E-Journal of Advanced Maintenance Vol.7-1 (2015) 8-13
Japan Society of Maintenology

Update on Implementation of Severe Accident Countermeasures in the
United States
Ryan VANSTON1*, Francis FERRARACCIO2, Nirmal JAIN2, Daniel FLAHIVE2, andLauren HELSTOSKY2
1
2

Westinghouse Electric Company LLC, 1000 Westinghouse Drive, Cranberry Township, PA, USA 16066
Westinghouse Electric Company LLC, 20 International Drive, Windsor, CT, USA 06095

ABSTRACTThe progress of the nuclear industry within the United States (U.S.) in responding to regulatory changes
following the events at Fukushima Dai-ichi is significant and rapidly evolving. The U.S. Nuclear Regulatory
Commission (USNRC) has issued questions related to submitted utility approaches in satisfying Order
EA-12-049. In addition to responding to these questions, the U.S. nuclear industry has continued to perform
plant modifications, procure equipment, establish national response centers, establish new plant procedures, andaddress other Regulatory Orders and Directives. In addition to these required activities, the U.S. nuclear industry
has also identified means to utilize these new capabilities to provide other benefits.
KEYWORDS

ARTICLE INFORMATION

U.S. FLEX, ELAP Response, Mitigating Strategies, EA-12-049

Article history:
Received 11 November 2014
Accepted 19 January 2015

1.

Introduction

The earthquake and subsequent tsunami which occurred near Japan and impacted the Fukushima
Dai-ichi nuclear site demonstrated how beyond-design-basis external events (BDBEE) could
challenge the defenses at nuclear power plants. In response, the USNRC established new
requirements for mitigation of BDBEE [1] which were issued on 12 March 2012. These
requirements direct licensees to implement and maintain strategies to maintain core cooling,
containment integrity, and spent fuel pool cooling following a BDBEE. The Nuclear Energy
Institute (NEI) developed a methodology to address these requirements [2a] which was endorsed by
the USNRC [3]. All U.S. utilities submitted Overall Integrated Plans (OIPs) for review in February
2013 based on this guidance. The purpose of this paper is to provide an update on activities
following submittal of those OIPs and includes both Regulatory and plant specific implementation
aspects.

2.

Activities Following the Initial Submittal of OIPs

2.1. Regulatory Interface on Submitted Material
After submittal of the initial OIPs, plants are required to submit updates on six-month intervals
to outline items important to the regulatory review process. These items include agreement withinternal schedules, open item generation or closure, and significant milestone completions. In orderto provide a cohesive Regulatory review, items applicable to all plants were identified and have been
considered generic items.
The review by the USNRC has been considered informal by nature considering the
beyond-design-basis aspects of the Order, the extensive number of parallel activities, and the*Corresponding author, E-mail: vanstort@westinghouse.com

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

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R. Vanston, et al. / Update on Implementation of Severe
Accident Countermeasures in the United States

challenging review schedule. To make the most efficient use of resources, both within the USNRC
staff and plant personnel frameworks, electronic document sharing interfaces were established. This
allows for timely review of higher risk items to allow for resolution early in the review process andestablishes a positive closure pathway directed toward the time at which the plant is required to be incompliance with the Order.
2.1.1. Generic Regulatory Issues
The USNRC initially performed reviews on selected lead plants for a range of nuclear power
plant design types. These lead plant reviews resulted in a large number of Regulatory questions
which were issued as generic audit questions to the individual utilities. The lead plants coordinatedtheir responses to the regulator with other sites through the Nuclear Energy Institute (NEI). Throughthis mechanism, NEI and the USNRC identified several key generic issues. Several U.S. industry
groups including NEI, the Pressurized Water Reactor Owners Group (PWROG), the Boiling Water
Reactor Owners Group (BWROG), and the Electric Power Research Institute (EPRI) coordinatedresponses to the USNRC on these issues and submitted industry white papers in response to thegeneric questions. These generic issues include questions on both the scope and methods included in
the OIPs.
The Regulatory inquiries on the scope of the OIPs pertained most significantly to the required
engineering rigor for mitigation of events initiated while the plant is in a shutdown state. On 30
September 2013 the USNRC issued a letter endorsing a more detailed position regarding theserequirements closing the issue [4]. The PWROG has subsequently been authorized to perform work
consistent with the scope of these requirements in supporting shutdown conditions. The Regulatory
questions related to applied methods are more varied and include the qualification and validation of
the thermal hydraulic codes used to predict the extended loss of alternating current power (ELAP)
event transient, the mechanisms and timing of the mixing of boric acid in the reactor coolant system(RCS), the modeling of reactor coolant pump (RCP) seal leakage, the use of anticipatory containment
venting in boiling water reactor (BWR) strategies, and the reliability of station direct current (DC)batteries.
The importance of the qualification of thermal hydraulic codes used to predict the timeline of the
ELAP is directly related to the validation of the timing and deployment activities expected to occur
during the postulated event. Utilities commissioned generic thermal hydraulic analysis supporting
strategy development. BWR stations used the Modular Accident Analysis Program (MAAP) to
predict both containment response and RCS thermal hydraulic response. Several pressurized water
reactor (PWR) stations also used MAAP to predict containment response. The USNRC issued an
endorsement of these uses of MAAP with specific limitations concerning the code versions and themanner in which the code has been applied [5]. The USNRC position did not accept the use of
MAAP for prediction of thermal hydraulic response for PWR nuclear steam supply systems (NSSS).
The PWR fleet, through the PWROG, typically applied codes consistent with previous station
blackout experience varied by NSSS type. Specifically, Westinghouse NSSS plants applied
NOTRUMP, the Combustion Engineering fleet applied CENTS, which has been endorsed [6], and theBabcock and Wilcox (B&W) NSSS applied RELAP5. The USNRC requested that additional
justification be provided for the use of these codes to predict the transient thermal hydraulicconditions. The USNRC endorsed the use of CENTS for prediction of the ELAP transient response
prior to entry into the reflux cooling condition. The USNRC has not yet endorsed the use of
NOTRUMP for Westinghouse NSSS; however such endorsement is expected within 2015 withconstraints similar to those identified in the CENTS endorsement.
The results of the thermal hydraulic codes for PWR NSSS depend on assumed RCP seal leakage
and the mixing of boron in the NSSS. The USNRC questions on boron mixing were addressed withrespect to ensuring both the continued transport of boron through the RCS and changing velocities in
mixing zones. This position, with limitations, was endorsed by the USNRC [7]. The questions on
RCP seal leakage and its impacts to coping timing continues to be a challenge and are still being
pursued for all seal types. Closure is anticipated in the near-term for the BDBEE application.
In BWRs, it has been recognized that anticipatory venting of containment is an effective means
to remove heat from containment, limits containment pressure, requires minimal support systems, and9R. Vanston, et al. / Update on Implementation of Severe
Accident Countermeasures in the United States

is readily available. Using lessons learned from the events at Fukushima Dai-ichi, earlier venting
could have provided more time for operator actions to prevent core damage and ensure containment
integrity. The USNRC issued an endorsement of the generic venting strategy in January 2014 [8]while still requiring plant-specific implementation results on such items as the capabilities of the ventpathway, net positive suction head for the reactor coolant system injection pumps, and guidance toprevent negative pressure in containment.
NEI developed a position paper to demonstrate that adequate DC power is available to ensure
successful implementation of a site’s mitigation strategies, required per NEI 12-06 guidance.
Because the initial assumed failure of onsite diesel generators, site batteries play a key role during theinitial coping phases of the postulated event. The position outlines the actions required to extend theFlexible and Diverse Coping Strategies (FLEX) mission time of the batteries by load shedding tomaintain power to key instruments. Because the batteries are safety-related, existing maintenanceand surveillance programs are adequate to trend battery capacity and ensure degradation is addressed
before design basis capacity is challenged. The USNRC endorsed the approach in September 2013[9] for demonstrating that the batteries can perform as credited.
2.1.2. Overall Integrated Plan Maintenance
After regulatory review of the OIP, closure of open items, and completion of site procurement,
installation, and programmatic items, the site will submit compliance documentation after the lastunit’s outage and Regulatory endorsement would follow. The process by which sites will maintaintheir respective Final Integrated Plans (FIP) would be in similar to the structure of a fire protection
plan. The sites would maintain an overall program document available for audit. This document
would be required to be in agreement with the NEI 12-06 guidance document. Minor changes would
not constitute a regulatory review; however, major changes, or those seen to deviate from theguidance in NEI 12-06, would require Regulatory review.
2.2. Plant Modifications and Equipment Procurement
Plants performed site-specific hydraulic calculations to confirm flow requirements relating tocore cooling and spent fuel pool cooling and are in the process of modifying plant systems to reflectrequired configurations for their FLEX strategies. In addition to fluid system modifications, electricalmodifications are ongoing to provide the necessary power supply connections to keep required
available equipment operational. These modifications allow for diversity in the event of a BDBEE
and support both the onsite and off-site support strategies. Equipment procurement is proceeding
both at the plant-specific level to allow for short-term coping capabilities and at the national support
level to maintain a long-term coping strategy.
Utilities have purchased portable equipment to meet key functions. In PWRs high flow, lowpressure pumps have been purchased to provide makeup to steam generators. High pressure low
flow pumps have been purchased to maintain RCS inventory. New connections have been added toexisting systems to connect the new portable equipment. These connection points to plant systems
satisfy both deployment time constraints and functional requirements.
Instrumentation, critical to monitoring the event, was identified and load shedding strategies aredeveloped. Low voltage generators have been purchased to power installed battery chargers.
Modifications to electrical systems have been made to provide connections for portable equipment.
Onsite storage structures have been erected to safely store and protect the equipment deemed to
be used during the BDBEE response. These structures provide protection through diversity against
the applicable events for each site as well as redundancy. Plants may have chosen to design a lesser
number of storage structures designed to a higher design pedigree for all applicable events or choose alarger number of storage structures that are designed for lesser protection considerations against
individual external events provided that at least the base quantity of equipment is postulated to survive
the external event.
In addition to utilities purchasing equipment to be stored onsite, national response centers havebeen established at two locations in the U.S. These locations have backup equipment for eachplant’s onsite equipment as well as large portable equipment such as medium-voltage generators to10

R. Vanston, et al. / Update on Implementation of Severe
Accident Countermeasures in the United States

support plants recovering from the ELAP event. Some equipment has been identified to be
applicable to all plants whereas some equipment is only specific to certain plants. Plants will maintainspecific deployment guidance with these two national response centers that outlines delivery items
and protocol that could vary depending on plant status and external event to provide an optimal
response. Tests of the capabilities of these centers have occurred and are considered to be
operational as of September 2014 [10]. Full implementation supports the interface between thenational response centers and federal, state, and local government and the utilities.
2.3. Development of Operational Guidelines and Plant Procedures
The PWROG initiated two projects to better understand the plant response to an ELAP event.
The first project was created to understand plant response with minimal operator action. The second
project explored additional questions pertaining to the best procedural practices to prevent or mitigate
undesirable plant conditions and better lead to plant recovery. The output of these projects was used
to develop operational guidelines and plant procedures. In December of 2012, the PWROG
developed generic FLEX Support Guidelines (FSGs) to assist utilities in the development ofsite-specific OIPs. In the spring of 2013, the PWROG developed NSSS-specific FSGs. Plants arein the process of using the information to create site-specific FSGs and revise existing site procedures
to incorporate the FSGs.
NEI is in the process of developing emergency response procedures and guidelines for extreme
events and severe accidents [11]. The purpose of this document is to provide guidance for ensuring
the emergency operating procedures, extensive damage mitigation guidelines, FSGs, and severe
accident management guidelines (SAMGs) are integrated in a cohesive, effective, and usable manner.
This guidance will also address the Regulatory basis of procedural integration, requirements for
SAMGs and supporting guidelines, and a command and control structure for responding to an extreme
event or severe accident.
2.4. Validation of Event Strategies
The structured anticipated timeline of the event was provided within the OIP of each plant. This
timeline outlines actions considered to be relevant to coping actions required for a successful strategy.
This timeline also identifies time constraints or the maximum time period associated with a FLEX
strategy for which an action or actions can be completed and still allow for a successful strategy.
The ability of a licensee to validate these steps is dependent on the relative timing of the action andthe impact on normal plant activities. As an example, it is not expected that a utility will connect andoperate FLEX equipment with plant systems during the validation process; however, adequate
justification that the action of deploying, connecting, and operating a pump can be performed within
the timeframe by which flow from that pump is required. Much like the pump deployment time,
analyses have been developed using analytical tools, such as thermal hydraulic codes, and procedural
support to develop timings for all related actions.
To assist the industry with this process, NEI has developed a Validation Process [2b]. This
process is required to validate the plant-specific timing and deployment analysis and provide
reasonable assurance that the plan will be successful. Because not all tasks are considered equalwith respect to the relationship to success, a grouping rubric was developed that categorizes actions
based on timing and importance. The groupings allow for variations in the processes required tovalidate a particular action and the validation methods are commensurate with the level of importance.
As an example, an early action may require a simulated scenario, whereas an action that occurs laterin the event requires reasonable judgment by subject matter experts at site. Validation of each action
is required and the summary of this review would be documented in the FIPs.
2.5. Maintenance of FLEX Equipment
NEI 12-06 guidance required that standard industry templates and associated bases are to be
developed to define specific maintenance and testing associated with equipment. As part of theregulatory requirement to define maintenance and testing programs for FLEX equipment, EPRI

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R. Vanston, et al. / Update on Implementation of Severe
Accident Countermeasures in the United Statesdeveloped a preventative maintenance basis for FLEX equipment which was endorsed by the USNRC
in October 2013 [12]. Consideration was taken to recognize that the equipment was to be used for
BDBEE coping and the equipment may be of a commercial grade. There was also consideration thatthe use of equipment was unusual compared to equipment currently in use. The FLEX equipment
will experience long durations of standby service followed by potentially high intensity usage during
disaster response. The templates categorize equipment based on critical and noncritical use, highand low duty cycles, and mild and severe service conditions. The approach is economical,
organizing appropriate tasks and intervals to address more significant or likely degraded states forvarying equipment types, while accounting for the influences of functional importance, duty cycle,
and service conditions.
2.6. Incorporation of FLEX into Other Accident Mitigation Capabilities
In addition to Order EA-12-049, the USNRC Near-Term Task Force (NTTF) [13] recommended
reevaluation of seismic and flooding conditions at sites as well as implementation of post-fuel damage
response capabilities. In the case of seismic requirements, the U.S. approach requires that plantcomponents credited in the ELAP response be assessed against potentially higher seismic conditions.
The Expedited Seismic Response Program (ESEP) [14] was created to identify such components
and determine their susceptibility, enabling sites to rapidly evaluate strategies prior to final
implementation of the respective programs. A similar program is in place to incorporate potential
updates to changing flooding requirements. A plan to integrate the EA-12-049 requirements with theflooding re-evaluations has been recommended by the NRC Commissioners [15]. Also, work isunderway to incorporate new plant capabilities into the severe accident guidelines supporting
post-core damage response capabilities.
2.7. Investigation of Ancillary Benefits of Plant Upgrades
As described in the previous sections of this paper, the U.S. based utilities are expending
significant resources in both labor and capital to address the Regulatory requirements which inprinciple will result in significant plant modifications, purchase and long term maintenance andstorage of significant equipment to support public safety and plant recovery resulting from a BDBEE
condition. In consideration of this, some utilities have, in parallel to their required efforts, initiatedconcerted efforts to find ancillary uses for their FLEX equipment or modifications. Of course, theseuses will need to be consistent with the required availability of the dedicated equipment and planned
response strategy; however, a few utilities have identified several ways to expand the benefits of thisequipment into routine Operational or Outage Support activities. The benefits can be associated withdirect economic benefits such as reduced outage time, or less direct such as improved risk state.
FLEX generators can be pre-deployed during an outage to provide temporary or backup power toredundant installed spent fuel pool (SFP) cooling pumps. This particular example provides animproved risk state for the plant as a parallel success path for protecting the SFP cooling safetyfunction.
FLEX pumps can be pre-deployed and aligned to the unit’s refueling water storage tank and used
to refill portions of systems needing borated inventory following scheduled piping or valve
maintenance that resulted in system draining; also refill of inventory to borated passive injection tanks.
This function specifically reduces operating cycles and wear on installed safety injection pumps
which otherwise would be used to support post maintenance and startup needs.
Given the significant expenditure of resources required to address the requirements described,
some utilities have initiated programs to identify uses for the upgrades which provide more direct
benefits, such as reduced outage time or reductions in risk. The ability to benefit from these ancillaryfunctions has to be carefully aligned with both the requirements to maintain availability of the FLEX
equipment and with the interconnection of this equipment with installed plant equipment. By
regulation, the FLEX equipment must remain available for imminent use; however, some equipment
is operating Mode dependent, allowing for its availability for other purposes when the plant is not in
the corresponding Mode. With regard to plant system interconnection, the typical non-nuclear safety
grade status of the FLEX equipment makes it unacceptable to interconnect with the safety grade plant

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R. Vanston, et al. / Update on Implementation of Severe
Accident Countermeasures in the United Statesequipment when that plant equipment is in its operable state. As noted with the prior examples,several opportunities have been identified where these challenges can be addressed by design and realtime plant economic benefit can be realized.

3.

Conclusion

Significant progress has been made in implementing strategies to maintain or restore corecooling, containment, and spent fuel pool cooling capabilities following a BDBEE in the U.S.
Several lessons learned during the implementation of these processes have been presented. Keyelements, successes, reference materials, and potential issues have been highlighted providing
guidance to those performing similar activities in other Regulatory environments.

References
[1]
[2a]
[2b]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]

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Owners Group (PWROG),” January 8, 2014. (ADAMS Accession Number ML13276A183)
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(ADAMS Accession Number ML13358A206)
USNRC Letter to NEI Endorsing NEI White Paper “Battery Life Issue,” September 16, 2013. (ADAMS
Accession Number ML13241A188)
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Operational Status,” September 11, 2014. (ADAMS Accession Number ML14259A222)
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and Guidelines for Extreme Events and Severe Accidents,” March 2014. (ADAMS Accession Number
ML14049A005)
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Maintenance Basis for FLEX Equipment,” October 7, 2013. (ADAMS Accession Number
ML13276A224)
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