ML20259A517

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Summary Report - 675th and 676th Meetings of the Advisory Committee on Reactor Safeguards, July 8-10 and July 21-24, 2020
ML20259A517
Person / Time
Issue date: 09/21/2020
From: Matthew Sunseri
Advisory Committee on Reactor Safeguards
To: Kristine Svinicki
NRC/Chairman
Burkhart, L, ACRS
References
Download: ML20259A517 (8)
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Text

UNITED STATES NUCLEAR REGULATORY COMMISSION ADVISORY COMMITTEE ON REACTOR SAFEGUARDS WASHINGTON, DC 20555 - 0001 September 21, 2020 The Honorable Kristine L. Svinicki Chairman U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001

SUBJECT:

SUMMARY

REPORT - 675th AND 676th MEETINGS OF THE ADVISORY COMMITTEE ON REACTOR SAFEGUARDS, JULY 8-10 AND JULY 21-24, 2020

Dear Chairman Svinicki:

During its 675th and 676th meetings, July 8-10 and July 21-24, 2020, which were conducted virtually due to the COVID-19 pandemic, the Advisory Committee on Reactor Safeguards (ACRS) discussed several matters. The July 21-24, 2020, meeting was convened as a special meeting for the purpose of discussing the NuScale design certification application (DCA) review and completing documentation to support the Phase 5 ACRS review of the DCA. The ACRS completed the following correspondence:

LETTER REPORT Letter report to Kristine L. Svinicki, Chairman, U.S. Nuclear Regulatory Commission (NRC),

from Matthew W. Sunseri, Chairman, ACRS:

  • Report on the Safety Aspects of the NuScale Small Modular Reactor, dated July 29, 2020, Agencywide Documents Access and Management System (ADAMS) Accession No. ML20211M386 LETTERS Letters to Margaret M. Doane, Executive Director for Operations (EDO), NRC, from Matthew W. Sunseri, Chairman, ACRS:
  • Safety Evaluation, Revision 2 for the Topical Report, Uranium Oxycarbide (UCO)

Tristructural Isotropic (TRISO) Coated Particle Fuel Performance: Topical Report EPRI-AR-1(NP), dated August 4, 2020, ADAMS Accession No. ML20210M241

K. Svinicki MEMORANDA Memoranda to Margaret M. Doane, EDO, NRC, from Scott W. Moore, Executive Director, ACRS:

  • Regulatory Guides, dated July 28, 2020, ADAMS Accession No. ML20209A338 HIGHLIGHTS OF KEY ISSUES
1. Report on the Safety Aspects of the NuScale Small Modular Reactor The Committee completed its review of the NRC staffs advanced Safety Evaluation Report (SER) with no open items for the NuScale applications for design certification and standard design approval during the special Full Committee meeting held July 21 - 24, 2020. The Committee conducted its initial review of the staffs SER with open items on a traditional chapter-by-chapter basis, along with applicable topical reports (TRs), and issued letter reports accordingly. To complete its review in an expeditious manner the Committee implemented a two-step process. Individual members conducted a detailed technical review of each chapter of the advanced SER with no open items for deliberation and approval by the Committee as a whole. The Committee also conducted focus area reviews, in cooperation with the staff, in which the Committee concentrated its attention on potentially safety-significant issues that were cross-cutting over multiple SER and design certification application (DCA) chapters. These included:
  • Boron Dilution and Return to Criticality;
  • Source Term; and

The Committees correspondence on the PRA and ECCS valve performance topics may be found in a letter dated June 1, 2020 (ADAMS Accession No. ML20149K596); the correspondence on the helical-tube steam generator design topic may be found in a letter dated March 24, 2020 (ADAMS Accession No. ML20091G387); the correspondence on the source term topic may be found in a letter dated December 20, 2019 (ADAMS Accession No. ML19354A031); and the boron dilution and return to criticality topic may be found in a letter dated July 29, 2020 (ADAMS Accession No. ML20210M890).

The Committee concluded that there is reasonable assurance that the NuScale small modular reactor can be constructed and operated without undue risk to the health and safety of the public. The Committee also stated that NRC staffs final SER for the NuScale design should be issued, and that a design certification and standard design approval for the NuScale

K. Svinicki applications should be issued, subject to the staffs proposed exclusions regarding the finality of design requirements. The Committee noted that it looks forward to working with the staff and applicants on any future combined license (COL) applications and reviewing new information on the exclusions and a number of potentially risk significant items discussed in this report.

This letter report fulfills the requirements of Title 10 of the Code of Federal Regulations (10 CFR) Sections 52.53 and 52.141 that the ACRS shall report on those portions of the application which concern safety.

Committee Action The Committee issued a letter on July 29, 2020, with the following conclusions and recommendations:

a) The NuScale small modular reactor is a natural-circulation, pressurized water reactor that incorporates unique design and passive safety features, providing enhanced margins of safety. There is reasonable assurance that it can be constructed and operated without undue risk to the health and safety of the public.

b) The NRC staffs final SER for the NuScale design should be issued.

c) A design certification and standard design approval for the NuScale applications should be issued, subject to the staffs proposed exclusions regarding the finality of design requirements: shield wall design, containment leakage from combustible gas monitoring, and steam generator tube structural and leakage integrity.

d) The Committee identified in this letter report several potentially risk-significant items that are not completed at this time. The Committee requested the opportunity to review the qualification of emergency core cooling system (ECCS) valve performance, the identification of a successful recovery strategy to prevent potential reactivity insertion accidents associated with boron dilution sequences, and the updated probabilistic risk assessment (PRA).

2. NuScale Area of Focus - Boron Redistribution The potential for boron dilution to occur in the downcomer of the NuScale Power Module (NPM) under design-basis event conditions, for both anticipated operational occurrences and design basis accidents, was raised during our area of focus review of boron redistribution. This occurs when the NPM riser is uncovered, interrupting natural circulation flow during passive cooldown transients or loss-of-coolant accidents (LOCAs). In these transient and accident conditions, boiling will cause distillation leading to condensation of the steam in the downcomer, which dilutes the concentration of boron over extended periods of time. In March 2020, NuScale determined that, under some conditions, the downcomer may deborate before the ECCS valves actuate, at which time a large ingress of water from the containment could push the deborated water in the downcomer into the core. This creates the potential for the reactor core to return to criticality and increase power with either the addition of makeup coolant water, or the reestablishment of natural circulation flow as the deborated water from the downcomer enters the core.

The applicant concluded that the associated risk was unacceptable, implemented corrective actions, and provided design modifications consisting of riser holes at the midplane level of

K. Svinicki the steam generators that maintain a flow of borated water to the downcomer, even if the riser is uncovered. Further, changes were made to ensure earlier ECCS actuation by adding a low reactor coolant system pressure setpoint and lowering the containment vessel level setpoint, hence preventing buildup of deborated water inventory in the downcomer at the time of ECCS actuation.

However, for most LOCA and ECCS actuation events, once the riser holes are uncovered, the downcomer and lower plenum begin to deborate because of condensation on the steam generator tubes, the reactor vessel inner wall, and the containment wall. Also, for steam-space LOCAs (e.g., failure or inadvertent opening of an ECCS reactor vent valve), deborated water continues to accumulate in the containment. NuScale and staff calculations show that, after ECCS actuation, boron concentration in the downcomer could fall below the critical boron concentration within hours - a potentially metastable system configuration from a reactor physics standpoint. Should this colder, deborated coolant insufficiently mix as it reaches the core, there exists the possibility of a recriticality and return to power event. However, the staff has asserted that mixing will occur as the deborated coolant enters the core preventing significant power excursions. The staff assertion is based on three-dimensional TRACE calculations supported by open literature experimental data indicating internal recirculation occurs in pool boiling conditions with fuel bundle geometries similar to the NPM core.

A focused effort by the COL applicant is needed to develop recovery strategies that will lead to effective operating procedures. Given the inability to measure the distribution of boron in the NPM during these events, these strategies should have a stronger technical basis than is currently documented that demonstrates a path to successful recovery to prevent core damage. The PRA should be updated accordingly at the COL stage to appropriately reflect the risk of boron dilution events, including associated operator actions.

Committee Action The Committee issued a letter on July 29, 2020, with the follow conclusions and recommendations:

a) NuScale has incorporated design and setpoint changes to the NuScale Power Module (NPM) to mitigate the effects of boron dilution in the downcomer for design basis uncontrolled passive cooling events and loss-of-coolant accidents (LOCAs) up to the time of emergency core cooling system (ECCS) actuation.

b) The applicant has demonstrated for these scenarios, through a conservative analytical approach, that the design modifications maintain the boron concentration in the downcomer above the critical boron concentration level necessary to prevent recriticality and a return to power. The staffs evaluation confirms the applicants analyses out to 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.

c) However, ECCS actuation events result in water levels below the new riser holes and render them ineffective; thus, coolant in the downcomer will deborate for a range of design basis accidents, including small-break LOCAs. The estimated time for the boron concentration to drop below the critical boron concentration in the downcomer for these events is within a few hours.

K. Svinicki d) Operator recovery actions raise the possibility of an influx of deborated water into the core, which may result in recriticality, return to power, and the potential for core damage.

e) Detailed operator response and recovery procedures will be developed by the combined license (COL) applicant. The staff must ensure that these recovery strategies will prevent core damage with a high degree of confidence.

f) A focused effort by the COL applicant is needed to develop recovery strategies that will lead to effective operating procedures. Given the inability to measure the distribution of boron in the NPM during these events, these strategies should have a stronger technical basis than is currently documented that demonstrates a path to successful recovery to prevent core damage. The probabilistic risk assessment (PRA) should be updated accordingly at the COL stage to appropriately reflect the risk of boron dilution events, including associated operator actions.

3. Safety Evaluation, Revision 2 for the Topical Report, Uranium Oxycarbide (UCO)

Tristructural Isotropic (TRISO) Coated Particle Fuel Performance: Topical Report EPRI-AR-1(NP),

The Department of Energy (DOE) initiated the Advanced Gas Reactor (AGR) fuel development program in 2002 to establish U.S. capability to fabricate high-quality UCO TRISO fuel and demonstrate its performance. TRISO-coated particle fuel is used as part of the overall fuel design for gas-cooled reactor designs as well as fluoride-salt-cooled and potentially other reactor designs. The AGR program initially focused on developing manufacturing processes that yield consistent performance, verified by irradiation and safety testing. Results from the first two coated particle irradiation tests in the AGR program, designated AGR-1 and AGR-2, were designed to demonstrate UCO coated particle performance during irradiation and in post-irradiation high-temperature accident safety tests and are the subject of this topical report.

The topical report provides the technical bases for the functional performance of UCO-based TRISO-coated particles. It is intended that the data in the topical report provide a design data base so that these particles can be manufactured with consistent properties that can then be used as the basis for fuel designs in a variety of high-temperature reactor designs.

The topical report identified the following conclusions:

1. Testing of UCO TRISO-coated fuel particles in AGR-1 and AGR-2 constitutes a performance demonstration of these particle designs over a range of normal operating and off-normal accident conditions.
2. The UCO TRISO-coated fuel particles tested in AGR-1 and AGR-2 exhibited property variations and were fabricated under different conditions and different scales, with very similar irradiation and accident safety performance results.
3. The AGR-1 and AGR-2 fission product release data and fuel failure fractions, as summarized in the topical report, can be used to support licensing of reactors employing UCO TRISO-coated particle fuel that satisfy the parameter envelope defined by measured particle layer properties in Table 5-5 from AGR-1 and AGR-2.

K. Svinicki The staff has performed a thorough review of the topical report. Data gathering and assessment within this review included a site audit at the INL, development of Requests for Additional Information, and public meetings. Based on the above findings the staff identified two limitations and three conditions for approval of the topical report. The Committee agrees that the limitations and conditions are appropriate.

The Committee notes that Table 5-5 captures the physical specifications of the particle layers that conform to the data discussed in the topical report. Some parameters such as the carbon/oxygen ratio (which is dependent on burnup) are not captured within the table and were deemed important to ensure adequate particle fuel performance within the bounds set forth by the topical report.

The Committee agrees with the staffs overall findings that the conclusions of the topical report are applicable and acceptable subject to the limitations and conditions imposed by the staff as well as to those prescribed in the topical report itself. However, the Committee stresses that the topical report represents only a starting point for a fuel design. Fuel particles fabricated to the specifications in the topical report must then be incorporated into an overall fuel system design.

Care must be taken that the fuel fabrication process itself does not introduce phenomena such as particle layer cracking beyond that covered in the specifications or other unknown unknowns that result in behavior outside of the Table 5-5 parameters.

Committee Action The Committee issued a letter dated August 4, 2020, with the following conclusions and recommendations:

a) The SER is focused and detailed. The SER identifies appropriate expectations, conditions, and limitations on future use of the topical report by reactor or fuel designers to establish performance of TRISO-coated particles.

b) The topical report provides a valuable starting point and data base for future coated particle fuel designs. However, incorporating coated particles that meet the specifications in the topical report into an overall fuel design should be done with caution to avoid introducing degradation phenomena not accounted for in the irradiation program.

c) The transition from coated particle to overall fuel system will likely require additional coated particle and fuel system irradiation programs to validate the overall design.

d) The SER should be issued.

SPECIAL FULL COMMITTEE MEETING IN AUGUST In accordance with the ACRS bylaws, Section 1.2, the Committee voted to conduct a special meeting of the Full Committee on August 20-21, 2020, with the primary purpose of discussing, writing and completing a letter report on lessons learned from the ACRSs review of the NuScale design certification application and other recent design certification applications and license applications.

K. Svinicki SCHEDULED TOPICS FOR THE 677th SPECIAL ACRS MEETING AND 678th ACRS MEETING The following topics are on the agenda for the 677th special ACRS meeting scheduled for August 20-21, 2020:

  • Design certification application review lessons learned The following topics are on the agenda for the 678th ACRS meeting scheduled for September 10-12, 2020:
  • Staff White Paper on Title 10 of the Code of Federal Regulations Part 53 Advanced Notice of Proposed Rulemaking regarding licensing of advanced technology reactors,
  • GEH Topical Report NEDC-3391P, BWRX-300 Reactor Vessel and Overpressure Protection,
  • Topical Report ANP-10337, Supplement 1, Deformer Spacer Grid Element, and
  • Future-focused research projects Sincerely, Matthew W. Sunseri Digitally signed by Matthew W. Sunseri Date: 2020.09.21 11:19:07 -04'00' Matthew W. Sunseri, Chairman

K. Svinicki September 21, 2020

SUBJECT:

SUMMARY

REPORT - 675th AND 676th MEETINGS OF THE ADVISORY COMMITTEE ON REACTOR SAFEGUARDS, JULY 8-10 AND JULY 21-24, 2020 Accession No: ML20259A517 Publicly Available (Y/N): _Y___ Sensitive (Y/N): N If Sensitive, which category?

Viewing Rights: NRC Users or ACRS only or See restricted distribution OFFICE ACRS SUNSI Review ACRS ACRS NAME LBurkhart LBurkhart SMoore (SWM) MSunseri DATE 8/18/20 8/18/20 9/15/20 9/21/20 OFFICIAL RECORD COPY

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