JP2017072480A - Fuel pellet, nuclear fuel rod, fuel assembly and fuel pellet manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel pellet capable of reducing environmental load by curtailing TRU generation amount in a light water reactor, a nuclear fuel rod, a fuel assembly and a fuel pellet manufacturing method.SOLUTION: A fuel pellet 10 has coated fuel particles 26 in each of which a central nucleus 28 containing a fuel material that is formed by adjusting its diameter is coated with a coating layer holding fission gas generated by nuclear fission and a matrix 27 containing the coated fuel particles 26 dispersed.SELECTED DRAWING: Figure 2

Description

  Embodiments described herein relate generally to a fuel pellet, a nuclear fuel rod, a fuel assembly, and a fuel pellet manufacturing method applied to a light water reactor.

  The radiotoxicity of high-level radioactive waste generated at nuclear power plants has been higher than the natural world for over 100,000 years in the once-through cycle. After about 500 years of this radiotoxicity, the toxicity due to the transuranium element (TRU) becomes dominant. The long-term toxicity of TRU nuclides is due to its long half-life.

  At present, high-level radioactive waste is stabilized and treated as a vitrified material, and is disposed of in geological formations. However, since it is necessary to secure a disposal site for disposal of these materials and to manage them for a long time, the environmental burden is extremely large.

  In order to shorten the management period of high-level radioactive waste and reduce the burden on the environment, research on making TRU nuclides short-lived nuclides by transmutation has been conducted. As typical technologies for transmuting TRU nuclides to short-lived nuclides, annihilation treatment is performed using a transmutation method using fast neutrons generated in a fast reactor or a transmutation facility having an accelerator such as an accelerator-driven reactor (ADS). Technology is being considered.

  However, light water reactors are considered to be the main reactor types used in current and future nuclear power plants. For this reason, a considerable amount of TRU may accumulate until the operation of TRU transmutation using a fast reactor or an accelerator-driven reactor starts, and the number of facilities necessary for transmutation may increase. In addition, construction of facilities necessary for nuclear transmutation requires enormous capital investment.

  For this reason, a technique for suppressing the amount of TRU generated in a light water reactor rather than transmuting the generated TRU has been studied. For example, by adjusting the ratio of plutonium and the core flow rate to increase the burnup of nuclear fuel, A technique for reducing the amount of generation of is disclosed.

JP 2013-33065 A Japanese Patent No. 4936906 JP 2008-216009 A JP 2006-64678 A

  By the way, in order to suppress the amount of TRU generated in a light water reactor, it is known that it is effective to increase the burnup of nuclear fuel and to increase the deceleration performance for decelerating neutrons generated by the nuclear reaction.

  However, when the burnup is increased by adjusting the ratio or amount of the nuclear fuel material contained in the fuel pellet, the fission product gas accumulates in the cladding tube and the pressure in the tube increases. Since the upper limit value of the pressure in the tube is determined by the design conditions of the cladding tube, there is an upper limit on the burnup that can be increased. In addition, if the burnup is too high, there is a problem that neutron irradiation damage of the cladding tube increases.

  On the other hand, in order to improve the deceleration performance, it can be realized by reducing the number of nuclear fuel rods arranged in the fuel assembly and adding water rods. The safety margin and economic efficiency of the reactor may be impaired. Although it is conceivable that the structure of the fuel pellet itself is a hollow pellet or a low density pellet, there is a limit to the line power density that can be reached.

  Thus, with the conventional technology, it has been difficult to simultaneously achieve high burnup and high speed reduction without impairing the soundness and economic efficiency of the reactor.

  The present invention has been made in view of such circumstances, and provides a fuel pellet, a nuclear fuel rod, a fuel assembly, and a fuel pellet manufacturing method capable of reducing the environmental load by suppressing the amount of TRU generated in a light water reactor. With the goal.

  In a fuel pellet according to an embodiment of the present invention, a particulate coated particle fuel in which a central core containing a nuclear fuel substance and having a diameter adjusted is coated with a coating layer that holds a fission product gas generated by fission And a matrix containing the coated particle fuel in a dispersed manner.

  In the fuel pellet manufacturing method according to the embodiment of the present invention, a step of forming a central core containing nuclear fuel material and adjusting the diameter, and covering the central core with a coating layer that holds a fission product gas generated by fission A step of forming a particulate coated particle fuel, and a step of mixing the base material and the coated particle fuel, adding a sintering aid, and performing a sintering process.

  Embodiments of the present invention provide a fuel pellet, a nuclear fuel rod, a fuel assembly, and a fuel pellet manufacturing method that can reduce the environmental load by suppressing the amount of TRU generated in a light water reactor.

(A) is a longitudinal cross-sectional view of a fuel assembly according to the present embodiment, and (B) is a longitudinal cross-sectional view of a nuclear fuel rod loaded with fuel pellets according to the present embodiment. (A) is explanatory drawing which shows the fuel pellet which concerns on this embodiment, (B) is sectional drawing of a covering particle fuel. (A) is a graph of analysis results showing the relationship between the particle diameter of the core of the coated particle fuel and the hydrogen to heavy metal atom ratio, and (B) is the relationship between the particle diameter of the core of the coated particle fuel and the burnup. The graph of the analysis result shown. The flowchart which shows the manufacturing method of the fuel pellet which concerns on this embodiment. The graph of the analysis result which shows the relationship between a hydrogen to heavy metal atom number ratio and radiotoxicity when changing a burnup as a parameter. The graph which compared the radioactive toxicity of the waste in the fuel pellet which concerns on this embodiment, and the fuel pellet (comparative example) in a normal light water reactor.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1A shows a longitudinal sectional view of a fuel assembly 10 according to the present embodiment, and FIG. 1B shows a longitudinal sectional view of a nuclear fuel rod 11 loaded with fuel pellets 20 according to the present embodiment. Show.

  In the fuel assembly 10, a plurality of nuclear fuel rods 11 loaded with fuel pellets 20 are arranged inside a cylindrical channel box 12. An upper tie plate 13 that supports the upper end of the nuclear fuel rod 11 is provided at the upper portion of the channel box 12, and a lower tie plate 14 that supports the lower end of the nuclear fuel rod 11 is provided at the lower portion. The spacers 15 are spaced apart from each other between the lower tie plate 14 and the upper tie plate 13, and maintain the horizontal spacing between the nuclear fuel rods 11.

  The upper tie plate 13 is provided with a handle 16 that is gripped during normal transport of the fuel assembly 10 and a channel fastener 18 that is fixed to the plate via a screw 17. The channel fastener 18 adjusts the interval between the fuel assemblies 10 arranged adjacent to the reactor core (not shown).

  As shown in FIG. 1B, the nuclear fuel rod 11 is loaded with a plurality of fuel pellets 20 stacked in a hollow cylindrical cladding tube 21. An upper end plug 22 and a lower end plug 23 are provided at the upper end and the lower end of the cladding tube 21, and the fuel pellet 20 is sealed in the cladding tube 21 by the both end plugs.

  The cladding tube 21 is made of a material having sufficient stability under high temperature conditions, such as a Zircaloy alloy, a stainless alloy, or a SiCf / SiC composite material (a composite material of silicon carbide fiber and silicon carbide). In particular, by using a SiCf / SiC composite material, generation of hydrogen and heat of oxidation due to an oxidation reaction with high-temperature water can be suppressed, and the progress thereof can be delayed at the time of a serious accident that causes core melting or the like.

  A plenum 24 is provided in the upper portion of the cladding tube 21 as a fixed space where the fuel pellets 20 are not stacked. The plenum 24 serves as a gas reservoir that stores the fission product gas released from the fuel pellet 20.

  A spring-like plenum spring 25 is provided in the plenum 24. One end of the plenum spring 25 is connected to the upper end plug 22, and the other end is locked to the fuel pellet 20, so that the fuel pellet 20 is covered. It is fixed in the tube 21.

  FIG. 2A is a view for explaining the configuration of the fuel pellet 20 according to this embodiment, and FIG. 2B is a cross-sectional view of the coated particle fuel 26.

  The fuel pellet 20 according to the present embodiment is a particulate coated particle fuel in which a central core 28 containing a nuclear fuel material and having a diameter adjusted is coated with a coating layer that holds a fission product gas generated by fission. 26 and a matrix 27 containing the coated particle fuel 26 in a dispersed manner.

  The coated particle fuel 26 is a particulate nuclear fuel element composed of a plurality of concentric spherical regions, and a central core 28 (kernel) including a nuclear fuel material is formed in a central portion, and a plurality of coatings are formed so as to cover the central core 28. A layer is formed.

As the nuclear fuel material, uranium oxide (UO ₂ ) enriched with uranium 235, plutonium, or the like is used. Moreover, transuranium elements, minor actinides, etc. separated and recovered from radioactive waste generated in light water reactors may be reused.

  The central core 28 containing the nuclear fuel material is formed by adjusting the diameter R of the particles to a small diameter by using a dry method or a wet method.

Here, the diameter of the central core 28 to be adjusted will be considered.
3A is an analysis result showing the relationship between the particle diameter R of the central nucleus 28 and the hydrogen to heavy metal atom number ratio (H / heavy metal atom ratio), and FIG. It is an analysis result which shows the relationship between the particle diameter R and a burnup.

  The atomic ratio of hydrogen to heavy metal is one of the indices indicating the neutron deceleration performance, and the higher the numerical value, the higher the deceleration performance. Here, calculation is performed using uranium as a heavy metal.

  The burnup is the energy generated per unit uranium weight by the nuclear fuel material loaded in the core and expressed by the energy generated per unit uranium weight. One gigawatt (GW) of heat is emitted per day (d). The unit is the amount of heat when continued.

  As shown in FIG. 3A, it can be seen that as the particle diameter R decreases from 5 mm, the hydrogen to heavy metal atom number ratio increases and the deceleration performance increases. Further, as shown in FIG. 3B, it can be seen that the burnup increases as the particle diameter R decreases from 5 mm. That is, by reducing the particle size of the particle size R, it is possible to simultaneously achieve high burnup and high speed reduction.

  The particle diameter R of the central core 28 is preferably set to 2 mm or less from the viewpoint of realizing high burnup and high deceleration, and is set to 1 mm or less because the burnup and deceleration performance are higher. It is more preferable.

Returning to FIG. 2, the description will be continued.
The plurality of coating layers covering the central nucleus 28 is for holding (confining) the fission product gas generated by fission in the central nucleus 28 and is composed of low-density carbon in order from the central nucleus 28 toward the outside. The first coating layer 29, the second coating layer 30 made of pyrolytic carbon, and the third coating layer 31 made of SiC. These coating layers are formed, for example, by vapor deposition and coating around the central core 28 by thermal decomposition of vapor deposition gas in a fluidized bed.

  Note that FIG. 2B shows an example of the coating layer, and is not limited to a coating layer composed of three layers, and a coating layer such as a pyrolytic carbon layer may be added. good.

  The matrix 27 is a metal ceramic containing the coated particle fuel 26 in a dispersed manner. Examples of the base material of the matrix 27 include silicon carbide, a titanium alloy, and aluminum oxide.

  The matrix 27 is formed into a cylindrical shape by mixing a powdery base material and the coated particle fuel 26, adding a sintering aid to the mixture, and sintering the mixture. The coated particle fuel 26 is sintered so as to be dispersed in the matrix 27. As a sintering method for forming the matrix 27, an existing method for forming ceramics such as a liquid phase sintering method is used.

As the sintering aid, a material having a melting point lower than that of the base material is selected. For example, a mixed material of aluminum oxide (Al ₂ O ₃ ) and yttrium oxide (Y ₂ O ₃ ) can be used.

  When the coated particle fuel 26 is loaded in the cladding tube 21 in the form of particles, the possibility of mixing in the cladding tube 21 is high, and a technique of providing a concentration distribution in the axial direction normally performed in a boiling water reactor is used. It becomes difficult to apply. On the other hand, safety and economy can be improved by sintering and pelletizing using a base material. Moreover, by making into pellets, the thermal conductivity can be increased, and the operating temperature when the reactor is loaded can be lowered.

  FIG. 4 is a flowchart showing a method for manufacturing the fuel pellet 20 according to the present embodiment (see FIG. 2 as appropriate).

  In the molding step S10, the central core 28 containing the nuclear fuel material is molded with a small diameter. The particle diameter R of the central core 28 is preferably set to 2 mm or less, and more preferably 1 mm or less.

  In the particle forming step S11, the core core 28 is coated with a plurality of coating layers, and the particulate coated particle fuel 26 is formed.

  In the sintering step S <b> 12, the base material and the coated particle fuel 26 are mixed, a sintering aid is added, a sintering process is performed, and the cylindrical matrix 27 is formed. At this time, the coated particle fuel 26 is sintered so as to be dispersed in the matrix 27.

  FIG. 5 is a graph of analysis results showing the relationship between the hydrogen to heavy metal atom number ratio and radiotoxicity when the burnup is changed as a parameter. The burnup is changed to 45 (GWd / t), 60 (GWd / t), 250 (GWd / t), and 500 (GWd / t).

  As shown in FIG. 5, it is understood that the toxicity of the radioactive waste is suppressed by increasing the burnup and increasing the deceleration performance (hydrogen to heavy metal atom ratio).

  That is, by adjusting the particle diameter R of the central core 28 containing the nuclear fuel material to a small diameter to increase the burnup and deceleration performance, it is possible to suppress the amount of TRU produced that exhibits high toxicity in a light water reactor. For this reason, environmental load is reduced.

  FIG. 6 is a graph comparing the results of the radioactive toxicity of waste in the fuel pellet 20 according to the present embodiment and the fuel pellet (comparative example) in a normal light water reactor.

  The example is a result of the fuel pellet 20 in which the burnup is 500 (GWd / t) and the particle diameter R of the central core 28 is 1 mm. On the other hand, the comparative example is an ordinary fuel pellet used in a light water reactor, in which a homogeneous uranium fuel with a burnup of 45 (GWd / t) is sintered into a fuel pellet.

  From the results of FIG. 6, it can be seen that by setting the particle diameter R of the central core 28 of the coated particle fuel 26 to 1 mm, the radiotoxicity is reduced to about one third as compared with a normal fuel pellet. .

  According to the fuel pellet of the embodiment described above, by using the coated particle fuel as the nuclear fuel element in the fuel pellet, and adjusting the particle diameter of the central core containing the fuel substance to a small diameter, high deceleration performance and high burnup are achieved. And the amount of TRU generated in the light water reactor can be suppressed. Thereby, an environmental load can be reduced.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

10 fuel assembly 11 nuclear fuel rod 12 channel box 13 upper tie plate 14 lower tie plate 15 spacer 16 handle 17 screw 18 channel fastener 20 fuel pellet 21 cladding tube 22 upper end plug 23 lower end plug 24 plenum 25 plenum spring 26 coated particle fuel 27 Matrix 28 Kernel 29 First Cover Layer 30 Second Cover Layer 31 Third Cover Layer

Claims (8)

A particulate coated particle fuel in which a central core containing a nuclear fuel material and having a diameter adjusted is coated with a coating layer that holds a fission product gas generated by fission, and
A fuel pellet comprising: a matrix containing the coated particle fuel dispersed therein.   The fuel pellet according to claim 1, wherein a diameter of the central core in the coated particle fuel is adjusted to 2 mm or less.   The fuel pellet according to claim 1 or 2, wherein the nuclear fuel material includes at least one of uranium, plutonium, transuranium element, and minor actinod.   The fuel pellet according to any one of claims 1 to 3, wherein the matrix is ceramic of silicon carbide, titanium alloy, or aluminum oxide.   A nuclear fuel rod, wherein the fuel pellets according to any one of claims 1 to 4 are stacked and loaded inside a cladding tube.   The nuclear fuel rod according to claim 5, wherein the cladding tube is made of a SiCf / SiC composite material.   7. A fuel assembly comprising a plurality of nuclear fuel rods according to claim 6 arranged and loaded inside a channel box. Including a nuclear fuel material and adjusting the diameter to form a central core;
Forming a particulate coated particle fuel in which the central core is coated with a coating layer that holds a fission gas generated by fission;
Mixing a base material and the coated particle fuel, adding a sintering aid, and performing a sintering process.