Control rod assembly

Purpose: To arrange control rod assemblies in a high conversion type water cooled reactor so as to attain uniform power distribution and greatly reduce the risk of leading to reactivity accidents. Constitution: A plurality kind of control rods of different diameters are arranged such that those control rods of greater diameter with higher control rod worths are inserted into the control portion of a fuel assembly and those control rods of smaller diameter with lower control and worths are inserted at the peripheral range. Accordingly, the power at the central portion of the fuel assembly where the void reactivity coefficient becomes positive, a great amount of voids are generated and the power is increased can be suppressed more as compared with the peripheral portion thereof thereby flattening the power distribution. Further, it is possible to suppress the fuel can temperature of fuel rods disposed in the central position and reduce the reactivity accidents. (Horiuchi, T.)