Photon and electron energy deposition in CANDU reactor fuel channels: a study using SANDYL and EGS4

It is important to accurately quantify the energy deposition by different types of radiation: fast neutrons are mainly responsible for the production of undesirable oxidizing and reducing species, whereas fast electrons produced by gamma rays are mainly responsible for their recombination. Any chemical model of the core of PHTS must have accurate dose rate information in its input if it is to accurately calculate concentrations of species, and chemical or electrochemical potentials. The absolute energy deposition varies across the core as well as the relative proportion of fast-neutron and gamma-ray energy deposition. Different amounts of boiling are found at different places in the core. As a result of these factors, different regions of the core require very different quantities of dissolved hydrogen in order to suppress radiolysis. This paper describes work in the development of models and codes to accurately calculate energy deposition by photons and electrons. Models using the SANDYL code were tested against models using an independent photon-electron transport code, EGS4. The results showed that the two codes, although they use different models, formulae and methods, agree in their evaluation of energy deposition in the outer coolant zone of a CANDU reactor fuel channel. Various parameters important to accurate calculations were identified. An empirical model was developed which helped explain the effect of different models of material boundaries on energy deposition, in particular near the pressure tube/coolant surfaces. Calculations showed that it is important to use small coolant regions next to solid/liquid boundaries, in order to see steep gradients of energy deposition next to the fuel and pressure tube. It is also very important to use a model as close to the real geometry as possible, e.g., the outer fuel elements of CANDU fuel bundles must be modelled in detail; annular representations of the geometry cannot be used for photon-electron transport. (author) 17 refs., 4 tabs., 31 figs