Critical experiment with thorium at KUCA

Thorium cross sections show relatively large differences among recent libraries compared with other major nuclides such as Uranium and Plutonium. In order to check the validity of cross section data, a series of critical experiments with Thorium at Kyoto University Critical Assembly (KUCA) has been performed since 1985. In these experiments, the purpose is to get the criticality property of nine cores of which H/235U and/or Th/235U values are different. Seven of the nine cores have been assembled. This experimental purpose is to obtain the experimental results for Thorium cores with different composition which have never been assembled at the KUCA: H/235U=150 and Th/235U=15.2. In this experiment, approach to criticality, control rod calibration, and measurement of sample worth for Thorium were performed. Comparing the experimental sample worth to calculation results by continuous energy Monte Carlo code MVP by JENDL-4.0, JENDL-3.3 ENDF/B-VII.0, and JEFF-3.1, there are about 10% difference. And there is also difference between libraries. However the experimental results are based on delayed neutron fraction which is calculated by SRAC, so it's necessary to assess the impact of delayed neutron fraction by using different libraries. (author)