Section 1

SUMMARY OF MAIN COMMENTS AND RECOMMENDATIONS

The U.S. gas-cooled reactor program was initiated in the Late 1950s with the development of design concepts based upon earlier work done primarily in the United Kingdom on lower temperature gas-cooled reactors. Two high temperature plants have been constructed and operated in the U.S.: the 40 MWe Peach Bottom plant, completed in 1967 and operated until 1974, and the 330 MWe Fort St. Vrain plant, completed in 1974 and scheduled defueling/decommissioning in 1990.

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The U.S. high temperature gas-cooled reactor program is primarily funded and managed by DOE with additional funding and management support provided by utilities through GCRA. Private funding has also been provided by GA. The program is currently focused on the development of the MHTGR, a four-module, 540 MWe plant, in which enhancement of passive safety characteristics is emphasized. A base technology program supported by DOE includes fuels performance and fission product behavior, graphite behavior, and metals behavior, with concentration on fuels and fission products development. EPRI is also funding a modest technology effort, concentrating primarily on the development of magnetic bearings for the main helium circulator. Other EPRI nuclear research programs have application to HTGR development.

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The U.S. electric utilities have consistently endorsed and supported the development of the HTGR concept. important of the recent expressions of the utility support for HTGR development was included in the report of the Energy Research Advisory Board's Subpanel for Advanced Reactor Development (April 1986).

The MHTGR represents the most recent visualization of an HTGR that could be made available for utility deployment upon successful completion of a demonstration plant project. Overall, the utility industry evaluation team believes that the MHTGR should be considered a viable application of an advanced nuclear concept and deserves continuing development. This general conclusion should be remembered when reading the following specific comments. The comments and recommendations in this evaluation report are intended as constructive suggestions for improving the applicability of this very promising plant design to utility systems.

This section of the report extracts and summarizes the main conclusions and recommendations explained in more detail in the subsequent sections. The items included in this section are selected on the basis of their significance to the MHTGR and/or the expressed strength of the evaluation team's opinion on the subject. The subsequent sections include many additional comments and recommendations not highlighted here.

1. The MHTGR program has developed a plant design which offers several attractive characteristics, especially in the safety arena. The most significant of these characteristics is the non-reliance on plant protection systems for assuring public safety, thereby avoiding the cost of design, construction, opeation and testing of such systems and the burden of frequently reviewing their reliability and of sometimes upgrading them. The design promises to provide a passively safe response even to severe accidents in which the reactor shutdown system is assumed to fail.

Nevertheless, one design feature which may have to be reviewed to assure regulatory and utility acceptance is the use of the TRISO particle as the primary barrier in the MHTGR containment system. This application of the TRISO fuel is unique to HTGRS. It is the MHTGR program's position that

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these coated fuel particles maintain integrity during all normal and licensing basis events and that fission product release from MHTGR fuel is therefore dominated by contamination and manufacturing defects. The MHTGR program has calculated that, assuming stringent fuel process specifications are met, the fission product release even under extremely low probability, severe accident events will still meet the dose limiting regulations, without a need for a secondary leak-tight containment structure. Plant operation and out-of-pile test results, although limited in scope so far, confirm the above performance claim. However, until the fuel demonstration program has progressed further, some uncertainty remains that these particles can be manufactured and inspected on a commercial basis to the quality required and that they will consistently meet integrity requirements under severe accident conditions. Providing confidence that the fuel meets all fabrication and operational expectations is the fundamental key to utility acceptance and success of the MHTGR concept. A comprehensive testing program, possibly including confirmatory result from a prototype plant, is the mechanism which will provide this confidence.

The predicted frequency and consequences of bounding events is so low in the MHTGR that no low-leakage containment is provided around the primary circuit boundary. Confirming the predicted frequencies and consequences will require proving not only the fuel quality and integrity as noted above, but also the accident analysis methods and the fission product transport methodology. Even then, it will be difficult to obtain public and regulatory acceptance of the nontraditional containment approach. A fall-back position, specifically one enhancing containment, should be developed so that the design and cost impact can be understood.

Two areas where technical needs and dependent costs may have

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been significantly underestimated are in plant manning levels and in spent fuel storage and disposal. The number of operating crews and the number of total plant personnel are less than can be supported by extrapolation from US nuclear plant experience to-date. The spent fuel storage capacity does not allow for the real-life delays to governmental removal of the spent-fuel from the site, and the assumed for the storage facilty and for the storage casks seem inadequate.

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The refueling plan for the MHTGR is to replace half the fuel every 20 months. The MHTGR program indicates 21 days has been allotted for the refueling outage. Virtually the entire core is removed from the reactor vessel to a fuel-handling bay, where half of the fuel is replaced as the core is reloaded into the reactor vessel. The number of fuel handling operations is quite large (roughly 1000 fuel and reflector blocks must be handled in a typical refueling) and must be accomplished without misloading and fuel damage. The MHTGR program has spent considerable effort designing the equipment to achieve this high rate of handling by using highly automated equipment, providing limited built-in redundancy and spares, and has some experience base from Fort St. Vrain on which to extrapolate this performance.

Reliability of the fuel handling equipment will be essential to maintaining the refueling schedule and plant availability, and the MATGR program will need to provide additional assurance regarding adequacy of the fuel handling and refueling methods.

5. The MHTGR design offers several attractive characteristics in the safety arena which may justify a premium in the busbar costs. However, the MHTGR project must strive for a. competitive busbar cost with nuclear and coal-fired alternatives. Based upon the presentations given to the

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utility team, and recognizing the potential impact of issues discussed in this report, such as containment, staffing levels, spent fuel storage and disposal, it is unlikely that the visualized METGR will be economically competitive with contemporaneous light water reactor plants. Achieving an economically competitive design appears to be the greatest challenge facing the MHTGR project. Accordingly, the utility evaluation team recommends that the economic challenge facing the METGR be recognized and that more emphasis be given to potential cost reducing improvements.

Seismic capability has been a special concern for the prismatic cores, made up of large blocks of graphite. One feature of the MHTGR design which helps deal with the seismic concern is the relatively deep embedment of the plant, which may help alleviate the motions and loads caused by a seismic event. The MHTGR program should make the seismic design basis of the plant more fully explicit, i.e., how the various structures, particularly including the core, are designed to provide adequate margins for the seismic events postulated in the design basis.