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

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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.

Introduction

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 four primary heat transfer systems: the divertor (DIV), the first wall blanket (FW/BLK), the vacuum vessel (VV) and the neutral beam injector (NBI).

A preliminary hazards assessment (PHA) was performed on the first three primary heat transport systems (PHTS) to gain a better understanding of the hazards, initiating events, and defense-in-depth mechanisms associated with the TCWS. Because the same types of equipment with less capacity will be used to cool the NBI system, the analyst judged the hazards to be similar to the other three systems. This process was patterned after the PHA methodology performed on the U.S. Department of Energy (DOE) nonreactor nuclear facilities using DOE standards 1027 [1] and 3009 [2] and draws on U.S. Nuclear Regulatory Commission (NRC) regulations for radioactive byproduct materials detailed in the U.S. Code of Federal Regulations (CFR) 10 CFR 30 [3], and International Atomic Energy Agency Safety Guide NS-D-11 [4]. The PHA was performed using the following steps.

  • 1.

    Hazard identification and screening

    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). Hazards commonly found in industry were screened out during this process; the remaining hazards were termed “identified hazards”.

  • 2.

    Hazard categorization

    Identified hazards were categorized as to their potential for harm to the workers, the public, and/or the environment.

  • 3.

    Hazard evaluation

    The design of the TCWS was examined to identify those initiating events that could expose the public, environment, or workers to the hazards should they occur. In addition, the system was examined to identify mitigating features such as barriers that prevent exposure, although no credit is given for these features at this stage of the PHA. Finally, consequences to the public or workers, should the initiating event occur and one or more of the barriers fail, were qualitatively assessed. Frequency of occurrence of the initiating event and subsequent barrier failure were qualitatively estimated.

  • 4.

    Accident analyses

    Initiating events will be indentified and accidental sequences categorized so that accident analyses [e.g., failure modes and effects analyses (FMEAs)] will be performed during the preliminary design of the TCWS when detailed information is available such as piping layout, size of equipment, location/types of controls and monitors, etc. Particular attention will be paid to those hazards that ranked high in both likelihood of release and magnitude of the resulting consequences given a release. As part of this more detailed analysis, the consequences will be assessed based on the mitigating features in place, and then the need for additional barriers and other measures to further reduce either the likelihood of release or the severity of consequences given a release will be identified.

Section snippets

Hazard identification/screening/categorization

All four PHTS have similar types of hazards. These include the following.

  • High pressure

  • High temperature

  • Chemical [W and Be powder (only under accident conditions), acids and bases used to control water chemistry]

  • Electromagnetic field environments

  • Cryogenic temperature environments

  • Vacuum environments

  • High voltage

  • Kinetic energy

  • Radiation

All hazards with the exception of radiation were screened from further review because they were common industrial hazards or they were not directly related to the

Hazard evaluation

Several initiating events that could lead to the release of the radiation hazards were identified along with an estimated frequency range. These were divided into two categories, small breaks (<190 l/min based on nominal pressure and flow rates in the pipe) and large breaks (>190 l/min up to double ended guillotine pipe breaks). The initiating event frequencies were grouped into three designations shown in Table 4. They apply to the entire TCWS.

TCWS subsystems are similar in design, and thus, the

Conclusions

A hazard assessment of the TCWS has been performed. Radiation hazards were identified as the most serious hazard associated with TCWS. Several initiating events and their frequency ranges were identified that could result in the release of radiation into the tokamak building with possible exposure of on-site personnel. It was determined that the quantities of materials released from the failure of a single loop would be less than the allowable limit so that evacuation would not be required.

Acknowledgements

This manuscript has been authored by UT-Battelle LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The views and opinions expressed

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