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Boration modeling of the PWR biological shield using SCALE6.1 hybrid shielding methodology

The PWR biological shield concrete activation is analyzed using SCALE6.1 hybrid deterministic- stochastic shielding methodology. Since accurate geometry representation is a paramount step for this type of shielding calculation involving heterogeneous three dimensional regions (reactor core, thermal shield, downcomer, pressure vessel, cavity, and shield), the Monte Carlo method is the method of choice. A detailed model of a combinatorial geometry, materials and characteristics of a typical PWR reactor was based on the best available data. The sources of ionizing radiation included fission neutrons and photons originating from the HBR-2 benchmark critical core. The activation reduction of reactor biological shield materials was investigated via concrete boration, which is applied when shielding from ionizing neutron radiation is especially important. The concrete boration is the common method for activation reduction in nuclear shields since the 10B has especially high thermal neutron cross section which effectively captures thermalized neutrons and decreases their (n, γ) reaction in structural materials. The reduction of secondary gamma emissions from radiative capture of thermalized neutrons in biological shield has been examined via concrete boration with natural boron (B) and boron carbide (B4C). The possibility of neutron flux mitigation in a biological shield as well as impurity isotopes (59Co, 151Eu, and 153Eu) activity decrease was especially explored. Satisfactory boron concentration which leads to saturation of neutron flux attenuation has been proposed. The activation levels of impurities in concrete, activated above the limit of IAEA clearance for the free release limit, have been estimated. The obtained results showed that activation over IAEA limits is mostly present in a thin layer of borated shield facing the critical reactor core in cavity, while the bulk of the shield is activated below IAEA threshold. The saturation of the neutron flux attenuation was clearly demonstrated for several boron concentrations so even small amounts of boron in biological shield drastically benefit the overall flux reduction.

concrete boration; radiation shielding; SCALE6.1; Monte Carlo; FW-CADIS; variance reduction

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