JPS59188592A - Method and device for controlling burnup of 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 [Field of Application of the Invention] The present invention relates to a method and apparatus for controlling combustion in a boiling water nuclear reactor, and in particular to a method and apparatus for controlling combustion in a boiling water nuclear reactor, and in particular, to eliminate voids by adjusting the ratio of water to uranium in the fuel assemblies themselves or in their gaps. The present invention relates to a combustion control method and apparatus that can increase burnup by utilizing rate changes.

[Background of the invention]

Boiling water reactors have a void distribution in the axial direction, so neutrons heat up more in the lower core than in the upper core, and the power peak position is shifted toward the lower core.

However, in the current core design, from the perspective of ensuring fuel integrity and improving plant utilization, the power peak is reduced as much as possible and the linear power density is kept low. Countermeasures have been taken, such as inserting control rods shallowly into the reactors, and using fuel rods containing flammable GdgOs at the locations where peak output occurs.

Furthermore, recently developed fuel with two enrichment regions, upper and lower, has been proven to be particularly effective in flattening the axial power distribution than the above-mentioned measures, and has come to be widely adopted.

However, as a result of recent fuel technology development, as countermeasures against PCI (fuel-cladding effect) such as barrier fuels have been developed, the flattening of the power distribution as described above is no longer necessary, and the linear power density It can be increased within a range that maintains the health of the fuel. For such cores, a new core design that takes advantage of the features of boiling water reactors is desired.

Incidentally, the core of a light water reactor is constructed by arranging a plurality of fuel assemblies of the same shape as a unit to form a 1A core. The gaps between the fuel rods and between the fuel assemblies are filled with moderator (light water).

In the core of a light water reactor, fast neutrons immediately after nuclear fission are moderated by this moderator and become thermal neutrons, which become 235
Causes nuclear fission of U.

Generally, in such a light water reactor, the energy distribution of the neutron flux is such that the fission reaction (combustion) of 235 U is likely to occur, but in this state, 238 U is not necessarily fully utilized.

In order to effectively utilize this 2U, it is beneficial to adjust the moderator volume depending on the degree of combustion. However, in conventional light water reactor cores, there was no suitable means for adjusting the moderator volume.

[Purpose of the invention]

The present invention has been made in view of these circumstances, and its purpose is to increase the burnup by controlling the void fraction during the combustion period and increasing the reactivity in the latter half of the combustion.

[Summary of the invention]

In boiling water reactors, voids are generated during reactor operation, and fuel assemblies operated at high void rates are more susceptible to plutonium than those operated at low void rates due to hardening of the neutron spectrum. It is characterized by increased accumulation. This effect increases proportionally with the duration of combustion at high void fractions.

Therefore, during the loading period of the fuel mixture into the reactor, the first half is combusted with a high void ratio to promote the increase in plutonium production, and the second half is to reduce the void ratio to utilize the contribution of the generated plutonium to the reactivity. However, operating methods that increase reactor reactivity and burnup can be considered. This is an application of the spectral shift effect.

In the combustion control method according to the first aspect of the invention, in order to obtain the above spectrum shift effect, the water-to-uranium ratio is adjusted for the fuel assembly inter-assembly.

For example, in the first half of the loading period in the reactor, a member is installed that makes the water-to-uranium ratio in the upper region of the fuel assembly smaller than the water-to-uranium ratio in the lower region, thereby inflating the power distribution downward and achieving a high void ratio. In the second half, the water-to-uranium ratio in the upper region of the fuel assembly is replaced with a member whose water-to-uranium ratio is the same as the water-to-uranium ratio in the lower region, and a lower void ratio is achieved by raising the power distribution relatively upward compared to the first half. . In this way, the spectral shift effect is made possible.

Further, a control device according to a second invention is a combustion control device for controlling combustion of a fuel assembly of a boiling water reactor, and includes an adjustment section for adjusting a water to uranium ratio inside the fuel assembly. ing.

Furthermore, the control method according to the third invention is a fuel control method based on the spectral shift effect of fuel assemblies for boiling water reactors, in which processing is performed to change the water to uranium ratio in the gaps of each fuel assembly. In particular, it is intended to control combustion.

That is, the volume of water is adjusted by arranging tubes filled with spheres of a material having a small neutron absorption cross section, such as Z2, in the water gap between each fuel assembly.

When the volume of water decreases, the proportion of medium and fast neutron flux increases relatively, and the absorption of 238U neutrons increases, producing "try." Near the end of the cycle, the spheres of Z and Z are moved out of the core, and this Instead, by filling the pipe with water, the volume of water is increased and 23'Um is burned to improve burnup.

(This Zr sphere will be referred to as a hydroball hereinafter.) Furthermore, the control device according to the tth invention is a combustion control device for controlling combustion of a combustion assembly of a boiling water reactor, wherein the An adjustment member made of a material with a small neutron moderating ability and a small neutron absorption is arranged in the gap between the aggregates, so that the volume of the moderator in the target gap can be adjusted.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.

First, embodiments according to the first and second inventions are shown in FIGS. 1 to 9.
This will be explained using figures.

The fuel assembly is shown in Figures 1 and 2.

In the fuel assembly 11 according to this embodiment, a hollow rod body 3, in which a moderator is enclosed, is housed together with the fuel rods 2 in the central portion of the channel box 1. This rod 3 is internally divided into an upper region A and a lower region B in the axial direction, and an adjustment rod 4 (Zircaloy rod) as a small rod for reducing the water to uranium ratio is inserted into and removed from the lower region A. It is installed if possible. That is, as shown enlarged in FIGS. 3 and 4, in this rod 3 (hereinafter abbreviated as SS rod), the water and uranium in the upper region in the axial direction are adjusted by inserting the adjusting rod 4 and by pulling the pull plate. The ratio is adjustable. In other words,
The lower end of the adjustment rod 4 is supported by an adjustment rod holder 8 that protrudes from the inner surface of the axially intermediate portion of the SS rod 3, and the outer peripheral surface of the upper end of the adjustment rod 4 is supported by a take-out spring 5. It is fixed via the take-out spring bone 7. Therefore, when taking out the adjustment rod 4 from the 8S rod 3, use the take-out spring 5.
After the spring is removed from the spring frame 7, it is pulled out upward. Note that the adjustment rod 4 is inserted when the core is first loaded (when configuring the fuel assembly). In addition, the pulling operation should be performed during periodic inspections, etc. In addition,
This adjustment rod 4 has a small neutron moderating effect, and is made of, for example, a Zircaloy rod. Therefore, when the adjustment rod 4 is inserted, the reactor water that has flowed in from the lower part of the fuel bundle assembly flows out from the hole 6 drilled in the intermediate wall in the lower region of the SS rod 3, and flows out from the hole 6 bored in the middle wall in the lower region of the SS rod 3. Water will not flow into the area. In addition, when the adjusting rod 4 is pulled out, the reactor water that has flowed in from the lower part of the fuel assembly will not flow from the lower part to the upper part in the axial direction in the SS rod 3, and the a1 adjustment rod mentioned above is inserted. This means that the ml volume of the flow path has increased compared to the case, and therefore, A distortion of the water to uranium ratio can be achieved by inserting and withdrawing Adjustment 4JI4.

FIG. 5 shows the state of the core loaded with fuel assemblies. In the figure, the numbers on the labels (1, 2, 3, 4) indicate the number of cycles in which the particles are allowed to stay in the furnace. In this example, the fuel assemblies labeled 1 and 2 are loaded with the SS rod 3 into which the adjustment rod 4 is inserted (11). +The fuel assemblies labeled 3 and 4 are:
It is assumed that the SS rod 3 with the adjustment rod 4 pulled out is loaded.

FIG. 6 shows the axial void distribution when the adjustment rod 4 is inserted (broken line a) and when it is pulled out (solid line b). As the adjustment rod 4 is pulled out, an increase in the void distribution is observed.

FIG. 7 shows the characteristic of the infinite multiplication factor of the fuel assembly when the fuel assembly is arranged according to the core configuration shown in FIG. As shown in FIG. 7, according to this embodiment, it is possible to increase the infinite multiplication factor after pulling out the rA rod (third cycle), and the extraction burnup of the fuel assembly increases.

In the above example, two SS rods 3 are housed in the center of the fuel assembly, but they may also be loaded in the periphery of the fuel assembly, as shown in FIG. 8, for example. As shown in the figure, a large number may be accommodated in a central arrangement.

In addition, it is desirable for fuel assemblies with a low water-to-uranium ratio to be in the range of 25% to 80% (12) of all loaded fuel assemblies, and for other fuel assemblies to have a high water-to-uranium ratio in actual aircraft. .

Further, the operation of changing the water to uranium ratio according to the embodiment includes dividing the fuel assembly into an upper region and a lower region in the axial direction and creating a difference in uranium enrichment between the upper region and the lower region.
Alternatively, it is more effective if the concentration of burnable poison in the fuel rods is varied between the upper and lower parts of the fuel assembly. The uranium enrichment in the upper region is higher than the uranium enrichment in the lower region, and the number of fuel rods containing burnable poison is also divided vertically in the axial direction. The member is pulled out with the number of fuel rods containing burnable poison in the upper region being greater than the number of fuel rods containing burnable poison in the upper region.

In addition, the fuel assembly is divided into an upper region and a lower region in the axial direction with respect to uranium enrichment, and the uranium enrichment in the upper region is higher than the uranium enrichment in the lower region under i. The concentration of poison (13) is also divided into upper and lower parts in the axial direction, and the operation is performed with the concentration of burnable poison in the lower region being higher than that in the upper region.

Furthermore, the fuel assembly is divided into an upper region and a lower region in the axial direction with respect to uranium enrichment, the uranium enrichment in the upper region is higher than the uranium enrichment in the lower region, and The number of fuel rods is also divided into upper and lower parts in the axial direction, and the number of fuel rods containing burnable poison in the lower region is controlled to be smaller than the number of fuel rods containing burnable poison in the upper region.

Furthermore, in the fuel assembly, the uranium enrichment is divided into an upper region and a lower region in the axial direction, and the uranium enrichment in the upper region is higher than the uranium enrichment in the lower region, and The concentration of the burnable poison is also divided into upper and lower regions in the axial direction, and the concentration of the burnable poison in the lower region is controlled to be higher than the concentration of the burnable poison in the upper region.

Next, examples according to the third and fourth inventions are shown in FIGS. 10 to 1.
This will be explained using Figure 3.

(14) In the control device according to this embodiment, as shown in FIG. The other end is connected to a hydroball tank 70.

The tank 70 and the pump 80 are connected by a water circulation pipe via a valve 60 for regulating the inflow of the hydro ball 10. The hydroball 1-O can be moved by the pressure of water injected from one end of the hydroball tube 20. As a result, the hydroball 10 pushed out from the other end of the hydroball tube 20 is stored in the hydroball tank 70. When the valve 60 under the tank is turned 1D, the hydroball 10 falls and is circulated from one end of the hydroball pipe 20 into the case 30 by water pressure, and this valve 60 controls the amount of circulation of the hydroball 10. I'm trying to get it under control.

As shown in FIGS. 11 and 12, the hydroball circulation system is disposed (15) in the gap between each fuel assembly 40.

The hydroball 10 is a sphere made of a material with low neutron absorption and neutron moderation ability, such as Z, At, alloys or oxides thereof.

In this embodiment, the hydroball 10 is 21 mm in diameter with 8 diameters.
The hydroball tube 20 has an outer diameter of 10 -1 and a tube thickness of 0.8 mm, and the hydroball tube storage case 30 has a wall thickness of 1 cm. Note that the hydroball 10 may be hollow to reduce its weight.

When a nuclear reactor is operated with the hydroballs 10 loaded, neutron moderation is small in the region filled with the hydroballs 10, and the proportion of medium and fast neutrons is relatively large. Therefore, 239 pm from zasU can be generated and accumulated more easily than in the case where the hydroball 10 is not filled.

Therefore, if water is circulated through the hydroball tube 20 and the hydroballs 10 are discharged according to the combustion of the fuel, the volume of water increases by the amount of decrease in the hydroballs 10, and neutron moderation increases, so thermal neutrons (16 ) ratio increases, and the generated 239p can be burned.

FIG. 13 shows the reactivity gain state when the hydroball is operated with 100% insertion and the hydroball is gradually discharged when the fuel burnup is about 220 W d/t. The reactivity gain occurs when 238U is burned and 2
This is because 39p is generated and accumulated.

Therefore, it is desirable to discharge the hydroball 10 near the end of the cycle. It is possible to select the hydroball tube 20 to perform the discharge operation and adjust the discharge amount as desired. If the hydroball 10 is extracted near a fuel assembly with particularly high burnup, the reactivity gain will be increased. Therefore, it is also possible to increase the reactivity gain by gathering and localizing several high-burnup aggregates in the reactor core in advance and operating only the hydroballs 10 in these areas.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to increase the spectral shaft effect of a boiling water reactor by making more effective use of, for example, 238U contained in light water reactor fuel. Pw''e can be generated, burnup can be increased, and fuel economy can be improved.

[Brief explanation of the drawing]

Figures 1 to 9 show embodiments of the first and second inventions, with Figure 1 being an enlarged cross-sectional view of the fuel assembly, and Figure 2 being a partial cross-section showing the adjustment body. Figures 3 and 4 are enlarged sectional views showing the insertion and removal states of the small adjustment rods, Figure 5 is an explanatory diagram showing the core configuration, Figure 6 is a characteristic diagram showing the void distribution, and Figure 7 is an explanatory diagram showing the core configuration. The figure is a characteristic diagram showing the combustion characteristics of infinite multiplication factor, Figures 8 and 9 are cross-sectional views of the fuel assembly showing other arrangement examples of the adjustment rods, and Figures 10 to 13 are Since this is a book showing practical examples of the third and fourth inventions, Fig. 10 is a schematic longitudinal cross-sectional view showing the adjusting member and the circulation system of the hydroball at j-, and Fig. 11 is an enlarged cross-sectional view of Fig. 10. Figure, 1st
FIG. 2 is an overall layout diagram of the hydroball circulation system shown in FIG. 11 in the furnace, and FIG. 13 is a characteristic diagram showing reactivity gain. (18) 2...Fuel rod, 3...Adjustment rod (880 d),
4... Small rod body (adjustment rod), 10... Adjustment member (hydro ball), 11.40... Fuel assembly. Agent Patent Attorney Tatsuyuki Unuma rlQ) To++l 30th Dormitory Hand Sen 5th (a) 3rd Country Eel 9 Mouth 10th Prisoner 11th Country Dormitory 17th Country □

Claims (1)

[Claims] 1. A nuclear reactor combustion control method based on the spectral shift effect of a fuel assembly for a boiling water reactor, the method comprising controlling the water-to-uranium ratio in the upper region of a fuel assembly during the loading period into the reactor. A combustion control method for a nuclear reactor, characterized in that the combustion control method is changed by removing parts at a predetermined time. 2. The operation of changing the ratio of water to uranium in the fuel assembly is performed by adding a member to the fuel assembly that reduces the water to uranium ratio in the upper region of the fuel assembly as well as in the lower region in the first half of the loading period in the reactor. There is a process in which the fuel assembly is installed to expand the power distribution downward, and a process in which the power distribution is relatively moved upward by replacing the member with a member whose water-to-uranium ratio in the upper region of the fuel assembly is the same as in the lower region in the latter half. 3. The operation of changing the water-to-uranium ratio of the fuel assembly is performed when the fuel assembly is axially oriented in terms of uranium enrichment. It is divided into upper and lower regions, and the upper region has a higher uranium enrichment than the lower region, and the number of fuel rods containing burnable poison is divided into upper and lower regions in the axial direction, and the number of fuel rods containing burnable poison in the upper region is higher than that of the lower region. The combustion control method for a nuclear reactor according to claim 1, characterized in that the combustion control method is performed in a state where the fuel assembly has a large amount of uranium. Regarding enrichment, it is divided into upper and lower regions in the axial direction, and the uranium enrichment in the upper region is higher than that in the lower region.
The nuclear reactor according to claim 1, wherein the reactor is divided into upper and lower regions in the axial direction with respect to the concentration of burnable poison, and the nuclear reactor is operated in a state where the concentration of burnable poison in the upper region is higher than that in the lower region. combustion control method. 5. The operation of changing the water-to-uranium ratio of the fuel assembly is performed by dividing the fuel assembly into upper and lower regions in the axial direction with respect to uranium enrichment, and the uranium enrichment in the upper region is higher than that in the lower region;
The nuclear reactor according to claim 1, wherein the reactor is divided into upper and lower regions in the axial direction with respect to the concentration of burnable poison, and the nuclear reactor is operated in a state where the concentration of burnable poison in the upper region is lower than that in the lower region. combustion control method. 6. 1. A combustion control device for controlling combustion of a fuel assembly of a boiling water nuclear reactor, characterized in that an adjustment section for adjusting a water-to-uranium ratio is provided inside the fuel assembly. 7. The water-to-uranium ratio adjustment section includes a required number of rods housed in the fuel assembly and containing a moderator, and a small rod that is removably inserted into the upper part of each rod and has no moderator present. 7. The combustion control device according to claim 6, comprising a rod body. 8. Combustion control device according to claim 6, characterized in that the rod is disposed at the center or the periphery of the fuel assembly 9. Fuel assembly in which the water-to-uranium ratio is set to a small value is set to 25% to 80% of all loaded fuel assemblies, and the water to uranium ratio is set to be large in the remaining fuel assemblies. . 10. A combustion control method based on the spectral shift effect of a fuel assembly for a boiling water reactor, and each combustion control device. 11. A combustion control device for controlling combustion of fuel assemblies of a boiling water reactor, wherein an adjustment member made of a material with low neutron moderating ability and neutron absorption is disposed in a gap between the fuel assemblies, and the gap between the fuel assemblies is A combustion control device characterized in that the volume of a moderator in can be adjusted. 12. The combustion control device according to claim 11, wherein the adjustment member comprises a tube fluidly loaded with spheres.