Monte Carlo simulation of neutron transport in intense neutron fields

Common Monte Carlo (MC) codes for neutron transport are usually applied to neutron fields of low density under the assumption that the isotopic composition of the structure materials will not be changed in neutron reactions. This assumption is no longer valid in intense neutron fields, where an appreciable number of nuclei of the structural material may get transformed into other isotopes. Due to this time-dependent isotope- transformation; however, neutrons injected later into the system will see a different cross-section profile than the neutrons injected earlier. We have therefore a combination of two transport processes: (a) a geometrical transport of the neutrons in the (geometry of the) material structure, and (b) a transport of the isotopes on the isotopic grid of the nuclear chart. As the standard MC-transport codes deal with step (a) only, we propose in Section 2 an MC-procedure which allows for both steps. Effectively it is a method for solving the coupled nonlinear differential equations, the Boltzmann-transport equation for the neutrons and the rate-equations for the nuclei of the system, by Monte Carlo.