制御棒落下実験における遅れ積分計数法

Abstract

The integral counting method in the rod drop experiments has widely been used for determination of subcriticality of a reactor. The decay of the neutron density followed by initiating the insertion of the rod is obliged to be slowed down according to its insertion speed. It is proved by analysis based on the one-point reactor kinetics that, in such cases, the integral counting method hitherto used tends to significantly underestimate the values of negative reactivity, even if the insertion time is in the range of 1-2s. As for the High Temperature Engineering Test Reactor (HTTR), the insertion time will be lengthened up to 4-6s. In orden to overcome the difficulty, the delayed integral counting method is proposed, in which the integration of neutron counting starts after the rod drop has been completed and the counts before is evaluated by calculation using one-point reactor kinetics. This is because the influence of the insertion time on the decay of the neutron density disappears soon after the moment. A large negative reactivity of -20$ can be determined within the systematic error of 3% by the present delayed integral counting method.