Neutron and gamma-ray deep penetration calculation through biological concrete shield of VVER-1000 reactor by a new technique based on variance reduction

Reducing the computer run time of neutron and gamma-ray transport calculation as well as obtaining more accurate results are the main objectives of deep penetration problems. In MCNP code, the widely-used variance reduction method, weight window, is applied to reach the above objectives. In the present paper a new method based on the variance reduction technique is introduced. This method is applied to the multilayer shielding structures of serpentine and reinforced concrete which are employed in a VVER-1000 nuclear power plant as biological shields. For applying this method, the thickness and density of shields are reduced to unrealistic values to obtain a suitable estimate for weight windows. Next, the thickness and density are both returned gradually to their real values in iterative MCNP runs until the suitable weight windows for flux and dose rate calculations are obtained. Furthermore, for reducing computer run time, the equivalent fixed surface source is considered instead of critical source. The obtained result of this deep penetration problem is compared with the value which is reported in the Final Safety Analysis Report (FSAR) of this reactor. The comparison shows good consistence between the two outcomes. This consistency satisfies the accuracy of calculation methods.