Evaluation of biological dose rates around the ITER NBI ports by 2-D Sn/activation and 3-D Monte Carlo analyses

Shielding analyses for the International Thermonuclear Experimental Reactor (ITER) neutral beam injector (NBI) ports have been performed using two-dimensional discrete ordinates Sn method with activation analyses and three-dimensional Monte Carlo method. From the two-dimensional Sn/activation analyses, it was found that a conversion ratio relating fast neutron flux to the biological dose rates at 106 s after reactor shutdown is 1.5–2.0×10−5 μSv/h per (cm−2s−1) for the fast neutron flux with energies above 0.1 MeV. From the results of three-dimensional Monte Carlo analyses, the fast neutron flux above 0.1 MeV at the cryostat is about 3.6×106 cm−2s−1 for the case of a 50–60 cm thick NBI port wall (thin case), and 5.2×105 cm−2s−1 for a 60–65 cm thick port wall (thick case). The corresponding biological dose-rates at the cryostat are about 7 and 2×101 μSv/h for the thin and thick cases, respectively. These dose rate levels satisfy the tentative design target of 100 μSv/h specified for the ITER Engineering Design Activity (EDA).