EXPERIMENTAL STUDY ON ACCELERATOR DRIVEN SUBCRITICAL REACTOR BY USING THE KYOTO UNIVERSITY CRITICAL ASSEMBLY (KUCA)

A series of basic experiments for an accelerator driven subcritical reactor (ADSR) has been performed at the Kyoto University Critical Assembly (KUCA) by combining a critical assembly with a Cockcroft-Walton type accelerator in view of a future plan to establish a new neutron source for research. Fourteen MeV neutrons were injected into a subcritical system through the polyethylene reflector. By varying subcriticality, the neutron multiplication and the prompt neutron decay constant as well as the subcriticality were measured mainly by an optical fiber detector system. Calculations were executed by a continuous energy Monte Carlo code MVP on the basis of JENDL-3.2 to examine the accuracy of neutronics design for the ADSR at the present stage. A large discrepancy was observed between the measured subcriticality and the calculated one mainly because of an inadequate evaluation of 235 U nuclear data compiled in JENDL-3.2, which leads to a large difference among the measured and calculated nuclear parameters of the ADSR. Through the present study, it was strongly recognized that the present tools for the neutronics design calculation of ADSR are not accurate enough especially to predict the neutron multiplication in the ADSR. Although the accuracy of keff calculation is becoming better and better in the current neutronics design tools for nuclear reactors, one should be careful that the neutron multiplication in the ADSR does not depend on keff itself, but approximately on 1/(1-keff).