JP7569708B2 - Evaluation method, evaluation device, and program - Google Patents

Description

The present disclosure relates to an evaluation method, an evaluation device, a program, and a fuel rod.

Fuel rods containing gadolinium are used to suppress excess reactions during reactor operation. For example, Patent Document 1 discloses a fuel rod that can equalize thermal output in the axial direction and extend the operating cycle.

JP 2012-154861 A

In Patent Document 1, the content of burnable poison in the fuel rods is increased from both ends in the axial direction toward the center, thereby making the thermal output uniform. However, there are an infinite number of combinations of the content of burnable poison and the region lengths of both ends and the center, so it is not easy to determine a good combination.

The present disclosure aims to provide an evaluation method, evaluation device, and program that can appropriately set the gadolinium concentration in a fuel rod and the region lengths of the upper and lower regions.

The present disclosure also aims to provide a fuel rod in which the gadolinium concentration and the length of the upper and lower regions are appropriately set.

The evaluation method according to one aspect of the present disclosure includes the steps of: setting the length of the upper and lower regions in the axial direction of the fuel region of a fuel rod loaded in a nuclear reactor; setting the gadolinium concentration of the upper and lower regions in the axial direction of the fuel region to be lower than the gadolinium concentration of the central region; evaluating the fuel rod in the event of a main steam line rupture accident; comparing the evaluation of the fuel rod in the event of a main steam line rupture accident with an evaluation of a fuel rod in which the gadolinium concentration of the fuel region is uniform; evaluating the fuel integrity of the fuel rod; comparing the fuel integrity evaluation of the fuel rod with a fuel integrity evaluation of a fuel rod in which the gadolinium concentration of the fuel region is uniform; and confirming whether the evaluation of the fuel rod in the event of a main steam line rupture accident and the result of the fuel integrity evaluation satisfy a limit value that serves as a judgment criterion.

The evaluation device according to one aspect of the present disclosure includes a region setting unit that sets the region lengths of the upper and lower regions in the axial direction of the fuel region of a fuel rod loaded in a nuclear reactor, a concentration setting unit that sets the gadolinium concentration of the upper and lower regions in the axial direction of the fuel region to be lower than the gadolinium concentration of the central region, a first evaluation unit that performs an evaluation of the fuel rod in the event of a main steam line rupture accident, a first comparison unit that compares the evaluation of the fuel rod in the event of a main steam line rupture accident with an evaluation of a fuel rod in which the gadolinium concentration of the fuel region is uniform, a second evaluation unit that performs a fuel integrity evaluation of the fuel rod, a second comparison unit that compares the fuel integrity evaluation of the fuel rod with the fuel integrity evaluation of a fuel rod in which the gadolinium concentration of the fuel region is uniform, and a criterion confirmation unit that confirms whether the evaluation of the fuel rod in the main steam line rupture accident and the result of the fuel integrity evaluation satisfy a limit value that serves as a judgment criterion.

A program according to one aspect of the present disclosure causes a computer to execute the steps of: setting the length of the upper and lower regions in the axial direction of the fuel region of a fuel rod loaded into a nuclear reactor; setting the gadolinium concentration of the upper and lower regions in the axial direction of the fuel region lower than the gadolinium concentration of the central region; evaluating the fuel rod in the event of a main steam line rupture accident; comparing the evaluation of the fuel rod in the event of a main steam line rupture accident with an evaluation of a fuel rod with a uniform gadolinium concentration in the fuel region in the event of a main steam line rupture accident; evaluating the fuel integrity of the fuel rod; comparing the fuel integrity evaluation of the fuel rod with a fuel integrity evaluation of a fuel rod with a uniform gadolinium concentration in the fuel region; and confirming whether the evaluation of the fuel rod in the event of a main steam line rupture accident and the result of the fuel integrity evaluation satisfy a limit value that serves as a judgment criterion.

A fuel rod according to one aspect of the present disclosure is a fuel rod having a central region of a fuel region in the axial direction, and upper and lower regions of the fuel region having a lower gadolinium concentration than the central region, and the results of an evaluation in the event of a main steam line rupture accident and a fuel integrity evaluation are better than the results of an evaluation in the event of a main steam line rupture accident and a fuel integrity evaluation of a fuel rod having a uniform gadolinium concentration in the fuel region, respectively, and the evaluation in the event of a main steam line rupture accident and the results of the fuel integrity evaluation of the fuel rod satisfy the limit values that serve as the judgment criteria.

This disclosure allows for good control of the gadolinium concentration in the fuel rod and the length of the upper and lower regions.

FIG. 1 is a diagram illustrating a fuel assembly according to the first embodiment. FIG. 2 is a cross-sectional view of the fuel assembly according to the first embodiment, cut in a direction perpendicular to the axial direction of the fuel rods. FIG. 3 is a partially cutaway view of a fuel rod according to the first embodiment. FIG. 4 is a diagram for explaining the fuel region of the fuel rod according to the first embodiment. FIG. 5 is a block diagram illustrating an example of the configuration of the evaluation device according to the first embodiment. FIG. 6 is a diagram for explaining the results of a comparison between the DNBR of the first embodiment and the DNBR of the comparative example. FIG. 7 is a diagram for explaining details of the comparison results of DNBR between the first embodiment and the comparative example. FIG. 8 is a diagram for explaining the power distribution in the axial direction of the fuel rod. FIG. 9 is a graph showing local power after a reactor transient occurs. FIG. 10 is a graph showing the difference in local power before and after a reactor transient occurs. FIG. 11 is a graph showing the relative value of the local power at the axial upper part of the fuel rod. FIG. 12 is a graph showing the relative values of the corrosion thickness at the axial upper portion of the fuel rod. FIG. 13 is a table showing the results of comparison between the fuel rod according to the first embodiment and a comparative example. FIG. 14 is a flowchart showing an example of a process flow of the fuel rod evaluation method according to the first embodiment. FIG. 15 is a block diagram showing an example of the configuration of an evaluation device according to the second embodiment. FIG. 16 is a flowchart showing an example of a process flow of the fuel rod evaluation method according to the second embodiment.

Below, an embodiment of the present disclosure will be described in detail with reference to the attached drawings. Note that the present disclosure is not limited to this embodiment, and when there are multiple embodiments, it also includes configurations in which the respective embodiments are combined. In addition, in the following embodiments, the same parts are given the same reference numerals, and duplicated explanations will be omitted.

[First embodiment]
A fuel assembly according to the first embodiment will be described with reference to Figures 1, 2, and 3. Figure 1 is a schematic diagram of a fuel assembly according to the first embodiment. Figure 2 is a cross-sectional view of the fuel assembly according to the first embodiment cut in a direction perpendicular to the axial direction of the fuel rod. Figure 3 is a cutaway view of a part of a fuel rod according to the first embodiment.

The fuel assembly 1 is loaded into a pressurized water reactor (not shown), and the reactor is filled with a moderator. The fuel assembly 1 is composed of 17 x 17 cells 10, and has 264 fuel rods 15, 24 control rod guide tubes into which control rods 60 are inserted, one in-core instrumentation guide tube into which an in-core instrumentation detector 70 is inserted, and a grid 18 that bundles the fuel rods 15, the control rod guide tube, and the in-core instrumentation guide tube. The fuel assembly 1 has an upper nozzle 19 provided on the axial upper side of the fuel rods 15, and a lower nozzle 20 provided on the axial lower side. The upper nozzle 19 and the lower nozzle 20 fix the axial upper and lower ends of the fuel rods 15, the control rod guide tube, and the in-core instrumentation guide tube, respectively.

The fuel rod 15 has a fuel region 30 made up of multiple fuel pellets 25, and a cylindrical cladding tube 26 in which the multiple fuel pellets 25 are loaded. The fuel pellets 25 are made by enriching uranium 235 as nuclear fuel to a predetermined level, sintering it into uranium dioxide, and forming it into pellets. Although uranium 235 is used as the nuclear fuel, this is not limiting, and other fissile materials such as plutonium may also be used. Then, a predetermined number of fuel pellets 25 are filled into the cladding tube 26, and the position of the fuel pellets 25 is restricted by attaching a spring 27 to the top, and end plugs 28 and 29 are attached to the upper and lower ends, respectively, to form the fuel rod 15. The heights of the multiple fuel pellets 25 may be the same or different.

When the fuel assembly 1 is loaded into a pressurized water reactor and the operation of the pressurized water reactor is started, the fuel assembly 1 heats the moderator with the generated thermal energy while the nuclear reaction is controlled by the control rod 60. At this time, the inside of the reactor is pressurized, so the moderator is in a liquid phase over the entire axial length of the fuel assembly 1.

In the later stages of the operation cycle of a pressurized water reactor, the thermal output of the fuel assembly 1 increases at both ends in the axial direction, while the thermal output of the fuel assembly 1 decreases in the center in the axial direction, resulting in a large distortion in the power distribution in the axial direction. As shown in FIG. 2, the fuel rod 15 includes a fuel rod 40 and a fuel rod 50. The fuel rod 40 is a conventional fuel rod that does not contain gadolinium. The fuel rod 50 is a fuel rod that contains gadolinium. In the first embodiment, the fuel rod 50 contains gadolinium distributed in the axial direction and is loaded into the fuel assembly 1.

[Fuel rod]
4 is a diagram for explaining the fuel region of the fuel rod according to the first embodiment. As shown in FIG. 4, the fuel region 30 of the fuel rod 50 has an upper region 31, a lower region 32, and a central region 33 in the axial direction. The axial length L of the fuel region 30 is, for example, 360 cm. The axial length of the upper region 31 is L1. The axial length of the lower region 32 is L2. The gadolinium concentrations of the upper region 31 and the lower region 32 are the same. The gadolinium concentrations of the upper region 31 and the lower region 32 may be different. The gadolinium concentration of the central region 33 is, for example, 10 wt %. The gadolinium concentrations of the upper region 31 and the lower region 32 are different from those of the central region 33. In the first embodiment, the region length L1 of the upper region 31, the region length L2 of the lower region 32, the gadolinium concentration of the upper region 31, and the gadolinium concentration of the lower region 32 are optimally set to uniformize the power across the axial direction of the fuel rod 50.

[Evaluation device]
FIG. 5 is a block diagram showing a configuration example of the evaluation device according to the first embodiment. The evaluation device 100 performs three evaluations, DNBR (Departure from Nuclear Boiling Ratio) evaluation, PCI (Pellet-Clad interaction) evaluation, and corrosion evaluation, in the event of a main steam line rupture accident in a fuel rod. The evaluation device 100 is a device that evaluates the power distribution in the axial direction of a fuel rod. The evaluation device 100 can be realized by a computer. The evaluation device 100 is provided in, for example, a nuclear power plant. The evaluation device 100 may be provided in, for example, a facility different from the nuclear power plant.

As shown in FIG. 5, the evaluation device 100 includes an input unit 110, an output unit 120, a memory unit 130, a communication unit 140, and a control unit 150.

The input unit 110 accepts various inputs to the evaluation device 100. The input unit 110 is realized by an input device such as a keyboard, a mouse, or a touch panel, for example.

The output unit 120 outputs various information including the evaluation results by the evaluation device 100. The output unit 120 is realized by output devices such as a display and a speaker.

The storage unit 130 stores various types of information. The storage unit 130 is realized, for example, by a semiconductor memory element such as a random access memory (RAM) or a flash memory, or a storage device such as a hard disk or an optical disk.

The communication unit 140 transmits and receives information to and from external devices. The communication unit 140 is realized, for example, by a communication module.

The control unit 150 controls the operation of each part of the evaluation device 100. The control unit 150 is realized, for example, by a CPU (Central Processing Unit) or MPU (Micro Processing Unit) executing a program stored in a storage unit (not shown) using a RAM or the like as a working area. The control unit 150 may be realized, for example, by an integrated circuit such as an ASIC (Application Specific Integrated Circuit) or an FPGA (Field Programmable Gate Array). The control unit 150 may be realized by a combination of hardware and software.

The control unit 150 includes an area setting unit 151, a density setting unit 152, a first evaluation unit 153, a first comparison unit 154, a second evaluation unit 155, a second comparison unit 156, and a reference confirmation unit 157.

The region setting unit 151 sets the region lengths of the upper region 31 and the lower region 32 in the fuel region 30. The region setting unit 151 sets the region length L1 of the upper region 31 to a length equal to or greater than the region length L2 of the lower region 32. The region setting unit 151 may set the region lengths of the upper region 31 and the lower region 32 to be an integer multiple of the height of the fuel pellets 25 that make up the fuel region 30, as shown in FIG. 3.

The region setting unit 151, for example, sets the region length L1 of the upper region 31 to 24 cm and the region length L2 of the lower region 32 to 12 cm. The region setting unit 151, for example, sets the region length L1 of the upper region 31 to 24 cm and the region length L2 of the lower region 32 to 24 cm. The region setting unit 151, for example, sets the region length L1 of the upper region 31 to 36 cm and the region length L2 of the lower region 32 to 24 cm. The region setting unit 151, for example, sets the region length L1 of the upper region 31 to 48 cm and the region length L2 of the lower region 32 to 36 cm. The region setting unit 151, for example, sets the region length L1 of the upper region 31 to 60 cm and the region length L2 of the lower region 32 to 36 cm. The region setting unit 151, for example, sets the region length L1 of the upper region 31 to 72 cm and the region length L2 of the lower region 32 to 36 cm. For example, the region setting unit 151 sets the region length L1 of the upper region 31 to 84 cm and the region length L2 of the lower region 32 to 36 cm. For example, the region setting unit 151 sets the region length L1 of the upper region 31 to 96 cm and the region length L2 of the lower region 32 to 60 cm. The region setting unit 151 may adjust the region length L1 of the upper region 31 to another value as long as it is set to be equal to or greater than the region length L2 of the lower region 32.

The concentration setting unit 152 sets the gadolinium concentration of the upper region 31 and the lower region 32 in the fuel region 30. The concentration setting unit 152 sets the gadolinium concentration of the upper region 31 and the lower region 32 to the same concentration. For example, the concentration setting unit 152 sets the gadolinium concentration of the upper region 31 and the lower region 32 to 2 wt%. For example, the concentration setting unit 152 sets the gadolinium concentration of the upper region 31 and the lower region 32 to 4 wt%. For example, the concentration setting unit 152 sets the gadolinium concentration of the upper region 31 and the lower region 32 to 6 wt%. For example, the concentration setting unit 152 sets the gadolinium concentration of the upper region 31 and the lower region 32 to 8 wt%. For example, when the gadolinium concentration in the central region 33 is 10 wt%, the concentration setting unit 152 may set the gadolinium concentrations in the upper region 31 and the lower region 32 to 1 wt%, 3 wt%, 5 wt%, 7 wt%, and 9 wt%. In other words, the concentration setting unit 152 may arbitrarily set the gadolinium concentrations in the upper region 31 and the lower region 32 as long as they are lower than the gadolinium concentration in the central region 33.

The first evaluation unit 153 performs an evaluation at the time of main steam pipe rupture of the fuel rod 50. Specifically, the first evaluation unit 153 evaluates the DNBR at the time of main steam pipe rupture in the fuel rod 50. DNBR stands for departure from nucleate boiling ratio, and is a parameter expressed as a ratio value related to the heat flux at which a sudden transition occurs from nucleate boiling, which has good heat transfer efficiency, to film boiling, in which the heating surface is covered with a steam film, that is, the departure from nucleate boiling. Specifically, DNBR is expressed as the ratio between the critical heat flux and the actual heat flux.

The first evaluation unit 153 evaluates the DNBR for each combination of the region lengths of the upper region 31 and lower region 32 set by the region setting unit 151 in the fuel region 30 of the fuel rod 50 and the gadolinium concentrations of the upper region 31 and lower region 32 set by the concentration setting unit 152. The first evaluation unit 153 evaluates the DNBR at the time of main steam pipe rupture in a long cycle of, for example, 13 months or more. The first evaluation unit 153 evaluates the DNBR at the time of main steam pipe rupture in cycles of, for example, about 15 months, about 17 months, and about 19 months.

The first comparison unit 154 compares the evaluation results of the DNBR. Specifically, the first comparison unit 154 compares the evaluation results of the DNBR of a fuel rod (hereinafter referred to as the comparative example) in which the gadolinium concentration in the fuel region is uniform in the axial direction (for example, uniformly 10 wt%) with the evaluation results of the DNBR of the fuel rod 50 by the first evaluation unit 153.

Figure 6 is a diagram for explaining the comparison results of DNBR between the first embodiment and the comparative example. In the evaluation data 81, "region length of upper region and lower region" indicates the respective region lengths of the upper region 31 and the lower region 32 in the fuel region 30. "Gadolinium concentration" indicates the gadolinium concentration of the upper region 31 and the lower region 32. The evaluation results are indicated as "improved," "worsened," or "no change." "Improved" indicates that the evaluation results of the DNBR of the fuel rod 50 have improved compared to the comparative example. "Worsened" indicates that the evaluation results of the DNBR of the fuel rod 50 have deteriorated compared to the comparative example. "No change" indicates that there is no difference in the evaluation results of the DNBR of the fuel rod 50 compared to the comparative example.

For example, the DNBR evaluation result of a fuel rod 50 with an upper region 31 length of 24 cm, a lower region 32 length of 12 cm, and a gadolinium concentration of 2 wt% in the upper region 31 and lower region 32 is improved compared to the comparative example. For example, the DNBR evaluation result of a fuel rod 50 with an upper region 31 length of 24 cm, a lower region 32 length of 12 cm, and a gadolinium concentration of 4 wt% in the upper region 31 and lower region 32 is unchanged compared to the comparative example. For example, the DNBR evaluation result of a fuel rod 50 with an upper region 31 length of 48 cm, a lower region 32 length of 36 cm, and a gadolinium concentration of 2 wt% in the upper region 31 and lower region 32 is worsened.

Figure 7 is a diagram for explaining the details of the comparison results of DNBR between the first embodiment and the comparative example. Detailed data 82 is data evaluating DNBR at the time of main steam pipe rupture in cycles of approximately 15 months, approximately 17 months, and 19 months. The fuel region 30 of the fuel rod 50 of the first embodiment shown in Figure 7 has a region length of 36 cm for the upper region 31, a region length of 24 cm for the lower region 32, and a gadolinium concentration of 2 wt% for the upper region 31 and the lower region 32.

As shown in Figure 7, the DNBR at the time of main steam pipe rupture in a cycle of approximately 15 months is 1.40 for the comparative example and 1.50 for the first embodiment. The DNBR in a cycle of approximately 17 months is 1.34 for the comparative example and 1.54 for the first embodiment. The DNBR in a cycle of approximately 19 months is 1.44 for the comparative example and 1.66 for the first embodiment.

That is, in the first embodiment, the DNBR is improved by about 0.1 to 0.2 compared to the comparative example. In the first embodiment, the upper region 31 and the lower region 32 are provided in the fuel region 30, and the gadolinium concentration in the upper region 31 and the lower region 32 is made lower than that in the central region 33, thereby mitigating the DNBR.

Returning to FIG. 5, the second evaluation unit 155 performs a fuel integrity evaluation of the fuel rod 50. The second evaluation unit 155 performs, for example, a PCI evaluation of the fuel rod 50 and a corrosion evaluation of the fuel rod 50.

The second evaluation unit 155 performs PCI evaluation and corrosion evaluation for each combination of the region length of the upper region 31 and the lower region 32 set by the region setting unit 151 in the fuel region 30 of the fuel rod 50 and the gadolinium concentration of the upper region 31 and the lower region 32 set by the concentration setting unit 152. Specifically, the second evaluation unit 155 may perform PCI evaluation and corrosion evaluation only for the conditions of the fuel region 30 where the DNBR evaluation result was better than the comparative example, among the comparison results of the first comparison unit 154.

The second comparison unit 156 compares the evaluation results of the PCI evaluation and the corrosion evaluation. Specifically, the second comparison unit 156 compares the evaluation results of the PCI evaluation and the corrosion evaluation of the comparative example with the evaluation results of the PCI evaluation and the corrosion evaluation of the fuel rod 50 by the second evaluation unit 155. From the comparison results, the second comparison unit 156 extracts the conditions of the fuel region 30 that have better evaluation results of the PCI evaluation and the corrosion evaluation than the comparative example.

Before explaining the PCI evaluation, the power distribution in the axial direction of the fuel rod will be explained. Figure 8 is a diagram for explaining the power distribution in the axial direction of the fuel rod. Figure 8 shows the power distribution in the axial direction of the fuel rod at the beginning of the cycle, the middle of the cycle, and the end of the cycle. The horizontal axis of Figure 8 indicates the relative power, and the vertical axis indicates the axial position. The comparative example is a fuel rod in which the gadolinium concentration in the fuel region is uniformly 10 wt% in the axial direction. The first condition is a fuel rod 50 in which, in the fuel region 30, the region length L1 of the upper region 31 is 36 cm, the region length L2 of the lower region 32 is 24 cm, and the gadolinium concentration in the upper region 31 and the lower region 32 is 2 wt%. The second condition is that, in the fuel region 30, the region length L1 of the upper region 31 is 72 cm, the region length L2 of the lower region 32 is 36 cm, and the gadolinium concentration in the upper region 31 and the lower region 32 is 6 wt%.

8, the horizontal axis indicates the relative output, and the vertical axis indicates the axial height (cm). As shown in FIG. 8, in the early stage of the cycle, in the comparative example, the first condition, and the second condition, the output in the central part in the axial direction is the highest, and the output decreases toward the upper and lower ends. In the middle stage of the cycle, in the first and second conditions, the upper region 31 with a low gadolinium concentration burns out, so the output in the upper region 31 increases and the output distribution becomes distorted. Specifically, in the second condition, the output in the upper region 31 is greater than in the first condition, and the distortion is also greater. In the final stage of the cycle, in the first and second conditions, the output distortion is alleviated, and the output distribution in the central region 33 becomes flat. In the final stage of the cycle, the output distribution in the comparative example is such that the output is greater at the upper and lower parts and weaker at the center.

The PCI evaluation according to the first embodiment will be described with reference to FIG. 9 and FIG. 10. FIG. 9 is a graph showing the local power after a transient change occurs in the reactor. FIG. 10 is a graph showing the difference in local power before and after a transient change occurs in the reactor. In FIG. 9, the horizontal axis shows the local burnup (GWd/t), and the vertical axis shows the local power (P) (KW/m). In FIG. 10, the horizontal axis shows the local burnup (GWd/t), and the vertical axis shows the difference in local power (ΔP) (KW/m) before and after the transient change occurs. In FIG. 9, the "white circle (◯)" shows the local power of the fuel rod of the comparative example, and the "white triangle (△)" shows the local power of the fuel rod of the second condition. In FIG. 10, the white circle shows the difference in local power before and after the transient change occurs in the fuel rod of the comparative example, and the white triangle shows the difference in local power before and after the transient change occurs in the fuel rod of the second condition. FIG. 9 and FIG. 10 show the local power when the cycle length is 19 months.

Figure 9 shows the current threshold line TL1 and the new threshold line TL2. Figure 10 shows the current threshold line TL3 and the new threshold line TL4. Under the current standard, if the local power exceeds both the current threshold line TL1 and the current threshold line TL3 at the same local burnup, there is a possibility that the fuel rod will undergo PCI failure. Under the new standard, if the local power exceeds both the new threshold line TL2 and the new threshold line TL4 at the same local burnup, there is a possibility that the fuel rod will undergo PCI failure. The current threshold line shows the threshold when the cycle length is the current 13 months, and the new threshold line shows the threshold when the cycle length is 19 months.

As shown in Figure 9, in the comparative example, when the local burnup is P, the local power exceeds both the current threshold line TL1 and the new threshold line TL2. As shown in Figure 10, in the comparative example, when the local burnup is P, the local power is slightly below the new threshold line TL4 but exceeds the current threshold line TL3.

As shown in FIG. 9, under the second condition, the local power when the local burnup is P exceeds both the current threshold line TL1 and the new threshold line TL2. As shown in FIG. 10, under the second condition, the local power when the local burnup is P falls below both the current threshold line TL3 and the new threshold line TL4. In other words, the second condition has an improved PCI evaluation result compared to the comparative example. In this case, the second comparison unit 156 determines that the second condition has a better PCI evaluation result than the comparative example. In other words, the second comparison unit 156 extracts the second condition as a condition for a fuel rod that has a better PCI evaluation result than the comparative example.

The corrosion evaluation according to the first embodiment will be explained using Figures 11 and 12. Figure 11 is a graph showing the relative value of the local power at the upper axial portion of the fuel rod. Figure 12 is a graph showing the relative value of the corrosion thickness at the upper axial portion of the fuel rod.

11 and 12, the horizontal axis indicates time, and the vertical axis indicates the relative ratio of corrosion thickness. In FIG. 11 and FIG. 12, the corrosion thickness of the comparative example is relativized to 1. As shown in FIG. 11, the local power output is lower at the end of the cycle in the first and second conditions than in the comparative example. As shown in FIG. 12, the relative value of the corrosion thickness is lower at the end of the cycle. That is, the amount of metal loss is reduced. For example, the amount of metal loss at a cycle length of 19 months is 9.4% in the comparative example, 8.9% in the first condition, and 9.2% in the second condition. That is, in the first embodiment, the amount of metal loss due to corrosion of the fuel rod can be reduced by reducing the local power in the axial direction of the fuel rod at the end of the cycle.

Returning to Figure 5, the criteria confirmation unit 157 confirms whether the results of the evaluation at the time of the main steam line rupture accident and the evaluation results of the fuel rod fuel integrity evaluation satisfy the limit values that serve as the predetermined criteria for judgment. The predetermined criteria for judgment is whether or not the criteria for safe use of the fuel rod are satisfied.

Figure 13 is a table showing the results of a comparison between the fuel rod according to the first embodiment and the comparative example. In Figure 13, "Improved" indicates that both the evaluation of the fuel rod at the time of the main steam pipe rupture accident and the fuel integrity evaluation of the fuel rod are better than the comparative example. "No change" indicates that both the evaluation of the fuel rod at the time of the main steam pipe rupture accident and the fuel integrity evaluation of the fuel rod are no different from the comparative example. "Deterioration" indicates that at least one of the evaluation of the fuel rod at the time of the main steam pipe rupture accident and the fuel integrity evaluation of the fuel rod is worse than the comparative example.

As shown in FIG. 13, the evaluation device 100 performs a comprehensive evaluation of the fuel rod at the time of a main steam pipe rupture accident and a fuel integrity evaluation of the fuel rod for each combination of the region lengths of the upper region 31 and the lower region 32 and the gadolinium concentrations of the upper region 31 and the lower region 32. The evaluation device 100 comprehensively performs an evaluation of the fuel rod at the time of a main steam pipe rupture accident and an evaluation of the fuel integrity of the fuel rod, thereby extracting the conditions of the fuel rod that are better than the comparative example. In addition, the evaluation device 100 can extract fuel rods that are better than the comparative example in terms of the evaluation at the time of a main steam pipe rupture accident and the evaluation of the fuel integrity of the fuel rod satisfy the limit value that is a predetermined judgment criterion, among the fuel rods that are better than the comparative example.

In this way, the first embodiment can obtain a fuel rod 50 having a central region 33 in the axial direction of the fuel region 30, and an upper region 31 and a lower region 32 having a lower gadolinium concentration than the central region 33. The evaluation results of the fuel rod 50 of the first embodiment in the event of a main steam line rupture accident and in the fuel integrity evaluation are better than the evaluation results of the fuel rod in which the gadolinium concentration in the fuel region is uniform in the event of a main steam line rupture accident and in the fuel integrity evaluation. The evaluation results of the fuel rod 50 of the first embodiment in the event of a main steam line accident and in the fuel integrity evaluation satisfy the predetermined judgment criteria.

[Evaluation method]
The fuel rod evaluation method according to the first embodiment will be described with reference to Fig. 14. Fig. 14 is a flow chart showing an example of the process flow of the fuel rod evaluation method according to the first embodiment.

First, the region setting unit 151 sets the region lengths of the upper region 31 and the lower region 32 in the fuel region 30 of the fuel rod 50 (step S10). Next, the concentration setting unit 152 sets the gadolinium concentration in the upper region 31 and the lower region 32 (step S11). Next, the first evaluation unit 153 performs an evaluation in the event of a main steam line rupture accident under the conditions set in steps S10 and S11 (step S12). Specifically, the first evaluation unit 153 performs a DNBR evaluation.

The first comparison unit 154 determines whether the evaluation result at the time of the main steam pipe rupture accident is good or not (step S13). Specifically, the first comparison unit 154 determines whether the evaluation result at the time of the main steam pipe rupture accident is improved compared to the evaluation result of the comparative example. If it is determined that the evaluation result is good (step S13; Yes), proceed to step S14. If it is determined that the evaluation result is not good (step S13; No), proceed to step S16.

If step S13 is judged as Yes, the second evaluation unit 155 performs a fuel integrity evaluation (step S14). Specifically, the second evaluation unit 155 performs a PCI evaluation and a corrosion evaluation for the fuel rods 50 whose evaluation results at the time of the main steam pipe rupture accident were good.

The second comparison unit 156 determines whether the evaluation result of the fuel soundness evaluation is good (step S15). Specifically, the second comparison unit 156 determines whether the evaluation result of the fuel soundness evaluation is improved compared to the evaluation result of the comparative example. If it is determined that the evaluation result is good (step S15; Yes), proceed to step S17. If it is determined that the evaluation result is not good (step S15; No), proceed to step S19.

If step S13 returns No, the first comparison unit 154 outputs an evaluation result indicating that the characteristics have deteriorated compared to the comparative example (step S16). Then, the process proceeds to step S20.

If step S15 returns Yes, the second comparison unit 156 outputs an evaluation result indicating that the characteristics have been improved compared to the comparative example (step S17). Then, the process proceeds to step S18.

The criteria confirmation unit 157 checks whether the evaluation result satisfies the limit value (step S18). Specifically, the criteria confirmation unit 157 checks whether the evaluation in the main steam line rupture accident and the result of the fuel integrity evaluation satisfy the limit value that serves as the judgment criterion. Then, the process proceeds to step S20.

If step S15 returns No, the second comparison unit 156 outputs a comparison result indicating that the characteristics have deteriorated compared to the comparative example or that the characteristics have not changed (step S19). Then, the process proceeds to step S20.

The control unit 150 determines whether or not to end the processing of the evaluation method (step S20). Specifically, the control unit 150 determines to end the processing of the evaluation method when it receives an operation to end the processing of the evaluation method or when it has evaluated all combinations of conditions of the region length and concentration of the upper region 31 and the lower region 32. If it is determined to end the processing of the evaluation method (step S20; Yes), it ends the processing of FIG. 14. If it is determined not to end the processing of the evaluation method (step S20; No), it proceeds to step S10.

[Second embodiment]
Next, a second embodiment of the present disclosure will be described. In the second embodiment, an analysis of operability is performed on a fuel rod in a condition in which the evaluation in the main steam line rupture accident and the fuel integrity evaluation are improved compared to the comparative example in the first embodiment. In this way, the second embodiment extracts a condition for a fuel rod in which the operability is improved.

Specifically, when a disturbance occurs due to the insertion of a control rod into the reactor, a phenomenon called xenon oscillation can occur, in which the axial power distribution in the fuel rod 50 fluctuates. In the second embodiment, among the fuel rods with improved evaluations in the main steam line rupture accident and fuel integrity evaluations compared to the comparative example, the conditions for fuel rods that further suppress the occurrence of xenon oscillation are extracted.

An example of the configuration of the evaluation device according to the second embodiment will be described with reference to FIG. 15. FIG. 15 is a block diagram showing an example of the configuration of the evaluation device according to the second embodiment.

As shown in FIG. 15, the evaluation device 100A differs from the evaluation device 100 shown in FIG. 5 in that the control unit 150A includes a vibration analysis unit 158 and a third comparison unit 159.

The vibration analysis unit 158 evaluates the operability of the fuel rod 50. The vibration analysis unit 158 analyzes xenon vibration as an evaluation of operability. For example, the vibration analysis unit 158 analyzes xenon vibration for a fuel rod 50 in which the evaluation in a main steam line rupture accident and the fuel integrity evaluation are improved compared to the comparative example. The vibration analysis unit 158 analyzes xenon vibration using, for example, a two-dimensional multi-group fuel assembly transport calculation code and a three-dimensional two-group diffusion calculation code. An example of a two-dimensional multi-group fuel assembly transport calculation code is PHOENIX-P, but is not limited to this. An example of a three-dimensional two-group diffusion calculation code is ANC, but is not limited to this.

The third comparison unit 159 compares the analysis results of xenon oscillation. Specifically, the third comparison unit 159 compares the analysis results of xenon oscillation for the fuel rod according to the comparative example with the analysis results of xenon oscillation for the fuel rod 50 under conditions in which the evaluation in the main steam line rupture accident and the fuel integrity evaluation are improved compared to the comparative example. The third comparison unit 159 extracts the conditions for the fuel rod 50 under which the dissipation of xenon oscillation is improved compared to the comparative example. In other words, the third comparison unit 159 extracts the conditions for the fuel rod 50 under which xenon oscillation is suppressed compared to the comparative example.

[Evaluation method]
The fuel rod evaluation method according to the second embodiment will be described with reference to Fig. 16. Fig. 16 is a flow chart showing an example of the process flow of the fuel rod evaluation method according to the second embodiment.

The processing from step S30 to step S38 is the same as the processing from step S10 to step S18 shown in FIG. 14, so the description will be omitted.

After step S38, the control unit 150A performs an analysis of xenon vibration (step S39). Specifically, the vibration analysis unit 158 analyzes xenon vibration for fuel rods 50 in which the evaluation in the main steam line rupture accident and the fuel integrity evaluation are improved compared to the comparative example. Then, the process proceeds to step S40.

The control unit 150A determines whether the dispersibility of xenon oscillation has improved (step S40). Specifically, the third comparison unit 159 determines whether the dispersibility of xenon oscillation has improved by comparing the analysis results of the xenon oscillation of the fuel rod related to the comparative example with the analysis results of step S39. If it is determined that the dispersibility of xenon oscillation has improved (step S40; Yes), proceed to step S41. If it is not determined that the dispersibility of xenon oscillation has improved (step S40; No), proceed to step S43.

If step S40 returns Yes, the control unit 150A outputs the conditions of the fuel rod with improved xenon oscillation dispersion (step S41). Specifically, the third comparison unit 159 outputs the region lengths of the upper region 31 and lower region 32 of the fuel rod 50 with improved xenon oscillation dispersion compared to the comparative example, and the conditions of the gadolinium concentration. In other words, the third comparison unit 159 extracts the conditions of the fuel rod 50 with improved operability compared to the comparative example. Then, the process proceeds to step S43.

The processing in steps S42 and S43 is the same as the processing in steps S19 and S20 shown in FIG. 14, so the description will be omitted.

As shown in FIG. 16, in the second embodiment, a xenon analysis is performed on a fuel rod 50 in which the evaluation in the event of a main steam line rupture accident and the fuel integrity evaluation are improved compared to the comparative example, to evaluate the operability. As a result, in the second embodiment, it is possible to extract conditions for a fuel rod 50 in which the evaluation in the event of a steam line rupture accident and the fuel integrity evaluation are improved, and the operability is improved, compared to the comparative example.

The evaluation method, evaluation device, fuel rod, and program described in each embodiment can be understood, for example, as follows.

The evaluation method of the first aspect includes the steps of setting the region lengths of the upper region 31 and the lower region 32 in the axial direction of the fuel region 30 of the fuel rod 50 loaded in the reactor, setting the gadolinium concentration of the upper region 31 and the lower region 32 in the axial direction of the fuel region 30 lower than the gadolinium concentration of the central region 33, evaluating the fuel rod 50 in the event of a main steam line rupture accident, comparing the evaluation of the fuel rod 50 in the event of a main steam line rupture accident with an evaluation of a fuel rod with a uniform gadolinium concentration in the fuel region in the event of a main steam line rupture accident, evaluating the fuel integrity of the fuel rod 50, comparing the fuel integrity evaluation of the fuel rod 50 with the fuel integrity evaluation of a fuel rod with a uniform gadolinium concentration in the fuel region, and confirming whether the evaluation of the fuel rod 50 in the event of a main steam line rupture accident and the result of the fuel integrity evaluation satisfy the limit value that serves as a judgment criterion.

With this configuration, the region lengths and gadolinium concentrations of the upper region 31 and lower region 32 of the fuel rod 50 can be set, and evaluations in the event of a main steam line rupture accident and fuel integrity evaluations can be performed. This makes it possible to compare the results of each evaluation of the fuel rod 50 with the evaluation results of a fuel rod in which the gadolinium concentration of the fuel region is uniform in the axial direction, so that the region lengths and gadolinium concentrations of the upper region 31 and lower region 32 can be set appropriately.

The second aspect of the evaluation method includes a step of evaluating the fuel integrity of the fuel rod 50 and a step of comparing the fuel integrity evaluation of the fuel rod 50 with the fuel integrity evaluation of a fuel rod with a uniform gadolinium concentration in the fuel region, when it is determined that the evaluation of the fuel rod 50 at the time of the main steam pipe rupture accident is better in the step of comparing the evaluation of the fuel rod 50 at the time of the main steam pipe rupture accident with the evaluation of the fuel rod with a uniform gadolinium concentration in the fuel region at the time of the main steam pipe rupture accident. This allows the fuel integrity evaluation to be performed only on fuel rods 50 with good evaluation results at the time of the main steam pipe rupture accident, making the evaluation more efficient.

In the third aspect of the evaluation method, in the step of evaluating the fuel rod 50 in the event of a main steam pipe rupture accident, a DNBR evaluation is performed in the event of a main steam pipe rupture accident, and in the step of evaluating the fuel integrity of the fuel rod 50, a PCI evaluation and a corrosion evaluation are performed on the fuel rod 50. This allows specific evaluations such as a DNBR evaluation, a PCI evaluation, and a corrosion evaluation to be performed.

In the fourth aspect of the evaluation method, in the step of setting the region lengths of the upper region 31 and the lower region 32, the region length of the upper region 31 is set to be equal to or greater than the region length of the lower region 32. This allows evaluation to be performed under various conditions in which the region length of the upper region 31 is set to be equal to or greater than the region length of the lower region 32.

In the fifth aspect of the evaluation method, the gadolinium concentration in the upper region 31 is the same as the gadolinium concentration in the lower region 32. This allows for efficient evaluation by making the gadolinium concentration in the upper region 31 and the gadolinium concentration in the lower region 32 the same.

In the sixth aspect of the evaluation method, in the step of setting the region lengths of the upper region 31 and the lower region 32, the region lengths of the upper region 31 and the lower region 32 are set in proportion to the height of the fuel pellets. This makes it possible to adjust the region lengths of the upper region 31 and the lower region 32 according to the height of the fuel pellets, making it easier to set the region lengths of the upper region 31 and the lower region 32 when manufacturing the fuel rod 50.

The seventh aspect of the evaluation method further includes a step of performing a xenon vibration analysis on a fuel rod 50 for which the evaluation in the main steam line rupture accident and the results of the fuel integrity evaluation satisfy the limit values that serve as the judgment criteria, and a step of comparing the results of the xenon vibration analysis of the fuel rod 50 and a fuel rod in which the gadolinium concentration in the fuel region is uniform. This makes it possible to extract conditions for a fuel rod 50 that improves operability from among the fuel rods 50 for which the evaluation in the main steam line rupture accident and the results of the fuel integrity evaluation satisfy the limit values that serve as the judgment criteria.

The evaluation device of the eighth aspect includes a region setting unit 151 that sets the region lengths of the upper region 31 and the lower region 32 in the axial direction of the fuel region 30 of the fuel rod 50 loaded in the reactor, a concentration setting unit 152 that sets the gadolinium concentration of the upper region 31 and the lower region 32 in the axial direction of the fuel region 30 to be lower than the gadolinium concentration of the central region 33, a first evaluation unit 153 that performs an evaluation of the fuel rod 50 in the event of a main steam pipe rupture accident, and a second evaluation unit 154 that performs an evaluation of the fuel rod 50 in the event of a main steam pipe rupture accident and a second evaluation unit 155 that performs an evaluation of the fuel rod 50 in the event of a main steam pipe rupture accident and a second evaluation unit 156 that performs an evaluation of the fuel rod 50 in the event of a main steam pipe rupture accident. The system includes a first comparison unit 154 that compares the evaluation of the fuel rod 50 with the evaluation of a fuel rod with a uniform gadolinium concentration in the fuel region at the time of a main steam line rupture accident, a second evaluation unit 155 that performs a fuel integrity evaluation of the fuel rod 50, a second comparison unit 156 that compares the fuel integrity evaluation of the fuel rod 50 with the fuel integrity evaluation of a fuel rod with a uniform gadolinium concentration in the fuel region, and a criterion confirmation unit 157 that confirms whether the evaluation of the fuel rod 50 at the time of a main steam line rupture accident and the result of the fuel integrity evaluation satisfy the limit value that serves as the judgment criterion.

The program of the ninth aspect causes a computer to execute the steps of: setting the length of the upper region 31 and the lower region 32 in the axial direction of the fuel region 30 of a fuel rod 50 loaded in a nuclear reactor; setting the gadolinium concentration of the upper region 31 and the lower region 32 in the axial direction of the fuel region 30 to be lower than the gadolinium concentration of the central region 33; evaluating the fuel rod 50 in the event of a main steam line rupture accident; comparing the evaluation of the fuel rod 50 in the event of a main steam line rupture accident with an evaluation of a fuel rod with a uniform gadolinium concentration in the fuel region in the event of a main steam line rupture accident; evaluating the fuel integrity of the fuel rod 50; comparing the fuel integrity evaluation of the fuel rod 50 with the fuel integrity evaluation of a fuel rod with a uniform gadolinium concentration in the fuel region; and confirming whether the evaluation of the fuel rod 50 in the event of a main steam line rupture accident and the result of the fuel integrity evaluation satisfy the limit value that serves as a judgment criterion.

The fuel rod of the tenth aspect is a fuel rod 50 having a central region 33 of the fuel region 30 in the axial direction, and an upper region 31 and a lower region 32 of the fuel region 30 having a lower gadolinium concentration than the central region 33, and the evaluation results of the main steam line rupture accident and the fuel integrity evaluation are better than the evaluation results of the main steam line rupture accident and the fuel integrity evaluation of a fuel rod having a uniform gadolinium concentration in the fuel region, respectively, and the evaluation results of the main steam line rupture accident and the fuel integrity evaluation of the fuel rod satisfy the limit values that serve as the judgment criteria.

Although the embodiments of the present disclosure have been described above, the embodiments are not limited to the contents of these embodiments. The above-described components include those that a person skilled in the art can easily imagine, those that are substantially the same, and those that are within the so-called equivalent range. Furthermore, the above-described components can be combined as appropriate. Furthermore, various omissions, substitutions, or modifications of the components can be made without departing from the spirit of the above-described embodiments.

REFERENCE SIGNS LIST 1 fuel assembly 10 cell 15 fuel rod 18 grid 19 upper nozzle 20 lower nozzle 25 fuel pellet 26 cladding tube 27 spring 30 fuel region 31 upper region 32 lower region 33 central region 40, 50 fuel rod 60 control rod 70 in-core instrumentation detector 100 evaluation device 110 input section 120 output section 130 memory section 140 communication section 150 control section 151 region setting section 152 concentration setting section 153 first evaluation section 154 first comparison section 155 second evaluation section 156 second comparison section 157 reference confirmation section 158 vibration analysis section 159 third comparison section

Claims (9)

Setting the length of an upper region and a lower region in the axial direction of a fuel region of a fuel rod to be loaded into a nuclear reactor;
setting a gadolinium concentration in an axial upper region and a axial lower region of the fissile zone lower than a gadolinium concentration in a central region;
Evaluating the fuel rod during a main steam line rupture accident;
A step of comparing an evaluation of the fuel rod in a main steam line rupture accident with an evaluation of a fuel rod having a uniform gadolinium concentration in a fuel region in a main steam line rupture accident;
performing a fuel integrity assessment of the fuel rod;
comparing a fuel integrity assessment of the fuel rod with a fuel integrity assessment of a fuel rod having a uniform gadolinium concentration in the fuel region;
A step of confirming whether the evaluation of the fuel rod in the main steam line rupture accident and the result of the fuel integrity evaluation satisfy the limit value that is a judgment criterion;
An evaluation method that iteratively performs each step, including: performing a fuel integrity assessment of the fuel rod and comparing the fuel integrity assessment of the fuel rod with a fuel integrity assessment of a fuel rod having a uniform gadolinium concentration in a fuel region;
In the step of comparing the evaluation of the fuel rod at the time of a main steam line rupture accident with the evaluation of a fuel rod having a uniform gadolinium concentration in the fuel region at the time of a main steam line rupture accident, the step is performed when it is determined that the evaluation of the fuel rod at the time of a main steam line rupture accident is better than the evaluation of a fuel rod having a uniform gadolinium concentration in the fuel region.
The evaluation method according to claim 1 . In the step of performing an evaluation in the event of a main steam pipe rupture accident of the fuel rod, a DNBR evaluation in the event of a main steam pipe rupture accident of the fuel rod is performed,
In the step of evaluating the fuel integrity of the fuel rod, a PCI evaluation and a corrosion evaluation are performed on the fuel rod.
The evaluation method according to claim 1 or 2. In the step of setting the region lengths of the upper region and the lower region, the region length of the upper region is set to be equal to or longer than the region length of the lower region.
The evaluation method according to any one of claims 1 to 3. the gadolinium concentration in the upper region is the same as the gadolinium concentration in the lower region;
The evaluation method according to any one of claims 1 to 4. In the step of setting the region lengths of the upper region and the lower region, the region lengths of the upper region and the lower region are set in proportion to the height of the fuel pellets, respectively.
The evaluation method according to any one of claims 1 to 5. A step of performing a xenon vibration analysis on the fuel rods for which the evaluation in the main steam line break accident and the result of the fuel integrity evaluation satisfy the limit value that is a judgment criterion;
and comparing the results of a xenon vibration analysis of a fuel rod having a uniform gadolinium concentration in the fuel rod and fuel region.
The evaluation method according to any one of claims 1 to 6. A region setting unit that sets the region lengths of the upper region and the lower region in the axial direction of the fuel region of the fuel rod loaded in the nuclear reactor;
a concentration setting unit that sets the gadolinium concentration in an upper region and a lower region in an axial direction of the fission zone to be lower than the gadolinium concentration in a central region;
A first evaluation unit that performs an evaluation in the event of a main steam pipe rupture accident of the fuel rod;
A first comparison unit that compares an evaluation of the fuel rod in the event of a main steam line rupture accident with an evaluation of a fuel rod having a uniform gadolinium concentration in a fuel region in the event of a main steam line rupture accident;
A second evaluation unit that performs a fuel integrity evaluation of the fuel rod;
a second comparison unit for comparing a fuel integrity assessment of the fuel rod with a fuel integrity assessment of a fuel rod having a uniform gadolinium concentration in a fuel region;
A criterion determination unit that confirms whether the evaluation of the fuel rod in the main steam line rupture accident and the result of the fuel integrity evaluation satisfy a limit value that is a judgment criterion;
Equipped with
an evaluation device, wherein the area setting section, the density setting section, the first evaluation section, the first comparison section, the second evaluation section, the second comparison section, and the reference determination section execute processes iteratively; Setting the length of an upper region and a lower region in the axial direction of a fuel region of a fuel rod to be loaded into a nuclear reactor;
setting a gadolinium concentration in an axial upper region and a axial lower region of the fissile zone lower than a gadolinium concentration in a central region;
Evaluating the fuel rod during a main steam line rupture accident;
A step of comparing an evaluation of the fuel rod in a main steam line rupture accident with an evaluation of a fuel rod having a uniform gadolinium concentration in a fuel region in a main steam line rupture accident;
performing a fuel integrity assessment of the fuel rod;
comparing a fuel integrity assessment of the fuel rod with a fuel integrity assessment of a fuel rod having a uniform gadolinium concentration in the fuel region;
A step of confirming whether the evaluation of the fuel rod in the main steam line rupture accident and the result of the fuel integrity evaluation satisfy the limit value that is a judgment criterion;
A program for causing a computer to repeatedly execute each of the steps including

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