ML19088A210

Review of Nuscale'S Design Certification Application with Respect to Mitigation of Beyond-Design-Basis Events Final Rule

ML19088A210
Person / Time
Site:	NuScale
Issue date:	03/28/2019
From:	Bergman T NuScale
To:	Document Control Desk, Office of New Reactors
References
LO‐0319‐64989
Download: ML19088A210 (13)

Text

LO031964989 March 28, 2019 Docket No. 52048 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk One White Flint North 11555 Rockville Pike Rockville, MD 208522738

SUBJECT:

Review of NuScales Design Certification Application with Respect to Mitigation of Beyond DesignBasis Events Final Rule

REFERENCES:

1. NuScale Power, LLC, NuScale Standard Plant Design Certification Application, Rev. 2, October 30, 2018 (ML18311A006) Tier 2 Chapter 20, Mitigation of BeyondDesignBasis Events.

2. NuScale Power, LLC, Mitigation Strategies for Extended Loss of AC Power Event, TR 081650797 Rev. 0, November 2016.

3. U.S. Nuclear Regulatory Commission, Staff Requirements Affirmation Session, SRM M190124A, January 24, 2019, ADAMS Accession Number ML19023A038.

4. U.S. Nuclear Regulatory Commission, Summary of the February 21, 2019, Public Meeting with NuScale Power, LLC, ADAMS Accession Number ML19084A136.

NuScale Power, LLC (NuScale) understands that the U.S. Nuclear Regulatory Commission (NRC) Staff are developing a Commission issue paper (SECY) to discuss options for reviewing NuScales Design Certification Application (DCA) content pertaining to the mitigation of beyonddesignbasis events (MBDBE). The Staff requested that NuScale provide our position for consideration in that SECY, which is the purpose of this letter.

NuScales approach to MBDBE is provided in Chapter 20 of the DCA final safety analysis report (FSAR) and in technical report TR081650797, Mitigation Strategies for Extended Loss of AC Power Event in anticipation of NRCs final MBDBE rule. In a recent affirmation session, the Commission approved the final rule [Reference 3].

The final rule simplifies the MBDBE requirements, does not require a threephase coping strategy, and anticipates the potential to rely on installed plant equipment for both the initial and longterm response to a loss of all ac power.

The NuScale design justifies relying solely on installed plant equipment for the initial and longterm response to a loss of all ac power. NuScales design evolved, in part in response to Fukushima lessonslearned, to eliminate reliance on electrical power (both ac and dc) and operator actions to perform safety functions. While the DCA review is not complete, on the basis of the review todate including draft safety evaluation reports, it appears the NRC staff accepts both these features. Further, NuScale has unprecedented coping capability compared to designs considered in development of the rule. Using only safetyrelated design features, the stored inventory in the reactor building pools, and the automatic plant response, the coping duration exceeds 50 days for core cooling and containment, and 150 days for spent fuel cooling. Beyond these durations, which NuScale believes exceed those necessary to satisfy the new rule, only a small amount of water (approximately 28 gpm) added through a gravityfed piping system would maintain the remaining pool inventory.

Although the MBDBE rule will not apply to NuScale as a DC applicant, NuScale seeks finality and issue resolution concerning the equipment a licensee will rely on for their mitigation strategies and spent fuel pool level monitoring. NuScale and staff agreed during preapplication that NuScale could address the pending rule, and NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360-0500 Fax 541.207.3928 www.nuscalepower.com

LO-0319-64989 Page 2 of 2 03/28/2019 NuScale has invested substantial resources developing this portion of its DCA and in supporting the staff review. Early resolution of this issue is consistent with the intent of 10 CFR Part 52, and is also consistent with the approach taken on control room staffing requirements in 10 CFR 50.54(m), another requirement that does not apply to design certification applicants. NuScale has not seen the Staff SECY, but based on discussions in a recent public meeting [Reference 4] understands Staff intend to propose three options: (1) Postpone all review of NuScale mitigating strategies until the combined-license (COL) review, providing no finality during design certification, (2) provide finality for the design certification up to the first 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, and the COL applicant would be responsible for "Phase 2 and Phase 3" of the MBDBE strategies, or (3) NuScale would modify the design to include instrumentation beyond 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> for monitoring key safety functions and full finality could be achieved during design certification. As the first two options defer resolution until the COL stage, NuScale does not support them because it negates the investment made by NuScale on this issue. The third option presents the staff's interpretation of the rule: that instrumentation is required regardless of a design's capability.

Based on the final rule approved by Commission, NuScale believes that installed plant features as currently described in the application provide the only equipment necessary for satisfying the language and intent of the final rule, and therefore a licensee's mitigation strategies need not address the acquisition and use of portable and offsite resources. The key issue related to this outcome is whether monitoring capability is required where the plant's coping capability relies only on installed, safety-related design features to achieve and maintain core, containment, and spent fuel cooling for an extended period, and does not rely on any operator actions. As detailed in the enclosure, NuScale believes requiring such monitoring independent of the expected plant response is not warranted for safety reasons and runs counter to the intent of the final rule. Based on a determination that such instrumentation is not required, NuScale requests Staff review of FSAR Chapter 20 and TR-0816-50797 with respect to the full extent of equipment required under the forthcoming rule (i.e., the entire duration encompassed by the rule).

NuScale appreciates the engagement with NRC Staff on this issue, and Staff's solicitation of NuScale's position on the preferred path forward. NuScale understands that the staff is seeking to submit its SECY as soon as possible, and asks Staff to include this letter and its Enclosure as an enclosure to the SECY paper as an additional option to those currently planned.

This letter makes no regulatory commitments and no revisions to any existing regulatory commitments.

If you have any questions, please contact Carrie Fosaaen at 541-452-7126 or at cfosaaen@nuscalepower.com.

Distribution: Robert Taylor, NRC, OWFN-7H4 Samuel Lee, NRC, OWFN-8G9A Greg Cranston, NRC, OWFN-8G9A Omid Tabatabai, NRC, OWFN 8H12

Enclosure:

NuScale's Position on Staff Review of Design Certification Application with Respect to Mitigation of Beyond-Design-Basis Events Final Rule NuScale Power, LLC 1100 NE Circle Blvd . Suite 200 Corvallis, Oregon 97330 Office 541.360-0500 Fax 541.207.3928 www.nuscalepower.com

LO031964989 Enclosure Page 1 of 11

Enclosure:

NuScales Position on Staff Review of Design Certification Application with Respect to Mitigation of BeyondDesignBasisEvents Final Rule I. Introduction NuScale requests that NRC Staff recommend within their Commission issue paper (SECY) the full resolution, via the design certification (DC) rulemaking, of the portions of forthcoming 10 CFR 50.155 pertaining to equipment for the mitigation of beyonddesignbasis external events (MBDBE). NuScale believes that the installed features of the NuScale passive design fulfill the requirements and intent of the forthcoming final rule, such that a licensee would not need to provide or assure additional capabilities (installed equipment, portable equipment, or pre-staged offsite resources) in developing mitigation strategies for the assumed event (a loss of all ac power concurrent with a loss of normal access to the normal heat sink).

That the NuScale DC can resolve the full scope of equipment required for the new rule is based upon NuScales position on two key issues addressed herein. First, NuScales understanding of the need for instrumentation and monitoring capability under the new rule. The NuScale design provides for at least 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> of monitoring capability that would allow operators to verify the plant responds as expected.

However, by relying exclusively on passive, failsafe, safetyrelated design features to maintain core cooling, containment, and spent fuel cooling for an extended periodwith no reliance on electrical power or operator actionsNuScale concludes that monitoring capability is not required to maintain safety or to satisfy the mitigation strategies portion of the rule. Second, NuScale believes that maintaining the required safety functions with only installed plant equipment for a sufficient duration satisfies the requirements of the rule, without need for portable or offsite resources. Based on the NuScale design and precedent, NuScale believes a minimum of 14 days provides adequate time to establish an alternate means of removing heat. NuScale has unprecedented coping capability compared to designs considered in development of the rule. Using only safetyrelated design features, the stored inventory in the reactor building pools, and the automatic plant response, the coping duration exceeds 50 days for core cooling and containment, and 150 days for spent fuel cooling. Beyond these durations, which NuScale believes exceed those necessary to satisfy the new rule, only a small amount of water (approximately 28 gpm) added through a gravityfed piping system would maintain the remaining pool inventory.

Under NuScales approach, the DCA would address, and the Staffs Safety Evaluation Report (SER) would reach findings on, the sufficiency of the installed equipment to support a licensees mitigation strategies to the full extent necessary to satisfy the new rule. The NuScale FSAR and supporting technical report would be revised to align with the final rule language and the approach described herein. Finally, NuScale recommends that the review and finality outcome be reflected in the Applicable Regulations portion of the DC rule, which would describe the applicability of 10 CFR 50.155 to licensees referencing the NuScale DC.

NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360-0500 Fax 541.207.3928 www.nuscalepower.com

LO031964989 Enclosure Page 2 of 11 II. NuScale Power Plant design capabilities for core cooling, containment, and spent fuel pool cooling and monitoring Following a loss of all ac power (an extended loss of ac power, or ELAP) concurrent with a loss of normal access to the normal heat sink, automatic responses of safetyrelated equipment establish and maintain the key safety functions by placing the reactor modules into a safe, stable, shutdown state with passive core and containment cooling. Following the initial, automatic response of safetyrelated equipment which requires no operator action and no electrical power (ac or dc)the reactor modules and the spent fuel pool rely only on the large inventory of the reactor, refueling, and spent fuel pools, which comprise the ultimate heat sink (UHS), to maintain uninterrupted and longterm heat removal.

Core cooling:

During an ELAP, reactor coolant system inventory is preserved by containment isolation that occurs within the first minute of the event.

If dc power is available, the decay heat removal system (DHRS) passively removes decay heat for the first 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> following an ELAP. If dc power is not available or is lost earlier than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, ECCS valves automatically open to remove decay heat.

The emergency core cooling system (ECCS) cools the core for the remainder of an ELAP. Reactor coolant water accumulates in the containment vessel (CNV) and passively returns to the reactor vessel by natural circulation after ECCS valves open. There is no plausible mechanism by which the ECCS valves could be changed from their safe open position.

The reactor modules are submerged in the reactor pool, which is part of the UHS. Passive heat removal to the UHS using DHRS and ECCS maintains core cooling for more than 50 days without pool inventory makeup or operator action.

Containment:

Containment isolation valves (CIVs) and the CNV provide passive containment function. Without operator action or electrical power, the safetyrelated CIVs close to isolate the CNV.

Heat removal to the UHS passively controls temperature and pressure to ensure containment integrity. Peak pressure and temperature conditions for the CNV occur early in the event when the ECCS valves open and do not challenge containment integrity.

Containment cooling is maintained for more than 50 days without pool inventory makeup or operator action.

Spent fuel pool cooling:

The spent fuel pool (SFP), as part of the UHS, communicates with the refueling pool and reactor pool above the SFP weir wall. As such, the pools respond as a single volume during an ELAP, until UHS level lowers below the weir wall.

The UHS inventory maintains passive cooling of the spent fuel in the SFP for more than 150 days following initiation of an ELAP without pool inventory makeup or operator action.

NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360-0500 Fax 541.207.3928 www.nuscalepower.com

LO031964989 Enclosure Page 3 of 11 Monitoring:

No operator action is required to establish or maintain the required safety functions for at least 50 days following the onset of an ELAP. Therefore, no instrumentation is necessary to support operator actions.

Although not necessary because of the failsafe and passive design, monitoring instrumentation (safety display instrumentation system, SDIS) is maintained in the main control room for at least 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> to provide additional assurance that systems have responded as designed.

Although sufficient UHS level exists for at least 50 days, UHS level monitoring, which includes SFP level, is assured for at least 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> using installed equipment alone.

III. Key Issues

1. Instrumentation

a. Monitoring to support mitigation strategies Two requirements from 10 CFR 50.155 raise the issue of what instrumentation is required by the rule and for how long. First, NuScale understands that the key technical issue that must be resolved to achieve full resolution (i.e., not limited to 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />) of the equipment aspects of the forthcoming rule is the extent to which monitoring capability is necessary to satisfy the mitigation strategies requirements of 50.155(b)(1) and (c). Under 10 CFR 50.155(b)(1), a licensee must have strategies and guidelines to mitigate beyonddesignbasis external events to address the safety functions of maintaining or restoring core cooling, containment, and spent fuel pool cooling capabilities in response to the defined ELAP damage state. Per paragraph (c), the equipment relied on for the mitigation strategies and guidelines required by paragraph (b)(1) of this section must have sufficient capacity and capability to perform those functions.

The question for resolution is whether the NuScale design relies on instrumentation to maintain the required safety functions during an ELAP. NuScale believes monitoring is only necessary to the extent required to support required actions under the mitigation strategies; i.e., the need for monitoring is dependent on the coping strategy. Because the NuScale approach does not depend on operator actions during the installed equipment coping period, NuScale believes that no monitoring is relied upon as contemplated by the final rule. NuScale understands Staffs position is that monitoring of the required safety functions is required regardless of the operators expected actions; i.e., monitoring is a requirement independent of the designed coping capability. NuScale understands that the intent of this position is to support a licensees ability to verify key safety functions are maintained and to facilitate decision making, which may inform contingency actions beyond those that are necessary to respond to the prescribed MBDBE boundary conditions.

As detailed above, the NuScale plant response to an ELAP relies entirely upon safetyrelated design features that perform their safety function automatically with or without dc power available and, once actuated, have no plausible mechanism for changing them to an unsafe configuration. Depending upon the availability of dc power, the plant response is completed within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> after the onset of the ELAP, by which point the plant is in a longterm, safe, passivecooled state. No operator action is required at any point to establish or maintain safety functions during the 50plusday coping period, beyond which NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360-0500 Fax 541.207.3928 www.nuscalepower.com

LO031964989 Enclosure Page 4 of 11 point a small amount of water added to the UHS by gravity feed through an installed, external piping connection would maintain pool inventory.

It is important to note that some of these key features of the plant response to an ELAP were not always part of the NuScale design. The design evolved, in part in response to Fukushima lessonslearned, to eliminate reliance on electrical power and operator actions to perform safety functions. The ELAP boundary conditions are equivalent to a design basis event for the NuScale plant, and thus the initial design response is carefully analyzed in accordance with all the rules and conservatisms applied to a design basis analysis in Chapter 15 of the FSAR. While the ELAP event may endure longer than an analyzed designbasis event, NuScale does not believe there is a plausible mechanism for the plant to change state during the extended period.

In a February 21, 2019 public meeting (summary at ADAMS accession number ML19084A136), Staff expressed their belief that monitoring capability must be available to operators throughout a beyond design basis external event, because of uncertainties in thermal hydraulic phenomena and boron distribution during long term cooling. According to Staff, such instrumentation would be needed by operators to monitor module status and facilitate contingency actions beyond the installed coping capacity.

NuScale notes that concerns about the possibility for boron plateout, riser uncovery, and overcooling return to power relate to conditions postulated to develop during longterm cooling following a transient with conservative, designbasis analysis assumptions. In important respects, certain Chapter 15 conservatisms (e.g., a stuck control rod) exceed the defined ELAP damage state. For the purposes of FSAR Chapter 15, NuScale expects to resolve these concerns in the course of the DCA review. Thus, NuScale believes that such resolution also provides a degree of assurance that exceeds that demanded for the beyonddesignbasis coping analysis that supports Chapter 20. Therefore, NuScale does not believe that these mechanisms warrant contingency coping strategies that could imply a need for monitoring capability to support the mitigation strategies. NuScale further believes that such a position would run counter to the intent of the new rule, and the objectives of the Advanced Reactor Policy Statement with respect to design simplification and enhanced safety.

In addition to the safety and policy considerations, NuScale notes that the final rule requires sufficient capacity and capability for the equipment relied on for the mitigation strategies. NuScale understands that relied on is analogous to the term relied upon as used in the definition of safetyrelated under 10 CFR 50.2. Under that definition, an SSC is understood to be relied upon, e.g., if it is credited in the safety analysis of a designbasis event. In the NuScale design, postaccident monitoring is a nonsafety related function because it is not relied upon by operators in response to design basis events. While the DCA review is not complete, on the basis of the review todate it appears that staff accepts this position.

Likewise, instrumentation is not relied upon by operators in response to an ELAP for MBDBE mitigation strategies. Therefore, NuScale believes no instrumentation is within scope of the equipment relied on for the mitigation strategies and guidelines as addressed by future 10 CFR 50.155(c). To add instrumentation not required for DBEs solely for the purpose of the MBDBE rule seems illogical, and adds complexity to a design that has taken passive safety and simplicity to an unprecedented degree.

NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360-0500 Fax 541.207.3928 www.nuscalepower.com

LO031964989 Enclosure Page 5 of 11 While NuScale does not believe any monitoring is necessary, the ELAP coping capability does include the capability to monitor important safety functions (reactor module parameters and UHS level monitoring) for a minimum of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> using installed, highlyreliable dc power supplies. The ability to monitor reactor module variables and pool levels for 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> provides ample time for operators to confirm that the plant responded to the ELAP as anticipated and that the pool inventory is trending as expected.

Plant conditions more challenging than the postulated ELAP conditions, and contingency actions to respond to such conditions, are beyond the scope of the final 10 CFR 50.155(b).

In sum, NuScales position is that the plant coping period far exceeds that required to fully satisfy the rule (see next section), and that no operator actions are necessary during that coping period.

Accordingly, NuScale believes that monitoring capability is not required in order to meet the mitigation strategies portion of the rule because it would not be relied on by a licensee in performing executing those strategies. The NuScale design includes monitoring capabilities that complement the plant coping capability, but are not required by the mitigation strategies portion of the rule.

b. Spent fuel pool monitoring Monitoring during an ELAP is only explicitly addressed by the new rule with respect to SFP monitoring, for which 10 CFR 50.155(c) will require:

In order to support effective prioritization of event mitigation and recovery actions, each licensee shall provide reliable means to remotely monitor widerange water level for each spent fuel pool at its site until 5 years have elapsed since all of the fuel within that spent fuel pool was last used in a reactor vessel for power generation. . . .

A required duration for SFP level monitoring is not defined by the new rule or discussed in the statements of consideration. As stated within the requirement, though, the intent of the rule is to support effective prioritization of event mitigation and recovery actions. As NRC stated in rescinding the SFP instrumentation order, EA12051, from the permanently shut down Oyster Creek station:

In the event of a challenge to the safety of fuel stored in the SFP, the focus of the facility staff would be the SFP condition, without the possibility of a concurrent challenge to the reactor and primary containment safety functions. Thus, in the event of a beyonddesignbasis external event, effective prioritization of event mitigation and recovery actions would be simplified, and the application of the Order requirements would no longer be necessary to serve the Order's underlying purpose.

Similar reasoning applies to the NuScale plant. No operator actions are required to mitigate an ELAP, and UHS inventory assures core cooling, containment, and SFP cooling, and operators have at least 50 days to arrange for a small amount of water to be added to the UHS through an installed external connection to maintain sufficient pool inventory indefinitely. Accordingly, NuScale believes SFP monitoring is not needed to support effective prioritization of event mitigation and recovery actions.

However, the NuScale design assures at least 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> of UHS level monitoring, including the SFP, using installed instrumentation and power sources, thereby fulfilling the intent of this requirement.

NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360-0500 Fax 541.207.3928 www.nuscalepower.com

LO031964989 Enclosure Page 6 of 11

2. Coping Duration As required by new 10 CFR 50.155(b)(1)(ii), a licensees mitigation strategies will be required to address the acquisition and use of offsite assistance and resources to support the [MBDBE safety functions]

indefinitely, or until sufficient site functional capabilities can be maintained without the need for the mitigation strategies. NuScale expects future licensees to rely on only installed equipment for both the initial and longterm response to the ELAP event, with no reliance on portable equipment and offsite resources. Thus, a second issue that should be decided to achieve full resolution of the forthcoming rule is the total coping duration expected for the mitigation strategies.

NuScale seeks to establish, for licensees that will reference the NuScale DC, a design approach for supporting mitigation strategies different from the threephase coping strategy licensees have used to address the MBDBE Orders thus far (Order Modifying Licenses with Regard to Requirements for Mitigation Strategies for BeyondDesignBasis External Events, EA12049, March 12, 2012). As acknowledged by the Commission in the forthcoming rules statements of consideration:

New reactors may establish different approaches from those of operating reactors in developing strategies to mitigate beyonddesignbasis events. For example, new reactors may use installed plant equipment for both the initial and longterm response to a loss of all ac power with less reliance on portable equipment and offsite resources than currently operating nuclear power plants. The NRC would consider the specific plant approach when evaluating the SSCs relied on as part of the mitigation strategies for beyonddesignbasis events.

With respect to the coping duration, the Commission also explains:

The acquisition and use of offsite assistance and resources to support the functions required by

§ 50.155(b)(1)(i) of this section indefinitely, or until sufficient site functional capabilities can be maintained without the need for the mitigation strategies means that licensees need to plan for obtaining sufficient resources (e.g., fuel for generators and pumps, cooling and makeup water) to continue removing decay heat from the irradiated fuel in the reactor vessel and SFP as well as to remove heat from containment as necessary until an alternate means of removing heat is established. The alternate means of removing heat could be achieved through repairs to existing SSCs, commissioning of new SSCs, or reduction of decay heat levels through the passage of time sufficient to allow heat removal through losses to the ambient environment.

NuScale seeks to demonstrate that by relying exclusively on installed SSCs, a licensee can maintain the required functions of core cooling, containment, and SFP cooling until sufficient site functional capabilities can be maintained without the need for the mitigation strategies. In other words, if the installedequipment coping duration is sufficiently long, the licensees mitigation strategies would not address the acquisition and use of offsite resources because there is no period of time for which such resources are required.

NuScale requests that the Staff address in their SECY paper a minimum coping duration that would satisfy the purpose of the rule and would therefore obviate the need for offsite resources. Although the current NuScale design provides for more than 50 days of coping, that duration may fluctuate with NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360-0500 Fax 541.207.3928 www.nuscalepower.com

LO031964989 Enclosure Page 7 of 11 future design changes. As discussed next, based on the Fukushima event and recent precedent as applied to the NuScale plant design, NuScale believes that a 14day coping duration is sufficient to establish an alternate means of removing heat for the NuScale design.

In the Fukushima Daiichi accident, personnel began adding water to the spent fuel pool of Unit 4 via fire and concrete pump trucks 9 days after the tsunami, and began injecting water via the fuel pool cooling system 14 days after the tsunami (International Atomic Energy Agency, The Fukushima Daiichi Accident, Technical Volume 1). These actions occurred with no preplanning or prestaging of resources, without the benefit of a hardened pool makeup connection, and despite widespread destruction limiting access to offsite resources.

In evaluating requests to relax the Mitigation Strategies Order, EA12049, for permanently shut down reactors, NRC has addressed the issue of an acceptable duration to maintain safety functions. For example, Oyster Creek estimated that, following requested rescission of the Order, the SFP water inventory would last a minimum of 7 days prior to reducing to a level of less than 10 feet above the top of the spent fuel rack, which would ensure spent fuel cooling and shielding (see ADAMS Accession number ML18150A249). In withdrawing the Order as no longer necessary (see ADAMS Accession number ML18176A071), NRC agreed and concluded that the 7 days of inventory satisfied the intent of the SFP cooling requirement, such that mitigation strategies for the SFP were not needed. Staff explained: the low decay heat and the long boiloff period of the SFP provides sufficient time for the licensee to obtain offsite resources on an ad hoc basis to sustain the SFP cooling safety function indefinitely, eliminating the need for the final phase of the guidance and strategies for maintaining or restoring SFP cooling capabilities.

For the NuScale design, the same reasoning applies not only to SFP cooling, but also for core cooling and containment. All three functions are assured by maintaining UHS inventory alone. After an extended coping duration, the only action that may be needed to maintain pool inventory indefinitely is to add water by gravity feed to the SFP, which has a hardened makeup line to the exterior of the Reactor Building for that purpose. Thus, for the simplified NuScale design, 14 days is a reasonably conservative coping duration that would allow the licensee to establish alternate means of removing heat, including through repair of existing SSCs, commissioning of new SSCs, or obtaining offsite resources on an ad hoc basis.

NuScales technical report TR081650797, Mitigation Strategies for Extended Loss of AC Power Event, will demonstrate that core cooling and containment will be maintained by the UHS water inventory for greater than 50 days without any operator action, electrical power, or makeup water. A water level of 10 feet above the top of the spent fuel rack would be maintained for approximately 135 days, and the spent fuel would remain cooled for approximately 150 days. Because this design coping duration provides substantial margin to the proposed minimum duration of 14 days, a licensees mitigation strategies would not need to address the acquisition and use of offsite assistance and resources. After 50 days, makeup of less than 28 gallons per minute would maintain pool inventory indefinitely, an amount easily provided by gravity through the existing external piping connection, facilitating the use of offsite resources on an ad hoc basis should makeup be warranted.

NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360-0500 Fax 541.207.3928 www.nuscalepower.com

LO031964989 Enclosure Page 8 of 11 IV. Regulatory Assessment With the above issues resolved, NuScale believes that the above installed capabilities fully satisfy the equipmentrelated requirements for licensees prescribed by 10 CFR 50.155(b)(1), (c), and (e). NuScale seeks NRC Staffs conclusion that such equipment has sufficient capacity and capability to perform the core cooling, containment, and SFP cooling functions for a minimum of 14 days, or another minimum duration that the NRC determines is sufficient to allow the licensee to establish alternate means of removing heat, as discussed above. Each of these requirements is assessed below, assuming a minimum 14day coping duration is required.

(b)(1) A licensee must develop, implement, and A licensee referencing the NuScale design will maintain strategies and guidelines to mitigate develop, implement, and maintain MBDBE beyonddesignbasis external events from natural strategies and guidelines, relying on the installed phenomena that are developed assuming a loss plant features and equipment to respond to an of all ac power concurrent with . . . a loss of ELAP.

normal access to the normal heat sink.

(b)(1)(i) The strategies and guidelines must be The licensees strategies and guidelines will capable of being implemented sitewide and address the full site and the required safety must include maintaining or restoring core functions. No operator actions are required to cooling, containment, and spent fuel pool cooling restore or maintain the safety functions. Thus, capabilities. NuScale expects the strategies and guidelines to address supplementary actions such as confirming the expected plant response, placing the secondary systems in a safe state, and restoration of power to restore normal plant conditions.

(b)(1)(ii) The strategies and guidelines must The coping duration ensured by installed SSCs include the acquisition and use of offsite exceeds 14 days, which provides sufficient time assistance and resources to support the functions for the licensee to establish an alternate means required by paragraph (b)(1)(i) of this section of removing heat, such as obtaining offsite indefinitely, or until sufficient site functional resources on an ad hoc basis, through repairs to capabilities can be maintained without the need existing SSCs or commissioning of new SSCs, or for the mitigation strategies. heat removal through losses to the ambient environment. Therefore, there is no period of time for which offsite resources need to be acquired and used. The strategies and guidelines may address offsite resources beyond the plants minimum coping duration (e.g., greater than 50 days), but this would be beyond the requirements of the rule.

NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360-0500 Fax 541.207.3928 www.nuscalepower.com

LO031964989 Enclosure Page 9 of 11 (c)(1) The equipment relied on for the mitigation The installed equipment relied on to ensure core strategies and guidelines required by paragraph cooling, containment, and SFP cooling has (b)(1) of this section must have sufficient capacity sufficient capacity and capability to perform and capability to perform the functions required those functions for at least 50 days, exceeding by paragraph (b)(1) of this section. the 14 days required. Monitoring is not relied on for the mitigation strategies and guidelines, but installed instrumentation provides at least 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> of module and UHS monitoring as a supplementary capability.

(c)(2) The equipment relied on for the mitigation The installed equipment relied on for the strategies and guidelines required by paragraph mitigation strategies and guidelines is safety (b)(1) of this section must be reasonably related, and therefore reasonably protected from protected from the effects of natural phenomena the effects of natural phenomena that are that are equivalent in magnitude to the equivalent in magnitude to the phenomena phenomena assumed for developing the design assumed for developing the design basis of the basis of the facility. facility. Monitoring is not relied on for the mitigation strategies and guidelines, but the safety display and information system and the highlyreliable dc power systems are also Seismic Category I and environmentally qualified.

(e) In order to support effective prioritization of The NuScale design provides at least 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> of event mitigation and recovery actions, each UHS level monitoring, including the SFP, from the licensee shall provide reliable means to remotely control room using installed instrumentation and monitor widerange water level for each spent power sources. Because mitigation actions are fuel pool at its site until 5 years have elapsed not required to maintain core cooling, since all of the fuel within that spent fuel pool containment, or SFP cooling for an extended was last used in a reactor vessel for power duration, this capability provides sufficient time generation to support effective prioritization of event mitigation and recovery actions.

V. DCA Updates, Review, and Resolution Process NuScale believes 10 CFR 50.155 can and should be addressed within the NuScale DC rulemaking.

Although the new rule will not directly apply to a DC applicant such as NuScale, NRC precedent establishes that it is feasible and sometimes appropriate to resolve design aspects of a rule applicable to licensees at the DC stage. One of NRCs objectives of design standardization, including through design certification, is to enhance plant safety and reduce regulatory uncertainty through early resolution of design issues (see, e.g., September 15, 1987, 52 FR 34884, Nuclear Power Plant Standardization).

Accordingly, certain design features with a relationship to operational requirements are reviewed and resolved at the DC stage. For example, NuScale is seeking to resolve control room operator staffing requirements at the DC stage, a condition of licenses under 10 CFR 50.54 that is not applicable to DC NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360-0500 Fax 541.207.3928 www.nuscalepower.com

LO031964989 Enclosure Page 10 of 11 applicants, because of the close relationship between operator staffing and the control room design.

With respect to the MBDBE rule, in response to NuScale comments, NRC recognized in the draft final rule that a reactor designer may address, at least in part, the provisions of the MBDBE rule in the design certification application (SECY160142: Enclosure 2, NRC Response to Public Comments, ADAMS Accession No. ML16271A063).

Accordingly, NuScale believes it is allowable and appropriate to resolve the equipment aspects of the new 10 CFR 50.155 rule during the design certification process. Based on NuScales interpretation of the requirements presented in Sections III and IV, above, issue resolution and finality can be fully achieved (i.e. for the full duration covered by the rule) for the portions of the rule described.

With these matters resolved at the DC stage, a COLAs remaining scope, addressed by a COL Item at the DC Stage, would be limited to developing the mitigation strategies required by 10 CFR 50.155(b)(1), to address actions supplementary to the plants inherent coping response to an ELAP. A licensee would also address other portions of the rule not discussed herein, including the extensive damage mitigation guidelines required under 10 CFR 50.155(b)(2) and the training requirements of paragraph 10 CFR 50.155(d). These items would be addressed by COL Items in the NuScale FSAR.

NuScale proposes to resolve these equipment aspects of the forthcoming rule at the DC stage. NuScales preferred approach is to exclude portions of 10 CFR 50.155 as Applicable Regulations for license applicants and licensees. This approach is effectively an exemption from portions of the rule for facilities referencing the NuScale design, consistent with the approach being used for the control room staffing requirements of 10 CFR 50.54(m). NuScale would include an additional section within Part 7 of the DCA similar to an exemption request to support the request, and the licenseeexemption would be included as a paragraph in Section V, Applicable Regulations, of the NuScale DC Rule. For a technical basis for the request, NuScale would rely on the revised TR081650797 to support the necessary design description and safety findings, similar to those in the table above.

In order to achieve issue resolution and support Staffs review, NuScale intends to substantially revise FSAR Chapter 20 and TR081650797. Currently, the NuScale FSAR compares NuScales mitigation strategy to NEI 1206, which has led to some confusion. Under the forthcoming rule, NuScale believes that the Commission has clearly established that the NEI 1206 approach of threephase coping is not required for future designs. Accordingly, references to NEI 1206 will be removed and FSAR Chapter 20 and TR081650797 will be rewritten to directly address the new rules requirements in a manner similar to that described herein. Chapter 20 will describe the plant coping duration using only installed SSCs and no operator actions in response to the rules prescribed boundary condition of loss of all ac power and loss of normal access to the normal heat sink.

In the February 21 public meeting with NRC Staff, Staff identified an apparent inconsistency between FSAR Chapter 20 and TR081650797 regarding the use of monitoring instrumentation. The revisions to both documents will conform to the approach described herein, which will remove this inconsistency.

Specifically, the documents will describe the design capability of at least 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> of monitoring using installed instruments and power sources as a supplementary feature. The documents will make clear that this capability is not relied upon to maintain core cooling, containment, or SFP cooling, and therefore not required by the final rule.

NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360-0500 Fax 541.207.3928 www.nuscalepower.com

LO031964989 Enclosure Page 11 of 11 If the NRC agrees that the approach described here is an acceptable means to satisfy the MBDBE rule, then NuScale expects to submit the revisions to FSAR Chapter 20 and TR081650797 within no more than 60 days of that decision.

VI. Conclusion Based on the final MBDBE rule approved by Commission, NuScale believes that installed plant features currently described in the DCA provide the only equipment necessary for satisfying the language and intent of the final rule, and therefore a licensees mitigation strategies and guidelines will not need to address the acquisition and use of portable and offsite resources. The key issue related to this outcome is whether monitoring capability is required where the plants coping capability relies only on installed, safetyrelated design features to achieve and maintain core, containment, and spent fuel cooling for an extended period, and does not rely on any operator actions. As detailed above, NuScale believes that such monitoring is not relied upon within the meaning of the rule, and is thus not required, because the plants automatic response achieves the required safety functions and maintains them for a duration that exceeds a minimum coping duration appropriate for the NuScale design. Therefore, although the NuScale design provides at least 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> of module and pool level monitoring, this capability is complementary to, and not required to support, a licensees mitigation strategies and guidelines. With agreement on this interpretation, NuScale expects to revise FSAR Chapter 20 and TR081650797 to align with the final rules requirements, and seeks to achieve finality with respect to the full extent of equipment required under the forthcoming rule.

NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360-0500 Fax 541.207.3928 www.nuscalepower.com