Deterministic Multigroup Modeling of Thermal Effect on Neutron Scattering by Heavy Nuclides (original) (raw)

The principal physical phenomenon underlying the computation of neutron spectra is the nuclear reaction in which neutrons lose or gain energy, i.e., the neutron scattering process. As long as neutrons only lose energy they are "slowing down". The loss of energy by the neutrons is the dominant energy exchange mechanism for very fast (and hence very energetic) neutrons. In the past this fact led to the use of approximations in which the gain of energy by the neutron in collisions with the fuel lattice atoms or other materials, such as surrounding moderator, was assumed negligible in the energy range above thermal. This assumption was demonstrated to be inaccurate and unacceptable when scattering resonances are present at intermediate energies (the lower energy domain within the slowing down range). The purpose of this thesis is to contribute a method that allows the relaxation of the incorrect assumption. Namely, a method is developed that accounts for up-scattering by heavy nuclides in the resonance energy range. A multigroup formulation for the exact neutron elastic scattering kernel, taking into account up-scattering events, has been developed and verified. The formulation has been applied to elastic scattering cross section data of heavy nuclides for a very fine energy group structure and then supplied to a deterministic lattice physics code demonstrating its effects. Such resonance treatment provides a more accurate representation of the interaction between neutrons and nuclei and results in more realistic and higher fidelity neutron fluxes that reflect the effect of the temperature of the lattice. The correct accounting for the lattice effects influences the estimated values for the probability of neutron absorption and scattering, which in turn determine the core reactivity and consequent burnup characteristics. The slowing down process is important in thermal reactors because it results in the neutrons entering the thermal energy range in which the majority of fission events occur. Correctly modeling the slowing down and hence slowing down source into the thermal energy range and consequently allowing the correct modeling of iii Chapter 6 Conclusion 6.