Abstract
The TORT-coupled MCNP method established in this study is an implementation of the consistent adjoint driven importance sampling (CADIS) methodology for Monte Carlo variance reduction. It utilizes the deterministic TORT adjoint function to perform source biasing and consistent transport biasing with weight window technique in the MCNP Monte Carlo simulation. It has been proved to be very effective in accelerating the MCNP calculations especially for those cases involving distributed sources and/or deep-penetration problem. The method is first verified by a benchmark calculation for a small PWR pressure vessel problem. After that, we have successfully applied this method in two real world difficult shielding problems: one is the shielding evaluation of the BNCT treatment room at Tsing Hua Open-pool reactor (THOR), and the other is the surface dose rate calculation for a spent fuel storage cask. In the first case for the shielding evaluation of the BNCT treatment room at THOR, the radiation attenuation from beam exit to detector outside the treatment room is more than seven orders of magnitudes. This is why an effective variance reduction technique is very important for solving this problem by Monte Carlo simulation. We have demonstrated that, with this method, the computational efficiency can be improved significantly by two to three orders of magnitude compared to an analog MCNP calculation. Since the neutron-induced secondary gamma rays are the main contribution to the total dose outside the treatment room, we have found that further optimization of the photon weight windows can lead to additional 50-75% improvement in the overall computational efficiency. In the second case for the surface dose rate calculations of the NAC-UMS spent fuel storage cask, it involves difficult problems of deep penetration and radiation streaming resulting from a large volumetric source. Effective variance reduction techniques are indispensable for a Monte Carlo simulation to obtain results of small statistic errors within reasonable computing time. In addition to the TORT-coupled MCNP method, we also adopted two conventional techniques in this problem, i.e. the traditional cell importance method in MCNP and the popular SAS4 analysis sequence in the SCALE5.1 package. This study thoroughly compares the accuracy and computation efficiencies of MCNP and SAS4 in the surface dose rate calculation of the NAC-UMS storage cask. Due to the effective source biasing and consistent transport biasing, the TORT-coupled MCNP calculation shows overall a superior computation efficiency in this case study.