EA023549B1 - Fuel element of fuel assembly for light-water nuclear reactor - Google Patents

Abstract

The invention relates to fuel elements used in fuel assemblies for light-water reactors of PWR type (e.g., AP-1000, EPR, etc.). The fuel element of the fuel assembly for a light-water nuclear reactor comprises a kernel containing enriched uranium and plutonium, and surrounding cladding. The named fuel element has a four-leafed profile, the leaves of which form screw spacing ribs, the pitch of axial coiling of the screw spacing ribs being between 5 and 30% of the fuel element length. Such configuration of the fuel element allows its installation in a seed cluster of the fuel assembly which, on the one hand, generates a significant part of its power in the breeding blanket using thorium as fuel and does not produce any wastes that are proliferative materials, and, on the other hand, can be mounted in the existing light-water reactor of PWR type (e.g., AP-1000, EPR, etc.) without any significant modification.

Description

The invention relates to light-water nuclear reactors, in particular to designs of fuel cells in fuel assemblies.

State of the art

All currently available nuclear reactors create a large amount of material, which is commonly called reactor plutonium. For example, a conventional 1000 MW reactor produces about 200-300 kg per year of reactor-grade plutonium, which may be suitable for the manufacture of nuclear weapons. Thus, fuel discharged from the core of conventional reactors is highly proliferating material, and precautions are required to ensure that the discharged fuel does not fall into the hands of those who do not have the right to own it. A similar security problem also exists in connection with the huge stockpiles of weapons-grade plutonium that were created in the United States and in the countries of the former USSR during the dismantling of nuclear weapons.

Another problem associated with the operation of conventional nuclear reactors is the constant need for the disposal of long-lived radioactive waste, as well as the rapid depletion of world resources of natural uranium raw materials.

To solve the above problems, recent attempts have been made to create nuclear reactors that operate on relatively small amounts of non-proliferative enriched uranium (enriched uranium has an I-235 content of 20% or less) and does not produce significant amounts of propagating materials, such as plutonium. Examples of such reactors are disclosed in international applications \ UO 85/01826 and \ UO 93/16477, which show reactors with a double core containing an ignition zone and a reproduction zone, which receive a significant percentage of their power from reproduction zones with thorium as fuel. Reproduction zones surround the ignition zone in a circle in which fuel rods from non-proliferative enriched uranium are located. Uranium in the fuel rods of the ignition zone emits neutrons that are captured by thorium in the reproduction zones, which creates the nuclear fission capable I-233, which burns in place and generates heat for the reactor power plant.

The use of thorium as a fuel for a nuclear reactor is attractive because the world's thorium reserves far exceed uranium reserves. In addition, both of the aforementioned reactors are non-proliferative in the sense that neither the initial feed fuel nor the fuel discharged at the end of each fuel cycle is suitable for the production of nuclear weapons. This is achieved due to the fact that only non-proliferative enriched uranium is used as fuel in the firing zone, and moderator / fuel ratios are selected that minimize plutonium formation, and a small amount of non-proliferative enriched uranium is added to the reproduction zone in which the I-238 component homogeneously mixes with I-233 remaining at the end of the reproduction cycle and denaturing (changes the natural properties) of I-233, as a result of which it becomes unsuitable for the manufacture of nuclei weapons.

Unfortunately, none of the above reactor designs is truly non-proliferative. In particular, it was found that both of these structures lead to the formation level of proliferative plutonium in the ignition zone, exceeding the minimum possible level. The use of a circular ignition zone with an internal or central reproduction zone and an external surrounding reproduction zone cannot ensure the operation of the reactor as a non-proliferative reactor, since the thin circular ignition zone has a correspondingly small optical thickness, which leads to the fact that the spectrum of (neutrons) of the ignition zone will dominate over significantly more stringent range of internal and external zones of reproduction. This leads to the appearance in the ignition zone of a larger proportion of suprathermal neutrons and a greater than minimum amount of production of multiplying plutonium.

Both of these previous reactor designs are furthermore not optimized based on the standard point of operating parameters. For example, moderator / fuel volume ratios in the ignition zone and reproduction zones are especially critical for obtaining the minimum amount of plutonium in the ignition zone, so that an adequate amount of heat is released from the fuel rods of the ignition zone and the optimum conversion of thorium to I-233 in the reproduction zone is ensured. Studies have shown that the preferred moderator / fuel ratios indicated in these international applications are too high in the ignition zones and too low in the reproduction zones.

The previous reactor core designs are also not particularly effective when non-proliferative enriched uranium is consumed in the fuel elements of the ignition zone. As a result, the fuel rods discharged at the end of each fuel cycle in the firing zone contained so much remaining uranium that they needed to be reprocessed for reuse in another reactor core.

The reactor disclosed in the application \ UO 93/16477 also requires a complex mechanical control circuit of the reactor, which makes it unsuitable for re-equipping it with the active zone of a conventional reactor. Similarly, the reactor core disclosed in the application \ UO 85/01826 cannot be easily transferred to the conventional core, since its design parameters are not compatible with the parameters

- 1,023,549 conventional core.

Finally, both previous reactor designs were specifically designed to burn non-proliferative enriched uranium with thorium and are not suitable for consuming large amounts of plutonium. Therefore, none of these designs provides a solution to the problem of stored accumulated plutonium.

Known reactor according to patent KI 2176826 with an active zone, including many ignition-reproducing modules, each of which contains a central ignition zone, and the ignition zone includes fuel elements of the ignition zone made of nuclear fissionable material containing uranium-235 and uranium 238, a circular reproduction zone surrounding the ignition zone and including fuel elements of the reproduction zone, containing mainly thorium and 10% by volume or less enriched uranium, a moderator in the ignition zone, and m the ratio of the amounts of moderator to fuel is in the range of values from 2.5 to 5.0, and the moderator in the reproduction zone, and the ratio of moderator to fuel is in the range of 1.5-2.0. Moreover, each of the fuel elements of the ignition zone consists of a uranium-zirconium (I-Ζτ) alloy, and the ignition zone is 25–40% of the total volume of each ignition-reproducing module.

The known reactor provides optimal operation in terms of efficiency and is not proliferative. This reactor can be used to consume large quantities of plutonium with thorium, without creating waste, which is proliferative materials. At the same time, this reactor produces significantly smaller amounts of highly radioactive waste, which significantly reduces the need for long-term storage of waste.

However, the ignition-reproducing modules used in this reactor are not suitable for use in existing light-water reactors of the above type P \ UR (for example, AP-1000, EPK, etc.).

From the description of patent KI 2222837, a fuel assembly of a light-water reactor similar to that described above is known, which has, in particular, a square cross-sectional shape, which makes it possible to install said fuel assembly from ignition-reproducing modules into a conventional light-water reactor.

However, in addition to indicating the cross-sectional shape of the assembly, the description of the above patent does not contain information on the structural design of the assembly, allowing it to be installed in an existing P \ UR light-water reactor (for example, AP-1000, EPK, etc.) without making any changes to the design of the reactor.

Known fuel assembly of a light-water reactor according to patent KL 2294570, containing a bunch of fuel elements and guide channels placed in the spacer grids, a shank and a head, the spacer grids being connected to each other and with the shank with elements located along the length of the fuel assembly, and the head consists of a connected upper and the lower plates, the shell located between the said plates, and the spring block, while the shell of the head is equipped with external ribs interconnected by The protruding parts of the rim and the lower parts - perforated plates.

The well-known fuel assembly relates to the construction of caseless heat-generating assemblies, from which the active zones of water-cooled power reactors of the WWER-1000 type are formed, and has improved performance due to increased stiffness, reduced head length and increased free space between the beam of fuel elements and the head with simultaneous increasing the length of the fuel elements. This allows you to increase the load in the fuel assembly of fuel with a greater burnup depth and thereby increase the power of the reactor core, as well as the duration of operation of the fuel assembly.

However, in this assembly all the fuel elements are made of fissile material traditionally used in light-water reactors; therefore, a reactor with such assemblies has the drawback described above - the creation of a large amount of reactor plutonium. In addition, this assembly is adapted for VVER-1000 reactors, i.e. has a hexagonal cross-sectional shape, which does not correspond to the shape of the fuel assemblies used in P \ UR reactors (for example, AP-1000, EPK, etc.).

Known fuel cell according to patent KL 2170956, consisting of a shell with end caps and a core in the form of nuclear fuel particles distributed in the contact material. Inside the shell coaxially with it for the entire length of the active part of the fuel cell there is a displacer made in the form of a rod of structural material used in the active zones of nuclear reactors and having a cross-sectional area in the range from 0.3 to 0.8 of the internal cross-sectional area fuel cell, and nuclear fuel particles are made in the form of granules with porosity from 2 to 30%. The shell may have a four-blade profile. In this case, the shell is twisted relative to the longitudinal axis with a constant pitch. Stainless steel, nickel alloys, chromium alloys can be used as the sheath material, and stainless steels, zirconium alloys can be used as the displacer material.

However, in the known fuel cell, the pitch of the helix of the ribs is not defined, which would enable these ribs to function as spacing elements when installing a bundle of such fuel cells in a fuel assembly that would correspond to the shape of the fuel assemblies used in P \ UR reactors (e.g. AR-1000, EPK, etc.).

The objective of the invention is the creation of a fuel cell that can be installed in the ignition module of the fuel assembly, which, on the one hand, generates a significant part of its capacity in the reproduction zone with thorium as fuel and does not create waste using it as proliferative materials, but with on the other hand, it can be installed in an existing light-water reactor of the P \ UR type (for example, AR-1000, EPK, etc.) without the need for its substantial modification.

Disclosure of invention

This problem is solved in that the fuel element of the fuel assembly of a nuclear reactor contains a core including enriched uranium or plutonium and its envelope, and has a four-petal profile, the petals of which form screw spacing ribs, and the pitch of the axial curling of screw spacer ribs is from 5 to 30% fuel cell length.

Preferably, the shell is made of a zirconium alloy, and a displacer having a substantially square cross section is located along the longitudinal axis of the core.

The displacer is made of zirconium or its alloy, and the core is made of uranium-zirconium (I--γ) alloy with a volume content of uranium of up to 30%, while uranium is enriched up to 20% by uranium-235 isotope.

The core can also be made of plutonium-zirconium (Pu-Ζγ) alloy with a volumetric content of energy plutonium up to 30%.

Features and advantages of the present invention will be apparent from the further description of a preferred embodiment with reference to the drawings.

Brief Description of the Drawings

In FIG. 1 shows one possible embodiment of a fuel assembly with fuel cells according to the invention;

in FIG. 2 schematically shows a cross section of the fuel assembly shown in FIG. one; in FIG. 3 is a schematic perspective view of a fragment of a fuel cell according to the invention;

in FIG. 4 is a cross-sectional view of the fuel cell shown in FIG. 3.

Embodiments of the invention

In FIG. 1 shows the fuel assembly, indicated by the general reference number 1. The fuel assembly 1 comprises an ignition module 2, a reproducing module 3, a head 4, a shank 5 of the ignition module and a shank 6 of the reproducing module. As shown in FIG. 2, the ignition module 2 contains a bundle of fuel cells 7, and the reproducing module 3 contains a bundle of fuel cells 8. Each of the fuel cells 7 of the ignition module 2 has a four-leaf profile forming screw spacing ribs 9 along the length of the fuel cell (Fig. 3). In addition, each of these fuel cells 7 contains a core 10 (Fig. 4), including enriched uranium or plutonium, and a zirconium alloy shell 11 surrounding it. A displacer 12 is located inside the core 10. All fuel elements 7 are in contact with each adjacent fuel element 7 at the touch points of the screw spacer ribs 9. The touch points of the screw spacer ribs 9 are axially spaced apart by 25% of the screw pitch lines. Mostly the core 10 is made of a uranium-zirconium (I-Ζγ) alloy, and the volume content of uranium in the fuel composition is up to 30% when enriched in the uranium-235 isotope to 20% or from plutonium-zirconium (Pu-Ζγ) alloy with a volume content plutonium up to 30%. The displacer 12, located along the longitudinal axis of the core 10, has a practically square cross section. The pitch of the helical line of the screw spacing ribs 9 is from 5 to 30% of the length of the fuel cell.

Each of the fuel cells 8 (Fig. 1, 2) has a circular shape in plan and is made of thorium with the addition of enriched uranium. The fuel cells 7 and 8 in cross section are arranged in rows and columns of a square grid so that the entire fuel assembly 1 has a square shape in plan. In particular, the fuel elements 7 of the ignition module are arranged in rows and columns of a square coordinate grid of 13 rows and 13 columns, and the fuel elements of the reproducing module 8 are located in two extreme rows and columns of a square coordinate grid of 19 rows and 19 columns.

The profiles of each fuel element 7 have the same diameter of the circumscribed circle (FIG. 4), for example, 12.6 mm. The number of fuel cells 7 is 144 pieces. Fuel elements 8 have the same diameter, for example, of 8.6 mm, and are located on the sides of the square in two rows and columns of the square grid. The number of fuel cells 8 is 132 pieces.

In the center of the ignition module 2, a tube 13 is located, forming a guide channel for placement of control means in it. Guide channels are located within the firing module 2

- 3 023549 for introducing absorbing rods and emergency protection rods installed in the head 4 with the possibility of axial displacement and connected with the shank 5 of the ignition module 2 and the shank 6 of the reproducing module 3.

The fuel element bundle 7 of the ignition module 2 is surrounded by a casing 17 fixed in the shank 5. The lower end sections of the fuel elements 7 of the ignition module 2 are installed in the support grid 18, and their upper end sections are in the guide grid 19 (Fig. 1). The fuel element 7 of the ignition module 2 can be manufactured by the method of joint pressing (extrusion through the matrix) in the form of a single assembly unit. The pitch of the screw line of the screw spacer ribs 9 is selected from the condition of the relative position of the axes of the adjacent fuel elements 7 at a distance equal to the diameter of the circumscribed circle in the cross section of the fuel element and ranges from 5 to 30% of the length of the fuel element.

This embodiment of the fuel cell allows it to be used in the ignition module of the fuel assembly, which can be installed in an existing P ^ K light water reactor (for example, AP-1000, EPK, etc.) without the need for significant modification.

Claims (6)

1. A fuel cell of a fuel assembly of a light-water nuclear reactor comprising a core including enriched uranium or plutonium and a shell surrounding it, said fuel cell having a four-petal profile, the petals of which form screw spacing ribs, wherein the pitch of the axial curl of the spacer ribs is from 5 to 30% of the length of the fuel cell. 2. The fuel cell according to claim 1, in which the shell is made of zirconium alloy. 3. The fuel cell according to claim 1, containing a propellant located along the longitudinal axis of the core and having an almost square shape in cross section. 4. The fuel cell according to claim 2, in which the displacer is made of zirconium or its alloy. 5. The fuel cell according to claim 1, in which the core is made of uranium-zirconium (I-Ζγ) alloy with a volume content of uranium of up to 30%, while uranium is enriched up to 20% in the isotope of uranium-235. 6. The fuel cell according to claim 1, characterized in that the core is made of plutonium zirconium (Pu-Ζγ) alloy with a volume content of energy plutonium up to 30%.