Photon-electron transport in CANDU reactor channels

Photo-electron transport simulations were performed to calculate the spatial and energy distributions of ionizing radiation deposited in a CANDU reactor channel. Two general purpose Monte Carlo codes SANDYL and EGS4 were utilized, and their results were shown to be in agreement with each other except when pair production was dominant - which is not of importance to the CANDU reactor. Predictions of the gradients of the energy deposition distribution near the coolant/solid (fuel and pressure tube) interfaces were found to be sensitive to the size of the Monte Carlo scoring regions assigned in the adjacent coolant. It is demonstrated that the commonly used method of using annular cells of materials to represent the geometry of fuel channels tends to significantly (>50%) underestimate the amount of energy deposition in the coolant near the fuel. The surface area of the solids surrounding the coolant, and the geometry of the outer fuel pins in the CANDU fuel bundle were proved to be important parameters in predicting the spatial distribution of energy deposition and these therefore need to be simulated in detail. Results of calculations are presented for a number of monoenergetic photon sources, and for a typical CANDU reactor photon source. (author) 10 refs., 6 figs