Relative Speed Tabulation Method for Efficient Treatment of Resonance Scattering in GPU-Based Monte Carlo Neutron Transport Calculation

A target velocity sampling method named the Relative Speed Tabulation (RST) is proposed for the efficient treatment of resonance elastic scattering in the Monte Carlo simulation utilizing graphics processing units (GPU). The RST method samples the relative speed between a neutron and a target nucleus by employing pretabulated probabilities of relative speeds. The target velocity is then determined from the sampled relative velocity and the neutron speed. The motivation was to avoid the rejection process of the Doppler Broadening Rejection Correction (DBRC) method, which can incur a significant reduction in the parallel performance of vector processors, such as GPUs, due to its largely varying rejection rates. The RST can also overcome the weakness of large variance of the Weight Correction Method (WCM), which would involve drastic changes in neutron weights. The verification results obtained for the Mosteller benchmark problems demonstrate that the RST is equivalent to the DBRC in accuracy, while the calculation speed remains at the same level of the WCM.