 24 is preferably concentrated to U-235 / U-238 initially with a maximum non-diffusion ratio of 20:80.

The seed-blanket core 10 operates according to the following simplified equation 1 to share power between the seed region 18 and the blanket region 20.

P _b / P _s = ε (K _b / (1-K _b )) ((K _s -1) / K _s )

In Equation 1, K _s and K _b are multiplication ratios of the seed region and the blanket region, respectively. P _b and P _s are the power generated in the seed region and the blanket region, respectively, and epsilon is slightly above 1 as a high speed effect. Seed multiplication (K _s ) is greater than 1 and blanket multiplication (K _b ) is less than 1. Thus, the blanket is subcritical and the seed acts as a neutral resource for the blanket.

In order to maximize the amount of energy calculated from thorium, it is necessary to distribute the core power calculated in the blanket area 20 in as high a ratio as possible. This is K _s

Achieved by making it as high as possible, K _s was determined to be possible up to 1.7 and K _b is chosen between about 0.85 and 1.

The number of neutrons absorbed by U-238 in the seed region 18 should be minimized. Most neutrons absorbed by U-238 are in the so-called resonant energy region indicated by the tightly spaced energy intervals for very high absorption. On the other hand, most of the nuclear fission in U-235 occurs at low energy in the thermal zone where the average neutron energy is nearly equilibrium with the ambient temperature of the hard water moderator. By making the water content of the seed region 18 as high as possible, the number of neutrons in the resonance region is reduced, so that fewer neutrons are captured by U-238.

U-238 capture reduction has two good effects. First, the multiplication ratio (K _s ) of the seed region is increased, thereby increasing the ratio of core power calculated from the blanket as described above, and second, the multiplication ratio of the plutonium since it captures U-238 neutrons forming plutonium. Formation is minimized.

The amount of water that can be placed in the seed region 18 is limited by the need to have enough room for the fuel rods 22 to be able to remove adequate heat from the fuel rods 28. The volume and surface area of the fuel rod must not be reduced to the point where the power density of the core rises above the operating limits dictated by the reactor's cooling system. By making the seed fuel element 22 from a uranium / zirconium alloy having a higher thermal conductivity than oxide fuel, the water moderator / fuel volume ratio of the seed region 18 can be compared to two to one in a conventional uranium core. To a high ratio of 4 or 5 to 1. The water moderator / fuel ratio of the seed region 18 should therefore be chosen between about 2.5 and 5.0 and most preferably between 3.0 and 3.5.

The high moderator / fuel volume ratio advantage of the seed region 18 is that it significantly reduces the amount of high level radioactive waste generated in the seed region 18. In particular, since the seed spectrum is a high temperature resulting from a large number of water collisions, very small amounts of ultrauranium or small amounts of actinides will be produced. These half-life millions of years require long-term storage in these actinides. Thus, during the 10-year blanket life associated with reduced actinide yield by the non-diffusing core 10, less radioactive waste is produced and heat is generated for a shorter period of time. This will significantly reduce the underground storage space requirements. In addition, the low level waste is somewhat reduced because no boric acid is dissolved in the water moderator during normal operation, so no tritium is produced in the core. It should be noted that the reason that boric acid is not used in the water moderator is that it unacceptably lowers the multiplication factor in the blanket region 20.

The moderator / fuel volume ratio of the blanket region 20 is a very important parameter, but depends on different constraints. In particular, the situation of the blanket region 20 is more complicated because excess water reduces K _b by absorbing excess neutrons coming from the seed fuel element, thus removing these neutrons from thorium. On the other hand, excessive water in the blanket area increases damage to protactinium. When thorium absorbs neutrons, it forms protactinium and then collapses into fissable U-233 with a half-life of 27.4 days. During this interval, proactinium is vulnerable to neutron absorption and forms a non-fissile U-234. This is a double loss for the neutrons and the expected U-233 nucleus. Studies have shown that in order to minimize this loss, the optimal value of the water / fuel ratio in the blanket region 20 should be chosen between about 1.5 and 2.0, preferably 1.7.

Preferably, seed region 18 comprises 25 to 40 percent of the total volume in SBU 12. This value range is also determined based on competing considerations. First, because it is designed to burn as much as possible, the blanket area 20 must be substantially large. On the other hand, the seed region 18 should not be made small so that the power density in the seed region rises too high for the above reason. The 25-40 percent has been determined to provide the optimum balance of these performances in consideration.

Another important design aspect of the SBU 12 is the central seed / annular blanket configuration. In Applicant's previous International Patent Application Publication No. WO85 / 01826, a seed-blanket core using an annular seed having an inner, a central blanket portion and an outer, surrounding blanket portion is disclosed. Such an arrangement cannot be made non-diffusively because the thin annular seed has a corresponding small "optical thickness" which causes the seed spectrum to be suppressed by the high intensity spectra of the inner and outer blanket portions. This results in high thermal neutron energy and also increases output of Pu-239 in the seed region. The central seed arrangement of the SBU 12 overcomes this drawback by making the seed 18 thick enough to prevent excessive interaction with thermal neutrons crossing from the blanket 20 to the seed 18. do.

Reference core and fuel assembly parameters for the core 10 and each SBUs 12 are shown in Table 1 and Table 2 below, respectively. These parameters were chosen to provide full compatibility of SBU fuel assemblies with existing (typical) VVER-1000 plants.

Core parameters parameter Full power (MWth) 3000 Average power density (W / cm ³ ) 107 Average moderator temperature, ℃ 306 Average SBUs Number of Cores 163 Number of control rod clusters (CRC) 61 Number of control rods per CRC 12 Blanket fuel U + Th (O ₂ ) Seed fuel U / Zr Alloy Seed reload schedule 54 seeds / cycle 1 year) Blanket Reload Schedule 163 blanket / 9 cycles 10 years)

Subparameters parameter Seed Blanket Outer diameter of fuel pellets, cm 0.310 0.380 Outer diameter of gas gap, cm - 0.3865 Outer diameter of cladding, cm 0.370 0.4585 Cell radius, cm 0.6652 0.6731 Pitch, cm 1.267 1.282 Moderator / Fuel Volume Ratio 3.18 1.68 Number of fuel rods 156 162 Number of water tubes 12 0 Number of other tubes One 0 Seed total weight, th.M. 6.71 - Blanket total weight, th.M. - 35.82 U (on the blanket) t - 3.11

To provide additional reactivity control during each seed cycle, the SBU 12 may include a plurality of combustible poison rods 28 and 30 spaced apart in the seed region 18 as shown in FIG. 3. It can be modified as well. In the example illustrated in FIG. 3, the first group of combustible poison rods 28 and 30 includes standard Westinghouse Inc. combustible poison rods known as WABAs currently used in prior art PWR fuel systems. These rods are formed from a composite of boron-10, boron-11, carbon, aluminum and oxygen. The second group of combustible poison rods 28 and 30 includes uranium / zircaloy seed fuel rods modified to contain a small proportion of natural gadolinium. The number and combination of flammable poison rods 28 and 30 can be employed as needed. In the example shown in FIG. 3, each SBU 12 includes six gadolinium / fuel rods 30 and six WABAs 28.

Both types of flammable poison rods have advantages. WABAs provide more uniform control of reactivity until the end of each reactor fuel cycle, while gadolinium / fuel rod 30 provides a large negative reactivity input for the first third of the reactor cycle life. FIG. 4 shows SBUs as a function of maximum power uptime for each of the four seed control variations in the absence of toxins, for gadolinium toxins, for boron toxins, and for bound gadolinium and boron toxins. Reactivity level (K) is shown in each of (12). As shown, the combination of the two toxic control types results in the flattest reactivity curve.

Conventional control rods are also preferably employed to compensate for transient reactivity of the reactor core. In addition, control rods can be employed for power transition and moderator temperature transition compensation by Xe vibration and emergency shutdown of the reactor. The control rods are assembled into CRCs with 12 control rods per control rod cluster (CRC). As shown in Table 1 above, each of the SBUs 12 need not include a CRC, and the calculation is sufficient to place one of 61 CRCs for the 163 SBUs of the core.

In operation of the non-diffusion hard thorium reactor core 10, a one-complete fuel cycle may be employed in which all fuel rods in the seed and blanket regions 18 and 20 are used only once in the reactor core. However, specific fuel management schemes may be employed in which the seed and blanket fuel assemblies are performed along separate fuel management paths. In particular, each of the seed fuel rods 22 remains in the reactor core for at least one seed fuel cycle (about 13 months) and preferably for three seed fuel cycles, but only a portion of the seed (preferably at the end of each seed fuel cycle). 1/3) is replaced. Preferably, the location of the SBUs 12 of the core 10 is shuffled at the end of each seed fuel cycle to improve power distribution through the core. In contrast, each blanket fuel rod 24 remains in each SBU 12 for the entire life of the blanket region 20, preferably nine fuel cycles or about ten years.

This fuel management scheme, coupled with the seed-blanket arrangement and associated with the core parameters, allows about 80-90% of the uranium to be consumed in this element before the seed fuel element 22 is removed from the core 10. As a result, the spent seed fuel rods 22 are not economical or of no value as an atomic nucleus because little U-235 originally loaded remains.

In addition, this enlarged combustion degree of the seed fuel rods increases Pu-238 sufficient to completely denature the small amount of Pu-239 (about 30 kg per year) calculated in the seed zone 18. More specifically, about 8-9% of the total plutonium produced by the reactor core 10 is Pu-238. Because Pu-238 is a heat generator that produces about 300 times more heat than Pu-239, this high proportion of Pu-238 prevents the use of plutonium produced by nuclear reactor cores for nuclear weapons. In particular, reactor grade plutonium with a Pu-238 content of 4.9% by weight or more cannot be used for nuclear weapons even when nuclear weapons are cooled to 0 ° F. With this concentration, the heat generated by Pu-238 melts high performance explosives and ultimately melts the plutonium core or at least changes from its normal alpha phase to delta phase. Phase change decreases its density and substantially increases its critical mass. Since the non-diffusing core 10 yields a concentration of Pu-238 that is well above 4.9% by weight, this effectively makes the discharged plutonium essentially non-diffuse.

A number of batch fuel management schemes are shown in greater detail through FIGS. 5A-5I showing the pie slices of about one fifth of the SBUs 12 in the core 10. Each of FIGS. 5A-5I shows a loading map for each of the nine seed fuel cycles corresponding to one blanket fuel cycle. This fuel loading map represents the basic research employed, a three batch fuel management system. This means that in every cycle, except for transition cycles 1 and 2, there are three seed batches that are fresh, one-burn and two-burn. These are denoted F, O and T in the reload map, respectively. Another major factor influencing the reloading pattern is the overuse of nuclear reactable toxins that can suppress local power peaks. Most of the unburned fuel is not loaded on the core periphery, but is distributed mainly within positions 6, 8, 10 and 12 and close to positions 20, 21, 23, 26 and 32. The additional information shown in FIGS. 5A-5I shows the distribution of U-Gd and WABA toxic rods within the core. The sophisticated distribution of flammable toxic rods indicates the complexity of the low leakage configuration and reloading patterns used in the present reactor design. These SBUs with CRCs are also designated C.

At the start of core life, ie cycle 1, all unburned seed fuel assemblies are loaded. Three different uranium enrichments and weight ratios are used to achieve a reasonable radial power distribution. As shown in FIG. 5A, one third of the SBUs 12 have 9.5% by volume uranium enriched to 12% by weight U-235, and the other one third of the SBUs 12 is 17% by weight U-235. It has 14.5% by volume uranium enriched and another 1/3 of SBUs (12) have 17% by volume uranium enriched with 20% by weight U-235. A target unburned fuel enrichment of 20% by weight of U-235 was then used in Cycles 3-9. Thus, cycles 1 and 2 are transition cycles and cycles 3-9 are quasi-balance cycles. Unburned fuel enrichment was constant at 20% by weight U-235, but the uranium weight ratio in the U / Zr alloy was varied to ensure 300 maximum power uptime corresponding to one seed fuel cycle. Since the reactor generally does not run at full power for the entire fuel cycle, the actual duration of the seed fuel cycle is about 13 months.

B. plutonium incinerator

A second preferred embodiment of the present invention relates to another seed-blanket reactor design known as plutonium incinerator. As the name implies, the ultimate purpose of this second embodiment is to consume as much of the weapon grade or reactor grade plutonium as possible. This is contrary to the object of the first embodiment of the present invention which derives as much energy as possible from the thorium fuel of the blanket. As will be explained in detail below, the purpose of the plutonium incinerator, which is completely in contrast to the purpose of the first embodiment, specifies that the core parameters used must be completely different.

The preferred form of the plutonium incinerator illustrated in FIG. 6 is formed of a number of SBUs 102 and includes a reactor core 100. Core 100 generally has a circular cross section of 89 SBUs 102 each having a square cross section. It should be noted that the size and shape of the reactor core may be arbitrary and may be modified as needed to achieve the desired power output and / or to accommodate retrofitting to any type of conventional core.

Each SBUs 102 includes a central seed region 104 and an annular blanket region 106. The total volume ratio of the SBU occupied by the seed region 104 is chosen in this embodiment as large as possible, preferably between about 45 and 55%, so that as much plutonium can be combusted as possible in the seed region. The reflector 108 is made of any suitable material, such as thorium oxide, and surrounds the core 102.

One preferred form for SBU 102 is illustrated in FIG. 7. As shown, the seed region 104 consists of a first multi-seed fuel rod 110 formed of plutonium (inorganic or reactor grade) and zirconium alloy or alternatively cermet fuel. The plurality of water holes 112 are evenly spaced throughout the seed region 104 to accommodate the control rod pins. The first and second plurality of combustible poison rods 114 and 166 are uniformly disposed throughout the seed region 104. The flammable poison rod 114 is preferably formed from a mixture of seed fuel and gadolinium. These can be of two types, the first type having a gadolinium concentration of 0.36 g / cc and the second type having a gadolinium concentration of 0.72 g / cc. Combustible poison rods 116 preferably comprise a conventional WABA poison rod. Any combination of the two types of nuclear reactable poison rods 114 and 166 may be employed as desired.

The blanket region 106 includes a plurality of blanket fuel rods 118 formed primarily of thorium oxide. Preferably, a small proportion of plutonium oxide, less than 1 vol.%, Is mixed with thorium oxide in blanket fuel rod 118 to maintain high blanket multiplication rates during initial reactor operation. In addition, about 2-5% by volume uranium residue is added to thorium to denature U-233 formed in thorium during reactor operation by non-fissile isotopes such as U-232, U-234, U-236 and U-238. do.

The moderator / fuel volume ratio of the seed region 104 is chosen to be higher than a conventional reactor core, but the reason for this selection is different from the non-diffusion embodiment of the present invention. In particular, the moderator / fuel volume ratio is selected between about 2.5 and 3.5, preferably between 2.5 and 3.0. This selection creates a thermal neutron trap in the seed region and increases the value of the controlled toxic reactivity in the trap, thereby making it easier to control the reactor. As in the non-diffusing core embodiment, the moderator / fuel ratio of the blanket region is selected between about 1.5 and 2.0.

The main core and SBU parameter values for the plutonium incinerator of the present invention are shown in Tables 3 and 4 below.

Primary core parameter parameter value Power level, MWth 3250 Number of SBUs in the core 89 Equivalent diameter of core, cm 380 Active height of the core, cm 365 Average power density, w / cz 78.5

Additional Core Parameters parameter Seed Blanket Fuel Rod / SBU Number 264 384 Number of water holes / SBU 25 0 Flat length, cm 25.5 35.7 SBU Volume% 51 49 Fuel pin diameter, mm 8.7 8.7 Fuel rod diameter, mm 9.7 9.7 Pitch, mm 15.0 12.75 Moderator / Fuel Volume Ratio 2.54 1.49 Fuel type Metal oxide composites Fuel material 2.4 vol% Pu 0.55 volume% PuO ₂ 97.6% by volume zircaloy 94.45-97.45 volume% ThO ₂ 2.0-5.0 volume% U residue Core heavy metal load, kg 2300 Pu 60,700Th392 Pu100 U residue

In operation of the plutonium incinerator core 100, the seed fuel rods 110 and the blanket fuel rods 118 remain in the core for two years and are discharged simultaneously. This fuel reloading system is optimal in terms of plutonium inventory reduction rates, but may be the next best option in terms of thorium use. However, this should not be a problem because the purpose of the plutonium incinerator core 100 maximizes plutonium consumption.

Preferably, the fuel management system employs a two-batch core with a standard out-in pattern. Reload configurations and accumulated burnups for one- and two-combustion fuel assemblies are illustrated in the core map of FIG. 8. The accumulated burnup rate for the two-combustion fuel assembly is about 15 GWD / T with an average fuel emission of about 31 GWD / T. Three different types of fuel assemblies are shown in the core map of FIG. 8. The type A assembly comprises 20 gadolinium based on the flammable toxic rod 14, each having a concentration of 0.36 g / cc gadolinium, and the type B assembly has a flammable toxic rod, having a concentration of 0.72 g / cc gadolinium. 20 gadolinium based on 114, and a Type C assembly includes 20 gadolinium based on flammable poison rod 114 and WABA flammable poison rod 116 having a concentration of 0.72 g / cc gadolinium. .

The annual charge of Pu-239 in the plutonium incinerator core 100 is about 1350 kg. Since 500 kg of plutonium is released from the reactor, the remainder of the remaining plutonium is in the form of plutonium isotopes, Pu-240, 241 and 242, leaving a net decay rate of only about 850 kg of the total plutonium. An equilibrium cycle based on a standard size LWR fuel assembly using the seed-blanket concept results in an equilibrium result.

The advantage of using thorium fuel cycles for Pu-239 incineration of seed-blanket reactors is shown by the neutron properties of thorium, ie its large heat absorption cross section. This results in a large, large initial Pu stock, consuming high Pu per unit energy. Thorium blanket operation by Pu fissile material results in high Pu power sharing and thus efficiently incinerates Pu.

The use of conventional homogeneous light water reactor (LWR) core structures presents a problem with controllability. Excessive reactivity of a fuel cycle based on Pu is the same value as a cycle based on the same uranium, but the reactivity value is considerably lower. Pu-based fuels are characterized by a very large heat absorption cross section and are comparable to control toxicants for thermal neutrons. The result of conventional homogeneous assembly design is that the control rod efficiency, soluble boron, and nuclear reactable toxins are reduced by about two factors compared to conventional LWR values. An obvious solution to this problem is to improve the responsiveness control values of different control mechanisms, such as the use of stronger absorbers and / or an increase in the moderator / fuel volume ratio of the core. Unfortunately, this solution negatively affects the safety and economic performance parameters of the reactor.

Thorium based seed-blanket structures provide a unique solution to this problem that does not involve economic or operational drawbacks. Since the control rods and / or flammable poison rods are located only in the seed region of each SBU 102, their control effect is significantly increased because the power density of the seed portion is higher than the average power density of the core. Thus, the neutrons in the seed region The main function of is very large, increasing the response of the control and poison rods. The high moderator / fuel volume ratio in the seed zone also improves power distribution in the SBU and increases the control toxic reactivity values by generating thermal neutron traps in the seed zone.

C. Summary

In summary, the present invention provides two new thorium-based seed-blanket reactor core devices that are particularly important in providing an economical and reliable power source while at the same time providing an economical and viable solution to the problems of proliferation and weapons grade nuclear destruction. To provide. The non-proliferative embodiment of the present invention is ideal for underdeveloped countries because it eliminates the concern that nuclear reactor fuel or waste will be used for nuclear weapons production, since the reactor fuel or waste cannot be used for the purpose of the production. Plutonium incinerator embodiments are particularly useful to provide excellent means by which stockpile reactor grade and inorganic grade plutonium can be readily collapsed. In both embodiments, the seed-blanket core device is essential to provide the desired result. Without this core device, a non-diffusion embodiment produces inoperable, ie diffused waste. In plutonium incinerators, the seed-blanket core device is required to ensure proper reactor control and prevents the production of significant new Pu-239.

Although the present invention has been described with reference to a number of preferred embodiments, it will be understood that other modifications and variations to the embodiment can be made without departing from the scope of the invention as defined in the claims.

Claims (67)

In a reactor having a core comprising a plurality of seed-blanket units, each of the seed-blanket units includes: a) a central seed region comprising a seed fuel element formed of fissile material comprising U-235 and U-238; b) an annular blanket region comprising a blanket fuel element consisting predominantly of thorium and up to about 10 vol% enriched uranium and surrounding the seed region; c) moderator in said seed region having a volume ratio of moderator to fuel in the range of about 2.5 to 5.0; And d) a moderator in said blanket region having a moderator to fuel volume ratio of about 1.5 to 2.0. The reactor of Claim 1, wherein each of said seed fuel elements is comprised of a uranium-zirconium alloy. The reactor of claim 1, wherein the seed region comprises about 25% to 40% of the total volume of each of the seed-blanket units. The method of claim 1, wherein the first group of seed-blanket units comprises a seed fuel element containing uranium enriched to a first level and the second group of seed-blanket units is enriched to a second higher level. A seed fuel element containing uranium, wherein said seed fuel element of said first group of said seed-blanket unit is designed to remain in a core for one seed fuel cycle, and wherein said seed fuel element of said second group of said seed-blanket unit And the seed fuel element is designed to remain in the core for at least two seed fuel cycles. 5. The seed fuel element of claim 4, wherein the third group of seed-blanket units comprises a seed fuel element enriched to a third upper uranium level, wherein the seed fuel element of the third group of seed seed blanket units is at least three seeds. A reactor designed to remain in the core during a fuel cycle. The nuclear reactor of Claim 1, wherein the moderators of the seed region and the blanket region are hard water. The reactor of claim 1, wherein the volume ratio of moderator to fuel in the seed region is about 3.0 to 3.5. The nuclear reactor of Claim 1, wherein said seed region further comprises a plurality of combustible poison rods. 10. The reactor of Claim 8, wherein the plurality of combustible poison rods comprise gadolinium and a WABA poison rod comprising the poison rod. (delete) (delete) (delete) (delete) (delete) (delete) (delete) (delete) (delete) (delete) (delete) The blanket of claim 1 wherein the seed fuel element is formed of fissile material comprising U-235 and U-238 at a ratio of about 20% by weight or less of U-235 to about 80% or more by weight of U-238 and the blanket Wherein the fuel element comprises up to about 10 volume percent enriched uranium having primarily thorium and up to 20 percent by weight U-235. 22. The nuclear reactor of Claim 21, wherein each of said seed fuel elements is comprised of a uranium-zirconium alloy. The reactor of claim 21, wherein the seed region comprises about 25% to 40% of the total volume of each seed-blanket unit. 22. The method of claim 21, wherein the first group of seed-blanket units comprises a seed fuel element containing uranium enriched to a first level and the second group of seed-blanket units is enriched to a second higher level. A seed fuel element containing uranium, wherein said seed fuel element of said first group of said seed-blanket unit is designed to remain in a core for one seed fuel cycle, and wherein said seed fuel element of said second group of said seed-blanket unit And the seed fuel element is designed to remain in the core for at least two seed fuel cycles. 25. The system of claim 24, wherein the third group of seed-blanket units comprises a seed fuel element enriched to a third higher uranium level, wherein the seed fuel element of the third group of seed-blanket units is at least three. A reactor designed to remain in the core during a seed fuel cycle. 22. The nuclear reactor of Claim 21, wherein the moderator, the seed region, and the blanket region are hard water. 22. The reactor of Claim 21, wherein the volume ratio of moderator to fuel in the seed region is about 3.0 to 3.5. 22. The nuclear reactor of Claim 21, wherein the central seed region further comprises a plurality of combustible poison rods. 29. The reactor of claim 28, wherein the plurality of combustible toxic rods comprises gadolinium and a WABA toxic rod comprising a toxic rod. 22. The reactor of claim 21, wherein each of the seed-blanket units has a cross-sectional shape and size that allows for retrofitting of the seed-blanket unit in a conventional pressurized water reactor. The reactor of Claim 1, wherein each of said seed-blanket units has a cross-sectional shape and size that enables retrofitting of said seed-blanket unit in a conventional pressurized water reactor. 32. The reactor of claim 31 wherein each of the seed-blanket units has a hexagonal cross-sectional shape. 32. The nuclear reactor of Claim 31, wherein each of said seed-blanket units has a square cross-sectional shape. 31. The nuclear reactor of Claim 30, wherein each of said seed-blanket units has a hexagonal cross-sectional shape. 31. The nuclear reactor of Claim 30, wherein each of said seed-blanket units has a square cross-sectional shape. In a light-water reactor core each seed-blanket unit comprising a plurality of said seed-blanket units having a cross-sectional shape selected from the group consisting of hexagonal and square, wherein each seed-blanket unit comprises: a) a first group of said seed-blanket units comprising U-235 and U-238, a seed fuel element containing uranium enriched to a first level, and a seed fuel containing uranium enriched to a second higher level A central seed region comprising a seed fuel element formed of a fissile material comprised of a second group of said seed-blanket units comprising an element, wherein said seed fuel element of said first group of said seed-blanket units comprises: Designed to remain in the core for a seed fuel cycle, wherein the seed fuel element in the second group of seed-blanket units is designed to remain in the core for at least two seed fuel cycles; b) an annular blanket region surrounding the seed region and containing a blanket fuel element comprising predominantly thorium and up to about 10 vol% enriched uranium; c) moderator in said seed region having a volume ratio of moderator to fuel between about 2.5 and 5.0; And d) a moderator in said blanket region having a moderator to fuel volume ratio between about 1.5 and 2.0. 37. The light water reactor core of claim 36, wherein the seed region comprises about 25% to 40% of the total volume of each seed-blanket unit. 37. The fuel cell of claim 36, wherein the third group of seed-blanket units comprises a seed fuel element enriched to a third higher uranium level, wherein the seed fuel element of the third group of seed-blanket units is at least three. A light-water reactor core designed to remain in the core during a seed fuel cycle. 37. The method of claim 36, wherein the seed fuel element is formed of a fissile material comprising U-235 and U-238 at a ratio of about 20% by weight or less of U-235 to about 80% or more by weight of U-238, and the blanket The light water reactor core, characterized in that the fuel element mainly comprises about 10 volume percent enriched uranium with thorium and up to 20 weight percent U-235. In a light-water reactor core each seed-blanket unit comprising a plurality of said seed-blanket units having a cross-sectional shape selected from the group consisting of hexagonal and square, wherein each seed-blanket unit comprises: a) a central seed region comprising about 25% to 40% of the total volume of each said seed-blanket unit, said seed fuel element being formed of a fissile material comprising U-235 and U-238; b) an annular blanket region surrounding the seed region and containing a blanket fuel element comprising predominantly thorium and up to about 10 volume percent enriched uranium; c) moderator in said seed region having a volume ratio of moderator to fuel between about 2.5 and 5.0; And d) a moderator in said blanket region having a moderator to fuel volume ratio between about 1.5 and 2.0. In a light-water reactor core each seed-blanket unit comprising a plurality of said seed-blanket units having a cross-sectional shape selected from the group consisting of hexagonal and square, wherein each seed-blanket unit comprises: a) U-235 and U-238, comprising from about 25% to 40% of the total volume of the seed-blanket unit, at a ratio of up to about 20% by weight of U-235 to at least about 80% by weight of U-238; A seed fuel element formed of a fissile material comprising: a first group of seed-blanket units comprising a seed fuel element containing uranium enriched to a first level, a seed containing uranium enriched to a second higher level A central seed region comprising a second group of said seed-blanket units comprising a fuel element, such that said seed fuel element of said first group of said seed-blanket units remains in a core for one seed fuel cycle; The seed fuel element of the second group of seed-blanket units is designed to remain in the core for at least two seed fuel cycles; b) an annular blanket region surrounding the seed region and comprising a blanket fuel element comprising predominantly thorium and up to about 10 volume percent enriched uranium; c) a hard water moderator in said seed region having a volume ratio of moderator to fuel between about 2.5 and 5.0; And d) a light water moderator of said blanket region having a volume ratio of moderator to fuel between about 1.5 and 2.0. (delete) (delete) (delete) (delete) (delete) (delete) (delete) (delete) (delete) (delete) (delete) (delete) (delete) (delete) (delete) A seed-blanket unit fuel assembly for a nuclear reactor, a) the seed fuel element is configured such that the moderator to fuel ratio in the seed region is between 2.5 and 5.0 during the reaction with the moderator in the seed region and is formed of enriched uranium comprising U-235 and U-238 A central seed region consisting of; And b) the blanket fuel element is configured such that the volume ratio of moderator to fuel is between 1.5 and 2.0 during reaction with the moderator in the annular blanket region, surrounding the seed region and comprising mainly thorium and up to 10% by volume of concentrated uranium And said annular blanket region comprised of said blanket fuel element. 59. The seed blanket unit fuel assembly of claim 57, wherein each of said seed fuel elements is comprised of a uranium-zirconium alloy. 59. The seed blanket unit fuel assembly of claim 57, wherein each of the blanket fuel elements is comprised of mixed thorium-uranium oxide. 59. The seed-blanket unit fuel assembly of claim 57, wherein the central seed region comprises between 25% and 40% of the seed-blanket unit volume. 59. The seed-blanket unit fuel assembly of claim 57, wherein the seed fuel element is configured such that the volume ratio of moderator to fuel in the seed region is between 3.0 and 3.5 while reacting with the moderator in the central seed region. 59. The seed-blanket unit fuel assembly of claim 57, wherein the central seed region further comprises a plurality of combustible poison rods. 63. The seed-blanket unit fuel assembly of claim 62, wherein the plurality of combustible toxic rods comprise rods selected from the group comprising a toxic rod consisting of WABA toxic rods and gadolinium. 59. The blanket fuel element of claim 57 wherein the seed fuel element is formed from enriched uranium comprising U-235 and U-238 at a ratio of up to 20 weight percent U-235 to 80 weight percent U-238. The seed-blanket unit fuel assembly of claim 11, wherein the seed-blanket unit fuel assembly comprises up to 10% by volume of enriched uranium having primarily thorium and up to 20% by weight of U-235. 59. The seed-blanket unit fuel assembly of claim 57, wherein the assembly has a cross-sectional shape and size that enables retrofitting the seed-blanket unit in a conventional light water reactor. 66. The seed-blanket unit fuel assembly of claim 65, wherein the assembly has a hexagonal cross-sectional shape. 66. The seed-blanket unit fuel assembly of claim 65 wherein the assembly has a square cross-sectional shape.