Monte Carlo Code for Calculating the Doppler Effect in Fast Reactors

A Monte Carlo code for calculating the Doppler effect between specified energy limits in fast reactors is described. Neutron histories are followed through the energy interval in which temperature changes in the reactor affect the multiplication rate. The normalization of the results requires additional information which may be obtained from suitable multigroup calculations. The reaction rates and their changes with temperature are obtained from Monte Carlo estimates of the unperturbed flux, and adjoint weighted with group adjoint fluxes specified in the input. Allowance is made for the resonance fine structure of the importance functions within the narrow resonance approximation. Geometry routines have been prepared for multiregion spherical geometry, each region containing a homogeneous mixture, and for a multilayer cylindrical geometry with a number of regions in each layer. There is also an option for infinite lattice calculations with top and bottom blankets and reflectors. The resonance cross-sections are based on the Doppler broadened Breit-Wigner single level formula and contain ℓ = 0 and ℓ = 1 contributions, the resonance parameters being chosen from their average values and distribution functions. The code is of value in studying the influence of different temperature excursions on the Doppler effect, since different temperature changes can be specified in each region of the assembly, interference effects between different resonance sequences are automatically included. The ℓ = 1 wave contribution to the Doppler effect can be investigated and core heterogeneity effects studied. Results will be presented showing the type of accuracy of the estimates obtained. In a typical run for a-2500-litre oxide-fuelled spherical assembly the contribution to the Doppler effect in the 16th group of the YOM set was obtained with confidence limits of ± 8% on the 50% level in about 5 min computation time on the Philco 2000 computer, when only the ℓ = 0 contributions of the nearest resonances of uranium and plutonium to the neutron energy were included in the cross-section calculations. (author)