Modeling gamma-ray dose rate from a spent pressurized water reactor fuel assembly

Gamma-ray dose rate distribution around a pressurized water reactor spent-fuel assembly is studied using the Monte Carlo N-particle transport code (MCNP) version 4a. A detailed rod-by-rod modeling of the assembly is utilized, showing explicitly the fuel, cladding, control rod channels, and the instrumentation tube. A cylindrically distributed source of gamma rays, within every fuel rod, is considered with a seven-group energy spectrum. Dose rates are obtained by tallying the gamma rays at several axial and radial positions outside the assembly. The results indicate that the radial distribution of the dose rate can be represented by a power relationship of the form r^-n, where r is the radial distance from the assembly center. Another important conclusion from this study is that the dose rate close to the assembly surface is overestimated if a homogeneous assembly model is used.