Shield design for a hull neutron monitor using Monte Carlo techniques

Abstract

<span style="font-size:10.0pt;font-family: " times="" new="" roman";mso-fareast-font-family:"times="" roman";mso-bidi-font-family:="" mangal;mso-ansi-language:en-us;mso-fareast-language:en-us;mso-bidi-language:="" hi"="" lang="EN-US">In the head-end process of a reprocessing plant, the spent fuel is chopped using a mechanical chopper into small pieces and the heavy metal oxide is leached in nitric acid leaving behind the undissolved zircaloy clad (hull) in the basket. The hull mass needs to be estimated for the residual actinides and fission products to check for the completion of dissolution and also for the purpose of nuclear material accounting. This is normally carried out by gamma spectroscopy for the residual fission products. A new method is proposed based on active/passive neutron measurements of the residual actinides. The basket along with the hull is moved to a corner of the dissolution cell using remote tools, where the hull monitoring exercise will be carried out. Hence, shielding around this basket is required in this area to limit the dose rate to a value acceptable (i) for the operating personnel engaged in monitoring, and (ii) for ³He detectors used to measure neutrons. It has been proposed that the acceptable dose rate criteria at this detector location should be less than 1.74 mGy/hr (200 mR/hr). ORIGEN2 code is used to calculate the gamma and neutron emission rate from the residual spent fuel and the clad material in the hulls. Shielding code based on Monte Carlo technique is used for the gamma and neutron shielding calculations. In this paper, calculations are carried out for each of the shield material considered, and it is found that the shielding thickness required to bring down the dose rate to 1.74 mGy/hr is 12.5 cm for lead or 21 cm for iron. </span>