JPH03214097A - Operating method of reactor - Google Patents

Abstract

PURPOSE:To improve an average takeout burnup of an initially loaded fuel and to reduce the number of rods to be replaced, by a method wherein the initially loaded fuel taken out at the time of completion of the second cycle at least is replaced by a fuel located inside a core and having a low reactivity. CONSTITUTION:As to a boiling water reactor having an electrical output of 1,350,000KW and 872 fuel loading rods, for instance, the enrichment of an initially loaded fuel loaded in a core at the time of a rise of the reactor is set at about 3.1wt.%. According to the core of the reactor thus prepared, the reactor is stopped in a state of control rods being left inserted, at the time of completion of the first cycle. At the time of completion this first cycle, 146 rods out of those of the initially loaded fuel are replaced by those of a fuel to be substituted which has an enrichment of about 3.5wt.%, and a second-cycle operation is conducted. At the time of completion of the second cycle and each subsequent cycle, the reactor is stopped in a state of all the control rods being drawn out. After the completion of the second cycle, 146 rods of the initially loaded fuel are reloaded since they have still a remaining combustion energy, while the other 314 rods are replaced by the fuel to be substituted, and then an operation of the third cycle is executed.

Description

[Detailed description of the invention] [Purpose of the invention] (Industrial application field) The present invention relates to a method of operating a nuclear reactor.

(Prior art) In general, the enrichment of the fuel in the first loaded reactor core, which is loaded into the reactor core for the first time after a nuclear reactor is constructed, is determined in advance for a pre-planned fixed period (this is called one cycle) at which the reactor can operate. It is determined as follows. In a boiling water reactor, if the operating period is 15 months, the enrichment of the initially loaded fuel is approximately 2.5 wt%. In this way, the enrichment of the initially loaded fuel is set to 2.5w.
%, the surplus reactivity of the core will be zero after one cycle of operation of the reactor, and it will just become critical when all the control rods are withdrawn.

In such a boiling water reactor, after the first cycle of operation, approximately 1/3 of the initially loaded fuel is replaced with replacement fuel and a second cycle of operation is performed, and after one cycle, the initial loading is continued. Approximately 1/3 of the fuel is replaced with replacement fuel and a third cycle of operation is performed. In the core of such a nuclear reactor, for example, in the second cycle, about two-thirds of the initially loaded fuel used in the first cycle is loaded and used as is, so in the second cycle, the core It is necessary to make the enrichment of the replacement fuel loaded in the tank higher than that of the initially loaded fuel, and the enrichment of the replacement fuel loaded in this second cycle is approximately 3.5 wt%. . In addition,
The enrichment of this replacement fuel is determined by the fuel replacement plan, that is, how many replacement fuels are loaded into the core in each cycle.

(Problem to be solved by the invention) In this operating method, 1/3 of the initially loaded fuel stays in the reactor for only one cycle, and therefore the fuel is taken out of the reactor with a low burnup. It turns out. Therefore, in order to extend the removal burnup of the initial loading fuel and at the same time reduce the number of replacement fuel loads, the initial loading fuel that was pumped out from the reactor during the first fuel exchange is used for the second and subsequent fuel exchanges. At the time of fuel exchange, an operating method (reloading of the initially loaded fuel) is carried out in which the fuel is replaced with one that is in the core and has a lower reactivity, but this is not always sufficient to reduce the number of replacement fuels loaded. However, there is still room for improvement from the perspective of improving the average discharge burnup of the initially loaded fuel.

[Configuration of the Invention (Means for Solving the Problem) In order to achieve the above object, the present invention operates with the initially loaded fuel for a certain period of time, and then a part of the initially loaded fuel is used as replacement fuel. In a method of operating a nuclear reactor for refueling, at least the initially loaded fuel taken out for refueling after the end of the second cycle is replaced with fuel that is in the core and has a lower reactivity at the time of refueling after the end of the third cycle. The feature is that it can be exchanged with.

(Function) According to the nuclear reactor operating method of the present invention, at least the initially loaded fuel taken out for fuel exchange after the end of the second cycle is stored in the reactor core at the time of fuel exchange after the end of the third cycle. Since the fuel is replaced with a fuel having a low reactivity, not only the number of replacement fuels to be loaded is reduced, but also the average discharged burnup of the initially loaded fuel is improved.

(Example I) An example of the present invention will be described using a nuclear reactor with an electrical output of 1.35 million kW and a fuel loading of 872 bodies.

The enrichment of the initial fuel loaded into the reactor core at startup of a boiling water reactor is set to 3.1. Let it be wt%.

According to such a nuclear reactor core, at the end of the first cycle, the reactor is stopped with the control rods still inserted. At the end of the first cycle, the neutron infinite multiplication factor of the initially loaded fuel becomes a in the figure. Here, the figure shows the relationship between burnup and infinite neutron multiplication factor, where the solid line shows the case of the initially loaded fuel and the broken line shows the case of the replacement fuel.

At the end of this first cycle, 146 units of the initially loaded fuel are replaced with replacement fuel with an enrichment of 3.5 wt%, and a second cycle operation is performed. At the end of the second cycle and subsequent cycles, the reactor is shut down with all control rods pulled out.

When the fuel is replaced after the second cycle, the infinite neutron multiplication factor of the initial fuel in the reactor core becomes b in the figure, and 460 of the 726 initial fuels in the reactor are replaced in the following way. Ru. In other words, the neutron infinite multiplication factor of the 146 initially loaded fuels taken out after the end of the first cycle is a in the figure.
As shown in Figure 2, there is still a surplus of combustion power, so these 146 bodies are reloaded into the core, and the remaining 314 bodies' initially loaded fuel is replaced with replacement fuel, and a third cycle of operation is performed.

When the fuel is replaced after the third cycle, the neutron infinite multiplication factor of the initially loaded fuel in the reactor core becomes C in the diagram, and all 266 initially loaded fuels remaining in the core are replaced as follows. In other words, 5 of these 266 bodies were transferred to the second
At the end of the cycle, the initial loaded fuel (the infinite neutron multiplication factor is b in the figure) taken out is replaced with the fuel, and the remaining 261 bodies are replaced with replacement fuel, and the fourth cycle of operation is performed.

In the fuel exchange after the end of the 4th cycle, the neutron infinite multiplication factor of the initially loaded fuel for the 146 bodies reloaded at the end of the 2nd cycle and the 5 bodies reloaded after the end of the 3rd cycle is C in the diagram.
Since the neutron infinite multiplication factor of the 146 replacement fuels loaded after the end of the first cycle is g in the figure, these 297 fuels are taken out after the end of the second cycle as shown in b in the figure. is smaller than the infinite neutron multiplication factor of the initially loaded fuel. For this reason, the initially loaded fuel removed after the second cycle is replaced with 28 pieces of fuel with lower reactivity in the reactor core, and the remaining 269 pieces are replaced with replacement fuel.

From then on, in the same way, when changing fuel at the end of the cycle,
A portion of the replaced fuel is used to replace the initially loaded fuel taken out at the end of the second cycle with fuel that is in the reactor core and has a lower reactivity, and the rest is replaced with replacement fuel.

According to the method of operating a nuclear reactor as described above, the average discharge burnup of the initial fuel loaded at the start-up of the reactor is increased, and the number of replacement fuels is not reduced. can.

That is, in the reactor operating method in which only the initially loaded fuel removed at the end of the first cycle is reloaded, the average extracted burnup of the initially loaded fuel is 5.5% compared to the operating method in which the initially loaded fuel is not reloaded at all. 9% improvement, and the total number of replacement fuels loaded is reduced by only 54. However, unlike the present invention, not only the initially loaded fuel is removed at the end of the first cycle, but also the fuel removed at the end of the second cycle. If the initially loaded fuel is also reloaded, the average extracted burnup of the initially loaded fuel will improve by 28.2%, and the total number of replacement fuels loaded can be reduced by 102.

(Example 2) In a core where the enrichment of part or all of the rice bag 6 (j fuel is further increased, for example, to 3.5 wt%, which is the same as the replacement fuel,
At the end of the first cycle, the second cycle is operated without any exchange of the initially loaded fuel, and after the second cycle, part of the initially loaded fuel is replaced with replacement fuel, and the third cycle is operated. I can do it. In the fuel exchange after the end of the third cycle, the initially loaded fuel taken out after the end of the second cycle is replaced with fuel that is in the core and has a lower reactivity, and the remaining fuel is replaced with replacement fuel. In the above, in the fuel exchange after the end of the first cycle, the initially loaded fuel is not replaced at all, but as in Example 1, after the end of the first cycle, a part of the initially loaded fuel is exchanged with the replacement fuel and the second fuel is replaced. The cycle is operated, and in the fuel exchange after the second cycle, the initial fuel loaded at the end of the first cycle is replaced with fuel that is in the core and has a lower reactivity, and the remainder is used as replacement fuel. You may replace it and perform the third cycle operation.

[Effects of the Invention] As explained above, according to the present invention, in order to replace the initially loaded fuel taken out at least at the end of the second cycle with fuel that is in the core and has a lower reactivity, the initially loaded fuel is removed. Not only is the average extracted burnup significantly improved, but the number of replacement fuels can also be reduced, so fuel can be used efficiently.

[Brief explanation of drawings]

The figure is a graph showing the relationship between the burnup and the neutron infinite multiplication factors of the initially loaded fuel and the replacement fuel in the present invention. (8733) Agent Patent Attorney Yoshiaki Inomata (Baka
1 person) JEc 2ΔEc Burnup 3ΔEc jEc

Claims (1)

[Claims] (1) In a method of operating a nuclear reactor in which the reactor is operated for a certain period of time using the initially loaded fuel and then a portion of the initially loaded fuel is replaced with replacement fuel, the reactor is removed for fuel replacement after at least the second cycle. A method for operating a nuclear reactor, in which the initially loaded fuel is replaced with a fuel with lower reactivity within the reactor core during fuel replacement after the end of the third cycle.