JP2020118495A - Method for operating nuclear reactor - Google Patents

Abstract

To provide a nuclear reactor core relocation method securing a thermal margin of the core when relocating from an existing core to a core to which a fuel with different active fuel length is loaded.SOLUTION: When relocating from long fuel 1 provided in a core to short fuel 3, a process of replacing the long fuel 1 by an adjustment fuel 2 (fuel having an active fuel length between the long fuel 1 and the short fuel 3) is interposed therebetween. A nuclear reactor operation method comprises: a first operation step of performing operation using a nuclear reactor with a plurality of first fuel assemblies; a second operation step of taking out the first fuel assemblies from the core, loading second fuel assemblies whose active fuel length is shorter than that of the first fuel assembly to a position where the taken-out first fuel assemblies have been placed and performing the operation; and a third operation step of taking out the first fuel assemblies or the second fuel assemblies from the core, loading third fuel assemblies whose active fuel length is shorter than that of the second fuel assembly to a position where the taken-out first fuel assemblies or the second fuel assemblies have been placed, and performing the operation.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method of transferring a boiling water nuclear reactor core, in particular, from the core of the current boiling water nuclear reactor, mainly for the purpose of burning transuranium elements, rather than the fuel of the current boiling water nuclear reactor. The present invention relates to a method of transitioning to a core loaded with a fuel having a short effective length and densely arranged fuel rods.

In a boiling water reactor currently loaded with fuel assemblies and cruciform control rods, the fuel rods in the fuel assembly are arranged in a dense triangular lattice and voids are generated in the channel box during operation. Patent Document 1 proposes a boiling water reactor in which the neutron spectrum is hardened by the following method (hereinafter referred to as a reduced-speed spectrum boiling water reactor).

This is the reactivity that is injected when the void ratio in the channel box increases due to a decrease in the flow rate of water (cooling water), which is the coolant, because the neutron spectrum is hardened in the low-moderation spectrum boiling water reactor. There is a problem that the negative absolute value of the void reactivity coefficient decreases. In order to solve this, in the configuration of Patent Document 1, a fuel in which the length in the height direction in which the fissile material is loaded (hereinafter, fuel active length) is made shorter than the current fuel (hereinafter, short fuel) is used, and voids are used. The negative void reactivity coefficient is maintained by increasing the axial neutron leakage as the rate increases.

JP, 2018-66690, A

By the way, in order to realize the core described in Patent Document 1, it is conceivable to load an existing boiling water reactor with a short fuel. At this time, from the viewpoint of not wasting the existing fuel with a relatively long fuel (hereinafter referred to as long fuel) already loaded in the existing boiling water reactor, combustion of the long fuel progresses for each operation cycle. It is desirable that all the fuel loaded into the existing core is made into short fuel by taking out a part of the fuel and loading short fuel instead. However, at the time of transition where long fuel and short fuel are mixed, the amount of fuel loaded in the lower core is larger than that in the upper core, so the calorific value in the lower core becomes extremely large, and the thermal margin of the core becomes small. There is.

The above problem occurs because fuels having extremely different active fuel lengths are mixed in the core. Therefore, in order to solve the above problem, when the long fuel provided in the core is changed to the short fuel, the long fuel is replaced with the adjustment fuel (fuel having an effective fuel length between the long fuel and the short fuel). Insert the process.

According to the present invention, it is possible to mitigate the decrease in the thermal margin of the core when the core loaded with the fuel having a certain active fuel length is transferred to the core loaded with the fuel having a different active fuel length. As a result, the safety is improved from the existing boiling water reactor loaded with long fuel to the reduced-speed spectrum boiling water reactor core loaded with short fuel, whose main purpose is to burn transuranium nuclides. It is possible to make a transition while securing.

It is a conceptual diagram of the operating method of a nuclear reactor. It is a schematic block diagram of ABWR. It is a schematic block diagram of the fuel assembly of long fuel. It is a schematic block diagram of the fuel assembly of a short fuel. It is a schematic block diagram of the fuel assembly of adjustment fuel. FIG. 6 is a schematic configuration diagram of an ABWR core in which a plurality of fuel assemblies of FIGS. 3 to 5 are loaded. FIG. 5 is an explanatory diagram of a fuel loading pattern of the transition core of Example 1. 5 is a graph of maximum line power density showing relative values of Example 1 and Comparative Example 1. 5 is a graph of maximum line power density showing relative values of Example 2 and Comparative Example 2. 9 is a configuration of a fuel for adjustment of Example 3. 7 is a graph of maximum line power density showing relative values of Example 3 and Comparative Example 3. .. 9 is a configuration of a fuel for adjustment of Example 3. 7 is a graph of maximum line power density showing relative values of Example 4 and Comparative Example 4. It is another method of core transition.

In the present specification, the boiling water reactor to which the fuel assembly and the core of the boiling water reactor loaded with the fuel assembly are applied is a recirculation pump and water (cooling water) is used as a coolant for the reactor pressure vessel. A normal boiling water reactor (BWR) that circulates cooling water by circulating it outside and then into the downcomer inside the reactor pressure vessel, equipped with an internal pump, and cooling water inside the reactor pressure vessel Includes an Advanced Boiling Water Reactor (ABWR) that circulates in the water ..

Below, as described in Patent Document 1 as the core after the transition, an ABWR in which all long fuels using mixed oxides (hereinafter referred to as MOX) are loaded as the core before the transition, and a square lattice channel of the current fuel is described. The fuel rods are densely arranged in a triangular lattice in the box, the core loaded with short fuel whose effective length is half that of the long fuel, and the fuel active length between the long fuel and the short fuel as the adjustment fuel. A certain configuration will be described as an example. As described above, the fuel assembly and the core according to the present invention can be applied to other boiling water reactors in which the number of fuel assemblies loaded with control rods having a cross-shaped cross section (cross control rods) is different. Is also applicable.

FIG. 1 shows a conceptual diagram of a method of operating a nuclear reactor according to this embodiment. The pre-transition core (current core) of the Nth operation cycle is composed of only the fuel assembly of the long fuel 1. In the transitional core of the (N+1)th operation cycle, one fuel assembly of the long fuel 1 is taken out from the plurality of fuel assemblies of the long fuel 1 included in the current core of the Nth operation cycle, and the fuel assembly of the adjustment fuel 2 is taken out. Load your body. By repeating this process, in the transitional core of the (N+2)th operation cycle, one fuel assembly of the long fuel 1 is taken out from the fuel assemblies of the plurality of long fuels 1 included in the transitional core of the (N+1)th operation cycle and adjusted. The fuel assembly of the fuel 2 is loaded. Further, in the transitioning core of the (N+3)th operation cycle, the fuel assembly of the last long fuel 1 included in the transitioning core of the (N+2)th operation cycle is taken out and loaded with the fuel assembly of the adjustment fuel 2. As a result, the core during transition in the (N+3)th operation cycle is constituted only by the fuel assembly of the adjusting fuel 2.

In the transitional core of the N+4th operation cycle, one fuel assembly of the adjustment fuel 2 is extracted from the fuel assemblies of the adjustment fuels 2 included in the transitional core of the N+3th operation cycle, and the fuel assembly of the short fuel 3 is extracted. Load your body. By repeating this process, in the transitional core of the (N+5)th operation cycle, one fuel assembly of the adjustment fuel 2 is taken out from the fuel assemblies of the plurality of adjustment fuels 2 included in the transitional core of the (N+4)th operation cycle, Load fuel 2. Further, in the post-transition core of the (N+6)th operation cycle, the fuel assembly of the last adjusting fuel 2 provided in the core during transition of the (N+5)th operation cycle is taken out and loaded with the fuel assembly of the short fuel 3. As a result, the post-transition core of the (N+6)th operation cycle (shown as N+m in FIG. 1) is composed of only the fuel assembly of the short fuel 3.

By adopting such a transition method, the reduction of the thermal margin at the time of core transition due to the mixture of the fuel assemblies of the long fuel 1 and the fuel assemblies of the short fuel 3 having extremely different active fuel lengths is mitigated.

FIG. 2 is a schematic configuration diagram of the ABWR. ABWR will be described. An ABWR 20 including a core loaded with a fuel assembly (described later in detail) according to the first embodiment includes a cylindrical core shroud 26 in a reactor pressure vessel 21. A core 22 loaded with a plurality of fuel assemblies (not shown) is installed in the core shroud 26. A steam separator 28 and a steam dryer 29 are provided in the reactor pressure vessel 21. The steam separator 28 is attached to the shroud head 30 that covers the core 22, and extends upward. The steam dryer 29 is arranged above the steam separator 28.

The upper lattice plate 24 is located inside the core shroud 26, below the shroud head 30, and above the core 22. The core support plate 23 is located inside the core shroud 26 and below the core 22. A plurality of fuel support fittings 25 are installed on the core support plate 23.

Further, in the reactor pressure vessel 21, a control rod guide pipe 32 is provided which allows a plurality of control rods (not shown) having a cross-shaped cross section to be inserted into the core 22 for controlling the nuclear reaction of the fuel assembly. Has been. A control rod drive mechanism housing (not shown) installed below the bottom of the reactor pressure vessel 21 is provided with a control rod drive mechanism 33, and the control rods are connected to the control rod drive mechanism 33. A lower mirror 34 is provided at the bottom of the reactor pressure vessel 21, and a plurality of internal pumps 31 are installed so as to penetrate the reactor pressure vessel 21 from below the lower mirror 34.

The plurality of internal pumps 31 are arranged outside the outermost peripheral portion of the plurality of control rod guide pipes 32, are annularly spaced from each other at a predetermined interval, and are arranged in plurality. As a result, the internal pump 31 does not interfere with the control rod guide pipe 32 or the like. The impeller of each internal pump 31 is provided inside an annular downcomer 26 formed between the cylindrical core shroud 26 and the inner surface of the reactor pressure vessel 21. Water (cooling water) as a coolant in the reactor pressure vessel 21 is supplied from the lower mirror 34 side to the reactor core 22 via the downcomers 27 by the impellers of the internal pumps 31. The cooling water flowing into the core 22 is heated by a nuclear reaction of a fuel assembly (not shown) to become a gas-liquid two-phase flow, and flows into the steam separator 28. The gas-liquid two-phase flow that passes through the water-water separator 28 is separated into steam (gas phase) containing moisture and water (liquid phase), and the liquid phase again drops to the downcomer 27 as cooling water. On the other hand, the steam (gas phase) is introduced into the steam dryer 29 to remove moisture, and then supplied to the turbine (not shown) via the main steam pipe 35. The cooling water flowing into the reactor pressure vessel 21 through the water supply pipe 36 via the condenser or the like flows (falls) downward in the downcomer 27. In this way, the internal pump 31 forcibly circulates the cooling water to the core 22 in order to efficiently cool the heat generated in the core 22.

FIG. 3 is a schematic configuration diagram (a vertical sectional view and a horizontal sectional view) of a fuel assembly of a long fuel loaded in the core 22. In the fuel assembly 300 shown on the right of FIG. 3, 74 fuel rods 302 having a diameter of about 10 mm are arranged in a square lattice shape inside a channel box 301 having a square cross section. Each fuel rod 302 has an upper tie plate and a lower tie plate (not shown) at both upper and lower ends thereof, and a fuel spacer (not shown) that axially separates the fuel rod 302 in the middle thereof. Is held. A fuel rod 302 and a water rod 303 having an outer diameter of about 34 mm are arranged inside the channel box 301. The outer region of the fuel rod 302 and the water rod 303 and the inner region of the channel box are moderators called water regions. Filled with 304. Saturated water 305 called a gap water region is provided outside the channel box 301.

The fuel assembly 301 shown on the left of FIG. 3 has a fuel region having an active fuel length of about 3.7 m as an axial configuration. The fuel rod 302 has a cladding tube (not shown) filled with MOX pellets (not shown) enriched with an oxide of depleted uranium or a transuranic nuclide containing fission plutonium in depleted uranium.

FIG. 4 is a schematic configuration diagram (a vertical sectional view and a horizontal sectional view) of a fuel assembly of a short fuel loaded in the core 22. In the fuel assembly 400 shown on the right of FIG. 4, 243 fuel rods 402 are arranged in a triangular close-packed manner inside a channel box 401 having a square cross section. The upper and lower ends of each fuel rod 402 are an upper tie plate and a lower tie plate (not shown), and a fuel spacer (not shown) is arranged in the middle of the fuel rod 402 at regular intervals in the axial direction. Is held. On the outside of the channel box 401, a gap water region 403 which is saturated water and a cross-shaped control rod (cross-shaped control rod) 404 are inserted. With the above configuration, the reduction rate spectrum BWR in which the neutron spectrum in the core 22 is hardened (shifted to the high energy side) is realized by reducing the ratio of the cooling water to the fuel. Further, the fuel assembly 400 shown on the left of FIG. 4 has a fuel region filled with MOX pellets and having an active fuel length of about 1.8 m.

FIG. 5 is a schematic configuration diagram (a vertical sectional view and a horizontal sectional view) of a fuel assembly of the adjustment fuel loaded in the core 22. The fuel assembly 500 shown on the right of FIG. 5 has the same cross-section (horizontal cross-section) as that of FIG. 4 in which the fuel rods 502 are arranged in a triangular close-packed manner inside a square channel box 501. It differs from the configuration in Fig. 4 in that the effective length is about 2.8 m. The fuel rod 502 is held at the upper and lower ends by an upper tie plate and a lower tie plate (not shown), and by a fuel spacer (not shown) which is located in the middle of the fuel rod 502 at regular intervals in the axial direction. There is. A gap water region 503 of saturated water and a cross-shaped control rod 504 (cross-shaped control rod in cross section) are inserted outside the channel box 501.

FIG. 6 is a schematic configuration diagram (horizontal sectional view) of the ABWR core 22 loaded with a plurality of the fuel assemblies 300, 400, 500 shown in FIGS. 872 fuel assemblies 601 are loaded in the core 22 in a square lattice shape. Except for a plurality of fuel assemblies 601 arranged at the outermost periphery, four fuel assemblies 601 adjacent to each other are cross-shaped. The core 22 is loaded so as to surround the mold control rod 602. The schematic configuration diagram (horizontal cross-sectional view) of the fuel assemblies 300, 400, 500 shown on the right side of FIGS. 3 to 5 shows one fuel assembly 601 of the above four fuel assemblies. .. As shown in the right side of FIGS. 4 and 5, the channel block 401 having a square schematic configuration (horizontal cross-sectional view) and the two sides connected at one corner forming the channel box 501 are cross-shaped control rods 602. The core 22 is loaded so as to face the two blades with a slight gap therebetween.

In order to explain the effect of the core migration method of Example 1, a comparative example (from the core 22 in which all fuel assemblies 300 in FIG. 3 are loaded as long fuel to the core 22 in which fuel assemblies 400 in FIG. 4 are loaded as short fuel) In the case of shifting to 22 in the case where the long fuel is directly replaced by the short fuel), and in Example 1 (from the core 22 in which all the fuel assemblies 300 of FIG. 3 are loaded as the long fuel, the fuel of FIG. When the assembly 400 is transferred to the core 22 that is loaded with short fuel, the fuel margin 500 is once replaced with the adjustment fuel, and then the adjustment fuel is replaced with the short fuel. The maximum linear power density, which is an index shown, is compared.

FIG. 7: is explanatory drawing of the fuel loading pattern of the transition core of Example 1. As shown in FIG. The number of materials (indicated by a square frame) of the ABWR 1/4 core 700 showing the horizontal cross section is 218. The number “1” in the square indicates the fuel loading position of the stay cycle number 1, the number “2” in the square indicates the fuel loading position of the stay cycle 2, and the number “none” in the square indicates the stay cycle. The fuel loading positions after the number 3 are shown. The ABWR quarter core 700 has 50 fuel 701s with a stay cycle number of 1 and 50 fuel 702s with a stay cycle number of 2. The fuel loading pattern of FIG. 7 is characterized in that the fuel 701 having a stay cycle number of 1 and the fuel 702 having a stay cycle number of 2 are not adjacent to each other in the central portion of the core where the output is high. As will be described later, the fuel 702 having a stay cycle number of 2 is loaded with a long fuel that is not relatively advanced in combustion, and when the short fuel is loaded at the position of the fuel 701 having a stay cycle number of 1, This is to suppress a decrease in the thermal margin due to an increase in the output of the lower core of a long fuel having a high output.

Next, a fuel exchange method at the time of core transition according to the first embodiment will be described. At the timing when the operation cycle of the ABWR 1/4 core 700 shown in FIG. 7 ends, the fuel having the largest number of stay cycles is taken out. Next, the fuel having the largest number of stay cycles among the remaining fuel is moved to the position of the taken out fuel. At this time, if the number of fuels of the number of stay cycles to be moved next is larger than the number of fuels of the number of stay cycles to be taken out first, combustion occurs in the fuel of the number of stay cycles to be moved. Take out in order from the advanced one. This is repeated up to the fuel of the stay cycle number 1. That is, the fuel having the stay cycle number of 2 indicated by "2" in the square in FIG. 7 moves to the position where the fuel having the stay cycle number of 3 (not shown) is present, and is indicated by "1" in the square. The fuel having the stay cycle number of 1 moves to the position indicated by "2" in the square in FIG. Then, finally, the adjustment fuel is loaded at the position where the fuel having the stay cycle number 1 indicated by "1" in the square in FIG. 7 was present.

Next, the effect of Example 1 is shown. The inventors, when transitioning from a core composed of long fuel to a core composed of short fuel, mixed long fuel and short fuel, and the presence of the short fuel causes the axial power of the core to increase in the lower part of the core. It was found that the maximum linear power density, which is one of the indexes of the thermal margin of the transition core, becomes smaller on the long fuel side. When the core is composed of a short fuel, the number of fuel rods is set to be larger than that of the long fuel so that the short fuel can originally generate an output with an active fuel length corresponding to the lower part of the core. However, since it is not assumed that long fuel produces power at the active fuel length equivalent to the lower part of the core, there is no countermeasure against the increase in power distribution in the lower part of the core, and if long fuel and short fuel coexist, heat will increase. The margin is reduced.

In view of these problems, the essence of the invention according to the first embodiment is to prevent the output of the lower core from becoming extremely high by loading the adjustment fuel having the effective fuel length between the long fuel and the short fuel. Especially. The effect of this adjusting fuel is effective when the active fuel length of the long fuel assembly is at least twice the active fuel length of the short fuel assembly.

FIG. 8 is a graph of the maximum linear power density showing the relative values of Example 1 and Comparative Example 1. The maximum linear power density A of Example 1 was transferred to the core 22 in which the adjustment fuel was replaced with the short fuel after all the long fuel was replaced with the adjustment fuel in the core 22 loaded with the long fuel. In this case, it is the maximum linear power density for each operation cycle. The maximum linear power density B of Comparative Example 1 is the maximum linear power density when the core 22 loaded with the long fuel is transferred to the core 22 in which the long fuel is replaced with the short fuel. The polygonal line in FIG. 8 is the relative value of the maximum line power density A to the maximum line power density B. The relative value is less than 1 in any of the transition first cycle and the transition second cycle periods, and the operation method of the reactor adopting the core transition method shown in Example 1 is to reduce the thermal margin of the core. It can be seen that the effect is high in terms of mitigation.

In the present example, the method of transitioning from a core loaded with a long fuel to a core loaded with a short fuel was shown, but conversely, a transition from a core loaded with a short fuel to a core loaded with a long fuel was performed. It can also be applied as a method.

The inventors have further found that among the long fuels, the linear power density becomes higher in the fuels in which combustion has not progressed relatively, and it is effective to suppress the output increase of these fuels in the lower core. It was That is, in the transition first cycle and the transition second cycle of FIG. 8, the maximum linear power densities are generated with the long fuel having the stay cycle numbers of 2 and 3, respectively. Further, even after the third cycle of transition, the maximum linear power density is generated in the remaining long fuel, but since the long fuel has a stay cycle number of 4 or more, the fuel output itself is reduced. , It was found that the absolute value of the maximum line power density decreased and there was no problem from the viewpoint of thermal margin. From this point of view, in Example 2, only the transition first cycle and transition second cycle in which the maximum linear power density is particularly high in Example 1, the adjustment fuel is loaded instead of the extracted long fuel, and the subsequent operation cycle is performed. Then, instead of the long fuel taken out, short fuel is loaded.

FIG. 9 is a graph of the maximum line power density showing the relative values of Example 2 and Comparative Example 2. The maximum linear power density C of Example 2 is the same as that of the core 22 loaded with the long fuel after the long fuel is replaced with the adjusting fuel only in the first transition cycle and the second transition cycle, and thereafter, the long fuel is used. Is the maximum linear power density for each operation cycle when the fuel is transferred to the core 22 in which is replaced with short fuel. The maximum linear power density D of Comparative Example 2 is the maximum linear power density when the core 22 loaded with the long fuel is transferred to the core 22 in which the long fuel is replaced with the short fuel. The polygonal line in FIG. 8 is the relative value of the maximum line power density C to the maximum line power density D.

As shown in FIG. 9, the maximum linear power density decreases until the third cycle of transition. In the transition fourth cycle, there is a period when the maximum linear power density becomes equal to or higher than that when the fuel is not used for adjustment, but it has already shifted to short fuel and the absolute value of the maximum linear power density Is small and the thermal margin is relatively large, so there is no problem.

According to the second embodiment, as compared with the first embodiment, it is possible to replace all the short fuel with a smaller number of transition cycles. Therefore, it is possible to effectively achieve the original purpose of using the short fuel, that is, the burning of the transuranium nuclide.

The inventor has further found that when the adjusting fuel and the long fuel in FIG. 4 are adjacent to each other, the region where the output of the long fuel is the highest is near the center of the adjusting fuel. Since the position near the center of the adjusting fuel is about 1 m from the lower end of the long fuel, even if the adjusting fuel is used, the output of the long fuel still tends to be biased downward. From this, if the configuration of the adjustment fuel in FIG. 4 is modified so that the output can be adjusted to be larger in the upper region of the long fuel, the increase in the output of the long fuel in the lower core is more effective. Can be suppressed. From this point of view, in the third embodiment, the loading amount of the fissile material in the axial direction is changed with respect to the fuel composition for adjustment of the first embodiment.

FIG. 10 is a schematic configuration diagram (horizontal sectional view) of the adjusting fuel of the third embodiment. On the right side of FIG. 10, the channel box 1001, the fuel rod 10002, the gap water region 1003, and the control rod 1004 of the fuel assembly 1000 of the adjustment fuel have the same configuration as in FIG. The left side of FIG. 10 shows the configuration of the fuel in the axial direction. In Example 3, the fuel having a plutonium enrichment of 13w% is in a region of about 1.8m from the lower end of the fuel assembly 1000 of the adjustment fuel, and the plutonium enrichment is 16w in a region of about 1m from the upper end of the fuel. % Fuel composition. With this configuration, it is possible to increase the output of the adjustment fuel in the upper portion and suppress the increase of the output of the long fuel in the lower portion of the core.

FIG. 11 is a graph of the maximum line power density showing the relative values of Example 3 and Comparative Example 3. The maximum linear power density E of Example 3 shifts from the core 22 of the ABWR in which all the fuel assemblies 300 of the long fuel in FIG. 3 are loaded to the core in which the fuel assembly 1000 of the adjustment fuel in FIG. 10 is loaded. It is the maximum linear power density in the first cycle. The maximum linear power density F of Comparative Example 3 shifts from the ABWR core 22 in which all the fuel assemblies 300 of the long fuel in FIG. 3 are loaded to the core in which the fuel assembly 500 of the adjustment fuel in FIG. 5 is loaded. It is the maximum linear power density in the first cycle. The polygonal line in FIG. 11 is the relative value of the maximum line power density E to the maximum line power density F.

As shown in FIG. 11, according to the third embodiment, the maximum linear power density can be reduced by using the fuel assembly 1000 of the adjusting fuel at the initial stage of combustion when the linear power becomes the highest. At the end of the cycle, when the fuel assembly 1000 for adjusting fuel is used, the maximum linear power density becomes high, but at the end of the cycle, the absolute value of the maximum linear power density becomes small and the thermal margin becomes relatively large. There is no problem because

According to the third embodiment, by changing the axial fuel composition of the adjustment fuel, it is possible to suppress the output increase in the lower part of the core during core transition.

The inventor further focused on the difference in pressure loss between the adjustment fuel and the short fuel. That is, since the adjusting fuel is configured to increase the effective fuel length of the short fuel, the pressure loss of the adjusting fuel is larger than that of the short fuel. When the pressure loss increases, the flow rate of the cooling water distributed in the fuel decreases, which may cause a problem that the limit output ratio, which is another index indicating the thermal margin, decreases. Therefore, it is desirable that the adjustment fuel has a configuration that suppresses an increase in output below the core while correcting the difference in pressure loss with the short fuel. From this point of view, in the fourth embodiment, in addition to the adjustment fuel structure of the first embodiment, the number of fuel rods arranged in the adjustment fuel is reduced.

FIG. 12 shows the structure of the adjustment fuel used in Example 4. The right side of FIG. 12 shows a horizontal cross-sectional view of the fuel assembly 1200 of the adjustment fuel, and the channel box 1201, the fuel rod 1202, the gap water region 1203 and the control rod 1204 have the same configuration as that of FIG. The number of the fuel rods arranged in the fuel assembly 1200 is the square of the flow passage area (the area inside the channel box minus the area occupied by the fuel rods) in order to match the pressure loss with the short fuel. In order to obtain the ratio of the effective length of the adjusting fuel and the short length fuel, the number of fuel rods in FIG. The left diagram of FIG. 12 shows the axial configuration of the adjusting fuel, which has the same active fuel length as the adjusting fuel of FIG.

FIG. 13 is a graph of the maximum line power density showing the relative values of Example 4 and Comparative Example 4. The maximum linear power density G of Example 4 was transferred from the core 22 of the ABWR in which all the fuel assemblies 300 of the long fuel in FIG. 3 are loaded to the core in which the fuel assembly 1200 of the adjustment fuel in FIG. 12 is loaded. In this case, it is the maximum linear power density for each operation cycle. The maximum linear power density H of Comparative Example 4 is the maximum linear power density for each operation cycle when the core 22 loaded with the long fuel is transferred to the core 22 in which the long fuel is replaced with the short fuel. The polygonal line in FIG. 11 is the relative value of the maximum line power density E to the maximum line power density F.

FIG. 13 shows the transition first, second and third cycles in which the output increase in the lower core during transition is remarkable. As shown in FIG. 13, the maximum linear power density of the core becomes small in the transition first cycle to the transition third cycle period in which the core loaded with the long fuel shifts to the short fuel.

According to the fourth embodiment, even if the number of fuel rods of the adjusting fuel is different from that of the short length fuel, it is possible to suppress the increase in the output in the lower core at the time of the core transition as in the first embodiment.

FIG. 14 shows another method of transferring the core. The fuel assembly of the central long fuel is taken out of the three fuel assemblies of the long fuel arranged side by side, and the fuel assembly of the adjustment fuel is loaded at the position where the fuel assembly of the taken long fuel was located. And drive. Next, one of the remaining two fuel assemblies of the long fuel is taken out of the fuel assembly of the long fuel, and the fuel assembly of the short fuel is placed at the position of the fuel assembly of the taken out long fuel. Load and drive. Next, the fuel assembly of the remaining long fuel is taken out, and the fuel assembly of the short fuel is loaded at the position where the fuel assembly of the taken out long fuel was located to operate. Next, the fuel assembly of the adjustment fuel is taken out, and the fuel assembly of the short fuel is loaded and operated at the position where the taken out adjustment fuel assembly was.

As described above, disposing the fuel assembly for adjusting fuel only at a position sandwiched between the fuel assemblies for two long fuels allows for suppression of the thermal margin and efficient replacement of the fuel assembly. Then it is effective.

1...long fuel, 2...adjustment fuel, 3...short fuel, 20...ABWR,
21... Reactor pressure vessel, 22... Reactor core, 23... Core support plate, 24... Upper lattice plate 25... Fuel support fittings, 26... Core shroud, 27... Downcomer,
28... Steam separator, 29... Steam dryer, 30... Shroud head,
31... Internal pump, 32... Control rod guide tube, 33... Control rod drive mechanism, 34... Bottom mirror,
35... Main steam pipe, 36... Water supply pipe,
300... Fuel assembly, 301... Channel box, 302... Fuel rod,
303... Water rod, 304... Moderator, 305... Saturated water,
400... Fuel assembly, 401... Channel box, 402... Fuel rod,
403... saturated water, 404... control rod,
500... Fuel assembly, 501... Channel box, 502... Fuel rod,
503... Saturated water, 504... Control rod 601,... Fuel assembly, 602... Cross control rod,
700... ABWR 1/4 core, 701... Fuel with a stay cycle number of 1,
702... Fuel with stay cycle number 2; 703... Fuel with stay cycle number 3 or more;
1000... Fuel assembly, 1001... Channel box,
1001... Fuel rod, 1003... Saturated water, 1004... Control rod,
1200... Fuel assembly, 1201... Channel box,
1201... Fuel rod, 1203... Saturated water, 1204... Control rod,

Claims (8)

A first operation step of operating in a core including a plurality of first fuel assemblies;
The first fuel assembly is taken out of the core, and a second fuel assembly having a shorter active fuel length than the first fuel assembly is loaded and operated at the position where the first fuel assembly was taken out. 2 operation steps,
The first fuel assembly or the second fuel assembly is taken out from the core, and the active fuel length is longer than that of the second fuel assembly at the position where the first fuel assembly or the second fuel assembly was taken out. And a third operation step of operating by loading a third fuel assembly having a short length. A method of operating a nuclear reactor according to claim 1, wherein
A method for operating a nuclear reactor, wherein the active fuel length of the first fuel assembly is at least twice the active fuel length of the third fuel assembly. A method of operating a nuclear reactor according to claim 1, wherein
A method for operating a nuclear reactor, which starts the third operation step after all of the fuel assemblies included in the core have become the second fuel assembly by performing the second operation step. A method of operating a nuclear reactor according to claim 1, wherein
The central first fuel assembly of the three aligned first fuel assemblies is taken out, and the second fuel assembly is loaded and operated at the position where the taken out first fuel assembly was,
One of the remaining two first fuel assemblies is taken out, and the third fuel assembly is loaded and operated at the position where the taken out first fuel assembly was,
The remaining first fuel assembly is taken out, and the third fuel assembly is loaded and operated at the position where the taken out first fuel assembly was,
A method of operating a nuclear reactor, wherein the second fuel assembly is taken out, and the third fuel assembly is loaded and operated at a position where the taken out second fuel assembly was. A method of operating a nuclear reactor according to claim 1, wherein
After the replacement of the first fuel assembly and the second fuel assembly is carried out twice, and the core is in a state of having one or more first fuel assemblies and two second fuel assemblies. , A method of operating a nuclear reactor for starting replacement of the first fuel assembly and the third fuel assembly. A method of operating a nuclear reactor according to claim 1, wherein
A method for operating a nuclear reactor, wherein the loading amount of the fissile material loaded in the lower portion of the third fuel assembly is smaller than the loading amount of the fissile material loaded in the upper portion of the third fuel assembly. A method of operating a nuclear reactor according to claim 1, wherein
The ratio of the active fuel length of the second fuel assembly to the active fuel length of the third fuel assembly is the square of the channel area in the channel box of the second fuel assembly and the third fuel assembly. The method of operating a nuclear reactor is approximately equal to the ratio of the square of the flow passage area in the channel box. A method of operating a nuclear reactor according to claim 7, wherein
The method for operating a nuclear reactor, wherein the number of fuel rods loaded in the second fuel assembly is smaller than the number of fuel rods loaded in the third fuel assembly.

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