FEATURES
Nuclear fusion: Targeting safety and environmental goals
Analyzing fusion power's potential for safe, reliable, and environmentally friendly operation is integral to ongoing research
by I or some decades, people have looked to the plant relate to safety andeconomics. This article Franz-Nikolaus process powering the sun — nuclear fusion — as looks at safety aspects of fusion power plant Flakus, John C. an answer to energy problems on Earth. Whether designs, and reviews efforts in safety-related Cleveland, and nuclear fusion can meet our expectations re- areas that are being made through interna- T. J. Dolan mains to be seen: technological problems facing tional co-operative activities. a fusion power plant designer are complex and a fusion power plant has not yet been built. Re- markable progress has been made, however, to- Safety-related goals and considerations ward realizing fusion's potential. Research in fields of nuclear fusion has Reliable predictions of the cost of electric- been pursued in various countries for decades. ity from fusion power cannot be performed The efforts include the JT-60, which has pro- until design details of commercial fusion vided important results for improving plasma power plants have been established. Currently, confinement; the D-IIID tokamak experiment, this cost is not projected to be significantly less which has achieved record values of plasma than the costs of other energy sources. pressure relative to the magnetic field pres- In areas of safety, however, fusion holds sure; and the Tokamak Fusion Test Reactor potential advantages over other energy (TFTR), which has generated 10 million Watts sources. In nearly all studies related to the of thermal power from fusion. The Joint Euro- design of a fusion power plant, safety and pean Torus (JET) is expected to approach environmental considerations are being in- breakeven conditions, where the fusion power creasingly emphasized, and safety goals for generated exceeds the input power. Unre- fusion have been extensively discussed. The solved physics issues, such as plasma purity, safety and environmental goals of a fusion disruptions, and sustainment of current, should power plant design are to protect workers be resolved by the International Thermonu- from radiation, electromagnetic fields, and clear Experimental Reactor (ITER), which is other hazards; the public from radioactive being designed by experts of the European and toxic materials; the environment from Community, Japan, Russian Federation, and pollutants and waste; and the investor from the United States. (See related article begin- damage by accidents. ning on page 16.) The fusion process. At sufficiently high tem- There is confidence that the engineering perature, nearly all light nuclei undergo fusion re- design issues — including those concerning actions and could in principle be used to fuel a superconducting magnets, vacuum systems, fusion power plant. However, technical difficul- cryogenic systems, plasma heating systems, ties increase rapidly with the nuclear charge of plasma diagnostic systems, and blanket cool- the reacting isotopes. For this reason, only deu- ing systems — can eventually be solved. Other terium, tritium and isotopes of helium, lithium, important aspects in designing a fusion power and boron have been proposed in practice. The first generation of fusion power plants will very likely use deuterium-tritium (DT) Mr.Flakus is a senior staff member of the IAEA Department of fuel because it is the easiest to ignite. The main Nuclear Safety, Mr Cleveland is a senior staff member of the reaction product, helium-4, does not pose a IAEA Department of Nuclear Energy; and Mr. Dolan is Head of the Physics Section of the IAEA Division of Physical and health hazard. The principal energy output Chemical Sciences. from a DT fusion event is a 14 MeV neutron.
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Nearly all materials become activated to some issued giving preliminary results of the ITER degree by energetic neutron bombardment. safety analysis. Neutron reactions in DT fusion reactors will Fusion power plant safety studies have inevitably create radioisotopes. The principal been evolving for more than 20 years. They are radioactive materials present in a DT fusion steadily adapting to the evolution of interna- reactor will therefore be tritium and neutron- tionally agreed radiation safety concepts and activated structural materials surrounding the requirements. reaction volume. In 1994 the IAEA, jointly with five other Safety-related considerations. Specific international organizations issued revised In- fusion power plant safety studies, which are ternational Basic Safety Standards for Protec- complementary to many other safety studies, tion against Ionizing Radiation and for the include those related to tritium safety, the as- Safety of Radiation Sources. The Basic Safety sessment of tritium releases, activation product Standards — issued jointly with the Food and safety, radioactive waste disposal, and analyses Agriculture Organization (FAO), International of potential accidents and their consequences. Labour Office (ILO), Nuclear Energy Agency The release rate of tritium during plant op- of the Organization for Economic Co-opera- eration has to be kept well within an acceptable tion and Development (OECD/NEA), Pan- safe range. This release of tritium is modeled American Health Organization (PAHO), and by computer codes that account for tritium World Health Organization (WHO) — take permeation through the materials present in the account of new recommendations on radiation power plant. Major tritium research laborato- protection of the International Commission on ries are in Canada, Germany, Italy, Japan, the Radiological Protection (ICRP). A central part Russian Federation, and the United States. of the dose limitation system is the "optimiza- The generation of neutron activation prod- tion of protection" principle. Fusion is a good ucts is not a serious problem if they can be candidate for the successful application of this contained and if they have short half-lives. principle. Optimization is best achieved when They are a byproduct of the fusion reaction, safety assessment is already built into the not a direct reaction product. Therefore, their early design stages of a project. generation in the blanket and structure of the As pointed out earlier, the first fusion power reactor is under the control of the designer and plants will most likely use the DT fuel cycle. can be minimized by proper design and appro- Once a fusion power plant based on the DT priate choice of materials. The use of a variety reaction has been built, advanced fuels could be of low activation materials is being exten- further pursued for energy exploitation. This sively studied. would bring about a lower tritium inventory. There is no potential for a runaway fusion Later fusion power plants may evolve to fuels reaction; indeed, the problem is making the (such as deuterium + 3helium) that generate fusion reaction proceed adequately at all. Vir- fewer neutrons, and hence produce less radioac- tually all hardware problems lead to fusion tivity in surrounding materials. Thus, during the shutdown, and there are inherent limits in any evolution to advanced fuel cycles, the safety ad- case because of the limited amount of fusion vantages of fusion may increase with time. It fuel present and the nature of the fusion reac- may be possible in the future to design power tion. However, a particular focus of work in plants with low enough radionuclide inventories fusion safety is the analysis of various other so that emergency planning and preparedness are potential accidents, such as magnet accidents, unnecessary. and "consequence calculations" are per- formed. For categorization of accidents into event groupings and estimation of the fre- Practical realization of fusion quency of accidents, specific component reli- ability data are required. It has been estimated that an investment of The approach for conducting a general the order of US $50-100 billion is needed to safety analysis for fusion plants is similar to bring fusion power to fruition. The rate of that used for the design of other large nuclear progress in fusion research is limited by the installations. (See box, page 25.) The results funding rate, which is estimated to be about US of safety analyses indicate that fusion power $1.5 billion per year worldwide. plants can meet the desired safety goals. For example, the ESECOM study compared the safety and economic aspects of many fusion * See "Report of the Senior Committee on Environmental, Safety, and Economic Aspects of Magnetic Fusion Energy", reactor designs.* The general safety issues of by J.P. Holdren, D.H. Berwald, R.J. Budnitz, et.al., UCRL- ITER were discussed and a draft report has been 53766 (1989).
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Currently, expectations are that ITER could begin significant DT operation around Fusion Safety Philosophy 2005-2010, followed by construction of a dem- onstration power plant. A demonstration fusion The fusion safety philosophy now includes the power plant could then begin operation about following concepts: two decades later. If the demonstration reactor is successful, i.e. if sufficient operational experi- • passive systems and inherent safety fea- ence warrants financing of a commercial tures; power plant, then early commercial fusion • fail-safe design; power plants could begin operation by about • reliability (including redundancy of compo- 2050. nents (pumps, valves, etc.); diversity (such as two different ways of supplying back-up This estimated timetable could be delayed power); independence (if one component or or accelerated. It could be delayed by funding system fails, it does not cause an adjacent shortages or by unforeseen difficulties with one to fail); simplicity; and surveillance, to plasma phenomena or technology. The sched- detect flawed components before an acci- ule could be accelerated by a breakthrough in dent occurs); understanding of plasma behavior (such as, • consideration of human factors; perhaps, the recent success with the "reversed • remote maintenance capability; shear mode" of tokamak operation), by a new • safety culture in worker attitudes; invention that enhances plasma confinement, • quality assurance (including codes and and by providing an increased funding rate. standards; verification and validation; and Non-tokamak types of plasma confinement safety analysis); are also being studied, to develop reactors that • operational controls (fault detection, auto- can produce electricity at a lower cost. For matic corrective response); example, large stellarator experiments are un- • safety systems to reduce consequences of der construction in Japan and Europe. It is failures; • accident preparedness and management, to clear that safety studies will play a major role preserve confinement integrity; in earning and keeping public trust, desire, and • emergency planning to mitigate effects of acceptance of fusion power. radioactive releases, if needed.
IAEA activities in fusion safety gress,researchneeds,andfutureplans.Typically Guided by the International Fusion Re- about 50 experts from a dozen Member States search Council (IFRC), the IAEA is conduct- have attended these meetings, which were held ing a range of activities that promote interna- about every three to four years. The proceedings tional co-operation and help to enhance the of the latest meeting in this series, held in 1993 safety and environmental advantages of fusion in Toronto, Canada, were published in the Jour- power. They include supporting the ITER pro- nal of Fusion Energy in June 1993. The next ject, whose Engineering Design Activity has meeting is planned in October 1996 in Japan. passed the halfway point. The ITER experi- ment will have safety built into the design, to ensure that no fatalities can arise during a seri- Prospects and future directions ous accident by release of radioisotopes. In 1995, the Agency published a discussion of Fusion reactors have a significant potential safety in inertial fusion reactors. for safe, environmentally benign operation. Many IAEA activities in the area of radia- Safety aspects of fusion power plants, which tion safety are relevant to fusion safety. They have been designed on paper, cannot yet readily cover topics such as safety standards for radia- be compared with safety aspects of fission power tion protection, safe transport of radioactive reactors or other operating energy sources. In materials and management of radioactive fusion, the bulk of radioactive material is a sec- waste, guidelines for safe handling of tritium, ondary product resulting from neutron activa- and limitation of radioactivity releases into the tion, leaving room for optimization of protection environment. through materials development and selection, or Since 1973, fusion safety has been a special by using advanced fuels. item on the Agency's agenda of safety activi- To ensure that the potential safety and envi- ties. Over the past two decades, the Agency has ronmental advantages of fusion can eventually organized several technical committee meet- be utilized, safety engineering must be integrated ings on fusion safety which discussed pro- into fusion reactor designs. This is being done by
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Fusion Safety Analysis Steps of Fusion Power Plant Accident Analysis
I. Consideration of the potential hazards As in safety studies of other large nuclear installations, various steps are involved in These include: accident analysis of a fusion power plant. Each sequence of events may be represented • gamma radiation by an " event tree'', and each branch of the tree has • routine releases of tritium an associated probability of occurrence. For exam- • accidental releases of radioactive material from ple, if a valve is ordered to close, it has a finite - structure probability of failing to do so. For loss-of-coolant - coolant and loss-of-flow events, the temperature rise in the - corrosion products first wall and blanket must be calculated as func- -dust tions of time. Then the mobilization of various - tritium in walls, blanket, coolant, vacuum system, elements can be estimated, based upon data from fueling system laboratory tests.
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