United States Nuclear Regulatory Commission - Protecting People and the Environment

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 
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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.  
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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 
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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

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               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. 

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                              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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