On the Influence of Biological Shielding on Neutron Flux behind Reactor Pressure Vessel

The neutron flux behind the reactor pressure vessel (RPV) is formed by primary transmitted neutrons from the core and also by backscattered neutrons from biological shielding. In the evaluations of neutron flux in the RPV, the knowledge of the reflected neutrons' share on the total flux is important for correct assessment of possible discrepancies. This paper aims to determine the share of backscattered neutrons in the spectrum behind the RPV and presents new results obtained with an improved fixed source model based on more precise and validated power distribution. Measurements of fast and thermal neutron fluxes in various configurations of biological shielding are realized in a VVER-1000 mock-up in the LR-0 reactor. Additionally, the calculations of the VVER-1000 mock-up are supplemented by calculations of an Electricite de France heavy reflector benchmark that represents arbitrary pressurized water reactor geometry. Comparison of both types of results shows that the LR-0 mock-up measurements are directly applicable, in terms of reactor dosimetry, to other types of pressurized water reactors, regardless of the burn-up and type of fuel they use. It is shown that the amount of backscattered thermal neutrons behind the reactor vessel strongly depends on the distance between the vessel and the concrete shielding. The choice of the data library for calculations influences the results in the thermal region as well.