JP2577367B2 - Fuel assembly - Google Patents

Description

The present invention relates to a fuel assembly for a nuclear reactor, and more particularly to a fuel assembly suitable for a long-term operation cycle.

(Prior Art) Fuel assemblies used in current commercial reactors are:
Fuel rods obtained by filling fuel pellets obtained by sintering enriched uranium dioxide into pellets into zirconium alloy cladding tubes,
The fuel cells are arranged in a square lattice and are bundled while maintaining the spacing between the fuel rods with spacers. Further, a part of the fuel rods constituting the fuel assembly is filled with pellets of a mixed oxide of enriched uranium and gadolinium, which is a neutron absorbing substance, in order to control the excess reactivity of the fuel. (Hereinafter, this is called gadolinia rod).

FIG. 2 shows an example of a current fuel assembly for a boiling water reactor (hereinafter referred to as 8 × 8 fuel). This fuel is a fuel rod
21 are arranged in the form of an 8 × 8 square lattice, and two water rods 22 in which cooling water flows are arranged in the center. Eight of the 62 fuel rods, indicated by G, are Cadolinia rods, and the remaining 54 are ordinary fuel rods filled with enriched uranium oxide pellets. 23 is a channel box, and 24 is a control rod.

Fig. 3 shows the fuel change of the 8x8 fuel of Fig. 2 at infinite multiplication factor.
It shows 31, but increases with fuel from the time of non-combustion to the burnup of 10 GWd / st, and thereafter decreases. When this 8 × 8 fuel is loaded into a 3-batch replacement core with an operation period of 10 GWd / st (replacement with one-third of all cores for new fuel at the time of refueling), the portion where the infinite multiplication factor increases and decreases Are just balanced, and a substantially flat effective multiplication factor is achieved throughout the operation period. Such a fuel can be obtained by appropriately designing the number of gadolinia rods and the gadolinia concentration. The former determines how much the infinite multiplication factor is suppressed, and the latter determines how long it is controlled. In particular, the concentration must be determined such that gadolinia is completely burned out at the end of the driving cycle. If gadolinia remains at the end of the operation cycle, neutrons will be absorbed wastefully, and fuel economy will deteriorate.

(Problems to be Solved by the Invention) In recent years, there has been an increasing demand for extending the operation cycle of a nuclear power plant from the viewpoint of improving the utilization factor. For this purpose, it is necessary to control the excess reactivity over a longer period of time than before, so that the gadolinia concentration must be higher than before. However, fuel pellets to which gadolinia is added have a problem that the thermal conductivity is lower than that of ordinary fuel pellets. This is the fourth
Shown in the figure. When the thermal conductivity of the fuel pellets decreases, the temperature of the calorific pellets increases, and the fission product gas trapped in the fuel element is easily released, so that the internal pressure of the fuel rods increases and the cladding tube is damaged. The probability increases. From such a viewpoint, it has been desired that the gadolinia density be as low as possible.

In addition, high burnup of fuel is planned to improve fuel economy.However, if the enrichment is increased, the output peaking will increase and the linear power density, which is a design constraint, will increase. is expected. Therefore, as shown in FIG.
A design has been considered in which the number of fuel rods is increased by arranging the fuel rods in nine square lattices. In this example, a neutron spectrum is softened by installing a large-diameter water rod 62 occupying nine fuel rods at the center of the fuel assembly. 64 is a control rod.
In addition, the fuel layer load per fuel assembly is reduced to 8 ×
It is necessary to reduce the diameter of the fuel rod in order to make it the same as the eight fuels. However, if gadolinia having the same concentration is used, the 9 × 9 fuel will burn out faster. Therefore, it is necessary to increase the gadolinia concentration in 9 × 9 fuel to 8 × 8 fuel or more.
Cadolinia concentration becomes more problematic than fuel.

The present invention has been made in view of the above circumstances, and its object is to make it possible to control the reactivity for the same period at a lower gadolinia concentration than in the past, thereby making it suitable for a long-term operation cycle core and high burnup. It is to provide a fuel assembly.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention provides a fuel assembly in which a plurality of fuel rods filled with pellet-shaped fuel are arranged in a square lattice. In the body, the burnable poison-containing fuel rods in which the fuel pellets are entirely filled with fuel pellets, which are mixed oxides of enriched uranium and gadolinium, in the axial direction of the fuel rods, except for the outermost periphery of the fuel assembly And at least a part of the burnable poison-containing fuel rods is arranged at a position adjacent to each other in the X direction or the Y direction in the second layer from the outermost periphery of the fuel assembly. It is characterized by the following. Here, the position adjacent in the X direction or the Y direction means the position in the X direction or the Y direction of the square lattice array, that is, the position adjacent in the vertical or horizontal direction in the horizontal cross section of the fuel assembly. Diagonal adjacency of the array does not correspond to this.

(Operation) Gadolinium suppresses the infinite multiplication factor by absorbing not only neutrons generated in the gadolinia rod but also neutrons flowing from adjacent fuel rods. However, since the neutron absorption cross section of gadolinium is much larger than that of uranium, neutrons in the gadolinia rod are more easily absorbed by gadolinium than uranium, and the amount of neutron generated by the gadolinia rod is about half that of a normal fuel rod.
For this reason, if there is a gadolinia rod next to the gadolinia rod, the number of neutrons flowing in from the outside will decrease, so the neutron absorption amount of gadolinium will decrease, and as a result, the burnup of gadolinium burned out You will be late. Therefore, the fuel assembly of the present invention can control the reactivity for the same period at a gadolinia concentration lower than the conventional one, and can suppress a decrease in thermal conductivity.

(Example) The present invention will be described with reference to the drawings.

FIG. 1 shows a partial sectional view of one embodiment of the present invention. As shown in the figure, the present embodiment has the same shape as the conventional example of FIG. 2 already described, and the average enrichment is about 3
%. 11 is a fuel rod, 12 is a water rod, 13
Is a channel box and 14 is a control rod.

Comparing this embodiment with the conventional fuel (FIG. 2), the two fuels differ only in the arrangement, number and concentration of gadolinia rods. That is, the number, the concentration, the present example, 9 lines, 4.2%, the conventional example, 8 lines, 5.0% FIG. 3 shows the combustion change 32 of the infinite multiplication factor of the present embodiment. Shows almost the same behavior. This means that the gadolinia rod can be placed at an adjacent position to reduce the gadolinia concentration from 5.0% to 4.2%, thereby improving the thermal conductivity of gadolinia-containing fuel pellets by about 5%. Will be. In addition, since the adjoining gadolinia rod does not reduce the neutron absorption effect of the gadolinia rod, in order to secure the effect of suppressing the infinite multiplication factor, in this embodiment, the gadolinia rod is increased by one from the conventional eight gadolinia rods. And nine.

FIG. 5 is a cross-sectional view of another embodiment of the present invention, and shows a 9 × 9 fuel corresponding to high burnup as shown in FIG. The average enrichment is about 6%, and the neutron spectrum becomes harder as the enrichment becomes higher. Therefore, a large-diameter water rod 52 occupying nine fuel rods is installed at the center of the fuel assembly, and the neutron spectrum is increased. Is softened. Here, 51 is a fuel rod, 53 is a channel box, and 54 is a control rod.

In the present embodiment, only the number, arrangement and concentration of gadolinia rods are different from the conventional 9 × 9 fuel shown in FIG. That is, the number, the concentration, the present embodiment 23 7.0%, the conventional example 20 8.0%, and the present embodiment (FIG. 5) and the conventional example (FIG.
Figure) shows the infinite multiplication factor combustion changes 71 and 72 of each fuel.
Both showed almost the same behavior, and the gadolinia density could be reduced by 1% from the conventional example. This also improves the thermal conductivity by about 5%.

Here, the arrangement of the gadolinia rod will be described.
FIG. 8 shows the relative distribution of thermal neutron flux at each fuel rod position of 8 × 8 fuel. It is better to reduce the number of gadolinia rods in manufacturing, but it is important to arrange gadolinia rods at the same place of thermal neutron flux in order to control reactivity at the same gadolinia concentration for the same period. From such a viewpoint, it can be seen from FIG. 8 that when the fuel assembly is divided into square layers from the center, the thermal neutron flux is almost equal in the same layer. Therefore, it is desirable that the gadolinia rods be arranged in the same layer. The gadolinia rod in the embodiment shown in FIG. 1 is arranged by selecting places where the thermal neutron fluxes are almost equal as shown in FIG. Further, as can be seen from FIG. 8, output peaking tends to occur on the outermost fuel rods of the fuel assembly having a large thermal neutron flux. Therefore, by arranging the gadolinia rod in the second layer from the outermost periphery as in the embodiment of FIG. 1, it is possible to suppress an increase in output peaking of the outermost peripheral fuel rod adjacent thereto. Further, the gadolinia rod is not arranged at the corner portion of the second layer from the outermost periphery where the thermal neutron flux is large, so that an increase in output peaking can be further suppressed. In the case of 9 × 9 fuel, the thermal neutron flux distribution is the same as that shown in FIG. 8, but in the embodiment shown in FIG. 5, some gadolinia rods are disposed in the same layer but some gadolinia rods It is located at the corner of the body. However, as can be seen from FIG. 8, the thermal neutron flux is slightly larger at the corners, so that the gadolinia burning speed is faster than the others. However, the difference is small and the number
There are practically no problems because only three of the 23 tubes are used.

In addition to the above-described embodiment, a design in which four gadolinia rods are arranged in a square shape may be considered. In such an example, the gadolinia absorption effect is further reduced than in the above-described embodiments, so that the reactivity can be controlled for a predetermined period with a lower concentration of gadolinia. However, it is necessary to further increase the number of gadolinia rods.

In order to control the reactivity for a longer period at a lower concentration of gadolinia, it is important to maintain the effect of having the gadolinia rod adjacent to the gadolinia throughout the entire combustion period until the gadolinia burns out. For this purpose, as in any of the above embodiments, it is desirable that the gadolinia densities filled in adjacent gadolinia rods be the same. In addition, in the case of a fuel provided with a low-enriched natural uranium blanket at the upper and lower ends, it is not necessary to add gadolinia to the natural uranium portion for controlling the reactivity.

[Effects of the Invention] As described above, according to the present invention, at least a part of the gadolinia rods are arranged at positions adjacent to each other in the X direction or the Y direction. The neutron absorption capacity of gadolinia decreases due to the decrease in the amount of neutrons coming out, and the reactivity can be controlled for a longer period of time than the conventional design example with gadolinia of the same concentration. As a result, in order to control the reactivity for a predetermined period, it is possible to lower the gadolinia concentration than before, and it is possible to minimize the decrease in thermal conductivity due to the addition of gadolinia, Deterioration of fuel rod integrity can be minimized, and an excellent effect of providing fuel suitable for a long-term operation cycle core and high burnup can be provided.

[Brief description of the drawings]

FIG. 1 is a sectional view of an embodiment of the present invention, and FIG.
FIG. 3 is a cross-sectional view of the × 8 fuel, and FIG.
8 is a diagram showing a combustion change of the fuel at infinite multiplication factor, FIG. 4 is a diagram showing a relationship between gadolinia concentration and thermal conductivity in fuel pellets, FIG. 5 is a sectional view of another embodiment of the present invention, and FIG. FIG. 7 is a cross-sectional view of a conventional 9 × 9 fuel, FIG. 7 is a diagram showing a change in combustion of the 9 × 9 fuel of FIGS. 5 and 6 at infinite multiplication factor, and FIG. 8 is each fuel of the 8 × 8 fuel. It is a relative distribution figure of a thermal neutron flux in a rod position. 11,21,51,61 …… Fuel rods 12,22,52,62 …… Water rods 13,23,53,63 …… Channel boxes 14,24,54,64 …… Control rods

Claims (2)

(57) [Claims] 1. A fuel assembly comprising a plurality of fuel rods filled with fuel in the form of pellets arranged in a square lattice, wherein the entirety of the fuel pellets is enriched with uranium and gadolinium over most of the fuel rods in the axial direction. Fuel rods containing burnable poisons filled with fuel pellets of mixed oxides are arranged only at positions other than the outermost periphery of the fuel assembly, and at least a part of the fuel rods containing burnable poisons is In the second layer from the outermost periphery of the fuel assembly, the X direction or Y
A fuel assembly which is arranged at a position adjacent to a direction. 2. The gadolinium oxide concentration of a burnable poison-containing fuel rod filled with fuel pellets composed of a mixed oxide of enriched uranium and gadolinium arranged at positions adjacent to each other in the X direction or the Y direction, 2. The fuel assembly according to claim 1, wherein the fuel assemblies are identical in the same cross section.