BIOLOGICAL SHIELD AROUND THE NEUTRAL BEAM INJECTOR DUCTS IN THE ITER CONCEPTUAL DESIGN

There are gaps between the toroidal field coils and neutral beam injec tor (NBI) duct wall for the thermal insulator in tokamak reactors such as ITER (International Thermonuclear Experimental Reactor). Neutrons stream through the duct, and some of them penetrate the wall and strea m through the gaps. These neutrons activate the materials composing th e duct wall, toroidal field coil (TFC) case and cryostat wall surfaces . The dose rate is enhanced just outside the cryostat around the ducts in the reactor room after reactor operation by activation. We investi gated the gamma-ray dose rate just outside the cryostat after shutdown due to gamma-rays from activity induced by the neutrons streaming thr ough the gaps. By evaluating the difference between the dose rate in m odels with and without gaps, we decided whether the thickness of the c ryostat as biological shielding is sufficient or not. From these inves tigations, we recommend a cryostat design suitable for radiation shiel ding. Dose rates after shutdown at a point just outside the cryostat a round the NBI ducts in the model with gaps are two orders larger than those without gaps. The value at this point is approximately 400 mrem h-1 (4 mSv h-1), which is two orders larger than the design value for workers to enter the reactor room. In order to reduce the dose rate af ter shutdown, a method of providing the shielding function of the cryo stat is suggested.