Shield structure optimisation studies for the west beam port of the KAMINI reactor

Abstract

KAMINI is the Kalpakkam Mini Reactor and its main purpose is to cater to the experimental needs and for neutron radiography. It is a water-cooled reactor with 233 U as the fissile material. The reactor has three neutron beam ports for experimental needs, which are made of graded cylindrical aluminium channel. All the beam ports start from the core, pierce the biological shield and are 2 m long. The shield structure optimisation studies for the beam port towards the west side of the reactor are presented here. The diameter of the west beam channel at the core centre is 54 mm and at the other end is 250 mm. The west beam tube opening is 530 mm below the floor level and hence the pit housing the experimental cavity is below the floor level with dimensions 2 m × 2.5 m × 1.3 m. The beam tube opening into the experimental cavity serves as the neutron source for radiation physics experiments and is assumed as a surface source in the calculations. Rough estimate of the shield design is made based on the literature on dose-equivalent index transmission through concrete for average neutron energy of 1.5 MeV. Detailed radiation transport calculations are performed using Monte Carlo neutral particle transport code (MCNP) to optimise the shield design. Neutron and capture gamma dose rates at the accessible areas are estimated. The contribution of prompt fission gamma rays is found to be negligible compared to the dose rates due to capture gamma rays. The details of the optimised shield structure proposed for the west beam port are as follows. Fixed concrete shields of thickness 650 mm on the lateral sides and a composite shield (500 mm paraffin and 50 mm concrete) in the front side at a distance of 1 m from the beam tube opening are recommended inside the experimental pit. During reactor operation, a composite mobile shield (500 mm paraffin and 500 mm concrete) closes the experimental cavity at the floor level. Fixed concrete shields are recommended to close the pit fully. The shield structure proposed increases the experimental cavity volume from 0.2 to 1.4 m 3 with the dose levels at the accessible areas less than one μSv/h. The MCNP computed neutron and gamma dose rates are compared with the measured values with the existing shield structure to verify the source term used.