Lee C. Cadwallader

SAFETY ISSUES WITH HYDROGEN AS A VEHICLE FUEL

This report is an initial effort to identify and evaluate safety issues associated with the use of hydrogen as a vehicle fuel in automobiles. Several forms of hydrogen have been considered: gas, liquid, slush, and hydrides. The safety issues have been discussed, beginning with properties of hydrogen and the phenomenology of hydrogen combustion. Safety-related operating experiences with hydrogen vehicles have been summarized to identify concerns that must be addressed in future design activities and to support probabilistic risk assessment. Also, applicable codes, standards, and regulations pertaining to hydrogen usage and refueling have been identified and are briefly discussed. This report serves as a safety foundation for any future hydrogen safety work, such as a safety analysis or a probabilistic risk assessment.

Component Failure Rate Data Sources for Probabilistic Safety and Reliability

Probabilistic safety methods, which are being used in the chemical, manufacturing, and energy industries, create a basic need for input data on failure rates of the mechanical, electrical, instrumentation and control, and other components that comprise the engineering systems in a facility. Some companies have data stored and easy to retrieve. Other companies hire consultants who use their own databases to perform safety assessments. For analysts who do not have either of these options available, this article presents data sources that are retrievable from the literature. The accessibility of data documents via the internet is also described.

Safety and Environment

Tritium and tokamak dust are the main radioactive hazards of fusion reactors. Tritium emits a low-energy beta ray with a half-life of 12.3 years. It is hazardous if inhaled or ingested, but cannot penetrate the skin. The tritium inventory in the fuel system and walls should be well contained, minimized, and closely monitored, to keep the source term low in case of an accident. Neutron absorption will make reactor internal components radioactive, so their radioactivities will be minimized by design, with a goal of clearance or recycling most materials after a cooling period of 50–100 years. If many superconducting cables and coils are used in industry and in fusion reactors, shortages of materials such as He and Nb may occur. The ITER safety team is analyzing dozens of potential accident scenarios to prevent them or to mitigate their consequences, so that public safety will be assured without the need for an evacuation plan.

Comparison of Tritium Component Failure Rate Data

Abstract Published failure rate values from the US Tritium Systems Test Assembly, the Japanese Tritium Process Laboratory, the German Tritium Laboratory Karlsruhe, and the Joint European Torus Active Gas Handling System have been compared. This comparison is on a limited set of components, but there is a good variety of data sets in the comparison. The data compared reasonably well. The most reasonable failure rate values are recommended for use on next generation tritium handling system components, such as those in the tritium plant systems for the International Thermonuclear Experimental Reactor and the tritium fuel systems of inertial fusion facilities, such as the US National Ignition Facility. These data and the comparison results are also shared with the International Energy Agency cooperative task on fusion component failure rate data.

Materials-related issues in the safety and licensing of nuclear fusion facilities

Fusion power holds the promise of electricity production with a high degree of safety and low environmental impact. Favourable characteristics of fusion as an energy source provide the potential for this very good safety and environmental performance. But to fully realize the potential, attention must be paid in the design of a demonstration fusion power plant (DEMO) or a commercial power plant to minimize the radiological hazards. These hazards arise principally from the inventory of tritium and from materials that become activated by neutrons from the plasma. The confinement of these radioactive substances, and prevention of radiation exposure, are the primary goals of the safety approach for fusion, in order to minimize the potential for harm to personnel, the public, and the environment. The safety functions that are implemented in the design to achieve these goals are dependent on the performance of a range of materials. Degradation of the properties of materials can lead to challenges to key safety functions such as confinement. In this paper the principal types of material that have some role in safety are recalled. These either represent a potential source of hazard or contribute to the amelioration of hazards; in each case the related issues are reviewed. The resolution of these issues lead, in some instances, to requirements on materials specifications or to limits on their performance.

Recent accomplishments and future directions in the US Fusion Safety and Environmental Program

The US Fusion Program has long recognized that the safety and environmental (S&E) potential of fusion can be attained by prudent materials selection, judicious design choices and integration of safety requirements into the design of the facility. To achieve this goal, S&E research is focused on understanding the behaviour of the largest sources of radioactive and hazardous materials in a fusion facility, understanding how energy sources in a fusion facility could mobilize those materials, developing integrated state-of-the-art S&E computer codes and risk tools for safety assessment and evaluating S&E issues associated with current fusion designs. In this paper, recent accomplishments are reviewed and future directions outlined.

Power Supply Reliability Estimates for Experimental Fusion Facilities

Abstract This paper presents the results of a task to analyze the operating experience data for large, pulsed power supplies used at the DIII-D tokamak. This activity supports the International Thermonuclear Experimental Reactor (ITER) project by giving fusion-specific reliability values for large power supplies that energize neutral beams and magnets. These failure rate data are necessary to perform system availability calculations and to make estimates of the frequency of safety-significant events (e.g., power supply arcs or fires) that might occur in other fusion facilities such as ITER. The analysis shows that the DIII-D data results compare well with the results of similar data analysis work that the Italian National Agency for New Technologies, Energy and the Environment (ENEA) has performed on the JET tokamak and compare fairly with data from two accelerators.

Status of Safety and Environmental Activities in the U.S. Fusion Program

Abstract This paper presents an overview of recent safety efforts in both magnetic and inertial fusion energy. Safety has been a part of fusion design and operations since the inception of fusion research. Safety research and safety design support have been provided for a variety of experiments in both the magnetic and inertial fusion programs. The main safety issues are reviewed, some recent safety highlights are discussed and the programmatic impacts that safety research has had are presented. Future directions in the safety and environmental area are proposed.

Radiological Dose Calculations for Fusion Facilities

This report summarizes the results and rationale for radiological dose calculations for the maximally exposed individual during fusion accident conditions. Early doses per unit activity (Sieverts per TeraBecquerel) are given for 535 magnetic fusion isotopes of interest for several release scenarios. These data can be used for accident assessment calculations to determine if the accident consequences exceed Nuclear Regulatory Commission and Department of Energy evaluation guides. A generalized yearly dose estimate for routine releases, based on 1 Terabecquerel unit releases per radionuclide, has also been performed using averaged site parameters and assumed populations. These routine release data are useful for assessing designs against US Environmental Protection Agency yearly release limits.

Failure rate adjustment factors for high technology components

In nuclear fusion research for power plant applications, experiments have used tritium fuel. With the use of radioactive tritium, the safety issues have increased and so have the safety assessments to demonstrate that safety is incorporated into design. For the ITER International Project, the safety concerns of a high power, tritium-burning experiment are being addressed in design. ITER also has another issue of interest, to give a reasonable forecast of the operational availability of this largest fusion experiment ever built. Both of these needs can require component failure rate data for safety assessment and for reliability availability maintainability inspectability (RAMI) studies. ITER is often using components of greater size, at higher temperatures, and at higher radiation damage levels than past experiments. In some cases, the component failure rates from previous operating experiences can be modified with multiplicative factors, called k factors or adjustment factors, to account for the different operating environments. With the use of k factors, the component failure rates from past operations can be applied to this new design. When the environments are defined, the k factor approach can be used with good effect to give component failure rate estimates. This paper describes recent work in developing k factors to adjust some magnet component failure rates for the ITER International Project.