A Conceptual Design Study for Active Nondestructive Assay System by Photon Interrogation for Uranium-Bearing Waste with MVP Code and Evaluated Photonuclear Data

Abstract

A conceptual design study has been carried out with a Monte Carlo transport code on an active nondestructive assay system for low-level uranium-bearing wastes discharged from an uranium enrichment plant. Delayed neutrons from photofissions of uranium isotopes in the steel waste are counted with this system to confirm that the activity concentration of uranium in this waste is below the clearance level. The present design work needs a coupled neutron/photon transport code which can simulate the photonuclear reaction and the resultant emission of neutrons. We have modified a continuous-energy Monte Carlo transport code MVP to equip it with a function to simulate the emission of photoneutrons along with the delayed neutrons from the photofissions. The photonuclear data library for the modified MVP code was prepared on the basis of evaluated photonuclear data by IAEA. For the 235 U and 238 U, we employed evaluated data in the JENDL photonuclear data 2004, which stores complete delayed neutron emission data for these nuclides. With this code system, a time-dependent simulation of neutron counts was made for the assay system, which consisted of a 15 MeV end-point bremsstrahlung photon source, helium-3 neutron detectors and the waste. The present paper describes the modification to the MVP code and the results of design study.