Following the March 2011 events at the Fukushima Daiichi nuclear power plant, the US Nuclear Regulatory Commission (NRC) established a task force to determine whether safety improvements should be recommended for US commercial nuclear power plants in light of the lessons learned in Japan. One of the recommendations from this task force, issued on 12 March 2012, was US NRC order EA-12-049, requiring nuclear facilities to implement mitigation strategies for a beyond-design-basis external event (BDBEE) using a three-phase approach. The initial coping phase relies on installed equipment and resources to maintain or restore core cooling, containment and spent fuel pool (SFP) cooling capabilities. The second phase relies on portable, on-site equipment and consumables to maintain or restore these functions. The third and final coping phase will rely on off-site resources to sustain those functions indefinitely.

The US nuclear industry, working through the Nuclear Energy Institute (NEI), developed a framework for addressing this order, as documented in NEI 12-06 "Diverse and Flexible Coping Strategies (FLEX) Implementation Guide," published in August 2012. The NRC endorsed this document (with comments) via Interim Staff Guidance JLD-ISG-12-01. Commercial nuclear plants operating in the United States must submit an integrated plan for demonstrating compliance with the order and interim staff guidance. These plans must be fully implemented no later than two refueling cycles after submittal, or by 31 December 2016, whichever comes first. Finally, the US NRC intends to issue a Safety Evaluation Report (SER) for each site following the demonstration of successful implementation of the FLEX plan.

FLEX objectives

Within NEI 12-06, the industry defines the term "FLEX" as an approach to develop flexible and diverse strategies for increased defence-in-depth for extended loss of AC power (ELAP) with a loss of normal access to the ultimate heat sink (LUHS), which occurs simultaneously at all units on a given site. By providing multiple, diverse methods to supply power and water to support key safety functions, FLEX is intended to mitigate the consequences of a BDBEE.

The design basis of US plants provides protection against a broad range of extreme external hazards. FLEX is intended to protect against a BDBEE, but the potential scope of beyond-design-basis conditions makes it impossible to plan for all possible conditions. NEI 12-06 has identified five classes of hazards that must be evaluated on a site-specific basis to determine applicability and develop FLEX strategies. These five classes are seismic events, external flooding, storms (hurricanes, high winds, tornadoes), snow and ice storms and extreme cold, and extreme heat.

Each plant is required to evaluate the FLEX protection and deployment strategies with regard to site-specific external hazards. Depending on the challenges presented, the approach and specific implementation strategy will vary from site to site. However, regardless of the challenges presented, US plants are required to implement FLEX with specific attention to the following four key FLEX elements, requiring:

1. Portable equipment that provides a means of obtaining power and water to maintain or restore key safety functions for all reactors at a site

2. Reasonable staging and protection of portable equipment from a BDBEE applicable to a site

3. Procedures and guidance to implement FLEX strategies

4. Programmatic controls that ensure the continued viability and reliability of the FLEX strategies.

Because of the many elements of FLEX, implementing a FLEX programme is a complex undertaking that requires a strong understanding of the nuclear steam supply system (NSSS), knowledge of the generic industry FLEX activities undertaken by the Pressurized Water Reactor Owners Group (PWROG) and of the basis and intent of the FLEX requirements. An approach to developing a FLEX implementation plan is outlined here via implementation of the following four tasks.

Task 1: Gap analysis

As previously stated, the FLEX strategies for coping with a BDBEE involve a three-phase approach.

Phase 1: Rely initially on installed plant equipment

Phase 2: Transition from installed plant equipment to on-site FLEX equipment

Phase 3: Obtain additional capability and redundancy by using off-site equipment until power, water, and coolant injection systems are restored or commissioned.

A gap analysis is performed to define "pinch points" for each phase and identify analysis or modifications to provide margin (that is, increased time) for the FLEX strategy. This effort is completed by evaluating the plant’s current ability to respond (with installed systems and existing procedures) to an indefinite ELAP and LUHS to determine how long the plant would cope under such an event before losing one or more key safety functions (core cooling, containment integrity or SFP cooling). This assessment requires the review of coping studies that have been completed for the plant, as well as generic coping analyses that have been done for the industry. The gap analysis first determines how long plants can maintain the three key safety functions by using installed equipment such as plant batteries, steam-driven pumps and protected water sources. The gap analysis does credit actions such as load shedding to extend the life of the batteries (if a load-shedding strategy exists).

Also included are the effects of external hazards on the site and how such hazards impact coping times. For instance, many plants do not have a seismically-qualified or missile-protected source of feedwater (such as a condensate storage tank). Without that, a gap could result if a seismic event or a missile from a high-wind event, such as a hurricane or tornado, impacted feedwater availability at the beginning of an ELAP and LUHS event.

The gap analysis will also evaluate the potential to demonstrate that non-seismically qualified or non-missile protected equipment (for example, non-seismic tanks, condenser hotwell) can be seismically rigid and missile protected so it can be credited in the initial FLEX strategies. Techniques for demonstrating this include seismic fragility and nonlinear analyses.

The gap analysis determines how long a plant will be able to cope using only plant equipment that is permanently installed, seismically qualified, and flood-, wind-, tornado- and missile-protected. This establishes the initial phase of FLEX and how much time the plant will have before on-site portable equipment must be used.

Modifications and strategies can be used to help extend the initial coping phase of a plant. This could include procedure modifications to begin deep load shedding early in an event, installation of low-leakage seals to preclude the need for an immediate cooldown of the plant, or defined operator actions to protect critical assets like the turbine-driven auxiliary feedwater pumps and associated control cabinets from overheating because of a lack of ventilation during the ELAP event. These modifications would address potential pinch points that are currently limiting the coping time.

Once the initial phase is defined, the transition phase can then be scoped. It would include the strategies and requirements of the portable equipment that can maintain key safety functions. This also includes the scoping of the connection points, which would include the ability to use the connection points following an external event such as seismic, flooding, tornado, and so on. The protection of the portable equipment from the external event also must be considered when establishing storage locations and requirements. The FLEX guidelines require a N+1 configuration, which means that however many units are on site, there must be that many plus one additional piece of equipment, connection point, and so on, to provide defence-in-depth.

The final deliverable of the gap analysis is a report summarizing current operating conditions against the expectations of FLEX, the identification of actionable items for complete FLEX implementation, and any areas requiring additional analyses.

Task 2: FLEX assessment

The FLEX assessment provides analysis of the key safety functions (core cooling, containment integrity and SFP cooling), including:

  • Selecting and confirming the functional requirements of FLEX equipment
  • Establishing timing requirements for deploying FLEX equipment
  • Identifying and prioritizing water sources
  • Conducting electrical coping studies to prioritize equipment needs
  • Identifying any additional analyses required (for example, reflux cooling)
  • Identifying instrumentation solutions to monitor key parameters

An analytical basis is recommended for establishing the functional requirements and timing necessary for deployment and use of the FLEX equipment. To support the industry in implementing FLEX, the PWROG, through Westinghouse, has implemented a baseline project known as Project Authorization ASC-0916, which generated baseline thermal-hydraulic analyses (PA-PSC-0965) to provide insights and trends of the primary system response and plant cooldown to an ELAP and issued the results via Westinghouse document WCAP-17601. Plants can confirm the applicability of these generic analyses to their specific needs, or they can perform additional analyses to confirm the timing of their strategies.

These primary system analyses will establish such functional requirements as the flow rate and head required of a portable pump to inject into the steam generator (SG), necessary to maintain secondary cooling. A reactor coolant system (RCS) makeup strategy will also be established, including the ability to make up for coolant shrinkage (reduction in RCS inventory level due to the cooldown), to make up for any leakage through reactor coolant pump seals and to add boron to the RCS to prevent the reactor from going recritical. There are several options for the RCS make-up strategy, including adding connections for a low- or high-pressure portable injection pump, or relying on the accumulators. During the FLEX assessment, a primary strategy and timing will be developed, and these will form the basis for procedural and plant changes.

Containment analyses also are performed to determine the maximum pressure and temperature that would occur during the ELAP and LUHS event. These containment analyses will determine if modifications are required that would allow portable pumps to spray the containment to reduce long-term pressure and temperature.

The decay heat from the reactor is removed via steam production in the SGs; eventually plants will run out of clean water to feed the SGs. Since coping must be indefinite, unclean water sources that will provide injection into the SG should be evaluated to understand the impact of unclean water on the SG. The use of unclean water could impact the SG function and damage the intake of the turbine-driven auxiliary feedwater pump. An analysis will prioritize the various water sources available on-site to minimize this potential fouling, and will determine how long plants have until they will need to introduce a filtering system to prevent SG tube clogging and maintain indefinite core cooling.

In addition to the long-term water supply, a long-term supply of electricity must also be established. Plant power distribution systems are very complex, and a study should be performed to determine the existing battery life, assess the potential for extending the batteries through DC load shedding, and determine a strategy for repowering low and medium voltage busses by using portable generators.

Finally, key instrumentation will be identified that will allow the operators to monitor and control the plant indefinitely. The PWROG has established a generic instrumentation list that balances the need for the operators to understand the condition of the plant, along with the concern that too much instrumentation can drain the batteries during the initial stage of the event. A minimum set of instrumentation has been established for both the initial phase of the event and the transition phase when portable generators will be available to repower vital DC buses (also in ASC-0916).

Task 3: Modifications

This task identifies the specific modifications required to successfully implement the FLEX strategies established in Task 2. The modifications must provide for a primary and alternate means of accomplishing a given function (defence-in-depth) and must allow for the ability to maintain the function during a severe external event. For example, quoting the NEI guide, at least one connection point of FLEX equipment will only require access through seismically robust structures. This includes both the connection point and any areas that plant operators will have to access to deploy or control the capability. In addition to assessment for seismic events, the modifications must be assessed for flooding, severe storms with high winds, snow, ice, cold and extreme heat. FLEX system modifications can include items such as:

  • Auxiliary feedwater SG injection (primary and alternate)
  • Primary system injection (primary and alternate)
  • SFP makeup/spray (primary and alternate)
  • Containment spray (primary and alternate)
  • Tank modifications to facilitate refilling from portable pumps
  • Low-voltage electrical connections (primary and alternate)
  • Medium-voltage electrical connections (primary and alternate).
  • In addition to the modifications to the plant, one or more on-site storage facilities will be required to house and protect the FLEX equipment from severe external events.

Task 4: Licensing support

The PWROG initiated, through Westinghouse, the project that led to PA-PSC-0965, entitled "Emergency Response to Extended Station Blackout Events". The scope of this project includes developing generic FLEX Support Guidelines (FSGs), identifying interfaces to existing Emergency Operating Procedures, identifying key plant instruments to be used when applying battery load-shed strategies, and providing a generic basis to establish timing and type of portable equipment and strategies to respond to an ELAP.

The project will provide industry standardization by developing generic modifications to the Emergency Operating Procedures that incorporate FSGs and providing operator instructions regarding how and when to use the FLEX equipment. These procedures must be implemented on a plant-specific basis, taking into account the specific FLEX strategies and modifications established for each plant. The FSGs include instructions on how to realign from the FLEX equipment back to plant equipment once the event is over.

Since the design basis of the plant is being modified to incorporate FLEX strategies, there may be some impact on the licence of the plant. For example, in the United States, plant modifications that allow for portable equipment hook-ups would require at least a screening via the 10 CFR 50.59 process. The specific impact and means of addressing the changes depend on the country-specific licensing process.

FLEX integrated plan

Licensees were required to develop and submit integrated plans for FLEX implementation by the end of February 2013. The level of detail considered adequate for this plan is consistent with the level of detail contained in the licensee’s Final Safety Analysis Report. In addition, the US NRC staff expects the licensee’s submittal to provide the following information, based on NEI 12-06:

  • Description of the guidance and strategies to be developed to meet the requirements
  • Description of major system components and the applicable protection being incorporated for the associated equipment from external events, including applicable quality requirements
  • Demonstration of how the strategies will be implemented in all modes
  • Demonstration of the necessary procedures, guidance, training, acquisition, staging or installation of equipment needed for the strategies, including necessary modifications
  • Piping and instrumentation diagram or conceptual sketches, as necessary, to indicate equipment that is installed or equipment hookups necessary for the strategies
  • Update of implementation schedule milestones.

Once submitted, the US NRC will review each plan and will either generate Requests for Additional Information or issue a draft SER on the plan. This draft SER becomes a final SER once the FLEX plan is fully implemented at each site.

The four tasks discussed above can be summarized in an overall FLEX integrated plan that includes a technical basis document. The document provides the basis for the FLEX strategies and summarizing all conceptual designs for FLEX modifications, as well as staffing requirements for implementation.

A complex process

A plant-specific FLEX implementation is a complex undertaking that requires a strong understanding of the NSSS, generic industry FLEX activities undertaken by the PWROG and the FLEX requirements outlined in NEI 12-06. Many plants have purchased portable equipment to extend coping; however, implementing the FLEX approach requires a basis for the strategies so that the equipment, modifications and procedures are aligned to provide the best chance of success in the event of a BDBEE. Initially developing the analytical basis will mitigate the risk of developing a flawed FLEX strategy. A successful FLEX programme will provide multiple, diverse methods to supply power and water to support key safety functions and will mitigate the consequences of a postulated BDBEE.

 

Mike Powell is director, Fukushima initiatives, Arizona Public Service Company (an owner of Palo Verde NPP). Jeff Taylor is integrated programme manager, post-Fukushima products and services, Westinghouse Electric Company. Susan Baier is product manager, fluid systems engineering, Westinghouse.

While some of the specific products developed by Westinghouse for the PWROG only apply to pressurized water reactors (for example, the analytical basis and procedures), the approach outlined in this article has been applied to both PWRs and BWRs in the United States. The key difference for BWRs is the analytical basis established as part of Task 2 required plant-specific thermal-hydraulic models.

References

1. EA-12-049, "Issuance of Order to Modify Licenses with Regard to Requirements for Mitigation Strategies for Beyond Design Basis External Events," U.S. Nuclear Regulatory Commission, March 12, 2012.

2. NEI 12-06, Rev. 0, "Diverse and Flexible Coping Strategies (FLEX) Implementation Guide," Nuclear Energy Institute, August 2012.

3. JLD-ISG-12-01, "Compliance with Order EA-12-049, Order to Modify Licenses with Regard to Requirements for Mitigation Strategies for Beyond Design Basis External Events," U.S. NRC.

4. PA-ASC-0916, Rev. 2, "Evaluation of Westinghouse, C-E, and B&W Designed Plant Reactor Coolant System Response to an Extended Loss of All Off-site and On-Site Power," PWROG, Approved June 2012.

5. PA-PSC-0965, "Emergency Response to Extended Station Blackout Events," PWROG, Approved February 2012.

6. WCAP-17601, "Reactor Coolant System Response to the Extended Loss of AC Power Event for Westinghouse, Combustion Engineering and Babcox & Wilcox NSSS Designs," Westinghouse Electric Company, August 2012.