BWR type reactors

Purpose: To flatten the power distribution and the burning distribution in the core of a nuclear reactor during operation. Constitution: In a reactor core comprising fuel assemblies disposed regularly in a lattice-like manner within the reactor core and cross-like control rods each surrounded with four fuel assemblies, and inserted and withdrawn to and from the reactor core from above, the control rods comprise those having intense neutron absorpting performance over the entire length and those having intense neutron absorbing performance at the portion near the upper end and weak neutron absorbing performance therebelow. (Seki, T.)