ML23292A278

Enclosure - Completeness Determination Terrapower, LLC, Radiological Source Term Methodology Topical Report, Revision 0 (L-2023-TOP-0046)

ML23292A278
Person / Time
Site:	99902100
Issue date:	11/14/2023
From:	Stephanie Devlin-Gill NRC/NRR/DANU/UAL1
To:	George Wilson TerraPower
References
EPID L-2023-TOP-0046
Download: ML23292A278 (1)

Text

Enclosure BASIS FOR NONACCEPTANCE AND SUPPLEMENTAL INFORMATION REQUEST RELATED TO TERRAPOWER, LLC TOPICAL REPORT RADIOLOGICAL SOURCE TERM METHODOLOGY REPORT, REVISION 0 PROJECT NO. 99902100 Overview The U.S. Nuclear Regulatory Commission (NRC) staff has completed its completeness determination review of TerraPower, LLC (TerraPower) topical report (TR) Radiological Source Term Methodology Report, Revision 0 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML23223A235). The completeness determination review was performed to determine if the TR contains sufficient technical information in scope and depth to allow the NRC staff to begin its detailed technical review. The completeness determination review is also intended to identify whether the TR has any readily apparent information insufficiencies in its characterization of regulatory requirements.

Based on its review, the NRC staff concluded that the TR did not provide technical information in sufficient detail to enable the NRC staff to begin its detailed review of the TR and make an independent assessment regarding the acceptability of the proposed TR in terms of the applicable regulatory requirements. However, the NRC staff is providing TerraPower with an opportunity to supplement the TR submittal with additional information. The NRC staff has outlined the basis for nonacceptance and supplemental information needed in the discussion, below.

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Background===

In accordance with Office of Nuclear Reactor Regulation Office Instruction LIC-109, Acceptance Review Procedures for Licensing Basis Changes, Revision 3 (ML20036C829), the NRC staff performed a review to determine whether the TR submittal included sufficient information to begin a detailed technical review. Using the guidance provided in LIC-109, Guide for Performing Acceptance Reviews, the NRC staff determined that TerraPowers approach to qualification of the source term methodology using Regulatory Guide (RG) 1.203, Transient and Accident Analysis Methods Revision 0 (ML053500170), appears to be appropriate. However, the NRC staff determined that TerraPower is missing key details that are necessary to make determinations with respect to the criteria presented in the RG. In general, the NRC staff concluded that (a) TerraPower did not provide sufficient detail to clearly identify the methodology that is the subject of the NRC staffs review, and (b) many steps in the evaluation model development and assessment process (EMDAP) presented in RG 1.203 were either not completed or not presented in sufficient detail for the NRC staff to review.

In general, the minimum aspects of the radiological source term methodology which need additional detail are (a) identification and justification of the source term phenomena to be credited in radiological consequence analyses, (b) a description of the model(s) for each phenomenon (e.g., equations, inputs, and assumptions) accompanied by justification that they are appropriate for the scenarios analyzed with the methodology, and (c) identification of supporting empirical data and data needs for the phenomenological models. A discussion of any plans for differences in how uncertainty or lack of information is addressed for analyses to support a construction permit application versus analyses for an operating license application should also be included for clarity. Additional details regarding the supplement information needs are provided below.

While there is sufficient detail in certain aspects of the TR for the NRC staff to conduct a review, a review of the TR in its current state would be of limited scope. Therefore, it is not clear to the NRC staff whether such a review would allow the NRC staff to make determinations that would support TerraPowers desired outcome and further licensing actions.

Supplemental Information Needs for Completeness Determination The NRC staff identified the following acceptance review issues that must be addressed by TerraPower as part of a supplement to the TR for the NRC staff to accept the TR and begin a detailed technical review:

1. There is no clear methodology for which approval is sought. RG 1.203 describes that an evaluation model (EM) includes one or more computer programs, special models, and all other information needed to apply the calculational framework to a specific event

[emphasis added]. As such, the NRC staff expects the TR to clearly describe:

a. the codes intended for use for each analysis;

b. the quantities conserved in each code and the general process used for numerical solution;

c. key inputs for each code and where they come from, particularly including inputs that come from results of other analyses;

d. key parameters or options in each code, including a discussion of why other parameters or options are less important; and

e. the choices made for these key parameters and options, or how a user would make a choice among multiple possible options, with appropriate justification.

The NRC staff does not expect the TR to discuss in detail those inputs, parameters, or options that do not significantly affect the results, beyond appropriate justification that they are not key drivers of the EM results.

2. Section 3, Evaluation Model Assessment Base Development, of the TR provides an overview of the work done to establish an assessment base for the mechanistic source term (MST) EM, consistent with Element 2 of the EMDAP discussed in RG 1.203. While the assessment objectives have been developed, the assessment matrix - which identifies experiments and benchmarks that support the assessment of important phenomena identified through the phenomena importance and ranking table (PIRT) process - has not. To find the TR acceptable for further review, the NRC staff expects the TR to include:

a. a preliminary assessment matrix that identifies the existing experiments available to assess key PIRT phenomena, with a discussion on experimental pedigree and high-level assessment of experimental uncertainty;

b. a justification of those experiments applicability to the EM; and

c. a discussion of any gaps identified in the assessment base as a result of this process, with a proposed path forward for closing those gaps.

3. As discussed in RG 1.203, closure models are the key to successful EM development, and their basis, range of applicability, and accuracy should be known and traceable.

The discussions on closure models in the TR are very limited in scope and do not provide sufficient detail for the NRC staff to determine whether they are acceptable for use in the EM. The TR should discuss key closure relationships and justify why they are appropriate, including in this justification a discussion of the basis, range of applicability, and accuracy of each key closure model. The discussion regarding the basis of a closure model should discuss model pedigree, addressing the physical basis of the closure model, assumptions and limitations attributed to the model, and details of the adequacy characterization at the time the model was developed, consistent with RG 1.203.

4. While the NRC staff does not necessarily expect the TR to include complete assessments of the closure models and integrated performance of the EM at this stage (consistent with EMDAP Steps 14 and 18, respectively), the TR should include a justification of why it should be expected that the EM will produce conservative results.

The NRC staff notes that anything less than a full assessment under the EMDAP would likely result in a limitation or condition on the TR specifying additional justification for the conservatism or accuracy of the EM must be provided.

To ensure completeness, the NRC staff requests that TerraPower supplement the TR to address the four items above within 60 working days from the date of this transmittal. This will enable the NRC staff to begin its detailed technical review. If the information responsive to the NRC staffs request is not received by the above date, the TR will not be accepted for review pursuant to the regulations in Title 10 of the Code of Federal Regulations Section 2.101, Filing of application, and the NRC will cease its activities associated with the TR. If the TR is subsequently accepted for review, TerraPower will be advised of any further information needed to support the NRC staffs detailed technical review by separate correspondence.

Additional Observations In addition to the acceptance review issues identified above, the NRC staff assessed the TR against the EMDAP process provided in RG 1.203 and made the following observations. The NRC staff does not consider the following observations as acceptance review issues for the TR.

However, the NRC anticipates these items would be considered or addressed as part of future licensing activities (e.g., revisions to the TR, future licensing submittals, audits).

1. Section 3.1, Assessment Base Objectives, of the TR addresses EMDAP Step 5 for assessment objectives:

a. This section is missing details regarding what the assessment needs to show.

b. The assessment objectives appear to identify only high-ranked PIRT phenomena. The NRC staff would expect both high-and medium-ranked phenomena to be included in the assessment, particularly for medium phenomena with a low state of knowledge.

2. Section 3.4, Evaluation of IET Distortions and SET Scaleup Capability, of the TR addresses EMDAP Step 8:

a. No information is provided and the TR notes that this EMDAP step is deferred for later action. This is expected because EMDAP Steps 5, 6, and 7 appear to be incomplete. However, the NRC staff notes that this EMDAP step has the potential to identify inadequacies with existing data or the need for new experiments, which could be long-lead items.

3. Section 3.5, Experimental Uncertainties Determination, of the TR addresses EMDAP Step 9:

a. No information is provided and the TR notes that this EMDAP step is deferred for later action. This is expected because EMDAP Steps 5, 6, 7 and 8 appear to be incomplete. Without completing this EMDAP step, it is unclear whether the existing experiments have too much uncertainty to be used in the assessment.

4. Section 4.1, Evaluation Model Development Plan, of the TR addresses EMDAP Step 10:

a. This section follows an example provided in RG 1.203, Appendix B, Example Showing the Graded Application of the EMDAP. While RG 1.203 does explicitly allow for a graded approach, Appendix B illustrates how to apply the EMDAP to changes to approved EMs. Since there is no approved EM in this case, the discussion in Appendix B is not fully applicable.

b. The section is missing a necessary discussion as outlined in RG 1.203 Section 1.3.1, Step 10. Establish an [EM] Development Plan, for the documentation requirements. While the NRC staff would not necessarily expect full documentation to be included at this stage, it should be discussed at some level.

5. Section 4.3, Closure Models, of the TR addresses EMDAP Step 12:

a. The NRC staff found that the discussion provided in Sections 4.3.1, Functional Containment Modeling Strategy, and 4.3.2, Radionuclide Transport Modeling Strategy, is useful to outline some more details of the EM, but these sections do not particularly cover closure models.

6. Section 5.1.3, Assess Scalability of Models, of the TR addresses EMDAP Step 15:

a. This TR section notes that the action is deferred and is anticipated to start in the first part of 2024. The NRC staff notes that models must be found to be applicable to the transient scenarios identified prior to an overall determination regarding the acceptability of the EM.

7. Section 5.2.1, Determine Capability of Field Equations and Numeric Solutions to Represent Processes and Phenomena, of the TR addresses EMDAP Step 16:

a. This TR section refers to TR Section 4.2, Evaluation Model Structure, for discussions on the field equations and numeric solutions; no actual assessment is performed. The justification provided in the TR does not meet the expectations of RG 1.203 Section 1.4.4, Step 16: Determine Capability of Field Equations to Represent Processes and Phenomena and the Ability of Numeric Solutions to Approximate Equation Set, which states, A complete assessment within this step can only be performed after completing a sufficient foundation of assessment analyses.

8. Section 5.2.2, Determine Applicability of Evaluation Model to Simulate System Components, of the TR addresses EMDAP Step 17:

a. This section refers to TR Section 4.2, which appears appropriate for the discussions contained. However, additional discussion on what models are needed and how they were assessed is warranted in TR Section 5.2.2.

9. Sections 5.2.2.1, Prepare Input and Perform Calculations to Assess System Interactions and Global Capability, through 5.2.2.4, Model Acceptance Assessment, of the TR address EMDAP Step 18:

a. It is not clear to the NRC staff that that the verification discussed in TR Section 5.2.2.2, Code Verifications, addresses code/solution verification, as discussed in RG 1.203, and instead seems to address software verification.

While software verification is an important aspect of quality assurance, it is insufficient for the NRC staff to ensure that the code implements the models correctly and that any discretization/numerical issues are appropriately addressed.

10. Section 5.2.3, Assess Scalability of Integrated Calculations and Data for Distortions, of the TR addresses EMDAP Step 19:

a. This section notes that this action deferred and is anticipated to start in the first part of next year (assuming 2024). This EMDAP step may identify differences in EM performance with respect to certain tests, and therefore gives a better understanding of the true EM uncertainties and biases. It is an important factor in the NRC staffs overall determination regarding an EM.

Project No. 99902100