JPS6039194B2 - nuclear fuel assembly - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nuclear fuel assembly for a boiling light water reactor, particularly to nuclear fuel rods with optimized axial power distribution.

The core of a boiling water reactor consists of a large number of rod-shaped fuel elements arranged in a row, the spacing between the fuel elements maintained by a plurality of spacers, and the upper and lower ends tied together with a tie plate to form the fuel east. A large number of nuclear fuel assemblies are arranged in vertical rows by covering them with a channel box to secure a coolant flow path for cooling the fuel bundle.

Then, a cruciform control rod was placed in the leakage passage outside the channel box surrounded by the four nuclear fuel assemblies, one in every four nuclear fuel assemblies, so that they could be inserted from the bottom of the reactor core. There is.

In boiling light water reactors, these control rods are used to control the reactivity and power distribution of the reactor core.

In the fuel assembly of a boiling light water reactor, voids occur from the bottom, and the amount of voids increases as you move upward, so the power distribution takes the form of a bottom peak, as shown in Figure 1.

In order to improve this and suppress the output in the lower part, burnable poison (OD) is currently added to the lower part of the initially loaded fuel. However, with the current concentration of Gd added, the peak of the power distribution at the bottom becomes too large once the combustion of the fuel has progressed.
By arranging deep insertion control rods and shallow insertion control rods in an appropriate pattern, the deep insertion control rods mainly control the reactivity.
The shallow insertion control layer suppresses the output peak at the bottom. If shallow insertion control rods are used in this way, when the shallow insertion control rod is completely withdrawn during pattern exchange, the local heat output near the control rod tip and its increase rate are large, making operation complicated and difficult. It may become.

By improving this, it is now possible to control the reactivity and the power distribution in the direction of the nuclear fuel assembly using only deep control rods without using shallow insertion control rods. As a method of lowering the peak of the fuel element, instead of using a nuclear fuel assembly in which the lower half of the fuel element is enriched less than the upper half or changing the enrichment between the upper and lower halves, a uniform average enrichment is used, but the upper and lower A nuclear fuel assembly has been proposed in which the concentration of burnable poisons in the lower half is increased by changing the concentration of burnable poisons.

Both proposals have their points. When changing the average concentration between the upper and lower parts, it is easy to cause mistakes in mixing the filled pellets during production. This also applies when changing the concentration of burnable poisons. Furthermore, it is difficult to measure the concentration of combustible substances by non-destructive testing after the fuel element is completed. In particular, when it exceeds 2 to 3%, the sensitivity curve worsens and the accuracy drops significantly. Further, filling a large number of fuel elements with a plurality of types of fuel complicates the work process and reduces productivity.

The object of the present invention was made in view of the above-mentioned circumstances, and is to obtain a nuclear fuel assembly which itself has the ability to optimize the axial power distribution. Embodiments of the present invention will be described below with reference to the drawings.

As shown in FIG. 2, the nuclear fuel assembly of the present invention has a plurality of fuel elements 1 arranged in a square lattice, the upper and lower ends of which are bound by an upper tie plate 2 and a lower tie plate 3, and a A plurality of spacers 4 are provided to cover a fuel bundle having a structure in which the fuel element spacing is maintained. The feature of the first embodiment is that fuel elements (2) as shown in FIG. 3 are incorporated into the fuel assembly at three corners on the control arm side shown by diagonal lines in FIG. 6.

In this fuel element, a cylindrical zirconium alloy cladding tube 11 is filled with cylindrical fuel pellets 12 only upward from approximately the center. A lower end plug 15 is provided at the lowest end of the hollow portion continuing below the end plug 14 for sealing. Furthermore, a waste material outlet hole 16 is provided slightly below the intermediate end plug 14, and a coolant inlet hole 17 is provided slightly above the lower end plug 16, so that the coolant flows through the hollow portion in the direction of the arrow. do.
In this embodiment, a fuel element (2) as shown in FIG. 4 may be incorporated in place of the fuel element (2) as shown in FIG. In this fuel element, a cylindrical zirconium alloy cladding tube 11 is filled with cylindrical fuel pellets 12 only upward from approximately the center, and cylindrical, cylindrical, or coil-shaped reflectors or moderators are filled downward from the center. Fill 20.

Possible reflective materials or moderators include beryllium metal, beryllium oxide, graphite, etc., which are compatible with the cladding material and the fuel pellet. In this embodiment, the case of a cylindrical shape is shown in FIG. It is also conceivable to add a burnable substance such as Gd to the reflective material or moderator used in this embodiment. Next, a second example will be shown. The feature of this nuclear fuel assembly is that fuel elements B' as shown in FIG. 4 are assembled in the three corners of the control beam side shown by diagonal lines in FIG. 6. In this fuel element B', a cylindrical zirconium alloy cladding tube 11 is filled with cylindrical fuel pellets 12 only upward from the center, and cylindrical, cylindrical, or coiled zirconium alloy or cylindrical fuel pellets 12 are filled downward from the center. Fill with stainless steel 20'. In the second embodiment, a burnable substance such as Gd may be added to the zirconium alloy or stainless steel filled in the fuel element, or the fuel element may have a hollow lower part as shown in FIG. A third example is shown.

In this embodiment, as in the second embodiment, three of the fuel elements to be replaced at the corner (including the corner on the control side side
In addition to incorporating the fuel element B' shown in FIG. 4 into the nuclear fuel assembly, the fuel element B' shown in FIG. 3 is further installed in the center of the nuclear fuel assembly as shown in FIG. 7. In the first to third embodiments of the present invention, the average enrichment of the entire nuclear fuel assembly is set to about 0.71 to 4.2%, and the loading amount is reduced in the lower part, thereby reducing the average enrichment. It is best to set the amount of K to about 0.1 to 0.4% in terms of the horizontal distribution of K in fuel for boiling water reactors. In addition, in order to suppress excessive reactivity at the initial stage of combustion of a nuclear fuel assembly, one or more fuel elements filled with fuel of a single enrichment over the effective length of the reactor core are oxidized with 0.5 to 1OW% of Cb. It is best to add the substance uniformly over the entire length.

At this time, the arrangement of the fuel elements must take into account local power peaks within the nuclear fuel assembly. Next, the effects of the present invention will be described.

In a boiling water reactor, the void fraction distribution in the vertical direction in the fuel assembly cooling channel increases as it goes upward, as shown in Figure 1, so the average enrichment of the nuclear fuel assembly volume cross section increases in the vertical direction. If the nucleation concentration is uniform in all axial directions or is not included, the axial power distribution of the nuclear fuel assembly will have a downward peak as shown in FIG.

In order to control this downward peak so that it does not become extremely prominent, it is possible to reduce the enrichment of the fuel used in the nuclear fuel assembly only in the downward direction so that the (infinite multiplication factor) of K in the downward direction becomes small, or to reduce the enrichment of the fuel used in the nuclear fuel assembly only in the downward direction. All you have to do is add. Another method would be to reduce the fuel loading without lowering the lower fuel enrichment, thereby substantially lowering the average enrichment.

The present invention is based on this idea. As a method of reducing the amount of fuel loaded in the lower part of the fuel east, it is sufficient to simply use a hollow fuel element instead of filling the fuel pellet downward.

However, if it is made hollow, there is a possibility that the nuclear properties may change due to water entering the hollow part due to damage to the cladding.
Instead of making it hollow, it is better to actively flow coolant or fill it with a suitable material. Below, water is added to the nuclear fuel assembly, B
The effect of lowering the fuel loading amount by incorporating a plurality of Tokuju fuel elements filled with e, C, Zr alloy, Smelt 304, etc. will be described. (In addition, in the present invention, the part where water flows partially in the cladding tube of a fuel element such as the special fuel element (■) is specified and called "wa to rrod".) At this time, the fuel Instead, such a fuel element is incorporated,
The enrichments of other fuels remain fixed. wa to rro
If d is placed at the outermost periphery of the nuclear fuel assembly, as the number increases, K of the nuclear fuel assembly will decrease.

However, if it is placed at the center of the cross section of the nuclear fuel assembly, it increases, peaks, and then decreases as shown in FIG. skin terr
This also applies to the case where a fuel element filled with a reflective material or a moderator, such as Toshige metal, beryllium oxide, or graphite, is used instead of od. However, since graphite has a large free path for neutrons, even if it is placed in the center, it will not contribute to increasing K-gon as shown in FIG.
When incorporating a fuel element filled with zirconium alloy or stainless steel, K decreases to varying degrees depending on the location, as shown in FIG.

In addition to lowering the enrichment or increasing the additive concentration of combustible substances such as W, it is also possible to reduce the K in the lower part and suppress the lower output peak by lowering the fuel loading. can. Taking advantage of this, it is necessary to select the special arrangement of the fuel elements used in the present invention and the material to be filled in the lower part of the fuel element in order to minimize the local power peak in the cross section of the nuclear fuel assembly in the lower part. There is. In addition, the special fuel element used in the present invention does not have a structure in which the enrichment level differs between the upper and lower parts of one fuel element, or the concentration of burnable substances such as G material differs, so there are Does not cause mixing of fillings or mistakes in measuring filling length, etc.
The filling can be manufactured by clearly distinguishing it by its appearance. Furthermore, non-destructive inspection after completion of the fuel element can be easily performed by measuring X-rays, nuclear fission r-rays, or neutron beams. The present invention
By reducing the amount of fuel loaded in the lower part of the nuclear fuel assembly, it works to suppress the output peak in the lower part.

It also avoids filling multiple enrichment fuels into a single fuel element. However, other effects are also possible with the invention. In the first embodiment of the present invention, when graphite is used in the lower part of the fuel element, the output peak of the fuel at the three corners on the control trailing side can be reduced. In addition, in the second and third embodiments, the control side 3
It is possible to reduce the output of the fuel element at the corner. As a result, it is possible to reduce the amount of change in output when the control group moves and increase the movement speed of the control beam, which in turn can contribute to improving the operating rate. In particular, in the third embodiment, only the lower part has skin terro.
It has a special fuel element (d) in the center, and a neutron absorber (Zircaloy or S
Since the fuel element has a special fuel element that includes (US, etc.), it is actively trying to bring the local peak in the cross section of the nuclear fuel assembly to the center, and the amount of output change during control comb movement. will be significantly reduced. Furthermore, the first to
In Embodiment 3, the samurai fuel element whose lower part is filled with things other than fuel is located at the 3 corners on the control side or at the center, but this is a part of the fuel element that monitors the output of the fuel east in the reactor. This is because the measurement accuracy is not compromised since the mounting system is located at the fourth corner.

Next, still another embodiment of the present invention will be described.

(2) In the first to third embodiments of the present invention, it is assumed that the fuel enrichment is uniform within each fuel element.

However, due to the neutron economy of the reactor core, 1 at the upper and lower ends of the effective core length
It is known that efficiency is good if fuel with a lower K value is placed in the other parts by about 5 to 30 degrees. Therefore, this effect is incorporated into the present invention. This is the fourth embodiment shown in FIG. In this example, for fuel elements filled with fuel over the effective length of the core, short partial 15 to 30 skins at the upper and lower ends are filled with natural uranium or depleted uranium, and the fuel element is filled with fuel only in the upper part. Since the number of elements is small, either the enrichment remains uniform or only a short section at the top is filled with natural or depleted uranium. (2) In the first to fourth embodiments of the present invention, in the special fuel element whose upper portion is only filled with fuel, the length of the fuel-filled portion is unified and the element is incorporated into the nuclear fuel assembly.

Therefore, as shown in FIG. 12, a large step occurs in the output distribution at this boundary, which is disadvantageous in terms of accuracy of the neutron instrumentation that monitors the output of the fuel east. Therefore, by changing the lengths of the fuel-filled portions of the holding fuel elements as shown in FIG. 11, a smooth output distribution as shown by the curve B in FIG. 12 can be obtained.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the void distribution and power distribution in all axial directions of a nuclear fuel assembly of the present invention and a conventional nuclear fuel assembly, and FIG. 2 is a longitudinal section showing an embodiment of the nuclear fuel assembly of the present invention. 3 to 5 are longitudinal sectional views of the fuel element, FIGS. 6 and 7 are cross sectional views of the nuclear fuel assembly, and FIGS. 8 and 9 are explanatory diagrams showing the effects of the present invention. FIGS. 10 and 11 are explanatory diagrams showing other embodiments of the present invention, and FIG. 12 is an explanatory diagram showing the fuel east axis universal power distribution of the present invention. 1...Fuel element, 2...Upper tie plate, 3...Lower tie plate, 4...Subasa, 11......Coating Tube, 12...
...Fuel pellet, 14...Middle end plug. Figure 1 Figure 7 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 9 Figure 8 Figure 10 Figure 11 Figure 12

Claims (1)

[Claims] 1. A plurality of fuel elements arranged in a grid, an upper tie plate and a lower tie plate that bind the upper and lower ends of the fuel elements, and several tie plates that regulate the spacing between the fuel elements. In a nuclear fuel assembly composed of a spacer and a channel that covers all of the fuel elements, several of the fuel elements are divided into an upper part and a lower part, and only the upper part is filled with fissile material, and the lower part is hollow or filled with moderator. A nuclear fuel assembly characterized in that it is a special fuel element filled with one of the following: a reflective material, and a neutron absorbing material. 2 The upper and lower parts of the Toju fuel element are separated by an intermediate end plug provided in the middle, and the moderator charged in the lower part is supplied through the cooling water inlet hole drilled at the lower end of the Toju fuel element. Claim 1, characterized in that the water is led from the fuel element and discharged from a cooling water outlet hole drilled below an intermediate end plug provided in the intermediate part of the fuel element. The nuclear fuel assembly described. 3. The nuclear fuel assembly according to claim 1, wherein the moderator and the reflective material are one of beryllium metal, beryllium oxide, and graphite. 4. Claim 1, characterized in that the neutron absorbing material is a member of zirconium alloy or stainless steel.
Nuclear fuel assembly as described in section. 5. The nuclear fuel assembly according to claim 1, wherein the special fuel element is arranged at a corner of the nuclear fuel assembly.