Abstract:
The invention relates to light water reactor designs in which thorium is used as fuel and in particular to designs of jacketless fuel assemblies, which make up the cores of pressurized water reactors (PWRs) such as the VVER-1000. Nuclear reactor cores containing seed and blanket subassemblies that make up the fuel assemblies are used to burn thorium fuel together with conventional reactor fuel that includes nonproliferative enriched uranium, as well as weapons-grade and reactor-grade plutonium. In the first alternative, the reactor core is fully “nonproliferative,” since neither the reactor fuel nor the wastes generated can be used to produce nuclear weapons. In the second version of the invention, the reactor core is used to burn large amounts of weapons-grade plutonium together with thorium and provides a suitable means to destroy stockpiles of weapons-grade plutonium and convert the energy released to electric power. The cores in both embodiments of the invention are made up of a set of seed-blanket assemblies, which have central seed areas surrounded by annular blanket areas. The seed areas contain uranium or plutonium fuel rods, while the blanket areas contain thorium fuel rods. The volume ratio of moderator to fuel and the relative sizes of the seed area and the blanket area have been optimized so that neither embodiment of the invention generates wastes that can be used to produced nuclear weapons. A new refueling system is also used for the first embodiment of the invention to maximize recycling of the seed fuel; the system also ensures that the spent nuclear fuel cannot be used to produce nuclear weapons.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority under the Paris Convention to PCT/RU2007/000732, filed Dec. 26, 2007, and to U.S. provisional patent application Ser. No. 61/116,730, filed Nov. 21, 2008, the content of both of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates in general to light water reactor designs in which thorium is used as fuel and in particular to designs of jacketless fuel assemblies, which make up the cores of pressurized water reactors (PWRs) such as the VVER-1000. 
     BACKGROUND 
     Nuclear power remains an important energy resource throughout the world. Many countries that lack adequate indigenous fossil fuel resources rely primarily on nuclear power to produce electricity. In many other countries, nuclear power is used as a competitive source of electricity which also increases the diversity of the types of energy used. In addition, nuclear power also makes a very important contribution to the achievement of such goals as controlling fossil fuel pollution (such as acid rain and global warming) and conserving fossil fuel for future generations. 
     Although safety is certainly a major issue in the design and operation of nuclear reactors, another key issue is the danger of the proliferation of materials that could be used in nuclear weapons. This danger is especially relevant to countries with unstable governments, whose possession of nuclear arms could pose a significant threat to world security. Nuclear power therefore should be generated and used in a way that does not lead to the proliferation of nuclear weapons and the resulting risk of their use. 
     All current nuclear reactors create large amounts of material customarily referred to as reactor-grade plutonium. A typical 1000 MW reactor, for example, creates about 200-300 kg per year of reactor-grade plutonium, which can be suitable for producing nuclear weapons. Hence the fuel discharged from the cores of conventional reactors is highly proliferative material, and security measures are required to prevent the discharged fuel from falling into the hands of unauthorized individuals. There is a similar security problem with the enormous stockpiles of weapons-grade plutonium created in the U.S. and the countries of the former Soviet Union in the process of dismantling of nuclear weapons. 
     There are other problems in the operation of conventional nuclear reactors associated with the constant need to dispose of long-life radioactive waste and the rapid depletion of worldwide supply of natural uranium raw material. 
     To solve these problems, there have been recent attempts to develop nuclear reactors that use relatively small amounts of nonproliferative enriched uranium (enriched uranium has a U-235 content of 20% or less) and do not generate significant amounts of proliferative materials such as plutonium. Examples of such reactors have been disclosed in international applications WO 85/01826 and WO 93/16477, which disclose seed-blanket reactors that obtain a substantial percentage of their power from blanket zones with thorium fuel. The blanket zones surround a seed zone containing fuel rods of nonproliferative enriched uranium. The uranium in the seed fuel rods releases neutrons which are captured by the thorium in the blanket zones, thus creating fissionable U-233, which burns in place and releases heat for the reactor power plant. 
     The use of thorium as nuclear reactor fuel is attractive because worldwide thorium reserves are considerably larger than uranium reserves. In addition, both of the aforementioned reactors are “nonproliferative” in the sense that neither the initial fuel loaded nor the fuel discharged at the end of each fuel cycle is suitable for producing nuclear weapons. This result is achieved by using only nonproliferative enriched uranium as seed fuel, selecting moderator/fuel volume ratios to minimize plutonium production, and adding a small amount of nonproliferative enriched uranium to the blanket zone, where the U-238 component is evenly mixed with the residual U-233 at the end of the blanket cycle and “denatures” (changes the natural properties of) the U-233, as a result of which it becomes unsuitable for making nuclear weapons. 
     Unfortunately, neither of the aforementioned reactor designs is truly “nonproliferative.” In particular, it has been discovered that both of the designs result in a level of production of proliferative plutonium in the seed zone which is higher than the minimum possible level. The use of a circular seed zone with both an inner or central blanket zone and an outer, surrounding blanket zone cannot provide reactor operation as a “nonproliferative” reactor, since the thin, annular seed zone has a correspondingly small “optical thickness,” which results in a seed (neutron) spectrum which dominates the considerably harder spectrum of the inner and blanket zones. This results in a higher proportion of epithermal neutrons in the seed zone and production of a higher than minimum quantity of proliferative plutonium. 
     In addition, neither of the previous reactor designs has been optimized from the standpoint of operational parameters. For example, moderator/fuel volume ratios in the seed zone and blanket zones are particularly critical for minimizing the amount of plutonium in the seed zone, so that adequate heat is released by the seed fuel rods, and optimum conversion of thorium to U-233 in the blanket zone is ensured. Research shows that the preferred moderator/fuel ratios indicated in the international applications are too high in the seed zones and too low in the blanket zones. 
     The previous reactor core designs also are not especially effective in consuming 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 had to be reprocessed for reuse in another reactor core. 
     The reactor disclosed in application WO 93/16477 also requires a complex mechanical reactor control system which makes it unsuitable for refitting a conventional reactor core. Similarly, the reactor core disclosed in application WO 85/01826 cannot easily be transferred into a conventional core, because its design parameters are not compatible with the conventional core parameters. 
     Finally, both of the previous reactor designs were designed specifically to burn nonproliferative enriched uranium with thorium and are not suitable for consuming large amounts of plutonium. Hence neither design provides a solution to the problem of stockpiled plutonium. 
     A reactor with a core which includes a set of seed-blanket assemblies, each of which contains a central seed region which includes seed fuel elements made of a material capable of nuclear fission containing uranium-235 and uranium-238, an annular blanket that surrounds the seed region and includes blanket fuel elements containing primarily thorium and 10% by volume or less enriched uranium, a moderator in the seed region, with a volume ratio of moderator to fuel in the range of 2.5 to 5.0, and a moderator in the blanket region, with a ratio of moderator to fuel in the range of 1.5 to 2.0, is known according to patent RU 2176826. Each of the seed fuel elements is made of uranium-zirconium alloy, and the seed zone makes up 25-40% of the total volume of each seed-blanket module. 
     The known reactor provides optimum operation from the standpoint of economy and is not “proliferative.” This reactor can be used to consume large amounts of plutonium with the thorium without generating proliferative wastes. The reactor produces substantially smaller amounts of hot waste, which significantly reduces the need for long-term waste storage sites. 
     However, the seed-blanket assemblies used in the reactor are not suitable for use in existing light water reactors such as the VVER-1000. 
     A fuel assembly for a light water reactor similar to the reactor described above, which, specifically, has a hexagonal cross-sectional form, which makes it possible to install the fuel assembly from the seed-blanket modules in a conventional light water reactor, is known from the description for patent RU 2222837. 
     Other than the presentation of the cross-sectional form of the assembly, however, the description for the aforementioned patent contains no information on the configuration of the assembly which would allow installing it in an existing light water reactor such as the VVER-1000 without modifying the reactor design. 
     A fuel assembly for a light water reactor including a bundle of fuel elements and guide channels in spacer grids, a tailpiece and a head, wherein the spacer grids are connected to each other and to the tailpiece by elements arranged along the length of the fuel assembly, and the head is made up of upper and lower tieplates, cladding situated between the plates, and a spring unit, and wherein outer ribs on the head shell are connected to each other along projections of the rim and along the lower parts by perforated plates, is known according to patent RU 2294570. 
     The known fuel assembly is classified as a design for jacketless fuel assemblies, which make up the cores of pressurized water reactors (PWRs) such as the VVER-1000, and has enhanced operating properties due to increased rigidity, reduced head length and increased free space between the fuel rod bundle and the head, with a simultaneous increase in the length of the fuel rods. This design makes it possible to increase the fuel load in the fuel assembly with greater depletion depth and thereby to increase the reactor core power and the life cycle of the fuel assembly. 
     However, all the fuel elements in this assembly are made of the fissionable material traditionally used in reactors such as the VVER-1000; consequently, the creation of large amounts of reactor-grade plutonium is a characteristic drawback of reactors with such assemblies. 
     One object of one or more embodiments of the invention is the creation of a fuel assembly which, on the one hand, generates a substantial percentage of its power in a thorium-fueled blanket region and does not create proliferative wastes and, on the other hand, can be installed in an existing light water reactor such as the VVER-1000 without requiring substantial modifications. 
     SUMMARY OF EMBODIMENTS OF THE INVENTION 
     This object and/or other objects, according to one of the embodiments of the invention, are achieved by a fuel assembly for a light water reactor having in plan the form of a regular hexagon contains a seed subassembly, a blanket subassembly surrounding it, a head, a tailpiece and a frame structure, wherein the seed subassembly contains a bundle of fuel elements, each of which has a kernel comprised of enriched uranium or reactor-grade plutonium, with the said kernel being enclosed by a cladding made of zirconium alloy and having a three-lobed profile forming spiral spacer ribs; the tailpiece of the seed subassembly with a support grid attached to it to hold the fuel elements of the seed subassembly; a channel connected to the tailpiece of the seed subassembly having in plan the form of a regular hexagon, with channel placed around the fuel rod bundle; a guide grid attached to the upper part of the channel for placing fuel elements so as to allow their free axial movement; a central tube forming a guide channel to accommodate controls, and peripheral tubes attached to the support grid, which form guide channels for inserting absorber rods and control rods, and placed in the head with the capability of elastic axial displacement; the blanket subassembly includes a frame structure comprised of six lengthwise angle units with spacer grids attached to them, with an opening in the central area to accommodate the channel of the seed subassembly; a bundle of fuel elements comprised of thorium with an addition of enriched uranium situated in the frame structure; and the tailpiece of the blanket subassembly, to which fuel elements of the blanket subassembly are attached, and which can be coupled with the support tube of the light water reactor, with the said tailpiece of the blanket subassembly and the tailpiece of the seed subassembly being attached by a locking mechanism and forming the tailpiece of the fuel assembly. 
     The head can be equipped with a pressure element that is in contact with the channel of the seed subassembly. 
     In another embodiment of the invention, a fuel assembly having in plan the form of a regular hexagon contains a seed subassembly, a blanket subassembly surrounding it, a head, a tailpiece that can be coupled with the support tube of the light water reactor and a frame structure, wherein the seed subassembly contains a bundle of fuel elements, each of which has a kernel comprised of enriched uranium or reactor-grade plutonium, with the said kernel being enclosed by a cladding made of zirconium alloy and having a three-lobed profile forming spiral spacer ribs; the tailpiece of the seed subassembly with a support grid attached to it to hold the fuel elements of the seed subassembly; a channel connected to the tailpiece of the seed subassembly having in plan the form of a regular hexagon, with the channel placed around the fuel rod bundle; a guide grid attached to the upper part of the channel for placing fuel elements so as to allow their free axial movement; a central tube forming a guide channel to accommodate controls, and peripheral tubes, which form guide channels for inserting absorber rods and control rods, and placed in the head with the capability of elastic axial displacement; the blanket subassembly includes a frame structure comprised of six lengthwise angle units with spacer grids attached to them, with an opening in the central area to accommodate the channel of the seed subassembly; a bundle of fuel elements comprised of thorium with an addition of enriched uranium situated in the frame structure and attached to the bottom tie plate (the tailpiece); and several support tubes attached to the tailpiece, with the head equipped to allow elastic axial displacement of the support tubes. 
     Displacer of zirconium or zirconium alloy having the cross-sectional form of a regular triangle is situated primarily along the longitudinal axis of the kernel in at least one of the embodiments of the invention to promote more uniform temperature distribution in the kernel volume. 
     The axial coiling pitch of the spiral spacer ribs also ranges from 5% to 20% of the fuel rod length in at least one of the embodiments of the invention. 
     In addition, the fuel rods of the seed subassembly in at least one embodiment of the invention have a circumferential orientation such that the three-lobed profiles of any two adjacent fuel rods have a common plane of symmetry which passes through the axes of the two adjacent fuel elements in at least one cross section of the fuel rod bundle. 
     Also in at least one of the embodiments of the invention, the kernel preferably is comprised of U—Zr alloy with up to 30% by volume uranium, with up to 20% enrichment with the U-235 isotope, and the kernel is comprised of Pu—Zr alloy with up to 30% by volume reactor-grade plutonium. 
     In addition, an object of one or more embodiments of the invention is a light water reactor containing a set of fuel assemblies, at least one of which is construction according to one of the alternative configurations described above. Either some or all of the fuel assemblies placed in the reactor may conform to the alternatives described above. 
     One or more embodiments of the present invention provides a fuel element for use in a fuel assembly of a nuclear reactor. The fuel element includes a kernel comprising fissionable material. The fuel element has a multi-lobed profile that forms spiral ribs. There may be three ribs. The fuel element may include a cladding enclosing the kernel, and the cladding may include a zirconium alloy. The fuel element may include a displacer with a cross sectional shape in the form of a regular triangle, the displacer extending along a longitudinal axis of the kernel. The displacer may comprise zirconium or a zirconium alloy. 
     One or more embodiments of the present invention provides a fuel element for use in a fuel assembly of a nuclear reactor. The fuel element includes a central displacer extending along a longitudinal axis of the fuel element. The displacer includes projections that extend laterally outward. The fuel element also includes a kernel extending laterally outwardly from the displacer. The kernel includes fissionable material and includes a plurality of ribs that extend laterally outward. The projections are aligned with respective ribs. The projections and their respective ribs may have matching twists along their longitudinal axes. In a cross-section of the fuel element that is perpendicular to the longitudinal axis, the kernel may surround the displacer. The plurality of ribs may include circumferentially equally-spaced ribs, wherein a cross sectional shape of the displacer has the form of a regular polygon having a corner for each of said ribs. For example, the plurality of ribs may include three circumferentially equally-spaced ribs, wherein a cross sectional shape of the displacer has the form of a regular triangle. The apexes of the regular triangle may be aligned with the lobes of the kernel. 
     One or more embodiments of the present invention provides a fuel assembly for use in a nuclear reactor. The fuel assembly includes a seed subassembly comprising a seed frame and a plurality of seed fuel elements supported by the seed frame. The fuel assembly also includes a blanket subassembly comprising a blanket frame and a plurality of blanket fuel elements supported by the blanket frame. The fuel assembly further includes a locking mechanism that releasably locks the seed and blanket frames together. The seed assembly is detachable from the blanket subassembly when the locking mechanism is released. The blanket subassembly may laterally surround the seed subassembly. The blanket subassembly may include a central opening into which the seed subassembly fits. The plurality of seed fuel elements may include fissionable material, and the plurality of blanket fuel elements may comprise thorium. 
     One or more embodiments of the present invention provides a method of using a fuel assembly according to one or more of the above embodiments. The seed and blanket subassemblies are attached to each other. The method includes, sequentially:
         (a) placing the fuel assembly in a core of a nuclear reactor;   (b) burning at least some of the fissionable material in the core of the nuclear reactor;   (c) detaching the seed subassembly from the blanket subassembly; and   (d) attaching a new seed subassembly to the blanket subassembly, the new seed subassembly comprising additional fissionable material.       

     The method may also include:
         (e) burning at least some of the additional fissionable material in the core of the nuclear reactor.       

     Additional and/or alternative objects, features, aspects, and advantages of one or more embodiments of the present invention will become apparent from the following description, the accompanying drawings, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of various embodiments of this invention will be apparent from the following detailed description of the preferred embodiments thereof together with the attached drawings, in which: 
         FIG. 1  is a schematic cross-sectional illustration of a nuclear reactor core containing fuel assembles constructed according to an embodiment of this invention; 
         FIG. 2  is a general side view of a fuel assembly according to the first embodiment of the invention, including cutaway views; 
         FIG. 3  is the head of the fuel assembly as per  FIG. 2  in enlarged longitudinal section view; 
         FIG. 4  is the tailpiece of the fuel assembly as per  FIG. 2  in enlarged longitudinal section view; 
         FIG. 5  is a cross-sectional view of a seed fuel rod; 
         FIG. 6  is the A-A cross-sectional view of the fuel assembly as per  FIG. 2 ; 
         FIG. 7  is a general side view of a fuel assembly according to the second embodiment of the invention, including cutaway views; 
         FIG. 8  is the head of the fuel assembly as per  FIG. 7  in enlarged longitudinal section view; 
         FIG. 9  is the tailpiece of the fuel assembly as per  FIG. 7  in enlarged longitudinal section view. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  shows the a nuclear reactor core  1  containing a set of fuel assemblies  2  which include a seed region and a blanket region, which form a hexagonal configuration, wherein the fuel assemblies themselves have in plan the form of a regular hexagon. The core  1  has the same geometric configuration and dimensions as the core in a conventional VVER-1000 light water reactor, so that the reactor can be refitted with such assemblies to form a core of 163 fuel assemblies  2 . The difference between the core  1  and the core of the VVER-1000 reactor lies in the composition and structure of the fuel assemblies  2 , as will be disclosed in greater detail below. The core  1  and fuel assemblies  2  presented here have been developed for use in a conventional VVER-1000 light water reactor; however, a similar core and fuel assemblies can be created for use in other standard or specially designed reactors without going beyond the scope of this invention. 
     The core  1  is surrounded by a reflector  3 , which preferably is comprised of a set of reflector assemblies  4 . Each reflector assembly  4  preferably contains a mixture of water and metal of the core basket/high-pressure vessel. In addition, easy reflector assembly  4  may be comprised primarily of thorium oxide. 
       FIG. 2  shows a general view of the first alternative configuration for each of the fuel assemblies  2 . 
     A fuel assembly  2  contains a seed subassembly  5 , a blanket subassembly  6  surrounding it, a head  7 , and a tailpiece  8  with its supporting part  9  in contact with the support tube of the reactor (not shown). The fuel assembly has in plan the form of a regular hexagon. The seed subassembly  5  contains a fuel rod bundle  10  which includes a number of rods, such as 108, placed on a support grid  11 , which is attached to the tailpiece of the seed subassembly  5 . A channel  12  with a hexagonal cross section is connected to the tailpiece of the seed subassembly  5  and encloses the fuel rod bundle  10 . A guide grid  13  for placing fuel elements  10  so as to allow their free axial movement is attached to the upper part of the channel  12 . Each of the seed fuel elements has a kernel  14 , which includes enriched uranium or reactor-grade plutonium. The kernel is comprised primarily of U—Zr alloy, with a uranium concentration of 25% or less by volume in the fuel composition and 19.7% uranium-235 enrichment. The kernel  14  is enclosed by cladding  15  of zirconium alloy and has a three-lobed profile forming spiral spacer ribs  16  ( FIG. 5 ). A displacer  17  of zirconium or zirconium alloy with the cross-sectional form of a regular triangle is placed along the longitudinal axis of the kernel. The seed fuel rods  10  may be fabricated as a single assembly unit by joint pressing (extrusion through a die). The axial coiling pitch of the spiral spacer ribs  16  is selected according to the condition of placing the axes of adjacent fuel rods  10  with a spacing equal to the width across corners in the cross section of a fuel rod and is 5% to 20% of the fuel rod length. Stability of the vertical arrangement of the fuel rods  10  is provided: at the bottom—by the support grid  11 ; at the top—by the guide grid  13 ; relative to the height of the core—by a system of bands (not shown) spaced evenly in the channel relative to the height of the bundle. The seed fuel elements  10  have a circumferential orientation such that the three-lobed profiles of any two adjacent fuel rods have a common plane of symmetry which passes through the axes of the two adjacent fuel elements ( FIG. 5 ) in at least one cross section of the fuel rod bundle. 
     In addition, the seed subassembly contains a central tube  18  that forms a guide channel to accommodate controls, and peripheral tubes  19  attached to the support grid  13  which form guide channels for inserting control absorber elements based on boron carbide (B 4 C) and dysprosium titanate (Dy 2 O 3 .TiO 2 ) (not shown) and burnable absorber rods based on boron carbide and gadolinium oxide (Gd 2 O 3 ) (not shown) and are placed in the head  7  with the capability of elastic axial displacement. The peripheral tubes  19  that form the guide channels are made of zirconium alloy. 
     The head  7  ( FIG. 3 ) is comprised of a spring unit, which includes precompressed springs  20 , an upper plate  21 , cladding  22  and a lower plate  23 . The cladding  22  is comprised of two telescoped parts: the upper part  24  rigidly connected to the upper plate  21 , and the lower part  25  rigidly connected to the lower plate  23 . The spring unit including the springs  20  is placed inside the cladding  22 . The peripheral tubes  19  fit into sleeves  26  and are capable of acting on the bottom ends of the sleeves (due to the presence of a step on the outer surface of the tube  19 , for example). The sleeves  26  have flanges against which the compression springs of the spring unit  20  rest. The other ends of the springs  20  rest against the upper plate  21 . The upper ends of the tubes  19  pass freely through openings in the upper plate  21 , and the sleeves  26  pass through openings in the lower plate  23 . The tubes  19  have stops  27  at the top ends. The central tube  18  is installed in a manner similar to the peripheral tubes  19 , except that it passes freely through the lower plate without the use of a sleeve. The spring  20  through which the central tube  18  passes rests directly against the lower plate  23  of the head  7 . A stay  28  with a stop  29  at the upper end is attached to the lower plate  23  to limit the distance between the plates  21  and  23 ; the stay  28  passes freely through an opening in the upper plate  21 . A pressure element  30  in contact with the channel  12  of the seed subassembly  5  is attached to the lower plate  23 . Hence a load applied to the upper plate  21  with the channel  12  fixed against axial movement is transmitted to the support grid  11  both by way of the peripheral tubes  19  and directly through the channel  12 . 
     The head may be constructed without the sleeves  26 . In that case, all the springs  20  of the spring unit rest against the lower plate  23 , and the peripheral tubes  19  pass freely through matching openings in the lower plate  23  (similar to the central tube  18 ). The entire load applied to the upper plate  21  with the channel  12  fixed against movement is transmitted to the support grid  11  directly through the channel  12 . 
     The tailpiece of the seed subassembly  5  has a locking device  31  attached to the casing which includes a cylindrical wall  32  with openings  33 , balls  34  placed in the openings, and a locking element  35  with an annular slot  36  capable of axial movement. The locking device  31 , which provides connection of the seed subassembly  5  with the tailpiece  37  of the blanket subassembly, can be also be constructed in any other form; it is important only that it provide a detachable connection of the tailpieces of the seed and blanket subassemblies. 
     The blank subassembly  6  includes a frame structure  38 , a bundle of fuel rods  39  situated in the frame, and a tailpiece  40 . 
     The frame structure  38  is comprised of six lengthwise angle units  41  with spacer grids  42  attached to them by resistance spot welding. Each spacer grid  42  is a honeycomb grid forming a set of cells (specifically 228) attached to the rim in outer and inner hexagons. The spacer grid  42  provides the required spacing of the fuel rods  39  and the required length of contact with them to allow the fuel rods  39  to slide in the spacer grid cells when they expand in length due to radiation and heat, the minimum possible sliding forces for the fuel rods to reduce internal stresses in the bundle, and the required initial tightness to avoid fretting corrosion of the fuel elements during operation. The spacer grids  42  have an opening in the central area to accommodate the channel  12  of the seed subassembly  5 . 
     The angle units are rigidly connected in the lower part to the tailpiece  40  of the blanket subassembly  6 , to which the support grid  43  of the blanket subassembly to hold the fuel rods  39  is attached. The support grid  43  of the blanket subassembly  6  provides mechanical strength under loads in modes with normal operating conditions, modes with violations of normal operating conditions, and design accidents and also provides the hydraulic resistances required according to calculations. 
     The fuel rod bundle  39  of the blanket subassembly includes a set of fuel elements (specifically 228 elements) made of a composition including 12% by volume UO 2  and 88% by volume ThO 2  with 19.7% U-235 enrichment. 
     The ratio of the volume of all fuel elements of the seed subassembly V seed  to the volume of all fuel elements of the blanket subassembly V blank  is approximately 0.72. 
     The tailpiece  40  of the blanket subassembly  6  includes a support grid  43 , a casing  44  and a ring  46  rigidly connected to it by braces  45 ; the ring interacts with the locking device  31 . The ends of the blanket fuel elements  39  are attached to the support grid  43 . The support grid  43  provides mechanical strength under loads modes with normal operating conditions, modes with violations of normal operating conditions, and design accidents and also provides the required hydraulic resistance to the flow of coolant (water). The casing  44  can be coupled with the support tube (not shown) of the light water reactor and acts as a guide device for delivering coolant to the areas of the seed and blanket subassemblies. 
       FIGS. 7-9  show the second alternative for construction of each of the fuel assemblies  2 . 
     This alternative design differs from the design shown in  FIGS. 2-4  in that the seed and blanket subassemblies are not rigidly connected to each other. As shown in  FIG. 9 , the tailpiece of the seed subassembly has a cylindrical bottom tie plate  47  instead of the locking device  31 , and the casing  44  in the tailpiece of  40  of the blanket subassembly  6  lacks braces  45  and ring  46  shown in  FIG. 4 . The cladding  22  of the head  7  ( FIG. 8 ), in contrast to the version shown in  FIG. 3 , is constructed in one piece, and an additional spring unit  48  is rigidly attached (e.g., welded) to it. The additional spring unit  48  chiefly includes several (e.g., six) additional upper plates  49  evenly distributed around the circumference and rigidly connected to the cladding  22 , an additional lower plate  50  rigidly linked to the lower plate  23 , cladding  51  attached to the additional plates  49  and  50 , compression springs  52  and support tubes  53 . The support tubes  53  are attached by the bottom ends to the support grid  43  of the blanket module  6 . The upper parts of the support tubes  53  are constructed and positioned in the additional upper and lower plates  49  and  50  similar to the peripheral tubes  19 ; i.e., the tubes  53  fit into sleeves  26  and are capable of acting on the sleeves in an upward direction. The compression springs  52  of the additional spring unit  48  rest at one end against flanges of the sleeves  26  and at the other end against the additional upper plates  21 . The upper parts of the support tubes  53  pass freely through openings in the additional upper plates  49 , and the sleeves  26  pass through openings in the additional lower plate  50 . The support tubes  53  have stops  54  at the top ends. 
     Before a fuel assembly is placed in the reactor, the seed subassembly  5  and the blanket subassembly  6  are first assembled separately. 
     In assembly of the seed subassembly according to the first embodiment, the fuel elements  10  are connected to the guide grid  13  attached to the channel  12 , and the central tube  18  and peripheral tubes  19  are connected to the head, in addition to being attached to the guide grid  13 . The tubes  18  and  19  pass through sleeves  17  situated in openings in the lower plate, through the springs  20  and through openings in the upper plate  21 . Then the stops  27  are attached to the top ends of the tubes (by a threaded or bayonet joint, for example). 
     The fuel elements  39  of the blanket subassembly are placed in a frame structure  9  by passing them through spacer grids  42  and attaching them to the support grid  43 . 
     Then the assembled seed and blanket subassemblies are connected to form a single fuel assembly by passing the channel  12  of the seed subassembly  5  through openings in the central part of the spacer grids  42 . The configuration of these openings in the central part of the spacer grids  42  matches the cross-sectional shape of the channel  12 , so that the channel  12  passes freely through the openings. The locking element  35  in the tailpiece of the seed subassembly is shifted upward, so that the balls  34  situated in openings  33  of the cylindrical wall  32  are capable of movement in an annular groove  36 , thus allowing the cylindrical wall  32  to pass through the ring  46 . After the tailpiece of the seed subassembly is stopped against the upper end face of the ring  46 , the locking element  36  is shifted downward. The balls  34  are forced out of the groove  36 , shift outward in the openings  33  and jut out of the wall  32 . As a result, due to interaction of the displaced balls and the bottom end face of the ring  46 , the tailpiece of the seed subassembly cannot move upward in relation to the tailpiece of the blanket subassembly. Thus the seed and blanket subassemblies form a single fuel assembly  2 . 
     After a fuel assembly  2  is placed in the reactor  1 , and the tailpiece  8  is resting in the support tube (not shown) of the light water reactor, the fuel assembly  2  is held down by the upper plate of the reactor (not shown) by resting against the face of the cladding of the upper plate  21  of the head  7 . Then the force is transmitted to the spring unit with springs  20 , which is compressed by an amount designed to keep the fuel assembly  2  from floating up in the flow of coolant from below; the upper plate  21  of the head  7  moves downward in relation to the lower plate  23  by the amount of compression of the spring unit. The possibility of downward movement of the upper plate  21  relative to the lower plate  23  of the head  7  is provided by telescoping of the upper part  24  of the cladding  22 , which is rigidly connected to the upper plate  21 , and the lower part  25  of the cladding  22 , which is rigidly connected to the lower plate  23 . 
     Then the force from the bottom ends of the springs  20  of the spring unit is transmitted through the sleeves  26 , acting on the peripheral tubes  19  by their bottom ends, to the peripheral tubes  19  and then to the support grid  11  and through the tailpiece of the seed subassembly, the locking device  31 , the ring  46  and the braces  45  to the tailpiece  44  of the blanket subassembly  6 , which comes into contact with the support tube (not shown) of the light water reactor. 
     In addition, part of the compression force from the upper plate of the reactor is transmitted to the channel  12  of the seed subassembly by the action on the pressure element  30  of the force of a spring  20  enclosing the central tube  18  and resting directly against the lower plate  23 , which is rigidly connected to the pressure element. If the head  7  does not have sleeves  26 , the entire compression force is transmitted by way of the channel  12 . 
     Coolant passes into the fuel assembly  2  through the casing  44  of the tailpiece of the blanket subassembly  6 ; the coolant flow is divided into two parts, one of which runs inside the casing  12  of the seed subassembly and bathes the seed fuel elements  10 , while the other runs outside the case  12  and bathes the fuel elements  39  of the blanket subassembly. 
     The compression force of the head  7  acting from the upper plate of the reactor (not shown) keeps the fuel elements from floating up in the specified coolant flow. 
     The passage of the required (for extracting nominal power from the fuel assembly) coolant flow through the seed and blanket subassemblies at the nominal pressure gradient (used in existing VVER-1000 reactors) relative to the height of the fuel assemblies with preservation of the serviceability of the assemblies is provided:
         by the use of a channel  12  between the seed and blanket subassemblies;   by the shape of the seed fuel elements  10  (three-lobed profile), their mutual circumferential orientation and the axial coiling pitch of the spiral spacer ribs  16 , which promotes a well-developed heat-transfer surface and a significantly more even coolant temperature distribution in the cross section of the seed subassembly due to forced convective mixing of the coolant.       

     The complete hydraulic characteristics of the fuel assembly  2  practically coincide with the characteristics of a standard fuel assembly, which ensures maintaining the resistance of the core of a VVER-1000 reactor with fuel assemblies according to one or more embodiments of the invention at the nominal level. Hence installing fuel assemblies according to one or more embodiments of this invention in a VVER-1000 will not cause a change in the coolant flow rate in the primary loop of the reactor. 
     The fuel elements  10  of the seed subassembly, as they heat up during operation, begin to lengthen upward due to thermal and radiation expansion; the bundle of fuel elements expands independently of peripheral tubes  19 , since the latter pass through the cells of the guide grid  13  with a guaranteed clearance. Hence the bundle of fuel elements  10  has no effect on the load-bearing peripheral tubes  19  and does not deform them; consequently, geometric stability of the form of the fuel assembly  2  is preserved during operation. 
     The fuel elements  39  of the blanket subassembly expand in length during operation and begin to take up the free space between their ends and the head  7  due to radiation expansion. 
     The operation of a fuel assembly  2  according to the second embodiment of the invention is similar, except that the casing  44  of the blanket subassembly is pressed against the support tube of the reactor by transmission of the compression force from the upper plate of the reactor through the support tubes  53 , and the seed subassembly, which is not attached to the blanket subassembly, is prevented from floating up by the action of the springs  20  against the flanges of the sleeves  26 , which transmit the force to the support grid  11  of the seed subassembly. 
     The use of one or more embodiments of this invention makes it possible to achieve a saving of natural uranium due to the presence of a thorium part (blanket subassembly) in the fuel assembly design, since the thorium during the depletion process accumulates secondary nuclear fuel in the form of uranium-233, burning of which makes a substantial contribution to the power output of a reactor core with such fuel assemblies. This leads to an improvement in nonproliferation characteristics and simplifies the problems in handling spent fuel assemblies, since the accumulation of the traditional secondary nuclear fuel (reactor-grade plutonium, which can be used to produce nuclear weapons) for VVER-1000 reactors is reduced significantly (by 80%), and the new secondary nuclear fuel, uranium-233 (or more accurately, what is left after it burns “in place” in a thorium blanket module), is not usable for producing nuclear weapons due to contamination with the uranium-232 isotope and even isotopes of plutonium. Problems in handling spent fuel assemblies can be simplified by reducing the volume of waste by increasing the specified life cycle of the fuel and reducing the content of isotopes with long-term radiation toxicity in discharged fuel. 
     The fuel assembly design according to one or more embodiments of this invention makes it possible to use the fuel assembly in VVER-1000 reactors due to both mechanical and hydraulic and neutronic compatibility with the design of standard fuel assemblies. 
     Mechanical compatibility with the standard fuel assembly for the VVER-1000 reactor is ensured by:
         the presence of a frame structure that provides resistance to deformation during long-term operation and high fuel depletion levels;   identical connection dimensions;   the use of tailpiece, head and frame structure designs compatible with the corresponding parts of corner standard fuel assemblies;   compatibility of the seed subassembly design with standard control mechanisms and load-handling devices.       

     The complete hydraulic characteristics of a fuel assembly according to one or more embodiments of this invention practically coincide with the characteristics of a standard fuel assembly due to the presence of a system of two parallel channels formed by the seed and blanket subassemblies and joined by common distribution (delivery) and collection headers. The seed and blanket subassemblies are hydraulically connected in the inlet and outlet segments. This fuel assembly structure ensures maintaining the resistance of the core of a VVER-1000 reactor with fuel assemblies according to one or more embodiments of the invention at the nominal level. Hence installing fuel assemblies according to one or more embodiments of this invention in a VVER-1000 reactor will not cause a change in the coolant flow rate in the primary loop of the reactor. The ratio of hydraulic resistances between the inlet to the assembly, the active part of the blanket subassembly and the outlet from the assembly in fuel assemblies according to one or more embodiments of this invention and the standard fuel assembly are similar, which ensures hydraulic compatibility of fuel assemblies according to one or more embodiments of the invention with standard assemblies and the absence of coolant overflows between them. This makes it possible to use some fuel assemblies according to one or more embodiments of this invention in a reactor at the same time with standard fuel assemblies for the reactor. 
     Neutronic compatibility with the standard fuel assembly is provided by the following:
         the specified burn-up level is achieved by utilizing specific fuel compositions and compositions with burnable absorbers;   standard power output of the fuel assembly is achieved by utilizing specific fractions of fuel loading in seed and blanket fuel compositions;   satisfaction of requirements for an uneven profile of power output is achieved by utilizing specific fractions of fuel loading in various rows of seed rods and the composition of fuel loading in the blanket;   preservation of reactivity effects within the range typical for standard fuel assemblies is achieved by utilizing special characteristics of fuel compositions;   the ability to regulate the level of output and reduce the output using standard control systems is achieved by utilizing standard technological channels for guiding control rods in the peripheral tubes in the seed subassembly which are compatible with the subassembly.       

     Another advantage of one or more embodiments of the invention is that the seed-blanket fuel assembly according to one or more embodiments of this invention is sectional, which makes it possible to change the seed subassembly independently. Changing the seed subassembly more frequently produces more favorable conditions (with respect to neutron balance and irradiation time) for the thorium placed in the blanket subassembly of the fuel assembly. 
     The foregoing illustrated embodiments are provided to illustrate the structural and functional principles of various embodiments of the present invention and are not intended to be limiting. To the contrary, the principles of the present invention are intended to encompass any and all changes, alterations and/or substitutions within the spirit and scope of the following claims. Any one or more aspects of the various embodiments may be used without also using other aspects of such embodiments, and without deviating from the scope of the present invention. For example, while the illustrated fuel elements  10  have a spiral twist along their longitudinal axes, such spiral may be omitted. While the illustrated fuel elements  10  have a non-cylindrical cross-section, they may alternatively comprise a cylindrical cross-section. While the illustrated fuel elements  10  include a plurality of spacer ribs or lobes  16 , such ribs/lobes  16  may be omitted. While the illustrated fuel elements  10  include displacers  17 , such displacers may be omitted. While the illustrated fuel elements  10  are used in conjunction with a seed/blanket arrangement within a fuel assembly, the fuel elements  10  may alternatively be used in conjunction with a variety of other types of fuel assemblies and/or core designs. While the illustrated fuel assembly  2  utilizes a channel  12  and various other particular structures within a fuel assembly, such structures may be omitted and/or modified in a variety of ways to accommodate other assembly and/or core designs.