FAST REACTOR SAFETY

JOHN GRAHAM

Westinghouse Advanced Reactors Division Madison, Pennsylvania

This book is a step along the arduous path of developing the technology of nuclear safety as a fully recognized discipline with quantitatively defined standards and an accepted methodology of analysis, design, and test inter­pretation. Completion of this task is needed to stabilize the nuclear power industry and eliminate the elements of unfounded opinion, and even emo­tion, which too often characterize public discussions of nuclear power plant safety, particularly in the application of fast power reactors.

A book devoted to nuclear safety as a special and separate skill raises immediately a conflict for engineers who have been associated with the design, construction, testing, and operation of a total nuclear power plant, a conflict caused by the conviction that if the designer does not provide reliability and safety in each of the components and systems for which he is responsible, that plant will not be reliable or safe. In fact, the existence of a separate skill in nuclear safety opens up a potential risk that the system or component designer will leave the matter of safety to this new specialist and in so doing, miss providing the safety features which only he can suc­cessfully assure. Guidance in resolving this conflict can be obtained by examining other fields in which I feel a similar dilemma has been faced and resolved. Let us look at an analogous case.

The development of high performance equipment in fields such as avia­

tion, space, modern structures, and of course, nuclear power itself, has required the development of specialized skills in stress analysis and me­chanics. This technical specialty is so demanding that the full careers of highly trained people must be exclusively devoted to it. Yet, their skills must be utilized by the designer and highly effective technical communica­tion between the designer and the sophisticated practitioner of mechanics is necessary. The success of achieving such communication has meant the success or failure of many projects. Similarly, the skill and knowledge of the highly trained metallurgist, pursuing his experiments and analysis as a separate endeavor, must be absorbed by the designer to ensure the proper use of materials in the component he is designing. Here again the skill must be developed independently because of its demanding nature, but the designer must utilize results of such development if he is to make a success of his project.

Nuclear safety, although not as well recognized today in the curricula of universities as is the study of mechanics or metallurgy, fulfills an analogous role. This skill must be recognized in a manner similar to that of mechanics and metallurgy. Further, it should not simply be restricted to nuclear safety but the basic methodology must also be applied to safety of all modern, massive, “high-technology” devices. The forerunner of this book, in fact, has been a set of lecture notes developed by the author for a course given as part of the nuclear engineering curriculum at Carnegie-Mellon University.

The transfer of nuclear safety skills to the designer must involve more than discussion, consultation, and teaching. It requires the definition and acceptance of safety design criteria, and the development of design meth­odology, to which a substantial portion of this book is devoted. In modern application of analysis and design techniques, the computer code is ever present as a specific device for this methodology transfer. It should be apparent from the discussions in the text, however, that many of these codes are still in a state of development and should not be used by the designer except with a full appreciation of their limitations. A third, and ultimately most important, means of transfer of safety skills is the incor­poration into design of the actual experience gained by safety experiments and by plant operation. The former must be incorporated through the comparison of design methods against definitive experiments and the sub­sequent improvement of these methods to more accurately portray the accident, or the accident initiators. This area of accident analysis verifica­tion is still quite weak, as indicated by the relative paucity of such experi­mental comparisons the author of this book is able to make. It is not clear that there is enough emphasis today, in the planning of experiments and in the development of analytical methods to interpret such experiments, on satisfying the need of the designer to have design methods which can be compared with such experiments and then applied with prudent extra­polation to his reactor design. In the latter case of the incorporation of plant operational experience, the methods of reliability analysis reviewed in the book are a main vehicle of transfer. Fortunately, there are many common features between fast reactors and the present generation of thermal reactors, particularly in the areas of reactor protection and accident pre­vention. As a result, the operational data accumulated on present day nuclear plants and their components can be incorporated into the reliability analyses of the fast reactor designer.

Attention by specialists in nuclear safety to the above means of transfer of their skill will prevent the “separation” of design from safety expertise and will, in addition, assure the proper balance in all the activities needed to design, construct, and operate a safe nuclear power plant.

One must be concerned about the tendency of pursuing safety technology for its own sake rather than in close coordination with design, so as to overemphasize the study of highly unlikely accidents and means of coping with their consequences. Such overemphasis inevitably results in a less than desired effort on (a) accident prevention through the study of accident initiators and design approaches to eliminate them, (b) the study of relatively probable accidents and the design of protective systems to assure that no significant loss in reactor or plant integrity will occur as a result of the accident, and (c) the definition of a highly reliable design of the plant systems and components which achieves safety through such reliability.

The importance of keeping the proper balance is underlined by the realiza­tion that I believe is held strongly by those experienced in nuclear operation: namely, that a functionally complex or awkward plant or one with un­reliable components is an accident-prone plant. No matter how many engi­neered safeguards systems there are on such a plant to cope with accidents, an accident-prone plant is not a safe plant. Further, the goal of designing a reliable plant, if rigorous standards, quality assurance procedures, and thorough preoperational testing and diagnostic in-service testing are em­ployed, is more assured than the counterpart goal of designing reliable engineered safeguards. Experience tells us that when we need a system to provide protection against accidents we should, wherever possible, utilize equipment which is a continued operational part of the plant and thus will receive the ultimate test of reliability—continuous performance.

One other important contribution this book can make, particularly for those of us devoting our energies to the development of the fast reactor,

is to bring fast reactor safety into context. Those engaged in fast reactor development do not find substance in the popular image that there is some­thing especially dangerous about a fast reactor. I can do no better in this respect than to quote from a statement by F. R. Farmer in his Foreword to a summary report+ on fast reactor safety published last year with which I heartily agree. He states,

There has been a prolonged and excessive preoccupation with the ap­parent differences between fast and thermal systems and, for many years, a particular interest in the explosive disruption of a fast reactor core…. In general, it is shown that differences exist between all reactors and the fast reactor is not notable in this respect, nor in respect to explosive potential…. The facts which may initiate the various modes of destruc­tive failure will be different between fast and thermal systems and will call for different methods of detection, but it is not obvious that one deserves greater effort or leads to greater concern than the other.

This book, by examining as quantitatively as our present technological capability permits the specific features of safety of the fast system, makes it clear that the fast reactor can be designed to be safe. The sodium coolant in the Liquid Metal Fast Breeder Reactor, for example, has significant advantages from a safety standpoint. The system can operate essentially at atmospheric pressure where the maximum pressure is a result only of the pump head. The boiling point of sodium is 500-600°F above the peak operating temperature range of the sodium coolant, largely eliminating concern about boiling in the reactor core. The historic concern about the short, prompt neutron lifetime of a fast reactor has been allayed for the most part by the finding that the rate of power increase caused by a re­activity addition in excess of prompt critical is limited to safe levels by Doppler feedback. The historic concern relative to a core disruptive accident is being brought into perspective by focusing attention both in analysis and experimentation on defining the quantitative features of this accident rather than the qualitative upper limit possibilities of it. Effort is also being placed on study of the initiators of the core disruptive accident so that design steps can be taken to eliminate such initiators. Of particular importance in this regard is the experimental program on fuel element failure propagation which must either establish that failure propagation is limited to safe levels

+ F. R. Farmer et at. An Appreciation of Fast Reactor Safety (1970), AHSB(S)-R-188. Authority Health and Safety Branch, United Kingdom Atomic Energy Authority, Risley, Warrington, Lancashire, England.

or show the way to fuel and fuel assembly design modifications to limit propagation to safe levels. Thus, there is no unique barrier to providing this power generating device that opens up vast natural fuel resources to mankind without exposing him to undue risks to his life and property.

It would be consoling to be able to say that this book completes the task of placing fast reactor safety beyond opinion and emotional issues, and reduces the entire subject to that of professionally recognized skills and standards. It does not because there are still substantial technical develop­ments in nuclear safety which have yet to be completed, both in the analytical and experimental areas. Lest we be discouraged, however, we must remember that this is the first major industrial technological enterprise which is being subjected to rigorous, professional development of its safety characteristics.

A final word on one other aspect of the book’s contribution. There has been an increasing call in fast reactor breeder development to establish safety criteria and licensing standards on an international basis. Such a step would better assure the coordinated and constructive response of all workers in the field to achieve the best set of such criteria and standards. In addition, artificial barriers involving licensing requirements set up along national lines would hopefully be reduced or even eliminated. This book does not directly address this issue. But is assists the process of achieving a more uniform international approach by presenting the licensing position as it stands today in the U. S. Similar presentations of licensing positions in Britain and Germany are also making a contribution to this international cooperation. John Graham, the author of this book, has carried out in his own career a significant amount of safety work across national boundaries, having participated heavily in the fast reactor safety program in Great Britain and now, more recently, in the safety analysis and licensing activities associated with the liquid metal fast breeder reactor program in the United States.

J. J. Taylor General Manager Breeder Reactor Divisions Westinghouse Electric Corporation