JP2008170454A - Mox fuel assembly for pressurized water reactor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a MOX fuel assembly for a pressurized water reactor capable of restraining sufficiently an assembly output peaking coefficient, without reducing a Pu charge amount per assembly, by designing properly an array of fuel rods. <P>SOLUTION: This MOX fuel assembly is arranged with the first MOX fuel rods of Pu enrichment and Pu amount preliminarily determined as the MOX fuel rod, and the second MOX fuel rods of Pu enrichment same to and Pu amount different from those of the first MOX fuel rod, the sheath tubes of all the fuel rods have the same outside diameter including the first MOX fuel rods and the second MOX fuel rods, respective fuel pellets have the same Pu enrichment each other both in the first MOX fuel rods and the second MOX fuel rods, pellet weights are different each other therein, and the Pu enrichment is determined not to increase the number of replacement bodies for securing the same operation period, compared with a reactor comprising only a UO<SB>2</SB>fuel assembly set based on a reactor core operation period. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a fuel assembly for a pressurized water reactor, and more particularly to a MOX fuel assembly for a pressurized water reactor including a uranium-plutonium mixed oxide (hereinafter referred to as MOX) fuel rod.

  As a conventional MOX fuel assembly for a pressurized water reactor (hereinafter referred to as PWR), in order to suppress the maximum value of relative fuel rod output in the MOX fuel assembly (intra-body output peaking coefficient), A so-called three-enriched MOX fuel assembly having three kinds of Pu enrichment of rods has been put into practical use.

In this assembly, by arranging low enrichment and medium enrichment MOX fuel rods near the outer periphery of the assembly, and high enrichment MOX fuel rods inside, the thermal neutrons from adjacent UO ₂ fuel assemblies are made of Pu. Even if it flows into a low enrichment or medium enrichment fuel rod with a small content ratio, the output of the MOX fuel rod near the outer periphery of the assembly can be suppressed, and the inclusion of Pu inside the assembly is less affected by the flowing thermal neutrons. Since the higher enrichment fuel rods having a higher ratio are arranged, the output can be balanced and the output peaking coefficient within the assembly can be sufficiently suppressed.

However, this three-enriched MOX fuel assembly has a drawback that the manufacturing cost is increased for the following reason.
-Equipment for preparing three types of MOX fuel powders with different Pu content ratios (different enrichments) is required.
・ Three kinds of powders with different enrichments need to be molded into pellets and sealed in fuel rods.
・ Every time MOX fuel with different enrichment is molded, it is necessary to clean up the production line (decontamination so that MOX fuel powder and pellets with different enrichment are not mixed).

Also, a design has been devised in which a poisoned UO ₂ fuel rod is arranged in a part of the MOX fuel assembly to reduce the type of Pu enrichment of the MOX fuel rod (Patent Document 1). There are two types of enrichment, and no fundamental solution has been made.
JP-A-8-201555

On the other hand, for a 1 enriched MOX fuel assembly composed of one type of MOX fuel rod, the Pu content is decreased from the average enrichment of the 3 enriched MOX fuel assembly, It is conceivable to put it into practical use by lowering the multiplication factor as compared with the UO ₂ fuel assembly loaded in the core at the same time. In this case, the output peaking coefficient in the assembly cannot be sufficiently suppressed, but the gain factor as the assembly is low, so the output peaking coefficient viewed from the whole core can be in a range that can satisfy the safety of the core. It is done.

However, if such a low enrichment single enrichment MOX fuel assembly is adopted, the number of replacement bodies (new fuel) for ensuring the same operation period is compared to a core consisting of only UO ₂ fuel assemblies. Number of bodies) will increase. Accordingly, there is a drawback that the total fuel production cost is increased.

In addition, when a MOX fuel assembly is loaded up to about 1/3 of the number of fuel assemblies in the core in a PWR using light water (so-called pull thermal), the loaded MOX fuel assembly is a UO ₂ fuel. It is arranged in the core adjacent to the assembly.

Since this MOX fuel has a greater effect of absorbing thermal neutrons than UO ₂ fuel, the energy spectrum of neutrons in the MOX fuel is hardened, and the thermal neutron flux is lower than that of UO ₂ fuel. Therefore, when the MOX fuel assembly and the UO ₂ fuel assembly are arranged adjacent to each other in the core, thermal neutrons flow from the UO ₂ fuel to the MOX fuel. As a result, fission by thermal neutrons flowing in the MOX fuel rod adjacent to the UO ₂ fuel assembly becomes active, and the output of the MOX fuel rod becomes relatively high.

  That is, the output of the MOX fuel rod near the outer periphery in the MOX fuel assembly is relatively higher than that on the inner side. The maximum power output of the fuel rods in the core is limited by the viewpoint of the core safety (fuel integrity), but in order to satisfy this limitation, the relative maximum fuel rod output in each fuel assembly ( It is necessary to sufficiently suppress the output peaking coefficient within the assembly.

  The present invention relates to a MOX fuel assembly, and by appropriately designing the arrangement of each fuel rod, the MOX for PWR that sufficiently suppresses the output peaking coefficient in the assembly without reducing the amount of Pu loaded per assembly. The purpose is to obtain a fuel assembly.

An MOX fuel assembly for PWR according to the invention described in claim 1 is a MOX fuel assembly for PWR in which poisoned UO ₂ fuel rods and MOX fuel rods are arranged in a grid array of n rows and n columns. And arranged in the core adjacent to the UO ₂ fuel assembly,
As the MOX fuel rod, a first MOX fuel rod having a predetermined Pu enrichment and Pu amount, and a second MO enrichment with the same Pu enrichment and different Pu amount with respect to the first MOX fuel rod. MOX fuel rods are arranged,
The claddings of all fuel rods including the first uranium-plutonium mixed oxide fuel rod and the second uranium-plutonium mixed oxide fuel rod have the same outer diameter;
The fuel pellets in the first uranium-plutonium mixed oxide fuel rod and the fuel pellets in the second uranium-plutonium mixed oxide fuel rod have the same Pu enrichment and have different pellet weights. Is what
The Pu enrichment is determined so as not to increase the number of replacement bodies for securing the same operation period as compared with a core composed only of UO ₂ fuel assemblies set based on the core operation period. It is characterized by this.

The MOR fuel assembly for PWR according to the invention described in claim 2 is the same as that of the MOX fuel pellet of the second MOX fuel rod in the first MOX fuel rod according to claim 1. MOX fuel pellets with different degrees of pellet diameter are loaded,
The cladding thickness of the uranium-plutonium mixed oxide fuel rod packed with fuel pellets having a small diameter is characterized by being thicker than the cladding thickness of other fuel rods.

The MOX fuel assembly for PWR according to the invention described in claim 3 has poisonous UO ₂ fuel rods arranged at the four corner positions or at the four corner positions and the vicinity thereof in the grid array according to claim 1 or 2. It is characterized by being.

  The MOX fuel assembly for PWR according to the invention described in claim 4 is a MOX fuel rod having a small amount of Pu compared to the inner position at the outer peripheral position of the grid array according to any one of claims 1 to 3. Is arranged.

  As described above, according to the present invention, by appropriately designing the arrangement of the fuel rods, the PWR MOX fuel assembly that sufficiently suppresses the output peaking coefficient within the assembly without reducing the Pu load per assembly. There is an effect that can be obtained.

In the present invention, a plurality of types of MOX fuel rods having the same Pu enrichment but different Pu amounts are arranged in the fuel assembly together with the poisoned UO ₂ fuel rods. For example, a poison containing UO ₂ fuel rods filled with toxicant-containing UO ₂ pellets (1), first and second MOX fuel rods Pu enrichment is packed identical Pu different amounts of two types of MOX pellets respectively (2) (3) MOX fuel assemblies composed of (Pu amount is (2) <(3)) can be mentioned. As a matter of course, in the present invention, MOX fuel rods having the same Pu enrichment and different Pu amounts may be used in addition to the first and second.

  As the first and second MOX fuel rods (2) (3) of the present invention, it is preferable to cover the same number of MOX fuel pellets having the same height and the same Pu enrichment and different pellet weights. Use MOX fuel rods obtained by packing in a tube. Specifically, the MOX fuel pellet weight of the fuel rod is different by changing only the diameter of the MOX fuel pellet, reducing the pellet density, or adopting a hollow pellet with a hole in the center of the pellet. It is also possible to make the amount of Pu different. This makes it possible to produce on a production line that differs only in the molding process, and thus has an advantage that the production cost does not increase even if a plurality of types of MOX fuels (2) (3) are produced.

In the present invention, preferably, poisoned UO ₂ fuel rods (1) are arranged at the outer peripheral corner of the assembly or at the corner and the vicinity thereof. Further, in the present invention, more preferably, a position where the output becomes high when the MOX fuel is loaded in the core adjacent to the UO ₂ fuel assembly at a position other than the poisoned UO ₂ fuel rod (1) (for example, the highest The MOX fuel rod (2) with a small amount of Pu is disposed at the outer peripheral position), and the MOX fuel rod (3) with a large amount of Pu is disposed at the remaining positions. Note that the Pu enrichment degree of the MOX fuel rod is preferably set so that the multiplication factor over the life and the UO ₂ fuel assembly loaded in the core are equivalent at the same time.

Further, since the MOX fuel has a greater effect of absorbing thermal neutrons than the UO ₂ fuel, the energy spectrum of neutrons in the MOX fuel is hardened, and the thermal neutron flux is at a lower level than that of the UO ₂ fuel. Therefore, when the MOX fuel assembly and the UO ₂ fuel assembly are arranged adjacent to each other in the core, thermal neutrons flow from the UO ₂ fuel to the MOX fuel.

In the MOX fuel assembly of the present invention, the flowing thermal neutrons are absorbed by the poisoned UO ₂ fuel rods and not only the output of the MOX fuel rods arranged in the vicinity thereof is suppressed, but also the outermost peripheral position of the assembly, etc. By placing a MOX fuel rod (2) with a small amount of Pu at a position where the level of thermal neutron flux that flows in is high, the number of Pu fission by thermal neutrons is suppressed, and the output peaking coefficient in the assembly is increased by 3 It can be suppressed to the same degree as the MOX fuel assembly.

One embodiment of the present invention (17 × 17 array) is shown in FIG. As shown in the figure, the fuel assembly of this example is packed with UO ₂ fuel rods (1) packed with gadolinia-filled UO ₂ pellets using gadolinia as a poison, and MOX pellets with a smaller diameter than gadolinia-filled UO ₂ pellets. And a first MOX fuel rod (2) and a second MOX fuel rod (3) packed with MOX pellets having the same diameter as the gadolinia-filled UO ₂ pellets.

  In an actual core, the control rod guide tube position of the MOX fuel assembly does not contain nuclear fuel material such as uranium and is composed of neutron absorbing nuclides such as boron (Burnable Poison rod or BP rod) ) Is often inserted. An assembly diagram in this case is shown in FIG. Further, Tables 1 and 2 show the main specifications of the MOX fuel assembly of this embodiment.

  FIG. 3 is a view showing an example of the arrangement of fuel rods in the MOX fuel assembly of the conventional 3-rich MOX fuel. FIG. 4 is a diagram showing a calculation system of the output peaking coefficient of the MOX fuel assembly without the flammable poison rod (case 1) and the MOX fuel assembly with the flammable poison rod (case 2). FIG. 5 is a comparison of the output peaking coefficients in the assembly for the MOX fuel assembly shown in FIGS. 1 and 2 and the MOX fuel assembly having the typical three enrichment distribution shown in FIG. FIG. As shown in the figure, the output peaking coefficient in the assembly when calculated with a typical calculation system simulating the PWR core is equal to or less than that of the conventional 3-rich MOX fuel assembly (FIG. 3).

In addition, when 13 months operation is performed in a 900,000 kW class three-loop PWR plant, the assembly multiplication factor of the MOX fuel assembly is almost equal to that of the UO ₂ fuel assembly at an assembly burnup of about 28200 MWd / t. For example, even if MOX fuel is loaded, the number of new fuel bodies will not increase. Here, in this embodiment, the Pu enrichment of the MOX fuel assembly can be set so that the assembly multiplication factor over the life is the same as that of the UO ₂ fuel assembly ( ²³⁵ U enrichment: 4.1 wt%). . In other words, in this example, the Pu enrichment of the MOX fuel assembly is as follows: the assembly multiplication factor is UO ₂ fuel assembly ( ²³⁵ U enrichment: 4.1 wt%) and the assembly burnup is about 28200 MWd / t. Can be set to be the same. (Fig. 6)

The cladding tube thickness of the MOX fuel rods (2) packed with MOX pellets smaller in diameter than the gadolinia-filled UO ₂ pellets is thicker than the cladding tubes of the other fuel rods (1) (3). If they are all the same, there will be no thermal hydraulic problems.

On the contrary, it is also possible to make the outer diameter of the cladding tube (2) of the MOX fuel rod (2) packed with MOX pellets smaller in diameter than gadolinia-filled UO ₂ pellets thinner than other fuel rods.

  Besides reducing the MOX pellet diameter, it is also possible to reduce the Pu amount of the fuel rod by reducing the pellet density or adopting a hollow pellet having a hole at the center of the pellet.

Further, in the fuel assembly of this embodiment, the amount of Pu loaded per assembly is equivalent to the conventional three enrichment MOX.
-Conventional 3 enrichment MOX (Pu enrichment: about 9.7wt% Pu-total): About 45kg
-1 enrichment MOX in the example: about 41 kg

  As described above, by using one enriched MOX fuel assembly according to the present invention, the production line is cleaned up every time MOX fuel having a different enrichment is molded (MOX fuel powder having a different enrichment or This eliminates the need to decontaminate the pellets so that they do not mix, and the manufacturing cost can be reduced. Moreover, the safety and economy when used in a nuclear reactor are equivalent to those of the conventional 3-rich MOX fuel.

It is a figure which shows the fuel rod arrangement | positioning in the MOX fuel assembly in one Example of this invention. It is a figure (at the time of combustible poison rod use) which shows fuel rod arrangement in a MOX fuel assembly in another example of the present invention. It is a figure which shows an example of the fuel rod arrangement | positioning in the MOX fuel assembly of the conventional 3 enrichment MOX fuel. It is a figure which shows the calculation system of the output peaking coefficient of the MOX fuel assembly (case 1) without a combustible poison rod and the MOX fuel assembly with a combustible poison rod (case 2). FIG. 4 is a diagram comparing the output peaking coefficients in the assembly for the MOX fuel assembly shown in FIGS. 1 and 2 and the MOX fuel assembly having the typical three enrichment distribution shown in FIG. It is. In one embodiment of the present invention, the enrichment of the MOX fuel rods in the MOX fuel assembly is the same as that of the UO ₂ fuel assembly ( ²³⁵ U enrichment: 4.1 wt%) in the assembly multiplication factor over the lifetime. It is a figure which shows having set to.

Claims (4)

A poison-containing UO ₂ fuel rods, uranium - a MOX fuel assembly for a pressurized water nuclear reactor arranged and plutonium mixed oxide fuel rods in grid-like array of n rows and n columns, and UO ₂ fuel assemblies In what is arranged in the reactor core,
As the uranium-plutonium mixed oxide fuel rod, the first uranium-plutonium mixed oxide fuel rod having a predetermined Pu enrichment and Pu amount, and the first uranium-plutonium mixed oxide fuel rod. A second uranium-plutonium mixed oxide fuel rod having the same Pu enrichment and a different Pu content is disposed;
The claddings of all fuel rods including the first uranium-plutonium mixed oxide fuel rod and the second uranium-plutonium mixed oxide fuel rod have the same outer diameter;
The fuel pellets in the first uranium-plutonium mixed oxide fuel rod and the fuel pellets in the second uranium-plutonium mixed oxide fuel rod have the same Pu enrichment and have different pellet weights. Is what
The Pu enrichment is determined so as not to increase the number of replacement bodies for securing the same operation period as compared with a core composed only of UO ₂ fuel assemblies set based on the core operation period. A MOX fuel assembly for a pressurized water reactor characterized by the above. The first uranium-plutonium mixed oxide fuel rod has the same Pu enrichment and pellet diameter as the uranium-plutonium mixed oxide fuel rod of the second uranium-plutonium mixed oxide fuel rod. Loaded with different uranium-plutonium mixed oxide fuel pellets,
2. The pressurized water according to claim 1, wherein the cladding tube thickness of the uranium-plutonium mixed oxide fuel rod packed with fuel pellets having a small diameter is thicker than the cladding tube thickness of the other fuel rod. MOX fuel assembly for type reactor. 3. The MOX fuel assembly for pressurized water reactor according to claim 1, wherein poisoned UO ₂ fuel rods are arranged at four corner positions of the grid-like arrangement or at four corner positions and the vicinity thereof. 4.   The pressurized water mold according to any one of claims 1 to 3, wherein a uranium-plutonium mixed oxide fuel rod having a small amount of Pu as compared with an inner position is arranged at an outer peripheral position of the lattice arrangement. MOX fuel assembly for nuclear reactors.

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