Annals of Nuclear Energy, Vol. 2, pp. 33 to 35. Pergamon Press 1975. Printed. in Northern Ireland

RELIABILITY AND P E R F O R M A N C E OF U.S. LIGHT-WATER COOLED REACTORS

BRIAN K. GRIMES

Chief Accident Analysis Branch Directorate of Licensing, U.S. Atomic Energy Commission, Washington, D.C.U.S.A.

(Received 20 June 1974)

The information available from operating nuclear power plants in the United States continues to in- crease at a rapid rate. At the end of 1972, the United States had about 110 reactor years of power reactor operation. Approximately 30 reactor years of experience were added to this total in 1973. To give additional perspective, the largest nuclear power plant in operation before 1968 was about 250 MW. Over the last 3 years, a dozen plants have come into operation in the 800 MW range and plants as large as 1200 MW are under construction. As of May 1974, there were 44 plants with operating licenses, 54 plants under construction, and over 100 plants on order. Our data base for assessing the reliability and performance of light-water reactors will, therefore, continue to expand rapidly over the next few years.

Compiling and analyzing this data base is the continuing function of the Office of Operations Evaluation of the United States Atomic Energy Commission. I will summarize the results of 2 recent studies by this office; one on nuclear power plant availability for 1973, and one on the abnormal occurrences in 1973 which influenced these avail- ability statistics. (The relevant report numbers are 00E-0S-001 and 00E-0S-002, both dated May 1974.) The 27 plants selected for these studies ranged in capacity from 68 MW to 886 MW.

The availability of nuclear power plants has fallen short of what was expected. Plant availability is the per cent of total time in a given period that a unit was producing electricity. The availability data for the Calendar Year 1973 show an average plant avail- ability factor of 70 per cent for 27 light-water cooled plants which were in commercial operation for at least 3 months of the year. This is a decline from the 1972 average of 73.4 per cent for 19 plants in oper- ation 3 or more months of that year. It is also slightly less than the average for comparably sized

* A lecture presented at a meeting sponsored by the Nuclear Energy Group and the Southern Branch of the Institution of Mechanical Engineers at Harwell, U.K.

fossil fueled plants in 1972, the latest year included in Edison Electric Institute statistics.

There was an indication in the 1972 availability data that the problems of nuclear power plants were concentrated in the first several years of operation and that after that amount of time, continued per- formance at an availability factor of 80 per cent might be achieved. While individual plants had availability factors ranging as high as 95 per cent, the average 1973 availability factor for plants in this older age group was 67 per cent, less than the average for all plants. Numerous forced outages, primarily resulting from equipment malfunctions, have been responsible for the poor availability statistics. In 1973, for example, the 27 nuclear plants covered in our study experienced approximately 400 forced outages, resulting in about 36,000 h of plant down- time. This was a poorer performance on a per plant basis than we had in 1972 (211 forced outages for 19 plants). While the low availability factors for 1973 can be explained as being principally the conse- quences of extended shutdowns at 3 plants, the statistics are reflective of a general problem. This can be illustrated by removing the best 3 plants as well as the worst 3 plants from the data base. This results in an increase of less than 3 percentage points above the 70 per cent availability calculated with the full data base and is still slightly below the 1972 availability level. Incidentally, there does not seem to be any bias in the statistics with respect to power range or reactor type.

As a result of observing these conditions, we con- cluded that a systemized body of knowledge might focus attention on the problems being encountered in operating plants and help point the way to needed improvements. Accordingly, we have begun pub- lishing on a monthly basis a so-called "Gray Book" containing key performance data on all operating commercial nuclear power plants in the United States. Copies are being sent to all utility licensees.

In addition to data on such factors as hours of operation, power generated, availability and capacity

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34 B. K. GRIMES

factors, the Gray Books contain information on inspection and enforcement status, systems and component problems, plant shutdowns and abnormal occurrences.

A recent issue of the Gray Book discloses that during April there were 43 nuclear generating units licensed to operate. Of these, 31 were actually in commercial operation; the remaining 12 were in the power ascension phase. The data on outages con- firms our concern about the performance of nuclear plants since there were 35 forced outages during the month extending over 2,900 h and causing a loss of production estimated to be 1,042,000 MWh.

All of these forced outages had durations of more than 12 h. Of these extended outages, 25 were a result of equipment failure and three a result of operator error. The month of April also saw an unusually large number of scheduled outages with 8 outages for refueling, 12 for scheduled and un- scheduled maintenance or test, 2 for operator train- ing (license examinations), and 1 a result of a license restriction imposed during the course of a contested public hearing. The 51 scheduled and unscheduled shutdowns occurred at 21 of the plants in commercial operation (out of 31 total in this category).

Our principal concern about the frequency of forced outages resulting from component failures has been its effect on safety. Frequent reference is made to the fact that there has been no accident in a commercial nuclear power plant which has caused injury to a member of the public. This is an excellent record. But there have been minor incidents, many of which have had as their root cause poor quality assurance practice and performance, and a number of these have involved safety-related equipment or systems.

We have recently undertaken an analysis of ab- normal occurrences which took place at operating nuclear plants during 1973. There were 861 such incidents reported to the AEC. We considered 18, or about 2 per cent, to have been of direct safety significance. In 12 of these cases, radioactivity was released offsite at rates exceeding those permitted in the operating licenses, although total a m o u n t s re- leased did not in any case exceed permissible limits. Eleven of these 12 cases occurred at 1 plant, Quad Cities, and could legitimately be considered as a single event resulting from various small primary system leaks. In the other radioactivity release, at the Palisades plant, the high release rate resulted from an unfiltered purge of a steam generator prior to maintenance. Other events classed as directly signi, ficant, which are of more significance to safety and to

the analysis of forced outages than very small radio- activity releases included the following.

A fatigue failure of a primary coolant pump shaft at Surry Unit 1.

The discovery of wear on fuel channel boxes at Vermont Yankee.

An inadvertent criticality during fuel loading operations at Vermont Yankee.

A break in a feedwater line at Indian Point Unit 2. Also classed as of direct safety significance because

of a personnel injury was the case of a man injured when containment integrity was violated by opening of an emergency access hatch causing the man to be forced into the containment, which is kept at sub- atmospheric pressure. This occurred at Surry Unit 1.

The last case classed as of direct safety significance was the operation of 4 fuel bundles at maximum heat generation rate levels slightly exceeding those allowed by the technical specifications at Nine Mile Point Unit 1.

Of as much interest, although classed as of potential safety significance, were a number of failures of valves in safety systems to operate when tested and the occurrence of lightning strikes on several off-gas processing systems which led to hydro- gen explosions and minor radioactivity releases.

We classified 371, or about 43 per cent, of the ab- normal occurrences as having potential safety signi- ficance. These were events involving such incidents as unplanned or uncontrolled releases of radio- activity in amounts within permissible limits, mal- function of equipment related to safety, significant personnel or procedural errors, unusual transients, major natural events affecting safety, and violations of limits established in license conditions.

The remaining 471 incidents, or about 55 per cent, were considered to have had no safety significance.

A word of caution is appropriate at this point on use of the abnormal occurrence data contained in the reports previously mentioned to compare the per- formance or design adequacy of individual units or reactor types. While all abnormal occurrences reported to the AEC in 1973 were included in the study, there is a lack of uniformity in what is reported to the AEC. The definition of what con- stitutes an abnormal occurrence is set forth in the Technical Specifications for each individual nuclear power plant and varies from plant to plant. (The Technical Specifications are the technical rules within which the utility is legally bound to operate.) This non-uniformity has resulted from the evolu- tionary development of reporting requirements.

Reliability and performance of U.S. light-water cooled reactors 35

The reader of these reports is therefore cautioned not to draw conclusions regarding the quality of operation or design of individual plants solely on the basis of information contained in these reports. The AEC staff is currently working to standardize the definition of "abnormal occurrence."

Although not uniform enough in content at present to allow a comparative critque of particular designs or utility operating practices, the data do provide a feedback mechanism on the general types of prob- lems experienced--feedback which is valuable to both the safety analyst and the operations analyst.

Because of the significant number of abnormal occurrences, in particular equipment failures, which continue to be observed at nuclear power plants, increased emphasis has been placed on quality assurance programs over the last year. Several changes were initiated in quality assurance require- ments which we hope will bear fruit in the future.

New procedures with respect to the design and procurement phase were announced in June 1973. Under these procedures a utility must demonstrate that it has developed and is implementing an adequate quality assurance program for ongoing design and procurement before its construction permit application will be docketed for formal review.

A paper description of intent to have such a quality assurance program is not sufficient. The program must be in being, as evidenced by a written program, qualified personnel to carry it out, an organization for quality assurance with unambiguous and independent responsibilities and authority, a well-developed manual of procedures, an inspection program, a document control system of records attesting to quality, and an audit system to tell management whether the overall program is work- ing. An AEC inspection is conducted as a part of the predocketing review to verify the existence of all these aspects of implementation.

To assist each prospective utility applicant in meeting these quality assurance requirements for design and procurement, a meeting is held with the utility some six to twelve months before the con-

struction permit application is filed. The purpose of the meeting is to explain the quality assurance requirements early enough to enable the utility to include adequate provisions in its design and pro- curement contracts and generally to meet the require- ments without them becoming a delaying factor on the project's critical path.

Another change we have in prospect is having AEC inspectors undertake direct inspection of the quality assurance activities of nuclear steam system suppliers, component vendors, and architect-engi- neers. Until now we have been holding the utility customer of these organizations responsible for their quality assurance implementation. This is an un- wieldly procedure. Direct AEC inspection, in our opinion, should be a better procedure and we are prepared to test its usefulness.

Another innovation which we expect to implement soon is the concept of resident inspectors. Basing our inspection effort in the 5 regional offices may have led to a high degree of reliance on records, perhaps at the expense of actual observation of activities at the reactor sites. We believe that locating inspectors near a cluster of sites will provide greater opportunity for such observation and that quality assurance programs, among others, may benefit.

Finally, I would like to briefly compare the per- formance of nuclear power plants and large fossil units. The 1973 nuclear plant availability factors determined in the present study were compared to the average annual experience for the 13 year period 1960-1972 inclusive as compiled by the Edison Elec- tric Institute. The Edison Electric Institute report includes standby time in the computation of the plant availability factor resulting in figures for fossil units approximately 3 per cent higher than those that would have resulted using the same computational methods used in the AEC nuclear power plant study. When these differences are eliminated, the plant availability factors for large nuclear power plants are as good as or better than those for comparably sized fossil-fueled plants.