Optimization Method of Rod-Type Burnable Poisons for Nuclear Designs of HTGRs.

Abstract

In block-type HTGRs, control rod insertion depths into cores had to be maintained as small as possible at full power operations, to avoid a fuel temperature rise. Thus, specifications (poison atom density (NBP) and radius (r)) of rod-type burnable poisons (BPs) had to be optimized so that the effective multiplication factor (keff) would be constant at a minimum value throughout a planned burnup period.However, the optimization had been a time-consuming work until now since survey calculations had to be done for most possible combinations of NBP and r. To solve this problem, I have found a optimization method consisting of two steps. In the first step, approximation formulas describing a time-dependent relation among effective absorption cross sections (ΣaBP), NBP and r are used to select promising combinations of NBP and r beforehand. In the second step, the best combination of NBP and r is determined by a comparison between ΣaBP of each promising combination and expected one. The number of survey calculations was reduced to about 1/10 by the optimization method. The change in keff for 600 burnup days was reduced to 2%Δk by the method. Hence, it was made possible to operate reactors practically without inserting the control rods into cores.