Neutron Generation Time in Highly-Enriched Uranium Core at Kyoto University Critical Assembly

Abstract

At the Kyoto University Critical Assembly experiments on kinetics parameters are carried out at near-critical configurations, supercritical and subcritical states, in the thermal neutron spectrum made with a highly enriched uranium fuel. The main calculated kinetics parameters, the effective delayed neutron fraction (β_eff) and the neutron generation time (Ʌ), are used effectively for the estimation of experimental parameters, and the accuracy of experiments on prompt neutron decay constant (α) and subcriticality (ρ_$) in dollar units is attained by the numerical results of β_eff and Ʌ. Furthermore, the value of β_eff/Ʌ is experimentally deduced with the use of the experimental results of α and ρ_$, ranging between 250 and −80 pcm. Thus, the experimentally deduced values of β_eff/Ʌ that reveal good accuracy through a comparison with those by the MCNP6.1 calculations with JENDL-4.0 are then taken as an index of Ʌ by introducing an acceptable assumption of β_eff at near-critical configurations. From the results of experimental and numerical analyses, the experimental value of β_eff/Ʌ is important for the validation of Ʌ since kinetics parameters are successfully obtained from the clean cores of near-critical configurations in the thermal neutron spectrum.

Keywords:

• Neutron generation time
• highly enriched uranium core
• Kyoto University Critical Assembly
• near critical
• thermal neutron spectrum

Acknowledgments

The authors are grateful to all the technical staff at the KUCA for their assistance during the experiments.