ML23277A289
| ML23277A289 | |
| Person / Time | |
|---|---|
| Site: | 99902071 |
| Issue date: | 10/04/2023 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| Download: ML23277A289 (1) | |
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1 NRC Staff preliminary questions regarding Revision 2 of the X-energy Principal Design Criteria Topical Report (ML23181A172)
- 1. General: In Table 6 of the topical report (TR), each proposed principal design criteria (PDC) is compared with modular high temperature gas-cooled design criteria (MHTGR-DCs) in 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, Appendix C and the redline-strikethrough markup is for the comparison provided. However, the redline-strikethrough is not accurately reflected for some of the PDCs. For example:
PDC 2: Safety-significant structures, systems, and components important to safety shall be designed to withstand the effects of natural phenomena such as Safety-significant is a new addition and should be colored in blue.
PDC 3: Safety-significant structures, systems, and components important to safety shall be designed and located to minimize Safety-significant is a new addition and should be colored in blue.
PDC 10: It states that PDC 10 of the Xe-100 design uses the language of MHTGR-DC 10 of RG 1.232 [4] with the following changes. However, there is no markups for the changes.
PDC 44: In redline-strikethrough markup, normal operating and, currently being deleted, should be undeleted.
The NRC staff requests X-energy to perform a comprehensive review of the TR to identify and correct all such instances.
- 2. PDC 2: In Revision 2 of the TR, X-energy replaces terms postulated accident or accident conditions used in RG 1.232, Guidance for Developing Principal Design Criteria for Non-Light-Water Reactors, with those describing licensing basis events (LBEs) used in Nuclear Energy Institute (NEI) 18-04, Risk-Informed Performance-Based Technology Inclusive Guidance for Non-Light Water Reactor Licensing Basis Development. The LBEs in NEI 18-04 include Anticipated Operational Occurrences (AOOs), Design Basis Events (DBEs), Design Basis Accidents (DBAs) and Beyond Design Basis Events (BDBEs). X-energy does not include BDBEs in the scope of the Xe-100 PDCs to replace terms such as postulated accidents and accident conditions in MHTGR-DCs because those terms are characterized as DBAs for the Xe-100 design. X-energy further states that the Xe-100 PDCs that align with required functional design criteria (RFDCs) assure that those required safety functions (RSFs) and associated design limits are met during DBAs, which aligns with the historical precedent of the GDCs of 10 CFR Part 50, Appendix A. X-energy states that BDBEs are addressed in accordance with the NEI 18-04 process as endorsed in RG 1.233 and documented in accordance with NEI 21-07, Technology Inclusive Guidance for Non-Light Water Reactors.
Regarding PDCs, NEI 21-07 contains guidance for those associated with RFDCs and complementary design criteria (CDCs), corresponding to PDC-RFDCs and PDC-CDCs, respectively. In addition, at the time that the TR was submitted, the NRC staff had issued
2 DG-1404, Guidance for a Technology-Inclusive Content-of-Application Methodology to Inform the Licensing Basis and Content of Applications for Licenses, Certifications, and Approvals for Non-Light-Water Reactors, Revision 1, to be issued as RG 1.253 when finalized, to endorse NEI 21-07 with additions and clarifications. Review of both documents shows that PDCs establish the necessary design, fabrication, construction, testing, and performance requirements for safety-significant structures, systems, and components (SSCs). Thus, consistent with the discussion above regarding safety-significant SSCs versus SSCs important to safety under the RG 1.233 process, the scope of PDCs should include all safety-related (SR) and non-safety-related with special treatment (NSRST) SSCs corresponding to safety-significant SSCs.
NEI 21-07 also discusses the possibility that applicants could choose to specify a narrower set of PDCs than those discussed in the guidance, but it notes that a reduced scope approach may require that they request an exemption and that it is expected that an applicant would, at a minimum, describe its approach to exemptions in the licensing application.
Based on the above, the NRC staff requests X-energy to confirm that the proposed scope of its TR is limited (i.e., not including BDBEs) and that future licensing applicants referencing this TR will need to request an exemption from applicable NRC regulations.
In addition, X-energy uses the terminology licensing basis event in PDC 2. In the NEI 18-04 process, an LBE is an AOO, DBE, BDBE, or DBA. In other parts of the TR, X-energy states that BDBEs are not within the scope of the PDCs. The NRC staff requests that X-energy clarify whether it intends to include BDBEs when using this terminology for this PDC.
- 3. PDC 13: It states that For PDC 13 RFDC-CDC changed anticipated operational occurrences and accident conditions to anticipated operational occurrences, design basis events, and design basis accidents More accurately normal operation, for anticipated operational occurrences, and for accident conditions was replaced by anticipated operational occurrences, design basis events, and design basis accidents.
The NRC staff requests that X-energy correct this editorial error.
- 4. PDC 13: Regarding deleting normal operations, X-energy states that:
The phrase for normal operation was removed because the instrumentation and controls and associated systems to maintain the Xe-100 within its normal operating envelope does not provide any RSFs, nor NSRST PSFs [Probabilistic Risk Assessment Safety Function], nor any associated RFDC and CDC respectively. Design for instrumentation and control and associated systems will align with OCDC that provide NST PSFs during normal operations.
Section 1.1.3 of the TR also states that:
As such, the set of Xe-100 PDC that is limited to normal operations is effectively outside the scope of the NEI 18-04 methodology and NEI 21-07 structure.
However, PDC related to normal operations are necessary to demonstrate that
3 the Xe-100 will provide reasonable assurance of adequate protection during normal operations.
In addition, Section 1.2 of the TR states the following:
Owner Controlled Design Criteria (OCDC) - Design-specific design criteria that are necessary and sufficient to meet the NST PSFs.
The term OCDC is not defined in NEI 18-04 nor NEI 21-07. However, there is a need to define a term analogous to a CDC that describes the category of design criteria that support meeting a particular NST PSF. The impetus in using the term OCDC is driven by the need to separate design criteria that support normal operations from the CDC and RFDC that support NSRST PSFs during AOOs and RSFs during DBEs and DBAs. In NEI 18-04, Section 4.1, Task 6 describes, For those SSCs classified as NST, the reliability and capability targets are part of the non-regulatory owner design requirements and Task 7 describes, owner design requirements for NST-classified SSCs. As such, NEI 18-04, as endorsed by RG 1.233, provides guidance to support the development of PDC categorized as OCDC to align with normal operations and NST SSCs.
Furthermore, Table 3, PDC Associated with Normal Operations, of the TR lists the following PDCs to be associated with normal operations or conditions: PDCs 2, 4, 10, 13, 15, 19, 22, 44, 60, 61, and 64.
The NRC staff requests that X-energy address the following:
- 1. Table 3 of the TR states that PDC 13 is associated with normal operations but X-energy removes normal operations from PDC 13 and uses OCDC instead. The NRC staff requests that the discrepancy be reconciled.
- 2. Is X-energy proposing to limit the scope of PDC 13 by removing normal operations from the scope (as compared to MHTGR-DCs)? If yes, an exemption request might be necessary to support future licensing applications that reference this TR.
- 3. Section 1.1 of the TR states that PDCs for normal operations are necessary for providing reasonable assurance of adequate protection during normal operations.
What criteria or rationale is used to distinguish between PDCs and OCDCs for normal operations or conditions? According to the above excerpt from Section 1.1, the PDCs associated with normal operations or conditions should be necessary for the Xe-100 design to demonstrate the reasonable assurance of adequate protection.
PDC 14, Reactor helium pressure boundary X-energy deletes fabricated, erected, and tested from PDC 14 citing that PDC 1 sufficiently covers fabricated and erected and PDC 6 addresses tested. The NRC staff confirmed that deleting tested based on PDC 6 is reasonable. However, the NRC staff finds that PDC 1 is specific to quality standards to be applied for the subject activities and does not capture what is intended by PDC 14, which is to have an extremely low probability of abnormal leakage.
Therefore, it appears that the deletion of fabricated and erected is not justified. Accordingly,
4 the NRC staff requests X-energy to justify these proposed deletions or consider retaining them as part of PDC 14.
PDC 26: X-energy stated that it is removing normal operations because the OCDC associated with the reactivity control system that maintains normal operations supports an NST PSF not a NSRST PSF or RSF. In addition, X-energy states that specified acceptable system radionuclide release design limit and certain helium pressure boundary design limits will not be exceeded during normal operations as described in PDC 10.
Similar to the discussion above for PDC 13, the NRC staff request X-energy to clarify what criteria or rationale is used to distinguish between PDCs and OCDCs for normal operations or conditions and how it is applied to PDC 26.
PDC 34: X-energy stated that the phrase for normal operations was removed because design criteria for active heat to support normal operations will align with OCDC that provide NST PSFs.
Similar to the discussions above for PDCs 13 and 14, the NRC staff request X-energy to clarify what criteria or rationale is used to distinguish between PDCs and OCDCs for normal operations or conditions and how it is applied to PDC 13.