Evaluation of the CANDU 6 Neutron Characteristics in View of Application of the Resonance Dependent Scattering Kernel in MCNP(X)

Abstract

This study continues our investigation of the influence of the resonant scattering kernel on different reactor types and, in particular, different moderators and coolants. The importance of an advanced neutron scattering treatment for heavy nuclei with strong energy-dependent cross sections, such as the pronounced resonances of 238 U, has been discussed in various publications, where the impact of the full double-differential scattering kernel on the core characteristics was derived. In this study, we concentrate on the application of the resonant dependent kernel to heavy water reactors, namely, the CANDU ® 6. In modeling nuclear reactors with Monte Carlo methods, we take advantage of the stochastic implementation of the resonant scattering kernel directly in MCNP(X), the so-called Doppler Broadening Rejection Correction – DBRC, which allows the direct calculation of the differential part of the Doppler broadened cross section. The DBRC model is based on an additional rejection scheme within the procedure “sampling of the target velocity” in the subroutine tgtvel in MCNP(X). This means that the DBRC samples the Doppler broadening of the double-differential cross section “on-line” and consistently for each neutron undergoing a scattering interaction with a heavy nuclide. The introduction of the resonant scattering kernel within postulated modes of the CANDU 6 lattice cell simulation leads to a predicted decrease in criticality of about 50 pcm (= 0.5 mk) near the design conditions and up to 100– 150 pcm (= 1.0–1.5 mk) at higher fuel temperatures. The standard deviation of these estimates is 8 pcm (= 0.08 mk). We predict a decrease in the fuel temperature coefficient of reactivity (Doppler Effect) by ~ 3 to 10% for fresh clean fuel. We found no noticeable effect of the DBRC on the coolant void reactivity coefficient, the difference being less than 10 pcm (= 0.1 mk) near the design conditions. In addition, some artificial lattice cells were simulated showing that, by decreasing the cell pitch, the impact of the new resonant scattering kernel predictions increases significantly.