MCNPX modelling of the NPL manganese bath facility based on a converted CAD model

A detailed CAD model of the new manganese bath facility of the UK National Physical Laboratory has been converted into MCNPX format using the conversion program MCAM. The model has been used to evaluate a number of important parameters of the new facility such as the fraction of neutrons that escape from the manganese sulphate solution but subsequently return and are captured by it, and the degree of irradiation of the solution when a source is being transferred to and from the bath. It has also been used to recalculate the correction factors for the fraction of neutrons captured by nuclei other than manganese, the fraction captured by the source and the source mounting assembly and the fraction escaping from the bath itself, and compare these with values from an earlier simplified model.