Radiation Safety Considerations for LHD Experiments

For experiments with the Large Helical Device (LHD) which is now under construction at the Toki site in Japan, radiation safety issues were discussed. In the course of plasma experiments, radiations such as X-rays, induced γ-rays, and neutrons increase. From a safety point of view, these radiation exposures to the environment should be controlled to limit the annual dose to less than 50 μSv at the site boundary. In order to meet this, an area monitoring system named RMSAFE (Radiation Monitoring System Applicable to Fusion Experiments) has been developed and partly installed. This can discriminate and measure radiations including burst-like emissions due to plasma shots from natural radiations. For the present period, this system is operating to monitor the natural radiation levels before the LHD operation. It was observed that the radiation levels strongly depend on rain levels and ground state. An indoor area monitoring system has already been implemented on site, to measure radiations from NBI and ECH test shots, and it shows good monitoring ability. Also, thermoluminescence dosimetry has been applied. The importance of investigating the radiological behavior under natural conditions with continuous monitoring is shown.