Accident Management Strategies for Consideration in the Individual Plant Examination Process (Generic Letter No. 88-20, Supplement No. 2)
April 4, 1990 TO: ALL HOLDERS OF OPERATING LICENSES AND CONSTRUCTION PERMITS FOR NUCLEAR POWER REACTOR FACILITIES SUBJECT: ACCIDENT MANAGEMENT STRATEGIES FOR CONSIDERATION IN THE INDIVIDUAL PLANT EXAMINATION PROCESS - GENERIC LETTER 88-20, SUPPLEMENT NO. 2 1. SUMMARY Over the past several years, the NRC has performed and reviewed numerous probabilistic risk assessments (PRAs) and severe accident studies. From this experience, it has become evident that it is possible to implement certain actions, or accident management strategies, that have significant potential for recovering from a wide variety of accident scenarios. These accident management strategies typically involve the use of equipment that already exists at plants. The NRC staff has compiled a list of such accident management strategies. The purpose of this letter is to forward these strategies to industry so that licensees can evaluate these or similar strategies for applicability and effectiveness at each of their plants as part of conducting the Individual Plant Examination (IPE) called for in Generic Letter 88-20: "Individual Plant Examination for Severe Accident Vulnerabilities." This generic letter supplement also transmits for information the enclosed NUREG/CR-5474, which contains a technical assessment of these accident management strategies. This generic letter supplement does not establish any requirements for licensees to take the specific accident management strategies into account as part of the IPE or to implement any of the strategies. Adoption on the part of a licensee of any accident management strategies in response to this supplement is voluntary. 2. DISCUSSION Generic Letter 88-20 directs each utility to perform an IPE to identify any plant-specific vulnerabilities to severe accidents and report the results to NRC. One purpose of the IPE is to determine whether modifications to hardware and procedures are necessary to reduce the frequency of severe accidents or to mitigate their consequences. An effective way of achieving that goal may be through the implementation of accident management procedures, that is, procedures that promote the most effective use of available plant equipment and staff in the event of an accident. In parallel with the IPE program, NRC is preparing to issue a generic letter in 1991 forwarding guidance to each licensee on the development of an accident management framework. The guidance will address identification and implementation of accident management procedures and associated hardware, development Technical Contact: R. Palla, NRR (301) 492-1076 9003300127 . 2 of training programs, definition of decisionmaking responsibilities, and development of technical guidance for operational staff such as technical support center personnel. The staff will work with the Nuclear Management and Resources Council (NUMARC) to define the scope and content of a utility accident management framework or plan, and the means of implementing such a framework. It is expected that the accident management framework each licensee develops will ensure that procedures are implemented in an effective and integrated manner, and that due consideration will be given to potential negative impacts on plant safety. Generic Letter 88-20 states that in the course of the IPE, utilities "may identify operator or other plant personnel actions that can substantially reduce the risk from severe accidents and that should be immediately implemented in the form of emergency operating procedures or similar formal guidance," and encourages each licensee to "not defer implementing such actions until a more structured and comprehensive accident management program is developed on a longer schedule, but rather to implement such actions immediately within the constraints of 10 CFR 50.59." The staff guidance document concerning the IPE submittal (NUREG-1335, "Individual Plant Examination: Submittal Guidance") requests that licensees document any strategies that were developed as part of the IPE process to prevent or mitigate the detrimental effects of severe accidents. As a result of experience with numerous PRAs and severe accident studies, the NRC staff has identified several accident management strategies that have significant potential for reducing plant risk. These accident management strategies can be grouped into three categories: x Conserving and/or replenishing limited resources during the course of an accident. These resources would include, for example, battery capacity, borated water, and compressed air. x Using plant systems and components for innovative applications during an accident. This usage would include enabling crossties of support systems or the use of fire systems, or control rod drive (CRD) pumps in the case of a boiling water reactor (BWR), for decay heat removal. In addition, this category includes procedures to connect alternate electrical power sources to meet critical safety needs during an accident. x Defeating appropriate interlocks and overriding component protective trips in emergency situations. An example of this strategy would be the ability to reopen main steam isolation valves (MSIVs) in a BWR anticipated transient without scram (ATWS) event. These three categories, and others as appropriate, can be applied to each of the major safety functions of the plant such as reactivity control, coolant inventory control, heat removal, and containment performance, as well as to the principal support functions such as electric power, equipment cooling, and air systems. Table 1 contains a list of examples of strategies derived from PRAs categorized into one of the three categories above. The NRC believes that a significant risk reduction benefit can be achieved, with reasonable resource expenditure, by implementation of emergency procedures and/or operating guidance associated with selected accident management strategies. . 3 We encourage evaluation of accident management strategies in conjunction with the IPE that each utility is expected to perform pursuant to Generic Letter 88-20. The IPE process is expected to disclose plant-specific design and operational information that will guide the evaluation of candidate strategies and the implementation of corresponding procedures or guidance, if appropriate. It is not intended that the evaluation of potential accident management measures be limited to the specific strategies identified in Table 1. For instance, strategies to maintain containment function and to delay or prevent possible early containment failure can also be assessed in this context. Accident management strategies related to external events would be considered at a later date. As part of the strategy evaluation process, we encourage licensees to consider the potential drawbacks or negative aspects of each strategy as well as the risk reduction potential. A detailed technical assessment of several accident management strategies is provided in the enclosed NUREG/CR-5474. This document provides evaluation guidance and cautions for each strategy. Licensees may wish to review the information provided in this document as part of the strategy evaluation and implementation process to provide added assurance that use of the strategy will not detract from overall plant safety. In the course of evaluating potential operator actions and accident management strategies, licensees may identify certain aspects of existing regulations or regulatory guidance that preclude, conflict with, or otherwise hinder the implementation of effective accident management measures, and that may not be in the interest of overall safety. Licensees are encouraged to inform the NRC of such situations so that NRC can consider the need for further clarification or modification of these regulations or guidance. 3. LICENSEE RESPONSE Licensees are encouraged to consider accident management strategies, such as those identified in Table 1 and assessed in NUREG/CR-5474, for applicability and effectiveness as part of conducting the Individual Plant Examination called for in Generic Letter 88-20. In accordance with guidance provided in NUREG-1335, licensees "should document any strategies to further prevent or mitigate the detrimental effects of severe accidents that were developed as part of the IPE process and for which credit has been taken in the analysis." For those plants with an existing PRA or IPE study, licensees may wish to consider the accident management strategies described herein, and document any procedures adopted as a result, as a follow-on to submittal of the IPE documentation. No documentation beyond that requested by Generic Letter 88-20 is necessary unless the IPE analysis is modified as a result of this evaluation. This generic letter supplement does not establish any reporting requirements. If reports to the NRC result from actions suggested herein, they are covered . 4 by Office of Management and Budget Clearance No. 3150-0011, which expires on January 31, 1991. The estimate of burden on licensees is covered by and unchanged from that presented in Generic Letter 88-20. Sincerely, James G. Partlow Associate Director for Projects Office of Nuclear Reactor Regulation Enclosures: 1. Table 1, "Generic Accident Management Strategies" 2. NUREG/CR-5474, "Assessment of Candidate Accident Management Strategies" 3. List of Recently Issued NRC Generic Letters . Table 1 Generic Accident Management Strategies I. Conserving and Replenishing Limited Resources x Refill refueling water storage tank (RWST) with borated water, or condensate storage tank (CST) with condensate. Assure adequate supply of boron on site. x Maintain emergency core cooling system (ECCS) suction to condensate systems to avoid pump failure as a result of high suppression pool temperature. x Throttle containment sprays to conserve water for core injection. x Conserve battery capacity by shedding non-essential loads. x Use portable battery chargers or other power sources to recharge batteries. x Enable emergency replenishment of gas supply, or otherwise ensure operability of air-operated components. x Enable early detection, isolation, or otherwise mitigate the effects of an interfacing systems loss-of-coolant accident (LOCA). II. Using Systems and Components in Innovative Applications x Strategies to enable emergency use of available pumps to accomplish safety functions. - Use diesel-driven fire systems for injection to the containment sprays, a BWR core, or the PWR steam generators. - Use control rod drive (CRD) pumps in BWRs or charging pumps in PWRs for core injection. - Use alternate injection (e.g., hydro test pump) when reactor coolant pump seal cooling is lost. - Enable emergency crosstie of service water and component cooling water to residual heat removal in BWRs or feedwater in PWRs. - Use condensate or startup pumps for feedwater injection. . Table 1 (Cont'd) x Strategies to enable emergency connection of available ac power sources to meet critical safety needs. - Use diesel generator or gas turbine generator to drive CRD pumps for core injection. - Enable emergency crosstie of ac power between two units or to onsite gas turbine generator. x Strategies to enable emergency connection of injection systems to alternate water sources. - Ensure appropriate recirculation switchover and cope with the failure to switch over in LOCAs. - Enable emergency connection of service water or feedwater systems to rivers, reservoirs, or municipal water systems. x Strategies for Reactivity Control. - Initiate standby liquid control system (SLCS) in case of potential core damage and guard against boron dilution when core injection is restored. - Ensure abundant supply of borated makeup for long-term accident control. III. Defeating Interlocks and Component Protective Trips in Emergencies x Reopen main steam isolation valves (MSIVs) and turbine bypass valves to regain the condenser as a heat sink. x Extend reactor core isolation cooling (RCIC) availability by either raising the turbine exhaust pressure trip setpoint or overriding the trip function. x Enable emergency bypass of protective trips for diesel generators and injection pumps.
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