Variance reduction in hybrid CADIS calculations of radiation shielding for VVER

The possibility of decreasing the variance in Monte Carlo calculations of radiation shielding by using a hybrid CADIS method is analyzed. CADIS that does not touch the main blocks of the MCU program for solving the transfer equation by the Monte Carlo method is proposed. The value function is calculated using the KATRIN program, intended for solving the transfer equation in a three-dimensional geometry by the S N method, in combination with the ConDat converter for converting the combinatorial description of the geometry in the format of the MCU program to a difference grid of the problem with local mass balance. CADIS is implemented by using a weight window with respect to collisions and limitation of the maximum optical path length, which is equivalent to a total weight window. The computational gain in calculating the fast neutron fluence in a VVER-440 and -1000 vessel can reach ~1000-fold.