Evaluation of gamma dose rate for the ionization chamber tube in a reactor using the Monte Carlo method

Abstract

A former plutonium production reactor code-named 821 which is natural uranium fuelled, graphite-moderated and water-cooled is currently in the second stage of decommissioning (partial dismantling and core burial). Ionization chamber tubes made of stainless steel, which are part of the neutron flux detection device, are activated by neutrons leaking from the core during the reactor operation. Since it is the only monitoring aperture of activation levels after a fully sealed burial of the core in the reactor decommissioning stage, the ionization chamber tube needs to be completely pulled out periodically for reactor monitoring. In this case, its high radioactive dose has to be taken into account. The work involved modeling the reactor core and the load-bearing water tank to calculate the neutron flux distribution on the ionization chamber tube surface, with neutron activation and subsequent radioactive decay of the tube simulated. The tube was sub-divided into five axial zones in order to calculate the number of gammas produced by the activated nuclides within the stainless steel in each zone. Then by setting up a G-M detector array using Geant4 based on the Monte Carlo method, gamma dose rates were obtained for five zones, with a maximum value of 9.86 mSv/h. The gamma dose distribution indicated the highest dose at 11–13m of the ionization chamber tube, and a safety distance on site of 12 m was established with a gamma dose rate of 43.87 μSv/h. A comparison of the simulated gamma dose rate with the on-site gamma dose rate measured by an AD6150 gamma meter verified the accuracy of the model and the calculation results, and the model was in good agreement in the high dose rate region of concern with a relative error of less than 15%.