Design Analysis to Reduce the Equivalent Dose Rate of the Neutral Beam Injection System in a Fusion Experimental Reactor

Abstract

As applied to the common design of the neutral beam injection (NBI) system in the International Thermonuclear Experimental Reactor (ITER) Conceptual Design Activity, a design is proposed and examined that reduces the equivalent dose rate of the NBI system in order to enable access to the outside of the injector. Modifying the current system is necessary because the equivalent dose rate in the NBI room after reactor shutdown is higher than the design limit for radiation workers. The NBI maintenance concept is based on full-remote maintenance. There are, however, some problems that must be solved before full-remote maintenance could be realized—such as connection and disconnection of the electric power cables and complicated coolant pipes, and location of the maintenance equipment—this concept solves the aforementioned problem by enabling worker accessibility to the outside of the injector. The following design points are suggested to reduce the equivalent dose rate.

The vacuum vessel should be composed of aluminum to reduce the induced radioactivity. Polyethylene, which has high shielding ability for neutrons, should be installed between the vessel and magnetic shield located outside the vacuum vessel to reduce not only neutron flux coming to the magnetic shield but also gamma-ray flux, caused by in-vessel components, leaking to the NBI room. The equivalent dose rate in the NBI room 1 week after reactor shutdown can be reduced to 28 µSv/h by applying the foregoing measures. Thus, the prospect exists for realizing access to the outside of the injector.