JPH0680439B2 - Reactor start-up operation method - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a start-up operation method for a nuclear reactor.

[Technical background of the invention]

In general, in thermal neutron reactors such as boiling water reactors, the neutron absorption capacity of the fission product or the xenon generated as its daughter nucleus is large, so at the start-up when no xenon is present and when the xenon reaches saturated rated output. The reactivity effect contributed by Zenon differs from that during operation by about 2.5% ΔK / K, and the Zenon concentration plays a very important role in planning a reactor start-up or restart plan.

For example, at the start of reactor startup, if the control rod is withdrawn with almost no Zenon, the power output before pulling the control rod out of the core up to the control rod pattern required to achieve the rated power level. To rise significantly. Then, when the output rises in the stage where the control rod withdrawal amount is small in this way, heat is concentrated in the fuel assemblies near the withdrawn control rods because the proportion of the withdrawn control rods is small, and in this portion. A local overpower area occurs.

In addition, since the fuel is generally designed to have an appropriate power distribution in the saturated Zenon state at the rated output, there is a problem that in the absence of Zenon, the output of the lower core without voids is significantly increased. . Then, when the output sharply increases, the fuel pellet and the cladding tube may interact with each other due to the difference in expansion coefficient, and the fuel may be damaged. Therefore, in the operation of the nuclear reactor, the fuel leveling operation (PCIOMR) is carried out in order to ensure the integrity of the fuel, and a limit value (PC threshold value) is set for the fuel output. Therefore, if the output is rapidly increased in the absence of Zenon, the local increase in output occurs as described above, and it is considered that the above limit value is exceeded. Therefore, when the output of the nuclear reactor rises, it is necessary to increase the output sequentially with the accumulation of Zenon.

By the way, generally, in a boiling water reactor, the power output of the core is controlled by two methods, a method by the core flow rate (a method of changing the speed of the recirculation pump) and a method by the control rod operation. The rods are sequentially pulled out from the fully inserted state and transferred to the control rod pattern (target control rod pattern) for rated operation. However, depending on the control rod pattern executed during this period, the output distribution may be locally greatly distorted and exceed the above-mentioned limit value. In such a case, the output is lowered by utilizing the increase in the Zenon concentration. There is a need.

On the other hand, when the output is controlled by the core flow rate, there is little fluctuation in the local power distribution such as the above-mentioned control rod withdrawal, so a relatively fast power increase is possible. In order to prioritize the convenience of driving, the control rod can be pulled out at a sufficiently low output level. PCIOMR
However, there is a problem in that the time from startup to rated output is extremely long because the above is applied.

[Problems of background technology]

Therefore, recently, a fuel that does not require PCIOMR (fuel leveling operation) has been developed from the viewpoint of improving the operating rate of a nuclear power plant (see Japanese Patent Laid-Open No. 54-59600). This fuel forms a thin layer of high-purity zirconium with high ductility inside the fuel rod cladding tube to reduce the interaction between the fuel pellets and the cladding tube. By doing so, it is possible to rapidly increase the core power to the rated power at the time of startup.

In addition, even with fuel that requires PCIOMR, if the fuel line power density during startup does not exceed the limit value (PC threshold) that requires smoothing operation, short-time startup is possible. For example, if the axial power distribution of the fuel is flattened and the linear power density is reduced by devising the axial distribution design of the uranium-235 enrichment and the content of burnable poisons, the PC threshold is If the value is high (PC threshold depends on the burnup of the fuel), etc., and if the fuel line power density during startup does not exceed the PC threshold, there is no need for break-in operation, so a short time is required. It becomes possible to start.

However, if these fuels are loaded into the core and the core output is rapidly increased to the rated output without performing PCIOMR operation, the Zenon will increase rapidly after reaching the rated output, making it difficult to maintain the core output stable. There was a problem.

[Object of the Invention]

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a start-up operation method for a nuclear reactor capable of stably maintaining core power even if there is a rapid increase in Zenon after reaching rated power. To provide.

[Outline of Invention]

In order to achieve the above-mentioned object, the present invention forms a unit cell from four fuel assemblies arranged adjacent to each other and control rods inserted between these fuel assemblies. In a nuclear reactor having a core arranged in the Y direction, the power is once increased to the rated output while the target control rod pattern at the rated output at the time of startup and a plurality of control rods located on the outer periphery of the core are inserted. The control rod located on the outer periphery of the core is gradually withdrawn according to the concentration of Zenon accumulated thereafter.

Example of Invention

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

First, a start-up operation method for a nuclear reactor according to the present invention will be described with reference to FIGS. 1 and 2. FIG. 1 is a diagram showing the relationship between reactor power and time during start-up operation according to this method,
FIG. 2 is a diagram showing the relationship between the reactor output and the core flow rate during the startup operation.

In FIGS. 1 and 2, the point A in the figures indicates the re-entry point of the generator. From this point A, the control rod is sequentially pulled out from the fully inserted state, and the output is rapidly increased to the point B. The reactor output at this time is about 60% when the recirculation pump is at the lowest speed. This value is an intercept of the rod block line shown in FIG. 2, and the control rod is pulled up to this rod block line to rapidly increase the reactor power. As shown in FIG. 3 (a), the control rod pattern at this time is such that several control rods are inserted in the outer peripheral portion of the core in addition to the target control rod pattern at the time of rating. This is to compensate the reactivity of Zenon as described above. The numbers shown in FIG. 3 indicate the insertion amount of the control rods, 0 indicates the fully inserted state, and 10 indicates that the control rod is inserted up to 10/48 in the axial direction of the core. In addition, the control rods at positions without numbers indicate the fully withdrawn state.

Next, from the point B to the rated output from the point C, the reactor power is rapidly increased by increasing the core flow rate. The control rod pattern at this time is the same as that shown in FIG. In the fuel assembly loaded in the core in this embodiment, a thin layer of highly pure zirconium having high ductility is formed inside the fuel rod cladding tube to reduce the interaction between the fuel pellets and the cladding tube. Fuel (see JP-A-54-59600), which does not require PCIOMR operation.

When the reactor output reaches the rated output in this manner, the xenon concentration increases as shown in FIG. At this time, the reactor power is maintained during rated power operation by increasing the core flow, but the core flow is limited to the maximum operational limit (for example, 10
Beyond 2.5%), it becomes impossible to compensate the decrease in reactivity due to the increase in Zenon with the core flow rate. Therefore, in such a case, the control rods located on the outer periphery of the core (hereinafter referred to as outer periphery control rods) are withdrawn from the core, and the decrease in reactivity due to the increase in Zenon is compensated by the outer periphery control rod withdrawing operation.

Specifically, first, the core flow rate is returned from the operational maximum limit to the minimum limit (for example, 92.5%) (from point D to E in Fig. 1).
1), the outer peripheral control rods are pulled out from the core at points E to F in FIG. 1. At this time, the number of outer peripheral control rods pulled out from the core is proportional to the increase amount of Zenon, for example, reaching the rated output. When the xenon concentration later changes from point a to point b in FIG. 4, the four outer peripheral control rods shown by the shaded areas in FIG. 3 (b) are withdrawn from the core, and the xenon concentration in FIG. When the point is changed from the point a to the point c, eight outer peripheral control rods shown by the hatched portion in FIG. 3 (c) are pulled out from the core.

Then, after the F point, the operations from the C point to the F point are repeated to compensate for the decrease in the reactivity due to the increase of Zenon by the pulling operation of the outer peripheral control rod. The above operation is usually repeated multiple times.

In addition, ● in Fig. 4 indicates the minimum operational limit of the core flow rate (for example, 92.5%), and ○ indicates the maximum limit (for example, 102.5%).
2.5%).

By the way, the Zenon concentration at the time of reaching the rated output depends on the Zenon concentration at the time of starting, and the Zenon concentration at the time of starting differs depending on the shutdown period of the reactor. Therefore, even if the startup process is the same, the Zenon concentration when the rated output is reached changes depending on the reactor shutdown period.Therefore, when restarting the reactor after intermediate inspection during the cycle or reactor shutdown due to scrum, etc. The control rod pattern when the rated output is reached is determined by the following method.

First, the zenon concentration when the rated output is reached is obtained from Fig. 5 from the time from the reactor shutdown to the restart. Then, the point corresponding to the Zenon concentration when reaching the rated output obtained from FIG.
Plotted in the figure, and the plotted points and points a and b shown in FIG.
Check the relationship between the point and the c point. Here, if the plotted point is at the same position as point a in FIG. 4, for example, the control rod pattern shown in FIG. 3 (a) becomes the control rod pattern when the rated output is reached, and the plotted point is the fourth point. When the control rod pattern in the middle of the control rod pattern shown in FIG. 3 (a) and the control rod pattern shown in FIG. Control rod pattern.

As described above, in the present embodiment, after the core power is increased to the rated power, the control rods located on the outer periphery of the core are gradually withdrawn in accordance with the Zenon concentration, whereby the fluctuation of the core power due to the increase of Zenon is increased. Can be minimized. Therefore, the core power can be increased to the rated power in a short time without causing the power fluctuation due to Zenon.

In the above-described embodiment, the fuel assembly that is unlikely to cause damage to the cladding tube due to the interaction between the fuel pellets and the cladding tube is used as the fuel assembly loaded in the core. If the fuel line power density of does not exceed the limit value (PC threshold) that requires break-in operation, short-time startup is possible. For example, when the axial power distribution of the fuel is flattened by devising the axial distribution design of the Raun-235 enrichment and the content of burnable poisons, the linear power density is reduced, and the PC threshold is reduced. When the value is high (PC
(The threshold varies depending on the burnup of the fuel), etc., and if the fuel line power density during startup does not exceed the PC threshold, there is no need for break-in operation, so a short startup is possible.

〔The invention's effect〕

As described above, according to the present invention, a unit cell is formed by four fuel assemblies arranged adjacent to each other and control rods inserted between these fuel assemblies, and the unit cell is formed by X and Y. In a reactor having a core arranged in a direction, in addition to the target control rod pattern at the time of rated output at startup, while increasing the output power once to the rated output while inserting a plurality of control rods located on the outer periphery of the core, Since the control rods located on the outer periphery of the core are gradually withdrawn according to the concentration of xenon accumulated thereafter, the effect is extremely large, such as improving the operating rate of the nuclear power plant.

[Brief description of drawings]

FIGS. 1 to 5 are for explaining a startup operation method of a reactor according to the present invention, and FIG. 1 is a diagram showing a relationship between reactor output and time during startup operation according to the present method. Similarly, Fig. 2 is a diagram showing the relationship between the reactor output and the core flow rate during startup operation, Fig. 3 is an explanatory diagram showing the transition of the control rod pattern during startup operation, and Fig. 4 is Zeno after reaching the rated output. FIG. 5 is a diagram showing the concentration change with time, and FIG. 5 is a diagram showing the relationship between the xenon concentration when the rated output is reached and the reactor shutdown period.

Claims (2)

[Claims] 1. A unit lattice is formed from four fuel assemblies arranged adjacent to each other and control rods inserted between these fuel assemblies, and the unit lattice is arranged in the X and Y directions. In a nuclear reactor having a core, in addition to the target control rod pattern at the rated output at the time of start-up, the output is temporarily increased to the rated output with a plurality of control rods located on the outer periphery of the core being inserted, and then the xenon accumulated. A start-up operation method for a nuclear reactor, characterized in that control rods located on the outer periphery of the core are pulled out stepwise according to the concentration. 2. The method of starting operation of a nuclear reactor according to claim 1, wherein the fuel assembly is made of fuel that does not require PCIOMR.