Background contributions of NRF-based nondestructive assay for spent nuclear fuel

Shizuma, Toshiyuki; Hayakawa, Takehito; Angell, C.; Hajima, Ryoichi; Minato, Futoshi   ; Suyama, Kenya; Seya, Michio; Johnson, M.*; McNabb, D.*

It is required to measure spent nuclear fuel as accurate as possible for nuclear material management concerning nuclear security and safeguards. The system performance such as counting precision for the -ray nondestructive assay (NDA) based on nuclear resonance fluorescence (NRF) would be affected by radiation backgrounds from spent fuel. Generally, the -ray spectrum of radiation backgrounds from spent fuel has characteristic shape with an exponential fall-off as a function of -ray energy. Therefore, the NRF measurement at higher energies is preferable to obtain a better signal-to-noise ratio. The counting precision is also affected by backgrounds from the coherent scattering such as Rayleigh, nuclear Thomson, and Delbrck scattering. In order to minimize the Coherent scattering contribution, we propose a method in which NRF transitions to the first excited state is measured. We calculated the radiation background counts from spent fuel as well as the coherent scattering background. Assuming various cross sections for NRF in the range of 3 to 100 eV barn at excitation energies of 2 to 5 MeV, we have estimated the statistical uncertainties of the proposed NDA system. We also discuss the NRF strength based on the previous experimental data as well as theoretical prediction by quasi-particle random phase approximation (QRPA).