WO2012097029A1

WO2012097029A1 - Locking device for nuclear fuel sub-assemblies

Info

Publication number: WO2012097029A1
Application number: PCT/US2012/020878
Authority: WO
Inventors: Alexey Glebovich Morozov; Sergey Mikhailovich Bashkirtsev; Valery Vladimirovich Kevrolev; Aaron Robert TOTEMEIER
Original assignee: Thorium Power, Inc.
Priority date: 2011-01-14
Filing date: 2012-01-11
Publication date: 2012-07-19

Abstract

A locking mechanism (95, 125, 130) for a nuclear fuel assembly (2) having seed and blanket (6) subassemblies is configured to selectively lock the seed subassembly within the blanket subassembly, preventing separation thereof. The locking mechanism contains one or more detents (95) on one of the seed subassembly or the blanket subassembly, that are configured to engage with a surface feature on the other of the seed subassembly or the blanket subassembly. The locking mechanism contains a locking member movable between two positions. In a first position, the locking member (100) is configured to prevent the one or more detents from disengaging from the surface feature (125, 130), thereby holding the blanket subassembly and the seed subassembly together. In a second position, however, the detents are able to move into a non- locking position, such that the one or more detents may disengage from the surface feature as the seed subassembly is separated from the blanket subassembly.

Description

LOCKING DEVICE FOR NUCLEAR FUEL SUB-ASSEMBLIES

RELATED APPLICATIONS

[0001] This application claims the benefit of U.S.S.N. 61/432,745 filed January 14,

2011, the content of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

[0002] 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).

BACKGROUND

[0003] Nuclear power remains an important energy resource throughout the world.

Many countries that lack adequate indigenous fossil fuel resources rely, or may rely in the future, 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.

[0004] 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.

[0005] 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. Such material, which can in theory be used for producing nuclear weapons or so called "dirty bombs", is both hazardous and long lasting. Thus, stringent security measures are required to prevent the discharged fuel from falling into the hands of unauthorized individuals. [0006] 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.

[0007] To solve these problems, there have been recent attempts to develop nuclear reactors that use fuels with enhanced proliferation resistance by using low enriched uranium (uranium with a U-235 content of 20% or less) and generating smaller quantities of proliferative materials such as plutonium. Examples of such fuel designs have been disclosed in international applications WO 85/01826 and WO 93/16477, which describe 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 fissile U-233, which burns in place and releases heat for the reactor power plant.

[0008] The use of thorium is attractive due to the relative abundance of worldwide thorium reserves. In addition, such reactors are proliferation resistant 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 enriched uranium as seed fuel, selecting moderator/fuel volume ratios to minimize plutonium production, and adding a small amount of enriched uranium to the blanket zone, where the U-238 component is homogeneously distributed 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.

[0009] Unfortunately, neither of the seed-blanket reactor designs referenced above is truly "nonproliferative." In particular, it has been discovered that both of the designs result in a proliferative level of plutonium production in the seed zone. 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 is dominated by 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.

[0010] 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 generally high in the seed zones and low in the blanket zones.

[0011] The previous reactor core designs also are not especially effective in consuming enriched uranium in the seed fuel elements. As a result, the fuel rods discharged at the end of each seed fuel cycle contained sufficient residual uranium that it may prove economically viable to reprocess them for reuse in another reactor core.

[0012] 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.

[0013] Finally, both of the previous reactor designs were designed specifically to burn enriched uranium with thorium and are not optimized for consuming large amounts of plutonium. Hence neither design provides a solution to the problem of stockpiled plutonium.

[0014] 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.

[0015] The known reactor provides enhanced operation from the standpoint of economy and has enhanced proliferation resistance compared to conventional commercial nuclear reactors. This reactor can be used to consume large amounts of plutonium while reducing the amount of used fuel generated at the end of the fuel cycle. The reactor produces substantially smaller amounts of transuranic elements, which require long-term waste storage sites.

[0016] However, the seed-blanket assemblies used in the reactor are not suitable for use in existing light water reactors, such as but not limited to the VVER-1000. [0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] However, all the fuel elements in this assembly are composed of the fissile 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.

[0022] 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 increases the proliferation resistance of the used fuel and, on the other hand, can be installed in an existing light water reactor such as the VVER-1000 without requiring substantial modifications to the reactor internals.

SUMMARY OF EMBODIMENTS OF THE INVENTION [0023] According to an aspect of the present disclosure, a fuel assembly for use in a nuclear reactor 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 seed subassembly comprising a seed frame and a plurality of seed fuel elements supported by the seed frame. The seed subassembly is movable relative to the blanket subassembly between engaged and disengaged positions. The fuel assembly further includes a locking mechanism that releasably locks the seed subassembly in the engaged position. The locking mechanism includes a surface feature operatively positioned on one of the blanket frame and the seed frame, and at least one detent operatively connected to the other of the blanket frame and the seed frame. The at least one detent is movable between a locking position and a nonlocking position, such the at least one detent engages the surface feature when the seed subassembly is in the engaged position and the at least one detent is in the locking position so as to prevent the seed subassembly from disengaging from the blanket subassembly. The at least one detent does not prevent the seed subassembly from disengaging from the blanket subassembly when the seed subassembly is in the engaged position and the at least one detent is in the non-locking position. The locking mechanism further includes a locking structure movably connected to the at least one detent. The locking structure is movable between a first position and a second position, such that (i) when the locking structure is in the first position and the at least one detent is in the locking position, the locking structure prevents the at least one detent from moving into the non-locking position, and (ii) when the locking structure is in the second position, the locking structure does not prevent the at least one detent from moving between the locking and non-locking positions.

[0024] Another aspect of the present disclosure provides a method of assembling a fuel assembly for use in a nuclear reactor core. The fuel assembly includes a blanket subassembly comprising a blanket frame and a plurality of blanket fuel elements supported by the blanket frame, and a seed subassembly comprising a seed frame and a plurality of seed fuel elements supported by the seed frame. The seed subassembly is movable relative to the blanket subassembly between engaged and disengaged positions. The fuel assembly further includes a locking mechanism adapted to releasably lock the seed subassembly in the engaged position. The locking mechanism comprises a surface feature operatively positioned on one of the blanket frame and the seed frame, and at least one detent operatively connected to the other of the blanket frame and the seed frame. The at least one detent is movable between a locking position and a non-locking position. The at least one detent engages the surface feature when the seed subassembly is in the engaged position and the at least one detent is in the locking position so as to prevent the seed subassembly from disengaging from the blanket subassembly. The at least one detent does not prevent the seed subassembly from disengaging from the blanket subassembly when the seed subassembly is in the engaged position and the at least one detent is in the non-locking position, and a locking structure movably connected to the at least one detent. The locking mechanism further includes a locking structure movable between a first position and a second position such that (i) when the locking structure is in the first position and the at least one detent is in the locking position, the locking structure prevents the at least one detent from moving into the non- locking position, and (ii) when the locking structure is in the second position, the locking structure does not prevent the at least one detent from moving between the locking and non-locking positions. The method includes moving the locking structure from the first position to the second position. The method also includes moving the seed subassembly from the disengaged position to the engaged position, and positioning the at least one detent in the locking position. The method further includes moving the locking structure into the first position such that the locking structure prevents the at least one detent from moving out of the locking position, thereby locking the seed subassembly in the engaged position.

[0025] Another aspect of the present disclosure provides a method for unlocking a seed subassembly from a blanket subassembly of a fuel assembly. The blanket subassembly includes a blanket frame and a plurality of blanket fuel elements supported by the blanket frame. The seed subassembly includes a seed frame and a plurality of seed fuel elements supported by the seed frame. The seed subassembly is movable relative to the blanket subassembly between engaged and disengaged positions. The fuel assembly further includes a locking mechanism releasably locking the seed subassembly in the engaged position. The locking mechanism includes a surface feature operatively positioned on one of the blanket frame and the seed frame, and at least one detent operatively connected to the other of the blanket frame and the seed frame. The at least one detent is movable between a locking position and a non-locking position. The at least one detent engages the surface feature when the seed subassembly is in the engaged position and the at least one detent is in the locking position so as to prevent the seed subassembly from disengaging from the blanket subassembly. The at least one detent does not prevent the seed subassembly from disengaging from the blanket subassembly when the seed subassembly is in the engaged position and the at least one detent is in the non-locking position. The locking mechanism further includes a locking structure movably connected to the at least one detent. The locking structure is movable between a first position and a second position such that (i) when the locking structure is in the first position and the at least one detent is in the locking position, the locking structure prevents the at least one detent from moving into the non- locking position, and (ii) when the locking structure is in the second position, the locking structure does not prevent the at least one detent from moving between the locking and non- locking positions. The method includes moving an unlocking structure relative to the locking mechanism into a release position that moves the locking structure from the first position to the second position, thereby permitting the at least one detent to move into its non-locking position. The method also includes moving the seed subassembly from its engaged position to its disengaged position, where said moving of the seed assembly causes the at least one detent to move from its locking position to its non-locking position. The method further includes moving the unlocking structure relative to the locking mechanism into a disconnected position that permits the locking structure to move from its second position to its first position.

[0026] These and other aspects of various embodiments of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. In one embodiment of the invention, the structural components illustrated herein are drawn to scale. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. In addition, it should be appreciated that structural features shown or described in any one embodiment herein can be used in other embodiments as well. As used in the specification and in the claims, the singular form of "a", "an", and "the" include plural referents unless the context clearly dictates otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] 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:

[0028] Fig. 1 is a schematic cross-sectional illustration of a nuclear reactor core containing fuel assembles constructed according to an embodiment of this invention; [0029] Fig. 2 is a general side view of a fuel assembly according to the first embodiment of the invention, including cutaway views;

[0030] Fig. 3 is the head of the fuel assembly as per Fig. 2 in enlarged longitudinal section view;

[0031] Fig. 4 is the tailpiece of the fuel assembly as per Fig. 2 in enlarged longitudinal section view;

[0032] Fig. 5 is a cross-sectional view of a seed fuel rod;

[0033] Fig. 6 is the A-A cross-sectional view of the fuel assembly as per Fig. 2;

[0034] Fig. 7 is a general side view of a fuel assembly according to the second embodiment of the invention, including cutaway views;

[0035] Fig. 8 is the head of the fuel assembly as per Fig. 7 in enlarged longitudinal section view;

[0036] Fig. 9 is the tailpiece of the fuel assembly as per Fig. 7 in enlarged longitudinal section view;

[0037] Fig. 10 is a general side view of a fuel assembly according to another embodiment of the invention, including cutaway views;

[0038] Fig. 11 is a cross-sectional view of a tailpiece and locking mechanism of the fuel assembly in Fig. 10;

[0039] Fig. 12 is a partial perspective view of the seed fuel rod of the fuel assembly in

Fig. 10, showing a protruding portion of the locking mechanism extending therefrom;

[0040] Fig. 13 is a side cross-sectional view of the protruding portion of the locking mechanism shown in Fig. 12;

[0041] Fig. 14is an enlarged cross-sectional perspective view of the fuel assembly in

Fig. 10, showing the protruding portion engaging with a surface feature on the tailpiece;

[0042] Fig. 15 is a side sectional view of an unlocking mechanism of the fuel assembly in Fig. 10 configured to allow the protruding portion to enter and lock into the tailpiece; and

[0043] Fig. 16 is a side sectional view of the unlocking mechanism of Fig. 15, where the protruding portion is inserted into the tailpiece.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0044] Figure 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.

[0045] 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, each reflector assembly 4 may be comprised primarily of thorium oxide.

[0046] Figure 2 shows a general view of the first alternative configuration for each of the fuel assemblies 2.

[0047] 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% or less uranium- 235 enrichment. In an embodiment, the kernel 14 may be enclosed by cladding 15 of zirconium alloy and may have 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. In other embodiments, different configurations of the kernel 14, cladding 15, and spiral spacer ribs 16 may be utilized (i.e. may form a four-lobe profile, or so on). 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.

[0048] 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 (B₄C) and dysprosium titanate (Dy2C>3-TiC>2) (not shown) and burnable absorber rods based on boron carbide and gadolinium oxide (Gd₂03) (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.

[0049] 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. [0050] 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.

[0051] The tailpiece of the seed subassembly 5 in Figure 4 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.

[0052] The blanket subassembly 6 includes a frame structure 38, a bundle of fuel rods

39 situated in the frame, and a tailpiece 40.

[0053] 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.

[0054] 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.

[0055] In an embodiment, the fuel rod bundle 39 of the blanket subassembly includes a set of fuel elements (specifically 228 elements) made of a composition including approximately 12% by volume U0₂ and 88% by volume Th0₂ with 19.7% U-235

enrichment. [0056] 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_bi_ank is approximately 0.72.

[0057] 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.

[0058] Figures 7 - 9 show the second alternative for construction of each of the fuel assemblies 2.

[0059] 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 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, 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, as shown in Fig. 7 and Fig. 8. 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.

[0060] Before a fuel assembly is placed in the reactor, the seed subassembly 5 and the blanket subassembly 6 are first assembled separately. [0061] 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).

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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 may be achieved by utilizing fuel compositions having various

characteristics;

- 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.

[0077] 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 to be converted into U-233.

[0078] Figures 10 - 16 illustrate another alternative embodiment of the fuel assemblies 2. [0079] This embodiment differs from those above by utilizing a different mechanism to selectively lock the seed subassembly 5 to the blanket subassembly 6 than the locking mechanism 35 previously described. Shown in Figure 10 is the fuel assembly 2, with the blanket subassembly 6 surrounding and obscuring the seed subassembly 5. The head 7 and the tailpiece 8 of the fuel assembly 2 are also shown, where the supporting part 9 of the tailpiece 8 would contact the support tube of the reactor (not shown). As above, the fuel assembly 2 may have a hexagonal frame structure 38, with six lengthwise angle units 41 with spacer grids 42 attached to them by resistance spot welding, permitting the fuel rods 39 to slide in the spacer grid cells.

[0080] Shown in Figure 11 is an enlarged cutaway view of the tailpiece 8, showing the seed subassembly 5 within the blanket subassembly 6. The seed subassembly 5 has a plurality of the seed fuel elements 10, as well as one of the peripheral tubes 19 spaced and interconnected by the support grid 11. The support grid 11 may be coupled to a top side of the lower plate 23. Protruding from the bottom side of lower plate 23 are one or more locking protrusions 60, the structure and operation of which is described in greater detail below. In an embodiment, such as that shown in the perspective view of the seed subassembly 5 depicted in Figure 12, more than one of the protrusions 60 may be spaced on the bottom of the lower plate 23. In an embodiment wherein the lower plate 23, and thus the fuel assembly 5 have a hexagonal cross section, as described above, the protrusions 60 may be spaced about the perimeter of the hexagon shaped lower plate 23. For example, each of the protrusions 60 may be positioned along one of the sides of the lower plate 23. In another embodiment, each of the protrusions 60 may be positioned at each corner of lower plate 23. In an embodiment, the protrusions 60 may be spaced on the bottom of the lower plate 23.

[0081] The structure and configuration of an embodiment of the protrusions 60 are depicted in Figure 13. As shown, a protrusion 60 may be connected to the lower plate 23 by a fastener 62 extending therethrough. The fastener 62 may be of any construction or configuration, including but not limited to a threaded screw or bolt. In other embodiments, the protrusion 60 may be secured to the lower plate 23 by any other suitable means, including but not limited to being formed together with the lower plate 23, or being welded thereto. In the illustrated embodiment, the peripheral tubes 19 are hollow, and a fastener 62 is inserted through the lower plate 23 into the protrusion 60 within them.

[0082] As shown, the protrusion 60 may comprise a body portion 65, and a head portion 70. In an embodiment, the protrusion 60 may be hollow, having an inner space 75. In an embodiment, the inner space 75 may extend within both the body portion 65 and the head portion 70. As shown, the inner space 75 may extend all the way to a forward end 80 of the protrusion 60. The head portion 70 of the protrusion 60 may comprise one or more slits 85 extending from the forward end 80 of the protrusion 60 back towards the body portion 65, such that the head portion 70 may form one or more collet chucks 90. As shown, outwardly disposed on each of the one or more collet chucks 90 are one or more detents 95. Although the detents 95 are shown as positioned on the collet chucks 90 in the illustrated embodiment, the collet chucks 90 are not present on some embodiments, and thus the detents 95 may be positioned on any suitable location of the protrusion 60.

[0083] The detents 95may be any body that is movable between locking positions and non-locking positions, described in greater detail below. In the illustrated embodiment, the movement of each of the detents95 between the locking and non-locking positions may be perpendicular to the protruding direction of the protrusion 60. Specifically, the detents 95 may be radially movable into and out from the inner space 75. In the illustrated embodiment, the movement of the detents 95 is enabled by the elastic flexibility of the collet chucks 90. In other embodiments, any other mechanism to inwardly and outwardly move the detents 95 may be utilized, including but not limited to having a motor driven locking and non- locking mechanism.

[0084] As shown in Figure 13, the protrusion 60 may include a locking structure 100 that is movably coupled to one or more of the detents 95. The locking structure 100 may be configured to move between a first, locked position and a second, unlocked position, such that when the locking structure 100 is in the first position, the one or more detents 95 are held in their locking position, and when the locking structure 100 is in the second position, the one or more detents 95 are movable between their locking and their unlocking position. Although the locking structure 100 may be of any construction or configuration, in the illustrated embodiment the locking structure 100 is a ball bearing that is movable in the inner space 75, between the body portion 65 and the head portion 70. Figure 13 shows the locking structure 100 in the first position, located in the head portion 70. When in the head portion 70, the locking structure 100 prevents inward movement of the collet chucks 90, and thus prevents movement of the associated detents 95 from their locking position into their unlocking position. As can be appreciated from Figure 13, if the locking structure 100 were moved within the inner space 75 to the body portion 65 (i.e. with the locking structure 100 in the second position), the collet chucks 90 may flex inward into the inner space 75, whereby the detents 95 may move from their locking position to their non-locking position. Although as described herein, the detents 95 (and the collet chucks 90) may default to the locking position, in another embodiment the detents 95 may be biased to their non-locking position, and the locking structure 100 may be configured to bias the detents 95 to their locking position when the locking structure 100 is in the first position.

[0085] Further shown in Figure 13 is a cross sectional view of a biasing member 105, that is configured to bias the locking structure 100 into the first position. The biasing member 105 may be of any suitable construction or configuration, and in the illustrated embodiment comprises a compression spring. In an embodiment, the locking structure 100 may be contained within the inner space 75, even against forces such as those that may be applied by the biasing member 105. As shown, near the forward end 80 of the protrusion 60 are internal ridges 110 that are configured to prevent the locking structure 100 from exiting the internal space 75. In combination with the biasing member 105, the internal ridges 110 participate in maintaining the biased position of the locking structure 100 between the collet chucks 90, restricting movement of the detents 95 from their locking position to their unlocking position.

[0086] In Figure 14, the protrusion 60 is shown in a perspective cross sectional view as mounted to the lower plate 23, and interacting with a portion of the tailpiece 37 of the blanket subassembly 6. In this view, the operation of the detents 95 may be appreciated. As shown, the support grid 43 of the blanket subassembly 6 in combination with the tailpiece 37 allows the bottom side of the seed subassembly 5 to be partially enclosed or framed. Also shown is an aperture 115associated with the protrusion 60. Although not shown in the view of Figure 14, there may be one aperture 115 associated with each of the protrusions 60. The apertures 115 extend through the tailpiece 37, and are configured to receive the protrusions 60 when the bottom plate 23 of the seed assembly 6 moves downwards towards the tailpiece 37. As shown, the aperture 115 may be of two sizes, defining a surface feature 120, described in greater detail below, demarking a narrow aperture portion 125 and a wide aperture portion 130. In an embodiment, the wide aperture portion 130 may be sized and configured to receive the detents 95 when the detents 95 are in the locking position. As shown, the surface features 120 may be configured to interfere with the detents 95 when the detents 95 are in the locking position, such that the protrusion 60 is locked within the tailpiece 37, thus preventing the seed subassembly 5 from moving relative to the blanket subassembly 6.

[0087] The narrow aperture portion 125 of the aperture 115 may be sized and configured to receive the protrusion 60 when the detents 95 are in the non-locking position. Again, for such an action to take place, the locking structure 100 is placed in the second position, such that the collet chucks 90 may retract into the inner space 75. The mechanism for placing the locking structure 100 in the second position, against the bias of the biasing member 105, is described in greater detail below. As illustrated, a forward side 135 of each of the detents 95, proximal to the forward end 80of the protrusion 60, may be slanted or curved with respect to the body portion 65 to form an incline/cam. In an embodiment, the forward side 135 of the detents 95 may, at a side proximal to the forward end 80 of the protrusion 60, be sized and positioned such that the protrusion 60 may begin to enter the narrow aperture portion 125. As the protrusion 60 continues to enter the narrow aperture portion 125, the forward side 135 compresses the collet chucks 90 into the interior space 75, allowing the detents 95 to be compressed into the non-locking position by the entrance to the narrow aperture portion 125. In embodiments where the detents 95 are biased into their non-locking positions, the forward side 130 of the detents 95 may be of any shape, as the movement of the locking structure 100 into the second position will result in the protrusion 60 compressing to fit into the narrow aperture portion 125, without need for a mechanism to compress the detents 95 into the non-locking position.

[0088] Once the detents 95 pass the surface feature 120, and enter the wide aperture portion 130, the sides of the tailpiece 37 may no longer constrain the detents 95 into the nonlocking position, and the detents 95 may return to the locking position in the wide aperture portion 130. The wide aperture portion 130 may be any width such that the detents 95 would interfere with the surface features 120 if the locking structure 100 is in the first position, such that the detents 95 are held in the locking position. In an embodiment, the wide aperture portion 130 might not exist in the tailpiece 37, and the surface feature 120 may simply be the intersection of the aperture 115 and a bottom side of the tailpiece 37, such that the protrusion 60 extends completely through the tailpiece 37, and all of the aperture 115 is substantially the size of the narrow aperture portion 130.

[0089] To allow the protrusion 60 to easily be removed from the aperture 115, which again would require the locking member 100 to be in the second position such that the detents 95 may move into their non-locking position according to one or more embodiments, a rear side 140 of the detents 95 may also be slanted or curved with respect to the body portion 65 to form another incline/cam. In such an embodiment, as the seed subassembly 5, and thus the protrusion 60, is lifted with respect to the tailpiece 37, the rear side 140 of the detents 95 compresses the collet chucks 90 into the interior space 75, allowing the detents 95 to be compressed into the non-locking position by the surface feature 120. Similar to above, in embodiments where the detents 95 are biased into their non-locking positions, the rear side 140 of the detents 95 may be of any shape configured to interfere with the surface feature 120, as the movement of the locking structure 100 into the second position will result in the head portion 70 compressing to fit into the narrow aperture portion 125, without need for a mechanism to compress the detents 95 into the non-locking position.

[0090] Turning to Figures 15 and 16, a non-limiting embodiment of an unlocking structure 145, configured to move the locking structure 100 from the first position to the second position is depicted. As shown in the illustrated embodiment in Figure 15, the unlocking structure 145 may comprise an unlocking protrusion 150 that is configured to extend through the tailpiece 37, to interact with the locking member 100 prior to any interaction between the head portion 70 and the tailpiece 37, moving the locking member 100 sufficiently away from the detents 95, so that the detents 95may flex inward (i.e. on the collet chucks 90) when the forward side 135 of the detents 95 begins to enter the narrow aperture portion 125 of the aperture 115. In an embodiment, the unlocking protrusion 150 may be of a sufficiently small width such that the detents 95 in their non-locking position may be compressed over the unlocking structure 145, allowing the head portion 70 to enter the narrow aperture portion 125. In an embodiment, the unlocking protrusion 150 may extend from a base plate 155, which may be positioned either spaced from or adjacent to the tailpiece 37, depending on whether the locking structure 100 is to be in the first position or the second position. In some embodiments, the base plate 155 may remain fixed below the tailpiece 37 or may be a part of the tailpiece 37. In some such embodiments, the unlocking protrusion 150 may selectively extend or retract into the base plate 155, to selectively move the locking structure 100 from the first to the second position.

[0091] In some embodiments, the position of the unlocking structure 145 associated with moving the locking structure 100 out of the first position may be characterized as a release position. Thus, where the base plate 155 with a fixed protrusion 150 is adjacent to the tailpiece 37, or where a retractable unlocking protrusion 150 is in the extended position, the unlocking structure 145 may be in the release position. In some embodiments, the position of the unlocking structure 145 associated with the locking structure 100 being in the first position may be characterized as a disconnected position. Thus, where the base plate 155 is appropriately spaced from the tailpiece 37, or where the retractable unlocking protrusion 150 is in an appropriately retracted position, the unlocking structure 145 may be in the disconnected position.

[0092] In an embodiment, once the detents 95 are within the narrow aperture portion

125 of the aperture 115, the constrained position of the detents 95 may narrow the shape of the inner space 75 and may prevent the locking member 100 from returning to the first position, even if the unlocking structure 145 is removed. Once the detents 95 clear the surface feature 120 of the aperture 115, as is shown in Figure 16, they may outwardly expand such that the rear side 140 of the detents 95 would interact with the surface feature 120 when the protrusion 60 is lifted out of the aperture 115. In an embodiment wherein the detents 95 are biased to the locking position, they may expand outward regardless of whether the unlocking structure 145 is holding the locking member 100 in the second position, as is depicted in the Figure. In another embodiment, wherein the detents 95 are biased to the non-locking position (i.e. inward into the inner space 75), the unlocking structure 145 may need to be removed, and the locking structure 100 may need to return to the first position (i.e. under the bias of the biasing member 105), in order to place the detents 95 in the locking position. Regardless, where the rear sides 140 of the detents 95 are in a position to interact with the surface feature 120, once the locking structure 100 returns to the first position, such as by removal of the unlocking structure 145, the protrusion 60 is configured to not be removable from the aperture 115, as the interaction between the rear sides 140 of detents 95 and the surface feature 120 will not result in the movement of the detents 95 into the non-locking position.

[0093] To remove the protrusion 60 from the aperture 115, and separate the lower plate 23 from the tailpiece 37, the unlocking structure 145 may be returned to place the locking structure 100 in the second position, again shown in Figure 16. By lifting the lower plate 23 from the tailpiece 37, the rear sides 140 of the detents 95 may slide along the surface feature 120, compressing the detents 95 into the inner space 75, allowing the head portion 70 to traverse the narrow aperture portion 125 of the aperture 115. In embodiments wherein the detents 95 are biased into the non-locking position, the movement of the locking structure 100 into the second position may immediately result in the head portion 70 being of a small enough size to traverse the narrow aperture portion 125. In such embodiments, the rear side 140 of the detents 95 may be of any appropriate shape, including but not limited to forming a right angle with the body portion 65.

[0094] The elements of this disclosure may be constructed of any suitable material. As the fuel assembly 2 will generally be located underwater, all elements may be constructed of materials that will not corrode or suffer from degraded operating performance in a wet or high humidity environment. Furthermore, being part of a nuclear reactor, the structural elements supporting the radioactive fuel assembly may be selected for their resistance to ionizing or other radiation related degradation. Due to the heats generated by the radioactive processes, the structural elements may further be selected for their resistance to heat degradation, including but not limited to having a high melting point. In some embodiments, the structural elements, including for example the protrusion 60, the locking mechanism 100, the biasing member 105, and so on, may be constructed from refractory metals having a high resistance to heat and wear. The elements of this disclosure may likewise be assembled or formed by any appropriate construction technique, including but not limited to being molded or cast together, being welded together, or fastened together by any appropriate fastener.

[0095] 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.

[0096] For example, in some embodiments, the protrusion 60 described above may be associated with the tailpiece 37, instead of extending from the lower plate 23, as is described above. In such an embodiment, the surface feature 120 and the aperture 115 may extend into or be associated with a portion of the seed subassembly 5, such as the lower plate 23, for example. In some embodiments, the unlocking structure 145 may be configured to extend through the seed subassembly 5, instead of through the lower plate 23 of the blanket subassembly 6. In some embodiments, other mechanisms for moving the locking structure 100 between the first and second positions may be utilized, including but not limited to motor driven locking mechanisms.

[0097] According to various embodiments, the locking structure 145 is disposed in a fuel pool of a nuclear power plant to facilitate separation and connection of different combinations of the seed subassemblies 5 and the blanket subassemblies 6. After a fuel assembly 2 is used in the reactor core for a predetermined period of time, the fuel assembly 2 is removed from the core and taken to the fuel pool. While the fuel assembly 2 is in the fuel pool, the locking mechanism is released via use of the locking structure 145 to enable separation of the seed subassembly 5 and blanket subassembly 6. The seed subassembly 5 is removed and a new seed subassembly 5is then locked to the blanket subassembly 6 or a new blanket subassembly 6is then locked to the existing seed subassembly 5, depending on the relative life cycles and points within the life cycle of the seed subassembly 5 and the blanket subassembly 6 respectively. The modified assembly 2 is then returned to the reactor to form part of the core.

[0098] As to other elements of the fuel assembly 2, 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 the spacer ribs 16 or lobes, the ribs 16 or the lobes may be omitted. While the illustrated fuel elements 10 include the displacers 17, such displacers 17 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.

[0099] Various embodiments of the present invention may be used in connection with the seed/blanket fuel assemblies and/or fuel elements disclosed in U.S. Provisional Patent Application No. 61/393,499, filed October 15, 2010, titled "Metal Fuel Assembly," U.S. Patent Application No. 12/340,833, filed December 22, 2008 (published as U.S. Patent Application Publication No. US2009/0252278 Al), PCT Patent Application No.

PCT/RU2007/000732, filed December 26, 2007, and PCT Publication No. WO2010074592 (Al), the entire contents of each of which are incorporated herein by reference.

Claims

WHAT IS CLAIMED IS:

1. A fuel assembly for use in a nuclear reactor, the fuel assembly comprising:

a blanket subassembly comprising a blanket frame and a plurality of blanket fuel elements supported by the blanket frame;

a seed subassembly comprising a seed frame and a plurality of seed fuel elements supported by the seed frame, the seed subassembly being movable relative to the blanket subassembly between engaged and disengaged positions;

a locking mechanism that releasably locks the seed subassembly in the engaged position, the locking mechanism comprising

a surface feature operatively positioned on one of the blanket frame and the seed frame,

at least one detent operatively connected to the other of the blanket frame and the seed frame, the at least one detent being movable between a locking position and a non- locking position, the at least one detent engaging the surface feature when the seed subassembly is in the engaged position and the at least one detent is in the locking position so as to prevent the seed subassembly from disengaging from the blanket subassembly, the at least one detent not preventing the seed subassembly from disengaging from the blanket subassembly when the seed subassembly is in the engaged position and the at least one detent is in the non-locking position, and

a locking structure movably connected to the at least one detent, the locking structure being movable between a first position and a second position such that (i) when the locking structure is in the first position and the at least one detent is in the locking position, the locking structure prevents the at least one detent from moving into the nonlocking position, and (ii) when the locking structure is in the second position, the locking structure does not prevent the at least one detent from moving between the locking and nonlocking positions.

2. The fuel assembly of claim 1, wherein the blanket subassembly laterally surrounds the seed subassembly, and wherein the blanket subassembly comprises a central opening into which the seed subassembly fits.

3. The fuel assembly of claim 1, wherein the plurality of seed fuel elements comprise fissionable material, and wherein the plurality of blanket fuel elements comprise thorium.

4. The fuel assembly of claim 1 , wherein the locking structure is biased toward the first position.

5. The fuel assembly of claim 4, wherein the locking structure comprises a spring biased ball.

6. The fuel assembly of claim 4, in combination with an unlocking structure that is movable relative to the locking structure between a release position and a disconnected position, wherein:

movement of the unlocking structure from the disconnected position to the release position forces the locking structure into the second position, and

movement of the unlocking structure from the release position to the disconnected position permits the locking structure to move into the first position.

7. The fuel assembly of claim 1, wherein the locking mechanism comprises a collet that includes at least one collet chuck, the at least one detent being disposed on the at least one collet chuck.

8. The fuel assembly of claim 7, wherein:

the at least one collet chuck comprises a plurality of circumferentially arranged collet chucks,

the at least one detent comprises a plurality of detents disposed on respective ones of the plurality of collet chucks, and

the plurality of detents engage the surface feature when the seed subassembly is in the engaged position and the plurality of detents are in the locking position so as to prevent the seed subassembly from disengaging from the blanket subassembly.

9. The fuel assembly of claim 8, further comprising an opening in the collet disposed radially inward of the plurality of circumferentially arranged collet chucks, wherein the locking structure is axially movable within the opening, between the first position and the second position.

10. A method of assembling a fuel assembly for use in a nuclear reactor core, the fuel assembly comprising:

a blanket subassembly comprising a blanket frame and a plurality of blanket fuel elements supported by the blanket frame; a seed subassembly comprising a seed frame and a plurality of seed fuel elements supported by the seed frame, the seed subassembly being movable relative to the blanket subassembly between engaged and disengaged positions; a locking mechanism adapted to releasably lock the seed subassembly in the engaged position, the locking mechanism comprising a surface feature operatively positioned on one of the blanket frame and the seed frame, at least one detent operatively connected to the other of the blanket frame and the seed frame, the at least one detent being movable between a locking position and a non-locking position, the at least one detent engaging the surface feature when the seed subassembly is in the engaged position and the at least one detent is in the locking position so as to prevent the seed subassembly from disengaging from the blanket subassembly, the at least one detent not preventing the seed subassembly from disengaging from the blanket subassembly when the seed subassembly is in the engaged position and the at least one detent is in the non-locking position, and a locking structure movably connected to the at least one detent, the locking structure being movable between a first position and a second position such that (i) when the locking structure is in the first position and the at least one detent is in the locking position, the locking structure prevents the at least one detent from moving into the non-locking position, and (ii) when the locking structure is in the second position, the locking structure does not prevent the at least one detent from moving between the locking and non-locking positions, the method comprising:

moving the locking structure from the first position to the second position;

moving the seed subassembly from the disengaged position to the engaged position; positioning the at least one detent in the locking position; and

moving the locking structure into the first position such that the locking structure prevents the at least one detent from moving out of the locking position, thereby locking the seed subassembly in the engaged position.

11. A method for unlocking a seed subassembly from a blanket subassembly of a fuel assembly, the blanket subassembly comprising a blanket frame and a plurality of blanket fuel elements supported by the blanket frame, the seed subassembly comprising a seed frame and a plurality of seed fuel elements supported by the seed frame, the seed subassembly being movable relative to the blanket subassembly between engaged and disengaged positions, the fuel assembly further comprising a locking mechanism releasably locking the seed subassembly in the engaged position, the locking mechanism comprising a surface feature operatively positioned on one of the blanket frame and the seed frame, at least one detent operatively connected to the other of the blanket frame and the seed frame, the at least one detent being movable between a locking position and a non-locking position, the at least one detent engaging the surface feature when the seed subassembly is in the engaged position and the at least one detent is in the locking position so as to prevent the seed subassembly from disengaging from the blanket subassembly, the at least one detent not preventing the seed subassembly from disengaging from the blanket subassembly when the seed subassembly is in the engaged position and the at least one detent is in the non- locking position, and a locking structure movably connected to the at least one detent, the locking structure being movable between a first position and a second position such that (i) when the locking structure is in the first position and the at least one detent is in the locking position, the locking structure prevents the at least one detent from moving into the non- locking position, and (ii) when the locking structure is in the second position, the locking structure does not prevent the at least one detent from moving between the locking and non- locking positions, the method comprising:

moving an unlocking structure relative to the locking mechanism into a release position that moves the locking structure from the first position to the second position, thereby permitting the at least one detent to move into its non-locking position;

moving the seed subassembly from its engaged position to its disengaged position, said moving of said seed assembly causing the at least one detent to move from its locking position to its non-locking position; and

moving the unlocking structure relative to the locking mechanism into a disconnected position that permits the locking structure to move from its second position to its first position.