Neutron and Gamma Ray Dose Studies in Cagr Instrumentation and Fuel Components

Abstract

In all reactor types, it is important to know radiation energy deposition or damage rates in various components so that the safe and efficient operation of the plant can be ensured. The size and geometric complexity of these components can vary greatly and Monte Carlo methods have been adopted in the UK for such dose assessments because of the flexibility they offer in accurate modelling. However as the detail of the modelling is Increased it is important to ensure that the dosimetric assumptions on which the calculations are based remain valid. In particular, for small regions comprising materials of different type secondary charged particle equilibrium is not always achieved and the effect of secondary electron processes must be included in any complete gamma transport analysis.

Within the core of Commercial Advanced Gas-Cooled Reactors (CAGR) there are a number of examples of such situations and two are illustrated. In the first, a dosimetric study of calorimeters used to monitor moderator energy deposition is described and the effect of secondary electron transport is shown to be an important part of this analysis. In the second example, electron transport and internal conversion processes are considered in the estimate of dose rates in gadolinium oxide burnable poison toroids which form part of some CAGR fuel. It is shown that the neglect of these effects can introduce systematic underpredictions of radiation doses in excess of 30% in specific circumstances.