JPS6232386A - Fuel aggregate for light water reactor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] [Technical field of invention] The present invention relates to a fuel assembly for a light water cooled nuclear reactor.

[Technical background of the invention and its problems] In general, in nuclear reactors, the power distribution within the fuel assembly is flattened as much as possible in order to extract more thermal energy from the core. In recent years, the power distribution within a fuel assembly has been flattened by appropriately arranging many types of fuel rods with different concentrations of fissile nuclides within the fuel assembly.

FIG. 6 shows such a fuel assembly, and in the figure, reference numeral 1 indicates a channel box that accommodates a large number of fuel rods 2. In this example, fuel rods 2 with a high concentration of fissile nuclides a are arranged in the central part of the fuel assembly, and fuel rods 2 with a low concentration of fissile nuclides are arranged in the peripheral part.

In other words, the symbols (a) to (d) attached to the fuel rods 2 in the figure indicate the magnitude of the fissile nuclide concentration, and the symbols (a) to (d) indicate the magnitude of the fissile nuclide concentration.
The concentration decreases in the order of (d). In the figure, the symbol G indicates a fuel rod containing a thermal neutron absorbing material, and the symbol W indicates a water rod.

Hereinafter, the same parts in the attached drawings will be denoted by the same reference numerals.

However, in a fuel assembly configured in this way,
There is a problem in that a considerable amount of fission nuclide remains remain inside the spent fuel assemblies that have been burned in the reactor core.

Additionally, in general, in order to increase the reactivity of a fuel assembly, it is desirable to place fuel rods with a high concentration of fissile nuclides in locations with a large thermal neutron flux distribution. Since fuel rods with a high concentration of fissile nuclides are arranged inside a fuel assembly with a small thermal neutron flux distribution, there is a problem in that the loss of reactivity becomes large.

In general, in the fuel assembly of a boiling water reactor, non-boiling water exists outside the channel box 1, so the neutron moderation effect increases in the periphery of the fuel assembly, and the thermal neutron flux distribution The distribution is large at the periphery of the fuel assembly and small inside the fuel assembly.

Therefore, in order to reduce such unburnt residue and obtain a high reactivity, as shown in Fig. 7, the distribution of the fissile nuclide concentration of all the fuel rods 2 in the fuel assembly is made uniform, and the fuel A fuel assembly has been developed in which fuel rods containing a thermal neutron absorbing material indicated by the symbol G are arranged at or near the outer periphery of the assembly.

However, in such a fuel assembly, it is difficult to equalize the power distribution within the fuel assembly, and the reactivity value per rod containing thermal neutron absorbing material is large, so the necessary initial In order to reduce the reactivity (q), there is a risk that the thermal limit value will not be satisfied.

[Objective of the Invention 1] The present invention has been made to address the above-mentioned problems, and is capable of controlling the initial reactivity of a fuel assembly using a thermal neutron absorbing material more precisely while satisfying thermal limit values. It is an object of the present invention to provide a fuel assembly for a light water reactor that can extract more heat energy by flattening the distribution, have less unburnt residue, and can obtain high reactivity.

[Summary of the Invention 1 The present invention provides a fuel assembly in which several Fi fuel rods containing a thermal neutron absorbing material are arranged in a large number of fuel rods arranged in a lattice pattern. A fuel rod containing a neutron absorbing substance is composed of two or more types of fuel rods with different concentrations of thermal neutron absorbing substances in the horizontal cross section of the fuel assembly, and contains the thermal neutron absorbing substance at least only in the shaft core. This is a fuel assembly for a light water reactor characterized by including fuel rods.

[Embodiment of the Invention] The details of the present invention will be described below with reference to the drawings and one embodiment thereof.

FIG. 1 shows a fuel assembly according to an embodiment of the present invention.
In the figure, a large number of fuel rods 2 are regularly arranged and housed in a channel box 1.

In this embodiment, fuel rods G containing a thermal neutron absorbing material uniformly in the axial direction are arranged at positions adjacent to the corners of the outermost periphery inside the channel, and adjacent to these fuel rods G are axial core portions. Fuel rod containing thermal neutron absorbing material only in
is located.

FIG. 2 shows a longitudinal cross-sectional view of such a fuel rod g. In the figure, reference numeral 3 indicates a fuel cladding tube. A large number of pellets 4 made of a nuclear fuel material containing a thermal neutron absorbing material and not containing a thermal neutron absorbing material are inserted into the outer peripheral portion 4b. In addition, as a thermal neutron absorption substance here, UO2-Gd203, Aj! 203-Gd
203, Si02-Gd203, fvlQ0
-Gd 203 (7) is used.

In addition, in the fuel assembly shown in FIG. 1, the diameter and concentration of the thermal neutron absorbing material in the core portion of the fuel rod 9 containing the thermal neutron absorbing material only in the core portion are all the same, and the nuclear fission of the fuel rod 2 is the same. The sexual nuclide concentrations are also all the same.

In the fuel assembly configured as described above, the fuel rods 2 adjacent to the channel box corners where the thermal neutron flux distribution is large are configured with fuel rods 9 as shown in FIG. Even if the concentration of radioactive nuclides is uniform, it is possible to suppress the occurrence of power peaking on the main side of the fuel assembly.

In addition, the concentration of fissile nuclides in the fuel rods 2 in the fuel assembly is uniform, and thermal neutron absorbing substances exist in the fuel rods around the fuel assembly at the initial stage of combustion. Combustion is suppressed, and the concentration of fissile nuclides in the fuel rods q disposed around the J,'c fuel assembly can be maintained high after the middle stage of combustion. As a result, the reaction content of the fuel assembly can be kept high after the middle stage of combustion.

Figure 3 shows a comparison of the combustion changes of the fuel assembly configured as described above and the conventional fuel assembly at an infinite multiplication factor, so the horizontal axis shows the burnup and the vertical axis shows the infinite multiplication factor. It is taken.

That is, the curve a in the figure represents the case of the fuel assembly of the present invention shown in FIG. 1, and the curve a represents the case of the conventional fuel assembly composed of many types of fuel rods with different fission nuclide concentrations shown in FIG. The case is shown below.

In addition, curve C represents the fuel rod Q of the fuel assembly shown in FIG.
This shows the case where the fuel rods are replaced with the same amount of thermal neutron absorbing material distributed uniformly in the axial direction, and the curve d shows the case of the conventional fuel assembly shown in FIG. In addition,
All of these fuel assemblies have the same fissile nuclide concentration, with an average of about 3.3 W10 of uranium-235. i() is.

As is clear from this graph, at a burnup of about 10 Gwd/st and above, at which thermal neutron absorbing substances such as gadolinia, which decrease with combustion, disappear, the 4B rate of infinite increase in the fuel assembly according to the present invention is lower than that of the conventional fuel assembly shown in FIG. It is more secluded than the example, and is almost the same as the example shown in FIG. As mentioned above, this is due to the effect that by making the concentration of fissile nuclides in the fuel assembly uniform, the concentration of fissile nuclides in the periphery of the fuel assembly, where the thermal neutron flux distribution is large, is higher than in the conventional case. It is something.

The gain in the extraction burnup of the fuel assembly due to such an increase in the infinite multiplication factor is as follows.

That is, when the operating period is about 7,800 Mwd/st, the average extraction burnup of the fuel assembly of the present invention is about 31,000 Mwd/st, which is lower than that of the conventional fuel assembly shown in FIG. , approximately 29,800Mwd/s
t, in the conventional fuel assembly shown in Fig. 7, approximately 31,
000 Mwd/st, which is equivalent to the present invention.

Therefore, with the fuel assembly of the present invention, a gain in average extraction burnup of about 4% can be obtained over the fuel assembly shown in FIG.

Figure 4 shows the combustion change in the local power beacon coefficient of the fuel assembly of the present invention shown in Figure 1 in comparison with the fuel assembly shown in Figure 7, with the horizontal axis plotting burnup. The vertical axis shows the local output peaking coefficient.

That is, curve 0 shows the case of the fuel assembly of the present invention, and curve r shows the case of the fuel assembly shown in FIG. Moreover, the curved line shows the case where the fuel rod g of the fuel assembly shown in FIG. 1 is replaced with the same fuel rod (n) in which the thermal neutron absorbing material (6) is uniformly distributed in the axial direction.

As is clear from the figure, in the conventional fuel assembly shown in Figure 7, the concentration of fissile nuclides was made uniform within the fuel assembly, so large power peaking occurred at the outer periphery of the fuel assembly, causing an excessive local power peaking coefficient. However, in the fuel assembly of the present invention, the increase in the local power peaking coefficient is suppressed because a thermal neutron absorbing substance is included in the axial core portion of the fuel rod 0 on the outer periphery of the fuel assembly. .

Therefore, in the fuel assembly of the present invention, even if the fissile nuclide concentration is uniform within the fuel assembly, the local power peaking coefficient does not deteriorate significantly, so problems such as not satisfying thermal limit values during reactor operation can be avoided. It can be resolved.

Furthermore, in the fuel assembly of the present invention, since combustion proceeds with a small local power peaking coefficient, that is, with a flat power distribution, it is possible to prevent combustion of only some fuel rods from proceeding excessively. The mechanical and thermal integrity of the fuel rods can be ensured.

Furthermore, in the fuel 11 assembly of the present invention, the initial reactivity and burn-up of the neutron absorbing material can be freely controlled depending on the cycle length and batch size.

That is, in the fuel assembly of the present invention configured as described above, the power distribution within the fuel assembly is flattened and the reactivity of the fuel assembly is increased, so that the fuel assembly can be burned for a long period of time. I can do it.

As a result, it is possible to reduce the amount of fissile nuclides remaining in the fuel assembly at the time the fuel assembly is spent.

The fuel assembly of the present invention is not limited to the fuel assembly shown in FIG. 1, but as shown in FIG. You can also use bar 'J+, Q2.

In this case, since the thermal neutron flux differs depending on the position of the fuel rod containing the thermal neutron absorbing material, at least either the shaft core diameter or the thermal neutron absorbing material concentration needs to be changed by -15.

For example, both fuel rods Q+ and (J2) contain 3 thermal neutron absorbing materials.
Assuming that the concentration of the thermal neutron absorbing material in the right-hand shaft core is 15% theoretical density, the constitutional proportion of the fuel rod (1+ and fuel rod
．． 8% and about 15%.

As another example, if the diameters of the core portions of the fuel rods g1 and g2 are made equal, and the proportion of the total to the bellet 1 is about 8.5%, thermal neutron absorption in the core portions is assumed. The material concentration is 12% theoretical density in fuel rod 1, fuel rod g2
Then, it can also be set to 20?6 theoretical density Tq.

In the above description, the fuel rods containing the thermal neutron absorbing substance are arranged only at the outermost periphery of the fuel bundle, but they may of course be arranged at other positions.

[Effect of the Invention 1 As is clear from the explanation above, according to the present invention,
The initial reactivity can be controlled finely while satisfying the thermal limit value, the power distribution within the fuel assembly can be flattened, and higher reactivity can be obtained.

Therefore, it is possible to burn the fuel assembly for a long period of time, and it is also possible to reduce the amount of unburned fissile nuclides remaining in the spent fuel assembly, greatly improving fuel economy compared to conventional methods. be able to.

Moreover, the flexibility regarding the cycle length and number of replacement batches of the fuel assembly can be increased.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view showing one embodiment of the fuel assembly of the present invention, FIG. A graph showing the relationship between the burnup and infinite multiplication factor of the fuel assembly of the present invention in comparison with a conventional example, etc. Figure 4 shows the relationship between the burnup and local power peaking coefficient of the fuel assembly of the present invention in a conventional example. FIG. 5 is a schematic sectional view showing another embodiment of the fuel assembly of the present invention, and FIGS. 6 and 7 are schematic sectional views showing conventional fuel assemblies. 1......Channel box 2...
......Fuel rod 3...Fuel cladding tube 4...
・・・・・・Bellet 4a・・・・・・・・・Axle part G containing thermal neutron absorbing substance・・・・・・・・・Fuel rod IJ uniformly containing thermal neutron absorbing substance , OI, Q2...Fuel rod W containing thermal neutron absorbing material only in the shaft core...Water Rod Applicant Japan Atomic Energy Corporation Toshiba Representative Patent Attorney Su Yamasa - Figure 1 Figure 2 Burn! 1. (G Wd/s □ Figure 3 Field μ & (G Wd/St ) Figure 4 Oni Figure 7

Claims (3)

[Claims] (1) In a fuel assembly in which a plurality of fuel rods containing a thermal neutron absorbing substance are arranged in a large number of fuel rods arranged in a grid, the fuel rods containing the thermal neutron absorbing substance are For a light water reactor, the fuel assembly is composed of two or more types of fuel rods having different concentrations of thermal neutron absorbing substances in the horizontal cross section, and includes fuel rods containing thermal neutron absorbing substances only in at least the shaft core. fuel assembly. (2) The fuel assembly for a light water reactor according to claim 1, wherein the fuel rod containing a thermal neutron absorbing substance only in the core portion is composed of two or more types of fuel rods having different diameters in the core portion. (3) A fuel rod containing a thermal neutron absorbing substance only in its axial core is comprised of two or more types of fuel rods with different concentrations of thermal neutron absorbing substances in their axial core.
A fuel assembly for a light water reactor as described in .