BWR type reactor

Purpose: To lengthen the operating period of a BWR type reactor and to enable the effective utilization of uranium. Constitution: Second control rods made of weak neutron absorber are disposed in the water gaps in which no control rods are disposed. The second control rod is formed, for example, of zircalloy having small neutron absorbing section as compared with a moderator. When the second control rods are withdrawn at the end of the operation, the amount of the moderators is increased and the critical state can be maintained due to the more fission product formed so far. Therefore, the operating period can be increased. Since the worth of the control rods is small, the distortion of the power distribution in the vicinity of control rod moving time is small and is effective in the control of the power distribution. (Kaminura, M.)