Monte Carlo Simulation of Atmospheric Neutron Transport at High Altitudes Using MCNP

Neutron transport calculations were performed using the Monte Carol code, MCNP. The transport problem considered has a point source at high altitude 40 km. Since atmospheric density decreases with altitude, two-dimensional effects cylindrical coordinates can be important. Results were compared to those obtained with the SMAUG-II computer code, which performs mass-integral scaling of approximate fits to one-dimensional spherical discrete ordinates solutions. These comparisons show the importance of two-dimensional computations. The report discusses practical issues of applying MCNP to this problem, without code modifications and includes example input files for MCNP and for SMAUG-II. Computational modelling of neutron transport in the atmosphere is complicated by the variation of air density with altitude.