共分散測定法に基づく実効遅発中性子割合の測定

Abstract

The effective delayed neutron fraction βeff is an important reactor physics parameter, because it plays a role of a conversion factor between the calculated reactivity in Δk/k unit and the measured one in dollar unit. In 1970's, central-sample-worth measurements were done to accumulate the basic data for fast reactor design. Calculated values to these measurements were 30% overestimate. It was pointed out that the 10% of this discrepancy was due to the βeff. Recently, as an origin of systematic discrepancy between the calculated and measured reactivity is due to the βeff, needs for experimental method obtaining the βeff with 1-2% accuracy have been stressed in order to re-examine delayed neutron data and calculational method. In this report, we describe the βeff measurements by using the method proposed by E.F. Bennett at the Kyoto University Critical Assembly (KUCA) of the Kyoto University Research Reactor Institute. This method is based on the measurements of covariance between neutron counts and the absolute fission rate. Measurements were performed with the 3/8″ P36EU thermal neutron system which was composed of 93.3% enriched uranium plates of 1/16″ thickness and 3/8″ polyethylene moderator plates, and the 1/8″ P80EU system which was composed of 1/8″ polyethylene plates and had slightly hard neutron spectrum. As the results of several measurements, the βeff of the 3/8″ P36EU system ranged from 7.60 ±0.18 to 7.96±0.19, and from 8.07±0.18 to 8.14±0.18 for the 1/8″ P80EU system. The latter is slightly large because of hard neutron spectrum. The experimental accuracy is ±2.2%- ±2.9% which did not clear the target accuracy of 1-2%. But this may be improved by means of an improvement of fission rate measurement. Comparing with these measurements and the calculated values based on the JENDL-2 nuclear data file, they agreed within the range form -4% to 1.5%. Then we concluded that the covariance method was usable to evaluate the βeff value of thermal neutron system.