Optimization study of BC filter thickness for shifting to fast spectrum at experimental position in NRAD core

Oizumi, Akito   ; Hummel, A. J.*; Marshall, M. A.*; Palmiotti, G.*; Youinou, G. J.*

The Idaho National Laboratory (INL) has proposed to perform measurements of the fission rate ratio and sample reactivity worth under a wide range of spectral indices created using several types of thermal neutron shielding filters in an experimental position in the Neutron RADiography (NRAD) reactor. The JAEA has a plan to perform integral experiments with fission chambers in the NRAD to further validate the nuclear data of MAs, and ultimately reduce the uncertainty in the cross-sections. In this study, the optimization of the BC filter thickness was conducted to acquire the high-accuracy and the beneficial experimental data. The optimized BC filter thickness was determined to be 3 mm based on the comprehensive evaluation from the viewpoints of the shielding ability of low energy neutrons, the level of the total neutron flux, and the influence of fabrication accuracy of the BC filter. The adoption of the tentative optimized model would be acquired the experimental data such as fission reaction rate and sample worth with the wider spectral indices because the tentative optimized model would be able to increase the ratio of the fast neutrons by almost completely eliminating the effect of the thermal neutrons.