Reference design of the experimental multi-purpose high-temperature gas-cooled reactor

Determination of the dimensions of control rods and their arrangement in the core is fundamental in reactor design from the viewpoints of reactivity, and power distribution control. It is not easy, however, because of the many restrictions and intuitive design of the control rods confuses the total design of reactor, itself. Nuclear parametric studies have been made in design of the control rods, preliminarily under two conditions, i.e., of fuel element resulting from the nuclear parametric study of fuel lattices and control rods inserted in the core uniformly. The points to be noted in control rod design are as follows; (1) Fundamental is to plan the detailed reactivity balance in the reactor and thereby determine the diameter of control rods and their arrangement in the core, in conforming with the structural design of fuel elements and reactor core. (2) The thickness and boron concentration of control rods must be flexible ones, because of readjustment after completion of the reactor design. (3) The reactivity to be compensated with twin-control-rods in the core is large, compared with that of the uniform single-control rod arrangement, for the fuel elements and reactor core acceptable in structural design. (4) The design of burnable poison must be performed, considering not only the reduction of reactivity to be compensated with the control rods but also the recovery of reactivity from the consumption of burnable poison with fuel burnup. (auth.)