4. METHOD OF SOLUTION
Monte Carlo simulation is used by tracing particle histories from fission birth down through the resonance region until absorption in the thermal range. The program is organised in three sections for fast, epithermal and thermal simulation, respectively; each section implements a particular model for both numerical techniques and cross section representation (energy groups in the fast section, groups or resonance parameters in the epithermal section, points in the thermal section).
During slowing down (energy above 1 eV) nuclei are considered as stationary, with the exception of some resonance nuclei whose spacing between resonances is much greater than the resonance width. The Doppler broadening of s-wave resonances of these nuclides is taken into account by computing cross sections at the current neutron energy and at the temperature of the nucleus hit.
During thermalisation (energy below 1 eV) the thermal motion of some nuclides is also considered, by exploiting scattering kernels provided by the library for light water, heavy water and oxygen at several temperatures.
KIM includes splitting and Russian roulette.
A characteristic feature of the program is its approach to the lattice geometry. In fact, besides the usual continuous treatment of the geometry using the well-known "combinatorial" description (adapted to planar domains), the program allows complex configurations to be represented by a discrete set of points, whereby the calculation speed is greatly improved. In this second approach configurations are described as the result of successive overlays of elementary figures over a basic domain.