Geometrical α- and β-dose distributions and production rates of radiolysis products in water in contact with spent nuclear fuel

A mathematical model for the dose distribution and production rates of radiolysis products in water surrounding spent nuclear fuel has been developed, based on the geometrical and energetic properties of radiation. The nuclear fuel particle is divided into layers, from which the radiation emits. The water is likewise divided into layers, where the doses are distributed. The doses are stored in vectors which are added to determine the total dose rate. A complete inventory with over 200 radionuclides has been used as input data for the model. The purpose of the model is to describe the geometrical dose distribution as a function of fuel age and burn-up, to be used as input data for kinetic modeling of the fuel dissolution. The results show that the β-dose contribution close to the spent fuel surface is negligible. Also, the variation in the relative α/β dose contribution between different ages and burn-ups is insignificant. The α- and β-dose rates vary between different burn-ups of the same age; the younger the fuel is, the larger is the difference. Exponential functions have been fitted to the relations between fuel age and average dose rate, giving useful expressions for determining average dose rates for fuel ages other than those covered in this work.