Monte Carlo Analyses for ITER NBI Duct by 1/4 Tokamak Model

In the ITER shielding design, the biological dose rates after shutdown in the region around the NBI ducts are critical. We have performed shielding calculations for the ITER/NBI ducts by 3-D Monte Carlo and 2-D SN codes with activation calculations. From comparison between calculated results by 3-D Monte Carlo and 2-D SN calculations, it has been found that the calculated results by the 2-D SN calculation overestimate by a factor of about eight at the cryostat in the case of the 91.5 cm high duct opening. From the 2-D SN calculation with activation calculations, we have deduced the conversion ratio relating fast neutron flux to the biological dose rates of ~1.5 − 2.0 × 10−5 μSv/hour/(cm−2sec.−1). The biological dose rates are about 7 × 10 μSv/hour in 50–60 cm thick duct wall from the fast neutron flux by the 3-D Monte Carlo calculation and the conversion ratio, and they can satisfy the design criteria.