Shielding Calculations For A Tank-In-Pool Reactor

ABSTRACT

The effectiveness of the water shield above the core and in the pool for shielding gamma rays

and neutrons was investigated by measurements and calculations for a tank-in-pool type reactor,

Ghana Research Reactor-1, GHARR-1. Experimental determination of y - dose rates were

made using Radiation Alert Monitor-4 and halogen counter type J613 y probe with readout on

y -radiation monitoring system of the control console and the microcomputer control system.

The PC version of the one-dimensional transport code ANISN was used to estimate the gamma

and the neutron dose-rates at various positions of the reactor. All computations were carried out

using the Sg angular quadrature in a cylindrical geometry pertaining to the reactor GHARR-1.

The P3 scattering order was assumed. The microscopic cross-sections of materials for various

regions were extracted from VITAMIN C master library consisting of 171 neutron and 36 photon

energies. Concentrations in various regions were computed based on procedures outlined in

WIMS-D/4. The calculational result of gamma dose rate was 25.8 fiSv / hr at the edge of the

pool for nominal power of 30kW which agreed with the experimental one of 25 juSv / h r . The

maximum and minimum percentage variation of the calculational and the experimental results

were 8.21 and 0.27 respectively with corresponding standard deviations of0.42 and 0.04. Based

on this satisfactory result, the dose profile along the slant tubes used to commission the reactor

was studied using the code and an assessment was also performed for the radiological

consequences of postulated accidents due to loss-of-coolant accidents. With the available

information on the variation of the neutron and the gamma doses along the slant tubes, high dose

theimolumniscent (TLD) badges could be used to confirm the calculated values in the future.

The tubes could then be mapped for studies on the effect of neutron and gamma irradiation on

mutation breeding of seeds in the field of agriculture. The effectiveness of the shielding in the

case of loss-of-coolant due to a break in the pipe of the reactor vessel or crack in the pool caused

by an earthquake was investigated. The results of the study indicate that the dose rates around

the reactor vessel and pool would be abnormally high. However, core melt-down is not expected

because the design features of the reactor are such that the core will remain covered with coolant

in the event of a break in a pipe line to the purification plant.