Safety issues and approach to meet the safety requirements in the tokamak cooling water system of ITER

Abstract ITER (Latin for the “way”) is an experimental tokamak fusion energy reactor that is being built in Cadarache, France, in collaboration with seven agencies representing China, the European Union, India, Japan, South Korea, the Russian Federation, and the United States. The main objective of ITER is to demonstrate the scientific and technical feasibility of a controlled fusion reaction that will allow the production of approximately 500 MW of fusion power for durations of several hundred seconds. As an experimental facility, ITER is intended to allow the exploration of physics scenarios, to conduct the technological tests essential to the preparation of a fusion reactor, and to demonstrate the favorable safety characteristics of fusion. The ITER tokamak cooling water system (TCWS) consists of several separate systems to cool the major ITER components—the divertor/limiter, the first wall blanket, the vacuum vessel, and the neutral beam injector. The ex-vessel part of the TCWS provides a confinement function for tritium and activated corrosion products in the cooling water. The vacuum vessel system also has a functional safety requirement regarding the residual heat removal from in-vessel components. A preliminary hazards assessment (PHA) was performed for a better understanding of the hazards, initiating events, and defense-in-depth mechanisms associated with the TCWS. The PHA was completed using the following steps. (1) Hazard identification . Hazards associated with the TCWS were identified including radiological/chemical/electromagnetic hazards and physical hazards (e.g., high voltage, high pressure, high temperature, and falling objects). (2) Hazard categorization . Hazards identified in the first step were categorized as to their potential for harm to the workers, the public, and/or the environment. (3) Hazard evaluation . The design was examined to determine initiating events that might occur and that could expose the public, the environment, or workers to the hazard. In addition, the system was examined to identify barriers that prevent exposure. Finally, consequences to the public or workers were qualitatively assessed should the initiating event occur and one or more of the barriers fail. Frequency of occurrence of the initiating event and subsequent barrier failure was qualitatively estimated. (4) Accident analyses . A preliminary hazards analysis was performed on the conceptual design of the TCWS. As the design progresses, a detailed accident analysis will be performed in the form of a failure modes and effects analysis. The results of the PHA indicated that the principal hazards associated with the TCWS were those associated with radiation. These were low compared to hazards associated with nuclear fission reactors and were limited to potential exposure to the on-site workers if appropriate protective actions were not used. However, the risk to the general public off-site was found to be negligible even under worst case accident conditions.