ML24232A227
| ML24232A227 | |
| Person / Time | |
|---|---|
| Site: | 99902100 |
| Issue date: | 01/07/2025 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| EPID L-2023-TOP-0046 | |
| Download: ML24232A227 (1) | |
Text
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION Enclosure OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION TERRAPOWER, LLC - AUDIT
SUMMARY
REPORT TOPICAL REPORT RADIOLOGICAL SOURCE TERM METHODOLOGY REPORT, REVISION 1 Applicant:
TerraPower, LLC Applicant Address:
15800 Northup Way, Bellevue, WA 98008 Plant Name:
Natrium Project No.:
99902100
1.0 BACKGROUND
By letter dated August 11, 2023, TerraPower, LLC (TerraPower) submitted topical report (TR)
TP-LIC-RPT-0003, Radiological Source Term Methodology Report, Revision 0 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML23223A234), which documents the radiological source term evaluation model (EM) development process for the Natrium reactor. On November 15, 2023, the U.S. Nuclear Regulatory Commission (NRC) staff determined that that additional information was needed for the staff to begin its detailed review and that TerraPower could supplement the submittal with the additional information (ML23292A269). On January 16, 2024, TerraPower supplied the supplemental information by submitting Revision 1 (ML24017A115) of the TR as a replacement to the information contained in Revision 0.
TerraPowers overall licensing approach for the Natrium reactor design follows the Licensing Modernization Project (LMP) methodology described in Nuclear Energy Institute (NEI) 18-04, Revision 1, Risk-Informed Performance-Based Technology Inclusive Guidance for Non-Light Water Reactor Licensing Basis Development (ML19241A472). Regulatory Guide (RG) 1.233, Guidance for a Technology-Inclusive, Risk-Informed, and Performance-Based Methodology to Inform the Licensing Basis and Content of Applications for Licenses, Certifications, and Approvals for Non-Light Water Reactors, Revision0 (ML20091L698) endorses the LMP methodology described in NEI 18-04.
On May 16, 2024, the NRC staff provided its audit plan for the subject TR to TerraPower (ML24127A048). The audit was conducted virtually from May 28, 2024, through August 13, 2024, using TerraPowers electronic reading room (ERR). The NRC staff held an audit exit meeting with TerraPower on August 13, 2024.
2.0 AUDIT REGULATORY BASES The basis for the audit includes the following regulatory requirements from Title 10 of the Code of Federal Regulations (10 CFR) 50.34(a) with respect to the following minimum information to be included in a preliminary safety analysis report submitted as part of a construction permit application:
10 CFR 50.34(a)(1)(ii)(D)(1): An individual located at any point on the boundary of the exclusion area for any 2 hour2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> period following the onset of the postulated fission product release, would not receive a radiation dose in excess of 25 rem[ ] total effective dose equivalent (TEDE).
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION 10 CFR 50.34(a)(1)(ii)(D)(2): An individual located at any point on the outer boundary of the low population zone, who is exposed to the radioactive cloud resulting from the postulated fission product release (during the entire period of its passage) would not receive a radiation dose in excess of 25 rem [TEDE].
10 CFR 50.34(a)(4) requires a preliminary analysis and evaluation of the design and performance of structures, systems, and components of the facility with the objective of assessing the risk to public health and safety resulting from operation of the facility and including determination of the margins of safety during normal operations and transient conditions anticipated during the life of the facility, and the adequacy of structures, systems, and components provided for the prevention of accidents and the mitigation of the consequences of accidents.
3.0 AUDIT PURPOSE AND OBJECTIVES The purpose of the audit was for the NRC staff to gain a more detailed understanding of TerraPowers EMs presented in the TR. A secondary purpose of the audit was to identify any information that will require docketing to support the NRC staffs safety evaluation.
4.0 SCOPE OF THE AUDIT AND AUDIT ACTIVITIES The audit followed the guidance in the Office of Nuclear Reactor Regulations Office Instruction LIC-111, Regulatory Audits, Revision 1 (ML19226A274). Audit activities included virtual meetings to discuss questions and review of submitted files.
Members of the audit team included the NRC staff listed below:
Zachary Gran, Reactor Scientist Michelle Hart, Senior Reactor Engineer Stephanie Devlin-Gill, Senior Project Manager, Audit Project Manager Roel Brusselmans, Project Manager, Audit Project Manager Reed Anzalone, Senior Nuclear Engineer, Natrium Technical Lead The participants from TerraPower for this audit were Bob Apthorpe, Jong Chang, Patrick Donnelly, Chris Forrest, Chad Pope, Matthew Presson, Joe Sinodis, and Hongbin Zhang.
On August 13, 2024, the NRC staff held an audit exit meeting with TerraPower and summarized the audit purpose, activities, and high-level results. The NRC staff did not acquire any documents during the audit. The NRC staff reviewed the following documents during the audit using TerraPowers ERR:
ANL/NSE-22/101, Revision 0A, "Natrium Source Term Analysis: [Sodium Processing System] and [Sodium Cover Gas System] Analyses," Argonne National Laboratory, December 2023.
ANL/NSE-22/100, Revision 2, Preliminary Neutron Damage and Activation Analysis of the Natrium Reactor, Argonne National Laboratory, October 2023.
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OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION
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NAT-5615, Revision 0, Releases from In-vessel Design Basis Accidents," TerraPower, LLC.
TerraPower audit meeting summaries for audit meetings dated June 6, 17, and 27, 2024.
5.0
SUMMARY
OF OBSERVATIONS As indicated in the NRC staffs audit plan, the audit was focused on specific inquiries pertaining to the content of the TR. The NRC staff reviewed information through the TerraPower ERR and held discussions with TerraPower staff to understand and resolve questions. The table below replicates the transmitted audit questions and summarizes the resolution of the questions.
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION Enclosure OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION Number Question Resolution of Question 1
How will the risk-significant radionuclides be determined by the methodology for use in the consequence analysis? Confirm if this is a process that will be addressed by the EM. If not addressed by the EM, describe how risk-significant radionuclides will be determined.
TerraPower clarified that two Sandia National Laboratories reports were reviewed to identify generic dose significant radionuclides. TerraPower also clarified that the inventory selections will be event-specific and will be described, and their basis documented in the analysis supporting a license application, not in the TR methodology.
However, TerraPower stated during the audit it would update the TR to provide clarification on the use of the Sandia reports and how their codes will be used regarding radionuclide inventory selection.
2 Will this methodology be used to characterize release locations? If so, describe how release locations will be characterized?
TerraPower clarified how source term methodology information will be used in downstream methodologies such as consequence analysis to characterize the release locations. TerraPower stated it would update the TR to summarize the connection between the source term and radiological consequences EMs.
3 How does this methodology determine the physical and chemical form of released radioisotopes? From review of information contained in TR Figure 4-1, Source Term Evaluation Model, the NRC staff understands that
((
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Confirm what codes are used in this EM to determine the physical and chemical form of released radioisotopes. In addition, make available the ((
))
user manuals so that the NRC staff can better understand the computer code methods.
TerraPower provided the requested user manual in the ERR for the NRC staff to audit. Review of the documentation enhanced staff understanding regarding the ((
)) computer code user inputs and capabilities, including the codes inclusion of details on the physical and chemical form of released radioisotopes.
4 How are time dependent isotopic release rates to the atmosphere handled (e.g., determination of timesteps, documentation of assumed timesteps, modeling a release over timesteps, number of timesteps for use in consequence assessment)? Will this aspect be addressed by the source term methodology EM or a TerraPower clarified that the selection of timestep size is not predetermined by this methodology and will be performed as part of analysis supporting a license application.
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION Number Question Resolution of Question different EM? TR section 4.2, Evaluation Model Structure, contains some discussion related to the various codes and their timestep abilities. Confirm what methods will be used in this EM to characterize time dependent releases to the atmosphere.
5 How is the heat content (or energy) of the carrier fluid determined? TR section 4.2 contains discussion related to the ability of ((
)) to model heat transfer of the carrier fluid. The NRC staff is confirming what codes the applicant will use in this EM to model the heat content of the carrier fluid.
TerraPower clarified that the heat content of the carrier fluid is not considered in the EM.
6 Is plume buoyancy determined by this EM (e.g. release rate, release height, plume heat content)? Describe plans for determining plume buoyancy to support the calculation of radiological consequences for license applications.
TerraPower clarified that plume buoyancy is not included in this source term EM and that the TerraPower "Radiological Release Consequences Methodology Topical Report," NAT-9391 (ML24208A181) discusses plume buoyancy.
7 How are radionuclide releases grouped into smaller representative source term groups or release categories? Are the representative events stated in TR section 2.4, Identification and Ranking of Phenomena and Processes (i.e., fuel handling accident, sodium processing system leak, and unprotected loss of flow with degraded pump coastdown) the release categories used to address section 4.3.16, Mechanistic Source Term Analysis (MS) of ASME/ANS RA-S-1.4-2021?
TerraPower stated that the radionuclide release groupings provided in TR section 2.4 are only intended to address the various phenomena being evaluated in the Phenomena Identification and Ranking Table (PIRT) and do not represent the final event release groupings. TerraPower clarified that radionuclide release groupings will be included in safety analysis documentation for a licensing application implementing the TR methodology and that event release grouping is not a part of the source term methodology. TerraPower stated it will update the TR to provide this clarification.
8 With regard to TR section 4.2.5, Normal Operation Methodology, is there a difference in source term development for different plant conditions (e.g., accident vs. normal operations)? Also, provide information related to the computer code ((
)) to enable the NRC staffs understanding of the EM inputs. Specifically, provide the output of this computer code and what TerraPower provided the requested computer code information. The information enhanced NRC staff understanding of the code capabilities, required user inputs, and output values for use as inputs to the source term methodology. TerraPower clarified that there will be differences in the assumed fuel failure between normal and accident conditions and that the
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION Number Question Resolution of Question information is used to determine ((
)). The NRC staff is looking to gain an understanding of how inputs into the ((
)) computer code differ between normal and accident releases.
normal operational source term will consider aspects such as ((
)) in the analysis.
TerraPower also stated it would update the TR to clarify how the source term will use GASPAR II and RG 1.109, Calculation of Annual Doses to Man-From Routine Releases of Reactor Effluents for the Purpose of Evaluating Compliance with 10 CFR Part 50, appendix I, Revision 1 (ML003740384) to quantify their normal effluent releases.
9 What will be the stated outputs of this radiological source term methodology TR? Describe how these outputs are to be used by the radiological release consequences methodology TR, TP-LIC-RPT-0005 (ML23311A138) and confirm that they provide sufficient information for the follow-on analyses. What does the source term documentation look like and are there any example results available for audit?
TerraPower clarified that the source term EM output is time dependent matrices of radionuclide releases to the environment for the various events. TerraPower stated that specific outputs from the ((
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computer codes depend on the event. TerraPower stated it would update the TR to reflect the data connection between source term and radiological consequences EMs.
10 In TR section 2.2, Figures of Merit, what is the difference between the following:
- a. inhalation dose potential and inhalation dose,
- b. submersion dose potential and submersion dose.
TerraPower clarified that the word "potential" was included only in TR section 2.2 to highlight that PIRT participants were informed that release scenarios associated with licensing basis events and operational events may involve different dominant isotopes.
TerraPower stated it would update the TR to clarify use of the term potential.
11 Provide an overview discussion of functional containment as it relates to source term scenario development (e.g., what aspects of the design are credited for functional containment for each source term). Clarify the aspects of functional containment considered in the source term for each type of event being assessed, as well as the difference between TerraPower summarized the Natrium functional containment approach, which involves selecting the barriers for each event being assessed and evaluating their performance using a mechanistic source term analysis. Functional containment barriers are thus event-specific. TerraPower also discussed the treatment of sodium, which is not part of the Natrium
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION Number Question Resolution of Question modeling of functional containment versus other potential radionuclide transport barriers.
functional containment, though its effect on radionuclide transport is modeled in the source term.
TerraPower stated it would submit a revision to the TR with a new section 2.3.5, "Functional Containment," to provide additional details on its functional containment strategy including definitions to describe the boundaries and barriers used in the analysis.
12 Provide additional discussion on the conservative approach ((
)) in TR table 3-2, Phenomena/Processes with High and Medium Importance Ranking - Conservative Approaches. Also provide additional discussion of the conservative approaches described in TR table 3-2.
TerraPower clarified that TR Table 3-2 is for information only, that the table discusses some of the potential approaches that could be used, and that the table is not a comprehensive list.
13 Provide and discuss an example calculation to illustrate the Design Basis Accident (DBA) source term modeling methodology.
TerraPower provided the calculations for two in-vessel DBA scenarios. The staff's audit of the documentation clarified how the TR methodology will be used to determine a DBA source term, including input and assumption information sources.
14 In TR table 4-7 Conservative Potential Approach Related to Phenomena / Processes, ((
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TerraPower stated it would submit a TR revision to correct the reference in table 4-7.
15 Provide the NRC staff access to the following TR references in the ERR: 21, 22, 23, and 24.
TerraPower made the requested references available for the NRC staff to audit.
16 In response to audit question 3, TerraPower made the
((
)) user manual available in the ERR. The NRC staffs reading of the contents of the user manual raised additional questions about the basis for user-selected parameter values described in both the TR and the user manual. If the expectation is that the user of the TR methodology would use a specific methodology or TerraPower provided the NRC staff access to a proposed updated version of the TR. The proposed updated TR revised the methodology such that information on methodologies or processes associated with user-inputs for which specified values are not provided in the TR would be justified through revisions
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION Number Question Resolution of Question parameter selection process to determine the value for a given user input, the TR should include information on such methodologies or processes. Therefore, provide additional clarification on the determination of user-defined event-specific analysis inputs in the EM.
Specific user-defined input examples include (see also TR Tables 4-5 and 4-6):
- 1. Aerosol modeling inputs for transport through sodium a) ((
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b) ((
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c) ((
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d) Sodium pool scrubbing decontamination factor (DF) e) ((
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f) ((
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- 2. The user option in ((
)) to allow radionuclides to switch classes (gas/vapor, aerosol) as they move from the cover gas to containment
- 3. Spent fuel pool DF to the TR or in documentation supporting a licensing application implementing the TR methodology.
The NRC staff noted that it would consider regulatory mechanisms (e.g., Limitation(s) and Condition(s)) to ensure justifications are available for NRC staff review.
OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION OFFICIAL USE ONLY - PROPRIETARY INFORMATION OFFICIAL USE ONLY - EXPORT CONTROLLED INFORMATION 6.0 REQUESTS FOR ADDITIONAL INFORMATION RESULTING FROM AUDIT As a result of the audit, the NRC staff did not identify any requests for additional information related to this TR. TerraPower revised the TR to address the NRC staffs audit items.
7.0 OPEN ITEMS AND PROPOSED CLOSURE PATHS There are no open items resulting from this audit.