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
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.
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,
Technical Contact: R. Palla, NRR
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
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
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
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 generic letter supplement does not establish any reporting
requirements. If reports to the NRC result from actions suggested herein,
they are covered
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.
James G. Partlow
Associate Director for Projects
Office of Nuclear Reactor Regulation
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
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
- 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
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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