1 INTRODUCTION
In the past, regulation and safety assessment of nuclear power plants concentrated on power operation as the most likely mode of operation in which a significant event could occur. However, recent events have brought about a new awareness of the risks present in other modes, specifically during shutdown operations. Lack of understanding of shutdown risks, compounded by an increase in non-routine activities, increases the probability of events occurring to an extent greater than previously considered. Furthermore, lack of correctional procedures and unavailability of mitigative equipment can reduce the effectiveness of the plant's corrective actions if an event should occur.
Cooling systems are needed to remove decay heat from a shut down plant. They require a water source and an electric power source. If sufficient cooling is not provided, the decay heat could cause the fuel temperature to rise enough to cause fuel damage and a radiological release. Maintenance activities can reduce the reliability of cooling systems if it is not fully understood how these activities affect the overall risk of radiological release.
Government and industry groups are working toward a better understanding of shutdown risk. Analyses of past industry experience and events as well as new probabilistic risk assessment (PRA) studies and individual plant evaluations (IPE) for shutdown conditions will help to locate weaknesses formerly overlooked and to assess the most appropriate corrective measures. The Nuclear Management and Resources Council (NUMARC), an industry group, has published NUMARC 91-06, "Guidelines for Industry Actions to Assess Shutdown Management". This document offers guidance on planning outages to keep risk minimal. The NRC has published NUREG 1449, "Shutdown and Low Power Operation at Commercial Power Plants in the United States". The report describes studies conducted by the NRC at several plants on operating experience, PRAs and utility programs for planning and control of shutdown operations. The report also discusses technical issues associated with shutdown operations and potential changes in NRC programs.
Through the study of electrical systems, decay heat removal systems, fire protection and containment closure issues, and outage planning methods, a more discernible image of the risks associated with shutdown operations is constructed.
7 CONCLUSIONS
These events, communications, and actions have increased awareness of shutdown risk. However, events continue to occur as a result of non-conservative approaches toward shutdown risk. There is still much to be researched to determine what changes in current practices are necessary to further enhance safety, specifically in the areas of plant activities and personnel attitudes.
Much has been done in improving the planning of outage activity schedules to reduce the coincidence of activities (e.g. switchyard activities, PWR midloop operations) which represent a significant risk to loss of cooling. However, some licensees' schedules are less conservative than others. This infers that the differences should be studied until a reasonable standard can be specified. An acceptable level of conservatism is yet to be determined, as well as the point at which plant resources would be better utilized in some other safety function.
Some non-conservative attitudes among personnel are observable both in the planning of outage activities and in their execution. The planning of those specific activities which are exclusive to shutdown operation is of concern since the activities have no power-operation safety standard basis. For example, the lack of procedures for some shutdown activities has, in part, caused some events. Since the consequences of such events would be more obvious during power operation, it is conceivable that, had the activities been scheduled for power operation, full procedures would have been written and implemented. This mode-based discrimination should be eliminated.
Non-conservative execution of activities is apparent specifically in the switchyard events. Stress reduction and training programs can minimize the possibility of personnel error, but large scale programs are yet to be fully implemented.
Although each event indicates a program failure, it presents an opportunity to learn and improve. Through research and analysis of the events, the industry has expanded its overall understanding of shutdown risk. However, a consistent approach to safety, along with the further development of programs, is necessary to ensure safety in shutdown operations.
Table of Contents
1 Introduction.................................................1
2 Electrical Issues............................................1
2.1 Switchyard Events......................................2
2.2 Non-standard Electrical Line-up Events.................5
2.3 Emergency Diesel Generators............................6
2.3.1 Emergency Diesel Generator Events...............7
2.3.2 NRC Guidance on Emergency
Diesel Generator Issues.........................8
2.4 Other Electrical Events................................8
2.5 NUMARC Guidance on Electrical Issues...................9
2.6 NRC Guidance on Electrical Issues.....................10
2.7 Industry Response to Guidance on Electrical Issues....10
3 Loss of Decay Heat Removal..................................11
3.1 Loss of Reactor Inventory.............................11
3.1.1 Loss of Reactor Inventory Events...............11
3.1.2 NUMARC Guidance on Loss of Inventory Issues....13
3.1.3 NRC Guidance on Loss of Inventory Issues.......13
3.1.4 Industry Response to Guidance on
Loss of Inventory Issues.......................13
3.2 Other Events That Cause Loss of Decay Heat Removal....14
3.3 Industry Guidance on Loss of
Decay Heat Removal Issues.............................14
3.4 NRC Guidance on Loss of Decay Heat Removal Issues.....15
4 Fire Protection Issues......................................15
5 Containment Closure Issues..................................16
6 Outage Planning.............................................17
6.1 Licensee Actions on Outage Planning Issues............17
6.2 NUMARC Guidance on Outage Planning Issues.............17
6.3 NRC Actions on Outage Planning Issues.................18
6.3.1 Inspections on Outage Planning Issues..........18
6.3.2 Guidance on Outage Planning Issues.............19
7 Conclusions.................................................19