Analysis of neutron streaming through the trenches at linac based neutron generator facility, IGCAR

Abstract

Shielded LINAC hall has been built to accommodate a radio frequency quadrupole (RFQ) linear accelerator (LINAC) based pulsed neutron generator at RSD, IGCAR. The concrete neutron shield wall thickness for the LINAC hall is finalized by adopting NCRP-51 methodology. The dimensions of the trenches and labyrinth are decided based on the neutron generator installation requirements. As per the AERB safety criteria, the radiation dose levels in the occupancy area should be less than 1μSv/h. However, the refined dose estimate with the inclusion of actual trench and labyrinth dimensions indicates an increased dose rate of ~3.10 μSv/h in LINAC control room. Hence, the control room is declared as controlled area. Additional shielding has been proposed for the trenches and labyrinth to make the control room as full occupancy area. For designing the additional shielding, experimental and theoretical analyses are needed to estimate the radiation streaming through the trenches and labyrinth. To start with, the neutron streaming through trenches have been studied using 185 GBq (5Ci) 241 Am-Be neutron source for qualifying the trenches. The 241 Am-Be source position is selected at the trench entrance such that it contributes the same neutron flux as that of neutron generator. Neutron dose rate and spectral measurements have been carried out at five locations along the trench from the entrance (LINAC hall side) to the exit (Control room side) of the trench.The experimental results are validated with the theoretical calculations using Monte Carlo N particle (MCNP) code. The analysis shows that the trenches are having a dose reduction factor better than 800 with respect to that of entrance dose. The observed dose rate at the trench exit is found to be less than 450nSv/h. In this paper, both the theoretical and experimental neutron streaming analyses through the trenches of LINAC hall are presented.