THERMAL NEUTRON SCATTERING SIMULATION USING FREE GAS MODEL AND SHORT COLLISION TIME APPROXIMATION

The paper is concerned with the research effort to develop the idea underlying the technique which enables the information from Evaluated Nuclear Data Files in Monte Carlo calculations to be used directly without processing. The technique is realized in terms of BRAND code system. The algorithms of thermal neutron scattering simulation using free gas model and short collision time approximation are described. These algorithms are based on the substitution of variables proposed by Eriksson for free gas model. The algorithm proposed by Eriksson was modified. In the algorithm proposed by Eriksson it is suggested that the scattering cross sections and some subsidiary functions should be tabulated. But in BRAND’s algorithm no tabulation is used. The algorithms described are compared with MCNP and MCU. The analytical formulas for the full scattering cross section calculation using free gas model and short collision time approximation are given. The examples of the calculation tasks on the spectrum of secondary neutrons scattered on the oxygen using free gas model and short collision time approximation are given. The results are in good agreement with numerically calculated spectrum results. The examples of the calculation tasks on energy spectrum calculation of the neutrons that penetrate the oxygen barrier and albedo neutron spectrum calculation are given. There are some disagreements between BRAND and MCNP results particularly in solving neutron penetration problem. It was assumed that these disagreements are possibly due to the fact that in BRAND code the calculations are made in strict accordance with standard algorithms ENDF-6, while NJOY and MCNP algorithms seem to be corrected for a better agreement with benchmarks.