Reactor Core

Abstract

A reactor core, comprising:an outermost region; a core region surrounded by said outermost region; a plurality of fuel support members, each of which is disposed at a lower end portion of said outermost region and said core region; and a plurality of fuel assemblies loaded in said outermost region and said core region and supported by said fuel support members,wherein a plurality of fuel assemblies disposed in said core region include a plurality of first fuel assemblies, each of which is inserted into a first coolant passage which is formed in said fuel support member and has a first resistor having an opening, and a plurality of second fuel assemblies, each of which is individually inserted into each of second coolant passage which is formed in said fuel support member and has a second resistor having an opening and a larger pressure loss than that of said first resistor; and,four fuel assemblies, each of which is adjacent to each of four lateral sides of each of a plurality of first fuel assemblies, include either three or four second fuel assemblies.

Description

CLAIM OF PRIORITY[0001]The present application claims priority from Japanese Patent application serial no. 2007-208255, filed on Aug. 9, 2007, the content of which is hereby incorporated by reference into this application.BACKGROUND OF THE INVENTION[0002]The present invention relates to a reactor core, and more particularly, to a reactor core suitable for the use in a Boiling Water Reactor having an apparatus for adjusting cooling water flow rate.[0003]A fuel assembly used for a Boiling Water Reactor (BWR) includes a plurality of fuel rods and a channel box. Each of the fuel rods is filled with a plurality of fuel pellets including fissile material in the cladding tube, are bundled in a square lattice form. The square channel box encloses the bundled fuel rods. The square channel box has an outer width of about 14 cm and a square cross-section. A core, which is disposed in the reactor pressure vessel of a BWR, is loaded with a plurality of fuel assemblies internally. For nuclear fue...

AI Technical Summary

Benefits of technology

[0007]It is necessary to improve thermal margin by providing minimum change of the core system so as to improve power density of the core at low cost. Furthermore, extended cycle operation of the reactor is necessary to improve economical efficiency of fuel; inevitably, a large number of fuel assemblies loaded in the core must be exchanged after the operation cycle has been completed. Roughly, at least 25% of total number of fuel assemblies loaded in the core must be exchanged, and the batch number of fuel exchange is equal to or less than 4. This leads to the increase in the rate of new fuel assemblies that the in-core fuel dwelling time is a first cycle. Therefore, since the number of the fuel assemblies that power is high relatively increases in the core, thermal margin of the core reduces. Accordingly, it is necessary to ensure thermal margin by reducing the power of the fuel assemblies that the in-core fuel dwelling time is the first cycle and becoming flattened the peak of the power in the radial direction of the core.

[0008]To reduce power generation cost, it is effective to improve thermal margin by minimizing the change of the core system and improve power density. On the other hand, extended cycle operation is effective for the improvement of the operating rate of the nuclear power plant. In both cases, the number of fuel assemblies to be exchanged is large. Roughly, equal to or more than 25% of fuel assemblies in the core (4 batches or less) must be exchanged under the condition in which enrichment is limited to 5 wt % or less. As stated above, this means that the percentage of high power fuel assemblies relatively increases in the core and the percentage of fuel assemblies which have low thermal margin increases. As stated above, it is necessary to ensure thermal margin by reducing the power of the fuel assemblies that the in-core fuel dwelling time is the first cycle and becoming flattened the peak of the power in the radial direction of the core.

[0009]As a method for improving thermal margin, it is contemplated to use an apparatus for adjusting the flow rate of the cooling water (coolant) per the fuel assembly and provide a relative flow rate of the cooling water per the fuel assembly. Specifically, by reducing the resistance value of the orifice of the cooling water inlet located in the fuel support fitting into which the bottom of the fuel assembly, the thermal margin of which is to be improved, is inserted, it is possible to increase the flow rate of cooling water in the fuel assembly, the in-core fuel dwelling time of which is the first cycle, thereby improving the thermal margin.

Problems solved by technology

However, if the flow rate of the cooling water supplied to a fuel assembly increases, void fraction of the fuel assembly decreases and neutrons are easily moderated.

Therefore, sufficient effects on the improvement of thermal margin were not obtained.

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