ML23199A316
| ML23199A316 | |
| Person / Time | |
|---|---|
| Site: | |
| Issue date: | 08/16/2023 |
| From: | Mallecia Sutton NRC/NRR/DANU/UAL1 |
| To: | George Wilson TerraPower |
| Sutton M | |
| Shared Package | |
| ML23199A318 | List: |
| References | |
| Download: ML23199A316 (1) | |
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Enclosure AUDIT QUESTIONS RELATED TO TERRAPOWER, LLC TOPICAL REPORT NAT-3056, PLUME EXPOSURE PATHWAY EMERGENCY PLANNING ZONE SIZING METHODOLOGY, REVISION 0 Introduction and Regulatory Basis (Chapter 1 and 2)
Section 1.1, Purpose, of the topical report (TR) states that the TR methodology provides an approach for determining a plume exposure pathway (PEP) emergency planning zone (EPZ) size. This size is based on the area within which public dose, as defined in Title 10 of the Code of Federal Regulations (10 CFR) Section 20.1003, Definitions, is projected to exceed 10 mSv (1 rem) total effective dose equivalent (TEDE) over 96 hours0.00111 days <br />0.0267 hours <br />1.587302e-4 weeks <br />3.6528e-5 months <br /> from the release of radioactive materials from the facility, considering accident likelihood and source term, timing of the release sequence, and meteorology. Section 2.1, Regulatory Requirements and Guidance Considered, of the TR indicates that this approach is based on the draft final rule that would provide alternative emergency preparedness (EP) requirements for small modular reactors (SMRs) and other new technologies (ONTs) (Agencywide Documents Access and Management System (ADAMS) Accession No. ML21200A064).
1.
The draft final rule includes a second EPZ criterion that the plume exposure pathway EPZ is the area where predetermined, prompt protective measures are necessary. Is the intent that TR methodology address this second criterion, or is it to be addressed separately in the facility license application which uses the methodology?
2.
Would an applicant use this TR methodology alone to determine PEP EPZ size, or is the evaluation described in the TR only one factor in the applicants establishment of the EPZ?
Accident Screening Methodology (Chapter 3) 3.
What is the relationship between the TR accident screening methodology to the selection of licensing basis events (LBEs) for the Natrium? Are they separate processes? Describe any differences and justification for the differences.
4.
Section 2.4, Previous NRC Considerations of Reduced Emergency Planning Zone Sizes, of the TR states that, It includes consideration of internal events, external hazards, and all modes of operation, as well as other PRA risks. Are all radiological sources at the plant (e.g., non-reactor accidents, multiple radiological sources) considered? Provide further clarification if all sources would not be considered.
5.
With respect to TR Section 3.7, Screening of Seismic Release Sequences, what is the relationship between the TR and the (still not submitted) Nuclear Energy Institute methodology on seismic event screening? What is the relationship to the beyond design basis event earthquake for the site?
6.
Section 3.7, Screening of Seismic Release Sequences, of the TR states that, The threshold for capturing most of the seismic risk will be on the order of 50% of the total seismic risk. What is the technical basis for the threshold of 50% of the total seismic risk?
7.
Will the seismic probabilistic risk assessment (PRA) for the Natrium design be available during the audit?
8.
TR Section 3.5.2, Security Events, states that security events will be considered for completeness, but that accidents resulting from security events may be eliminated from detailed consideration by meeting regulatory requirements to protect against threats. The NRC staff would like more information on how security events will be considered in EPZ sizing.
9.
Clarify how the discussion of the defense-in-depth evaluation in Section 3.5.5 of the TR, Defense-in-Depth, is related to the EPZ sizing methodology.
- 10. TR Section 3.6.1 compares the screening release frequency of 1E-7 to the NRC safety goals of a large release. The EPZ is not a design feature, therefore clarify how the discussion of safety margin relates to EP.
- 11. The NRC staff would like more information on how release timing of events, considering the need for prompt protective measures, is used for screening purposes as noted in TR Section 3.8.
Radiological Consequence Considerations (Chapter 5)
- 12. Clarify which methodology referred to TR Section 5.1, Meteorological Input is out of scope for the TR; is it the atmospheric dispersion methodology or the radiological consequence analysis methodology?
- 13. TR Section 5.3, Radiological Consequence Analysis, states that the radiological consequence analysis methodology is primarily based on use of the MELCOR Accident Consequence Code System (MACCS) computer code and will be addressed separately, so is out of scope for the TR. Is the use of MACCS a required part of the EPZ sizing methodology?
- 14. Clarify which exposure pathways are to be modeled for the EPZ sizing analysis (e.g.,
immersion in plume, inhalation of plume, groundshine).
- 15. Clarify whether evacuation is modeled in the consequence analysis methodology.
- 16. Provide an overview of the radiological consequence analysis methodology and whether it will include details on assumptions or modeling choices specific to EPZ sizing consequence analyses.
- 17. Clarify which methodology referred to in the last sentence in Section 5.4, Dose Estimation for Pathway Contributors, of the TR is out of scope for the TR; is it the radiological consequence analysis methodology or the entirety of the discussion in Section 5.4?
- 18. The NRC staff would like to understand whether evaluating the dose results at a specific distance, as described in TR Section 5.4, is expected to provide similar information on extent of potential dose consequences, as in the evaluations done in NUREG-0396, Planning Basis for the Development of State and Local Government Radiological Emergency Response Plans in Support of Light Water Nuclear Power Plants, Revision 0 (ML051390356). How does this compare to the development of dose-distance curves?
- 19. Regarding the TR Section 5.4 discussion of the iterative process; is this expected to result in site-specific designs, and how would these design modifications to support an EPZ size be noted and maintained through the life of the facility? What is the relationship between these design choices and the licensing modernization project (LMP) design and licensing basis, including 50.59-like change processes? How does this potential design dependency conform with the concept that EP is an independent last layer of defense-in-depth? [also see question 28]
- 20. TR Sections 6.1.2, Most Radiological Release Sequences, and 6.1.3, Worst Radiological Release Sequences, have an event categorization scheme based on LBEs which appears to be different than the LBEs determined for the facility using the LMP process. Please clarify this difference.
- 21. TR Section 6.1.3 describes the development of a dose-distance chart to map the reduction in dose as distance is increased from the EPZ boundary. Is there a criterion for meeting this evaluation? What is the basis for the criterion? Is TerraPower using information from NUREG-0396 for this purpose?
- 22. What are the criteria or considerations used to determine if any of the events require prompt protective measures, as described in TR Section 6.2, Necessity of Predetermined Prompt Protective Measures?
- 23. TR section 6.3, Uncertainty and Sensitivity Analysis Methodology, states that the mean release frequency and 95th percentile frequency will be looked against the protective action guide (PAG) levels using a Monte Carlo distribution to obtain the specific release frequencies. Considering that the EPA PAGs are dose levels used to consider whether actions should be taken given an ongoing or imminent release of radioactive materials and do not consider release likelihood, please clarify the use of probability in the comparison to PAG levels.
- 24. TR Section 6.3 states that cliff-edge events will be looked at, but it is not clear what specific criteria or other considerations (including their bases) would be used to screen out or include these events within the EPZ sizing methodology.
General
- 25. Are there example calculations using the methodology available for the NRC staff to audit?
- 26. The TR refers to the EPA PAGs as limits. The EPA PAGs are not limits or strict numerical criteria, but are guides to trigger protective action. Was it the intention to describe the PAGs as limits? If so, why?
- 27. This TR methodology and the discussion of a risk-informed approach to regulation appears to be more related to the design of the facility rather than the use of the EPZ as a planning tool for implementation of protective actions. Additionally, the discussion in Section 6 of the TR and comparisons of screening criteria to safety goals seems to imply the EPZ contributes to demonstrating acceptable risk of the facility design. The NRC staff would like more information on TerraPowers understanding of the relationship between the Commissions safety goal policy statement and approaches to risk-informed regulation as it pertains to emergency preparedness and this TR.