ML24193A214
| ML24193A214 | |
| Person / Time | |
|---|---|
| Site: | Hermes File:Kairos Power icon.png |
| Issue date: | 07/11/2024 |
| From: | Michael Orenak NRC/NRR/DANU/UAL1 |
| To: | |
| References | |
| EPID L-2023-CPS-0000 | |
| Download: ML24193A214 (50) | |
Text
SUMMARY
REPORT FOR THE REGULATORY AUDIT OF KAIROS POWER LLC HERMES 2 CONSTRUCTION PERMIT PRELIMINARY SAFETY ANALYSIS REPORT GENERAL AUDIT OCTOBER 2023 - JUNE 2024
1.0 BACKGROUND
AND PURPOSE By letter dated July 14, 2023 (Agencywide Documents Access and Management System (ADAMS) Package Accession No. ML23195A121), Kairos Power LLC (Kairos) applied for a construction permits (CPs) under Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing of Production and Utilization Facilities, for the Hermes 2 test reactor facility. The proposed facility contains two fluoride salt-cooled, high temperature test reactors with intermediate heat transfer loops and a common power generation system. The general audit plan was issued on October 3, 2023 (ML23268A446). This audit enabled the U.S.
Nuclear Regulatory Commission (NRC) staff (the staff) to gain a better understanding of the Hermes 2 CP application through review and discussion of underlying supporting documentation. Enhanced understanding and communications supported effective and efficient development of information needs.
2.0 AUDIT REGULATORY BASES The bases for the audit are the regulations of 10 CFR 50.34, Contents of applications; technical information, paragraph (a), Preliminary safety analysis report, 10 CFR 100.10, Factors to be considered when evaluating sites, and 10 CFR 100.11, Determination of exclusion area, low population zone, and population center distance.
3.0 AUDIT OBJECTIVES The primary objective of the audit was to enable a more effective and efficient review of the Hermes 2 preliminary safety analysis report (PSAR) through the staffs review of supporting documentation and discussion with Kairos.
4.0 AUDIT ACTIVITIES The audit was conducted from October 2023 to June 2024, via the Kairos electronic reading room (ERR). The staff conducted the audit in accordance with the Office of Nuclear Reactor Regulation (NRR) Office Instruction LIC-111, Revision 1 Regulatory Audits, (ML19226A274).
2 Members of the audit team, listed below, were selected based on their detailed knowledge of the subject. Audit team members included:
Pravin Sawant, NRR (Senior Nuclear Engineer, Lead Technical Reviewer)
Jason Schaperow, NRR (Senior Reactor Systems Engineer)
Andrew Bielen, Office of Research (RES) (Senior Reactor Systems Engineer)
Ben Adams, NRR (General Engineer)
Michelle Hart, NRR (Senior Reactor Engineer)
Clair Song, NRR (Mechanical Engineer)
Alexander Chereskin, NRR (Materials Engineer)
Alexandra Siwy, NRR (Senior Nuclear Engineer)
Amitava Ghosh, NRR (Physical Scientist)
Joe Ashcraft, NRR (Electronics Engineer)
Calvin Chung, NRR (Electronics Engineer)
Jenise Thompson, NRR (Geologist)
Steve Jones, NRR (Senior Safety and Plant Systems Engineer)
George Thomas, NRR (Senior Civil Engineer)
Sarah Tabatabai, NRR (Geophysicist)
Mike Orenak, NRR (Lead Project Manager)
Matt Hiser, NRR (Senior Project Manager)
Edward Helvenston, NRR (Project Manager)
Brian Bettes, NRR (Project Manager)
3 Prior to the audit, the audit team reviewed the PSAR and defined the range of topics in the audit plan to be addressed and focused on during the audit. The following table documents dates that the staff transmitted audit questions and when audit meetings were held:
Audit Questions (ADAMS Accession No.)
Audit Meetings October 13, 2023 (ML23296A220)
October 4, 2023 (entrance meeting)
October 31, 2023 (ML23304A209)
October 27, 2023 (structural design)
November 20, 2023 (ML23324A117)
November 6, 2023 (operating life)
December 4, 2023 (ML23338A235)
November 13, 2023 (instrumentation and control)
December 11, 2023 (ML23346A016)
November 15, 2023 (seismic shear wave measurements)
January 2, 2024 (ML24002B139)
November 28-29, 2023 (metallic materials crosswalk)
January 24, 2024 (ML24024A226)
November 30, 2023 (nearby industrial facilities)
February 2, 2024 (ML24033A181)
December 4, 2023 (human factors engineering)
February 12, 2024 (ML24043A217)
December 5, 2023 (geology)
February 15, 2024 (ML24046A173)
December 7, 2023 (materials and component replacement)
February 26, 2024 (ML24057A387)
December 12, 2023 (instrumentation and control)
December 15, 2023 (structural design)
December 19, 2023 (multiple subjects)
December 19, 2023 (control room hazards, cranes and rigging)
January 3, 2024 (steam line breaks, load combinations and fatigue analyses)
January 4, 2024 (steam line breaks)
January 8, 2024 (accident analyses)
January 12, 2024 (irradiation of reactor vessel)
January 12, 2024 (geology)
January 16, 2024 (instrumentation and control)
January 17, 2024 (accident analyses)
January 19, 2024 (heat transport systems)
January 22, 2024 (instrumentation and control)
January 23, 2024 (thermal embrittlement)
January 25, 2024 (effective areas for aircraft impact)
February 1, 2024 (accident analyses)
February 8, 2024 (rupture disks)
February 21, 2024 (rupture disks and intermediate heat transfer system (IHTS))
February 28, 2024 (instrumentation and control)
March 6, 2024 (rupture disks and IHTS)
March 21, 2024 (rupture disks and IHTS)
April 3, 2024 (spent fuel pool storage racks)
May 2, 2024 (rupture disks and IHTS)
May 7, 2024 (rupture disks and IHTS)
4 The staff reviewed the following documents via the ERR:
o Chapter 2 Data Report of Geotechnical Exploration K-33 Site Additional discussion of Boring B6 o Chapter 3 Hermes Reactor Building System Design Description o Chapter 4 Graphite Hermes 2 CPA Crosswalk Dec 2023 Metallics Hermes 2 CPA Crosswalk Dec 2023 Hermes Reactor Vessel Internals System Requirements Document Hermes Reactor Vesel System Requirements Document Hermes Reactor Vessel Support System Requirements Document Hermes Reactor Core System Design Description Hermes Reactivity Control and Shutdown System Design Description Hermes Reactor Vessel Internals System Design Description Hermes Reactor Vessel System Design Description Hermes Reactor Vessel Support System Design Description o Chapter 5 Kairos internal memo, BeNaF properties from open literature sources Hermes 2 Primary Heat Transport System Requirements Document Hermes 2 Intermediate Heat Transport System Requirements Document Hermes 2 Primary Heat Transport System Design Description Hermes 2 Intermediate Heat Transport System Design Description Hermes Primary Salt Pump System Design Description Briggs, R. B., 1963, Molten Salt Reactor Program Semiannual Progress Report for Period Ending July 31, 1963, ORNL-3529, Oak Ridge National Laboratory, Oak Ridge, Tennessee.
Draft Memo, A Qualitative Comparison Study of Construction Rules for Rupture Disks within ASME Section III, Division 5 and Section VIII, Division 2 Rupture Disk Design Code Justification o Chapter 6 Hermes Decay Heat Removal System Design Description o Chapter 7 Hermes Instrumentation, Controls & Electrical Description and Architecture o Chapter 9 Chemistry Control System Requirements Document Inert Gas System Requirements Document Inventory Management System Requirements Document Hermes Pebble Handling and Storage System Design Description Hermes Reactor Thermal Management System Design Description Hermes Chemistry Control System Design Description Hermes Inert Gas System Design Description Hermes Tritium Management System Design Description Hermes Inventory Management System Design Description
5 o Chapter 13 Tritium Inventory and Release Calculation Methodology for the Maximum Hypothetical Accident and Postulated Events Applicability of the Maximum Hypothetical Accident to Hermes 2 Applicability of HER-EP-RPT-000-0005 to Hermes 2 Expected changes to QRA for Hermes 2 Adaptation of the Hermes Safety Case Documentation for Hermes 2 Thermal Fluids Phenomena Identification and Ranking Table 5.0
SUMMARY
OF AUDIT OUTCOME The staffs audit focused on the review of supporting documents associated with the topics identified in the audit plan and subsequently transmitted questions. The staff reviewed information through the Kairos ERR and held discussions with Kairos staff to understand and resolve questions. In many cases, Kairos updated the Hermes PSAR to resolve items discussed in the audit. Gaining access to underlying documentation and engaging in audit discussions about various aspects of the Hermes 2 design facilitated the staffs understanding of the Hermes 2 CP application and aided in assessing the safety of the proposed test reactors. The tables below reproduce the transmitted audit questions and summarize the resolution of the questions.
6 General Description (PSAR Section 1.3)
Question Number Question Resolution 1.3-1 The proposed operating life for the Hermes 2 test reactor facility is 11 years, as compared to the 4-year operating life proposed for the Hermes 1 test reactor facility; this increased operating life could affect the safety functions of the Hermes 2 structures, systems, and components (SSCs). Upon review of the Hermes 2 preliminary safety analysis report (PSAR), the NRC staff identified limited information regarding analyses or other evaluations that were performed to account for or justify the longer operating life, as compared to the Hermes 1 test reactor facility. The NRC staff is seeking to understand Kaiross approach to account for the increased operational life and would like to discuss this topic with the Kairos staff. Additionally, the NRC staff requests that Kairos provide a list of any new design changes, analyses, evaluations, or additional data collection (e.g., experiments, testing) that were performed or are currently planned to support the longer operating life for the Hermes 2 facility. The NRC staff expects that this list would focus primarily on analyses or evaluations performed or planned to justify the longer operating life for the safety-related SSCs listed in Hermes 2 PSAR table 3.6-1, [Structures, Systems, and Components,]
Kairos acknowledged that additional materials qualification data will be required during the operating license (OL) application review period.
Kairos updated the PSAR (ML23355A161) to provide tables indicating the materials qualification tests required for an 11-year lifetime. Kairos stated during the audit that additional materials qualification data will not be provided during the CP application review because Hermes 2 PSAR does not request any final safety findings on SSCs.
7 Comparison with Similar Facilities (PSAR Section 1.5)
Question Number Question Resolution 1.5-1 On page 1-2 of the Hermes 1 safety evaluation (SE)
(Agencywide Document Access and Management System (ADAMS) Accession No. ML23158A265), the NRC staff identified several supplements to the Hermes 1 construction permit (CP) application that provide additional information on that docket beyond the latest versions of the preliminary safety analysis report and referenced technical reports. Please identify which of these Hermes 1 application supplements are applicable to and incorporated by reference into the Hermes 2 CP application.
Kairos clarified the applicability of the Hermes 1 CP application supplements to the Hermes 2 CP application by letter dated October 27, 2023 (ML23300A141). Kairos identified that 8 of the 10 supplements to the Hermes 1 application are applicable to Hermes 2.
1.5-2 For the Hermes 1 CP application, the NRC staff conducted five audits that are documented in audit summary reports transmitted to Kairos on April 25, 2023 (ML23115A480), and June 9, 2023 (ML23160A287), and referenced in the Hermes 1 SE. These audit reports contain discussions of audit questions, references to electronic reading room document reviews, and summaries of discussions held between NRC and Kairos staff.
For each such item documented in these five Hermes 1 CP audit reports, please identify if they are still applicable to the Hermes 2 CP application. If the response or information is no longer applicable to the Hermes 2 CP application, but the question is still relevant to the Hermes 2 CP application, please provide an updated response or information for the Hermes 2 CP application.
Kairos clarified the applicability of the Hermes 1 CP application general audit responses to the Hermes 2 CP application by letter dated October 27, 2023 (ML23300A144). Kairos identified that all but two of the Hermes 1 general audit responses are applicable to Hermes 2 and provided responses specific to Hermes 2 for those two items (1.3.9-1 and 9.6-2).
8 Siting Questions (PSAR Chapter 2)
Question Number Question Resolution 2.1-1 In Hermes 2 PSAR table 2.1-2 [Resident Population Distribution within 5 miles (8 km) of the Site in Morgan County],
please provide the base year used for population projection.
Compared to Hermes 2 PSAR table 2.1-1, [Resident Population Distribution within 5 miles (8 km) of the Site in Roane County,] it seems 2010 is used for Morgan County and 2020 is used for Roane County. In addition, there is a significant population increase at the 3-to-5-mile band for all years compared to Hermes 1 PSAR table 2.1-2. Please discuss this discrepancy.
Kairos stated that 2010 was a typo, which was corrected to 2020 by letter dated December 15, 2023 (ML23349A147). During the audit, Kairos explained that the census blocks were changed between the Hermes 1 and Hermes 2 CP applications; this change resulted in the observed population increases for the 3 to 5-mile band.
2.1-2 Hermes 2 PSAR section 2.1.1.1 [Specification and Location]
does not mention Hermes 1 as a prominent man-made feature within 5 miles. Please discuss why Hermes 1 is not included.
Kairos modified the PSAR by letter dated December 15, 2023 (ML23349A147), to include the Hermes 1 facility to the list of prominent man-made features within 5 miles of Hermes 2.
2.2-1 The effects of Hermes 1 accidents on the safe operation of the Hermes 2 facility are not discussed in Hermes 2 PSAR section 2.2.3 [Analysis of Potential Accidents at Facilities].
For example, the Hermes 1 facility is planned to include storage of 21,555 gallons of diesel fuel in one onsite fuel tank. The overpressure on Hermes 2 from a potential explosion of this tank is likely negligible, but no analysis is provided regarding other potential hazards from this storage tank (e.g., fire, smoke, and other BLEVE [boiling liquid expansion vapor explosion]
hazards).
Kairos modified the PSAR by letter dated December 15, 2023 (ML23349A147), to state that an analysis for a Hermes 1 fuel tank fire will be provided in the Hermes 2 OL application.
2.2-2 In Hermes 2 PSAR table 2.2-1 [Nearby Facilities], the Ultra Safe Nuclear Corporation Pilot Fuel Manufacturing Facility is 0.8 mi [miles] away from the Hermes 2 facility. The mention of this facility is new in the Hermes 2 PSAR, as compared to the information provided in the Hermes 1 construction permit application. The table notes that accidents at this facility are not expected to affect the Hermes 2 facility without any discussion or bases for this assertion. Please provide a discussion or justification for this note.
Kairos modified the PSAR by letter dated December 15, 2023 (ML23349A147), to state that an analysis for the potential impacts of the Ultrasafe facility will be provided during the Hermes 2 OL application review. In addition, Kairos clarified during the audit discussion that the Ultra Safe facility site was not yet chosen at the time of the Hermes 1 CP application submittal.
9 2.2-3 In Hermes 2 PSAR table 2.2-1, the TRISO-X Fuel Facility is 2.4 mi away from the Hermes 2 facility. The mention of this facility is new in the Hermes 2 PSAR, as compared to the information provided in the Hermes 1 construction permit application. The table does not provide any discussion of the effects of accidents at this facility on the Hermes 2 facility, if any. Please provide additional information regarding effects of accidents at the TRISO-X fuel facility on the proposed Hermes 2 facility.
Kairos stated during the audit that the 2.4 mi distance prevents any physical impacts from the TRISO-X facility on Hermes 2 operations; however, Kairos acknowledges the potential impacts on operation from a release at the TRISO-X facility. Kairos modified the PSAR by letter dated December 15, 2023 (ML23349A147),
to state that an analysis for the potential impacts from a release at the TRISO-X facility will be provided in the Hermes 2 OL application.
2.2-4 Regarding table 2.2-8 [Near-Airport and Helicopter Crash Frequency Inputs and Calculations], the projected aircraft hazards may have new developments since the Hermes 1 construction permit application was submitted (e.g., the Oak Ridge airport project has progressed). The NRC staff desires to review the aircraft crash hazard calculation and selection of the representative aircraft for structural design.
Through audit discussion with Kairos staff and further staff evaluation, the staff confirmed the accuracy of Kaiross analysis. Kairos modified the PSAR by letter dated February 9, 2024 (ML24039A191), to provide updated aircraft hazard data. The NRC staff will review the selection of the representative aircraft for structural design during review of the OL application.
10 2.5-1 From the response to Hermes 2 General Audit question 1.5-2 (ML23300A144), the NRC staff understands that the due diligence report on geotechnical investigation for the Hermes 1 site is applicable to the Hermes 2 site. However, site characterization was expected to continue after the construction permit application review for Hermes 1. Additionally, the strata at the overall site is inclined with a reactor foundation elevation of ~760 ft. [feet] for Hermes 1 vs. ~750 ft. for Hermes 2, and the overall site shows karst features. Therefore, significant spatial variations in subsurface geology may exist between Hermes 1 and Hermes 2. The NRC staff would like to:
a) Review the current due diligence report (irrespective of updates).
b) Understand the continuing site characterization that has happened since the submittal of the Hermes 1 construction permit application and whether this information resulted in any refinements to the understanding of subsurface geology at the Hermes 2 site.
During the audit, Kairos clarified that the due diligence report is the same for both facilities and is applicable to Hermes 1 and Hermes 2. The due diligence report was provided to the staff through the ERR for review. Kairos stated that additional site characterization is being performed for Hermes 1 in preparation for the Hermes 1 OL application. Kairos stated that additional site characterization has shown that solid rock is found across the site at some depth and the foundation rock will be at depths to ensure no evidence of karstic dissolution is encountered.
However, the staff observed karst activity in the borings detailed in the due diligence report for the Hermes 2 locations. As a result, the staff is proposing a license condition for Hermes 2 to ensure that the foundation materials are adequately characterized.
11 2.5-2 Section 2.5.5.2, Plant Layout and Foundation Interface, of the Hermes 2 PSAR states that the reactors will be located near Borings B-3 and B-6. In addition, section 2.5.5.2 states that the foundation mat will be placed directly over sound rock or over thin concrete fill. Figure 2.5-11, Location of the Facility at K-33, indicates that the reactor foundation level may be in the Pond Spring Formation, which is thin-to-medium bedded limestone, but also shows the Murfreesboro Limestone at the far southeastern corner of the Hermes 2 reactors. Figure 2.5-22, Foundation Interface, shows that the proposed foundation will be approximately at 745 ft. elevation and does not show the contact between the Murfreesboro Limestone and Pond Spring Formation in the subsurface or at the foundation level.
a) Clarify the specific lithology of the foundation level rock, stating if the foundation level for both units is entirely within the Pond Spring Formation or if the far southeastern portion of the foundation will be underlain by the Murfreesboro Limestone.
b) Discuss whether figure 2.5-22 needs to be updated to account for these foundation rock variations.
Kairos stated that additional borings will be conducted to support the Hermes 2 OL application and these borings will establish the foundation level geologic unit and any unit contacts in the foundation area. Kairos clarified that the specific lithology of the foundation rock is currently an approximation based on the information in the Elverton Quadrangle. Figure 2.5-22 will not be updated for the CP application but will be confirmed or revised for the final design in the OL application.
12 2.5-3 Borings B-3 and B-6 are the closest borings to the proposed Hermes 2 foundations that are at ~745 ft. elevation and ~20 ft.
deep (ground level is ~765 ft. elevation). The [Kairos] due diligence report shows the multiple formations in those borings.
B-3 terminated at 14.1 ft. deep where weathered rock was encountered; however, the formation is not documented in the borehole log. Boring B-6 is ~35 ft. deep. The borehole log of B-6 indicates moderately weathered and highly fractured rock at elevation 746 to 746.4 ft. (sample RC-3). Additionally, a clay-filled solution feature was observed at elevation 742.7 to 744 ft.
Sample RC-5 at elevation 731 to 733 ft. indicates slightly weathered rock with clay-filled fractures.
a) Section 2.5.5.2 of the Hermes 2 PSAR states, The bearing system for the safety-related structures is a foundation mat resting directly over sound rock
[emphasis added] or over a thin concrete fill. It is anticipated that sound bedrock [emphasis added] will be very close to the elevation of the bottom of the basemat.
Additionally, in the supplement regarding boring B-6 posted to the Kairos electronic reading room on December 21, 2023, Kairos characterized the material encountered at a depth of 25 ft in B-6 (elevation ~741 ft) as "adequate for foundation purposes. However, taking account for the dip of the rock units at the site, this section of suitable rock may not be continuous at this depth and elevation throughout the excavation footprint.
Discuss whether the observations from borings B-3 and B-6 are considered sound rock as described in section 2.5.5.2 and whether this sound rock is expected to occur continuously at the foundation elevation throughout the entire excavation.
b) Discuss the plan, if any, to characterize the weathered rock and clay-filled solution features and fractures observed at and below the proposed foundation level of the Hermes 2 reactors, including their spatial extents Kairos stated during the audit that additional borings will be performed in and around the Hermes 2 reactor footprints to better characterize the subsurface soil and rock layers, which will be provided in the Hermes 2 OL application. Kairos will determine the rock mass quality (using rock mass rating (RMR), Q, and/or geological strength index (GSI) methodologies). From the estimated rock mass quality, Kairos will estimate the rock mass strength, etc. and use those parameter values to determine the bearing capacity of the foundation and stability of the reactor foundation and the reactor when constructed.
For part (c) of the question, Kairos stated that the borehole description is repeated because it went to the next page. Both RC-4 and RC-5 samples are from the same five-foot boring interval.
13 and mitigation of their effects on the reactor foundation stability.
c) The description of sample RC-4 is the exact same as RC-5, despite being different elevations. However, the boring logs note that RC-4 has a recovery of 100% and RQD of 90% while RC-5 has a recovery of 96% and RQD of 68%. Clarify which sample (RC-4 or RC-5) is associated with the description given in the borehole log of B-6 and provide a description for the other. Also, clarify whether the lower RQD at RC-5 will affect the stability of the proposed foundation level closer to the elevation of RC-4.
14 Design Of Structures, Systems, and Components Questions (PSAR Chapter 3)
Question Number Question Resolution 3.5-1 KP-FHR PDC [principal design criteria] 4 [Environmental and dynamic effects design bases] as modified by the discussion in PSAR section 3.1, Introduction, states that safety-related structures, system, and components (SSCs) shall be designed to be compatible with the environmental conditions associated with accidents. Since Hermes 2 has a power generation system that results in steam piping in the non-safety related portion of the reactor building, protecting the safety-related reactor building from adverse effect of steam line break is important.
However, PSAR section 3.5.2, Design Bases, does not include PDC 4 within the design bases for the safety-related portion of the reactor building. Please clarify the design basis for the protection of safety-related SSCs from environmental effects of a steam break in the non-safety related portion of the reactor building (e.g., distance or barriers). Additionally, address the effects of potential pressurization of the non-safety related portion of the reactor building on the conditions within the safety-related portion of the reactor building.
Kairos modified the PSAR by letter dated January 24, 2024 (ML24025C685), to indicate that PDC 4 is applicable to section 3.5, Plant Structures. In addition, Kairos provided discussion in sections 3.5 and 9.9.1, Steam System, to clarify that the reactor building is designed to protect safety-related SSCs against high-pressure steam system leaks and breaks.
The staff and Kairos also discussed technical aspects of the seismic design of the safety-related portions of the reactor buildings related to PSAR section 3.5.3.3.1, Seismic Design of the Safety-Related Portion of the Reactor Building.
Specifically, Kairos stated the seismic design category 3 (SDC-3) structures seismic performance criteria, and corresponding limit state selection, defined based on its credited function during or following an earthquake. Kairos stated that the OL application will provide specific details and justification for the selected limit state(s) (i.e.,
A, B, C, or D in American Society of Civil Engineers 43-19, Seismic Design Criteria for Structures, Systems, and Components in Nuclear Facilities) for the seismic design of the safety-related portion of the reactor building that contains and protects all Hermes 2 safety-related SSCs.
15 Reactor Description Questions (PSAR Chapter 4)
Question Number Question Resolution 4.3-1 Table 4.3-2, Load Combinations for the Reactor Vessel System, and table 4.7-1, Load Combinations for the Reactor Vessel Support System, provide the load combinations for the reactor vessel (RV) system and reactor vessel support system (RVSS) and show that loads of service level A/B/C/D are considered. Additionally, the fatigue analyses for the RV system and RVSS might be impacted by the difference between Hermes 1 and Hermes 2 considering the longer operation life for Hermes 2:
a) Provide calculations or documents on the Kairos electronic reading room, if available, that discuss load development methodology for loads of different service levels.
b) Discuss how the fatigue analyses will be updated for 11 years of operation.
Kairos stated during the audit that the basis for the load development methodology is the same for Hermes 2 as for Hermes 1 and will be performed in accordance with the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel (BPV) Code,Section III, Division 5. The calculations will be provided as part of the Hermes 2 OL application.
Kairos also stated during the audit that the fatigue analysis method is the same for Hermes 2 as Hermes 1 and will be performed in accordance with the ASME BPV Code,Section III, Division 5.
In addition, Kairos indicated that the fatigue analysis calculations will be provided to the staff for audit during the Hermes 2 OL application and that the 11 years of operation for Hermes 2 will be considered in the fatigue analysis.
4.3-2 Since the Hermes design contains graphite components that can oxidize during an air ingress event, discuss whether Kairos plans to assess potential combustible gas generation due to oxidation of graphite as part of a future operating license application.
Kairos stated during the audit that the analysis of potential combustible gas generation will be addressed during the Hermes 2 OL review.
16 4.3-3 Although thermal aging is mentioned in the Kairos topical report KP-TR-013-P, Metallic Material Qualification for the Kairos Power Fluoride Salt-Cooled High-Temperature Reactor,
[(ML23062A727)] in the context of corrosion coupons, it is not clear how embrittlement due to thermal aging is incorporated into the Hermes 2 design as it is not explicitly discussed in PSAR section 4.3, Reactor Vessel System. Please discuss whether Kairos evaluated thermal embrittlement for an 11-year lifetime plant. If so, discuss whether it was judged to be a significant factor and describe how it may eventually be incorporated into design of metallic safety-related SSCs, which includes the effect on loss of tensile strength and embrittlement in welds from thermal aging.
Kairos stated that the OL application will assess the potential for thermal embrittlement in the 11-year proposed operational lifetime and will provide justification that the level of thermal embrittlement will not affect the ability of safety-related metallic components to perform their safety functions. Kairos believes that enough external and internal materials data (for the base and weld metals) exists to assess thermal embrittlement and will analyze the effects of thermal embrittlement on the mechanical performance of metallic components when the final designs operational conditions are known.
Kairos stated that they do not currently see any data gaps and that no additional research and development testing is expected to be necessary, but they will do so if warranted by the final design.
4.5-1 The vessel irradiation discussion in PSAR section 4.5.3.2, Nuclear Design Analysis Inputs to Other Sections, remains unchanged from the Hermes 1 PSAR. State whether the preliminary best estimate dpa [displacements per atom] plus uncertainty for Hermes 2 is still within 30% of the low-level irradiation value taking into account the Hermes 2 vessel lifetime of 11 years (as opposed to 4 years for the Hermes 1 design).
Kairos clarified that the preliminary best estimate dpa plus uncertainty for Hermes 2 is still within 30% of the low-level irradiation value when including consideration of the Hermes 2 vessel lifetime.
17 Primary Heat Transport System Design Questions (PSAR Section 5.1)
Question Number Question Resolution 5.1-1 PSAR section 5.1, Primary Heat Transport System, states that the primary coolant (Flibe) in the primary heat transport system (PHTS) is maintained at a higher pressure than the intermediate coolant (BeNaF).
a) In case of an intermediate heat exchanger (IHX) leak or break, please discuss if:
- a. the consequences of not maintaining the pressure differential are limited only to the contamination of Flibe,
- b. there are any nuclear safety concerns stemming from contamination of Flibe by BeNaF, and
- c. Kairos has established an acceptable level of contamination of the Flibe by BeNaF.
b) Provide the acceptable pressure differential between the primary and intermediate loops and the basis for this value.
a) Kairos stated that there are no radiological consequences from an IHX break, only contamination issues, and that there are no nuclear safety concerns from contamination and contamination of Flibe by BeNaF would increase the economy of sodium-24 for the core.
Kairos stated that the acceptable limit of the contamination of Flibe by BeNaF will be provided with the OL application. Flibe will have a purity level specification provided in the OL, not the level of contamination of BeNaF. The staff issued request for confirmation of information (RCI) 2 and Kairos responded on April 12, 2024 (ML24103A243), confirming that they will demonstrate that contamination of Flibe in the PHTS will remain within purity specifications.
Additionally, Kairos stated that topical report KP-TR-005-P-A, Reactor Coolant for the Kairos Power Fluoride Salt-Cooled High Temperature Reactor (ML20219A591), does address uncertainties in reactor coolant (Flibe) properties and those properties are accounted for in the design of the decay heat removal system (DHRS).
Kairos clarified that the DHRS uses radiative heat transfer, so changes in the Flibe properties due to contamination is not very impactful to DHRS operation. The NRC staff asked if contamination would affect natural circulation within the reactor vessel. Kairos stated, and confirmed in RCI 2, that the Hermes 2 design will maintain a specified range of Flibe properties consistent within the
18 range of purity specifications or justify potential deviations from the purity specifications in the FSAR. This ensures that any potential impact on natural circulation remains within the analyzed conditions.
b) Regarding pressure differential between the PHTS and IHTS, Kairos stated that they do not have the specific values of the pressure differences calculated at this time.
5.1-2 Paragraph 10 CFR 50.34(a)(3)(iii) requires, in part, that construction permit applicants provide information relative to materials of construction as part of the preliminary design information for a proposed facility. State the materials to be used to construct the PHTS and intermediate heat transport system (IHTS). Additionally, state whether the materials will be galvanically similar to each other and the reactor vessel system materials.
Kairos stated that the PHTS will be austenitic stainless steel which is specified in section 5.1.1.4, Primary Loop Piping, of the Hermes 2 PSAR. In addition, Kairos indicated during the audit that the IHTS is expected be austenitic stainless steel.
19 5.1-3 PSAR section 5.1 describes two heat removal systems: the heat rejection subsystem (HRS) and IHTS.
a) Discuss the relationship between the HRS, IHTS, and the power generation system (PGS) (e.g., when plant operations transitions from the HRS to the PGS and back).
b) Inadvertent actuation of the non-safety HRS could lead to overcooling. Discuss whether there are control system setpoints (e.g., minimum temperature) to avoid overcooling or freezing.
c) Discuss how the HRS impacts decay heat removal system (DHRS) operation and threshold power.
a) Kairos stated during the audit that the HRS is used during low power startup and normal shutdown. As reactor power increases, the heat rejection blower will be switched off to minimize heat losses through the heat rejection radiator.
Specific details regarding this transition from HRS to IHTS will be provided in the Hermes 2 OL application.
b) Kairos indicated that section 7.2.1.3, Primary Heat Transport Control System, of the Hermes 2 PSAR specifies that the non-safety related primary heat transport control system maintains the primary coolant in a liquid phase with control setpoints and uses auxiliary heating to prevent localized under-and overcooling.
c) During the audit, Kairos stated that the HRS will not impact DHRS operation or threshold power.
Threshold power is based on current reactor power level, operating history, and parasitic losses and does not depend on the heat rejection path being used for normal operation.
20 Intermediate Heat Transport System Questions (PSAR Chapter 5.2)
Question Number Question Resolution 5.2-1 PSAR table 3.6-1, Structures, Systems, and Components, appears to show several subsystems of the intermediate heat transport system (IHTS) that are new to the proposed Hermes 2 facility, as compared to the proposed Hermes 1 facility. Clarify whether these subsystems (e.g., intermediate chemistry control, intermediate inert gas system) are unique systems that are separate from the auxiliary systems described in chapter 9, Auxiliary Systems, of the preliminary safety analysis report (PSAR). For any subsystem that is separate and unique from those described in chapter 9, provide the following information (which is discussed for all other facility systems listed in table 3.6-1):
- a. Description of the system
- b. Design bases for the system
- c. A description for how the system meets its design bases
- d. Any testing and inspection requirements for the system
- e. Place the system requirements document on the portal for NRC staff audit Kairos clarified that the IHTS subsystems are separate from the systems described in PSAR chapter 9. Kairos modified the Hermes 2 PSAR by letter dated December 21, 2023 (ML23355A192),
to include additional information on the IHTS and its design bases. In addition, Kairos provided the IHTS design documents listed in section 4.0 of this report on their ERR for staff review. Kairos indicated that section 5.2.4, Testing and Inspection, of the Hermes 2 PSAR specifies that testing and inspection requirements for the IHTS will be provided in the Hermes 2 OL application.
21 5.2-2 PSAR section 5.2.1, Description, states that BeNaF is thermodynamically stable, is compatible with structural materials, and has analogous properties to the primary Flibe coolant. However, in PSAR section 1.3.9, Research and Development, a new research and development activity, Complete compatibility evaluation of the intermediate and reactor coolant chemical interaction (Section 5.1.3), is listed.
The NRC staff would like to understand the basis for the aforementioned statement in PSAR section 5.2.1 and the research activity scope discussed in PSAR section 1.3.9, including a high-level description of:
a) the compatibility evaluation process and the parameters/characteristics that will be quantified (e.g.,
corrosivity, radionuclide solubilities, thermophysical properties),
b) the interactions with structural materials (e.g., 316H SS
[stainless steel], graphite) that will be characterized and quantified, and whether the ingress of the BeNaF into the primary loop would alter thermophysical properties of the Flibe (e.g., viscosity) or change solubilities of radionuclides or salt components.
Kairos stated that the two salts are chemically compatible. The contamination of Flibe will still need to be bounded by its purity specification found in topical report TR-KP-005-P-A. Flibe contaminated by BeNaF is not expected to have a major effect on Flibe properties or structural materials.
a) As discussed in RCI 2, Kairos will either maintain their purity specifications from the referenced topical report or provide justifications for deviations from the specification. This allows the NRC staff to evaluate the potential impact of changes in the salt composition on properties of the reactor coolant.
b) Kairos added PSAR table 4.3-5, Environmental Compatibility Testing of Metallic Materials, that discusses corrosion testing for metallic materials in a supplement dated December 21, 2023 (ML23355A161). Kairos stated they will have compatibility data for BeNaF and graphite for the OL application.
The proposed research and development activities are confirmatory and may not be a quantification of interactions of the two salts.
Additionally, Kairos mentioned during audit discussions that natural circulation flow and heat removal are not particularly sensitive to changes in reactor coolant properties.
22 5.2-3 PSAR section 5.2.1 states that the IHTS is equipped with safety-related rupture disks located in the intermediate inert gas system to prevent over-pressurization of the IHTS during a postulated superheater tube leak or rupture event.
a) One of the stated functions of the intermediate inert gas system is to keep the intermediate coolant pressure in the heat exchangers lower than the pressure in the PHTS. Please discuss how the safety-related rupture disks, the non-safety related intermediate inert gas system, and IHTS interact to maintain this pressure differential.
b) State whether the IHTS rupture disk should be listed as a safety-related SSC in PSAR table 3.6-1, Structures, Systems, and Components.
c) State whether any preliminary calculations have been performed to size the safety-related rupture disks to maintain acceptable pressures within the IHTS. If so, please make these calculations available for NRC staff audit.
d) From the discussion in PSAR section 13.1.10.11, IHX Failure Due to Superheater Tube Rupture or Leak, it appears that preventing IHX failure is an important function of the safety-related rupture disks. However, this is not clear from the description of the design in PSAR section 5.2. Please describe the safety functions of the rupture disks.
e) Discuss the consequences of the IHTS rupture disks failing to burst.
f) Discuss whether inadvertent operation of non-safety related trace heating could lead to IHTS over-pressurization and rupture disk burst.
g) Although PSAR section 5.2.1 cites the need for safety-related rupture disks for overpressure protection, the remainder of the IHTS appears to be classified as non-safety related. State the consequences of IHTS failure from mechanisms besides overpressure a) Kairos updated section 5.2.3, System Evaluation, in Revision 1 of the PSAR (ML24144A090). The rupture disks are only meant to relieve pressure in case of a superheater tube leak.
b) Kairos updated PSAR table 3.6-1 to list the rupture disks as safety-related.
c) Kairos stated that calculations have not been completed yet but will be available in the OL application. Kairos confirmed in RCI 3 (ML24135A382) that it will provide a final design for the IHTS and the safety-related rupture disks that justifies that the rupture disks will reliably perform their safety function to provide overpressure protection.
d) Kairos updated PSAR section 5.2.1.2, Intermediate Inert Gas Subsystem, to state,
[t]he rupture disks preclude a gross failure of the IHX that could occur as a result of a postulated superheater tube leak or rupture event by relieving pressure in the IHTS and providing a relief path for the steam (see Section 13.1.10).
e) The consequence would be failure of the IHX due to over-pressurization. While exact design details have not been finalized, Kairos confirmed in RCI 3 that they will address the redundancy and independence of the rupture disks for the OL application.
f) Kairos stated that the trace heating will not have the heat transfer capacity to be able to over-pressurize the IHTS.
23 (e.g., corrosion, creep rupture) and discuss whether Kairos has assessed the consequences of these alternate failure mechanisms.
g) Kairos indicated that the consequences are bounded by the salt spill event discussed in PSAR section 13.1.3. Components near where BeNaF-concrete interactions could happen and prevent SSCs from performing their function are designed with features to preclude such interactions, such as steel liners.
5.2-4 Provide the currently available thermophysical properties (e.g., thermal conductivity, viscosity, density, heat capacity) of BeNaF.
Kairos provided the memo, BeNaF properties from open literature sources, on the ERR that contained data on BeNaF thermophysical properties collected from open literature sources for the staff to review. The NRC staff reviewed the memo to obtain a rough estimate of different property correlations and assess the quality of available data (e.g., uncertainties). In addition, Kairos stated that an evaluation of the purity requirements for BeNaF will be available during the Hermes 2 OL application review.
5.2-5 Anhydrous hydrogen fluoride [HF] is used for tritium management in the IHTS. Since anhydrous hydrogen fluoride is known to increase the corrosivity of the salt, please discuss if there is any limit on the addition of hydrogen fluoride in the IHTS. Additionally, please provide information on when and how the salt will be sparged with hydrogen fluoride.
This question was superseded by audit questions 5.2-8 and 5.2-9. Kairos indicated it will provide information in the OL application on how it plans to mitigate the effects of HF.
Additionally, depending on the outcome of the analyses performed for RCI 1, if the IHX is relied upon to remain functional during postulated accident scenarios, the NRC staff would take into consideration the potential corrosive effect of HF additions when evaluating IHX tube integrity.
24 5.2-6 Describe the preliminary design of the rupture disk(s). Given that rupture disk design and operation may vary based on particular system conditions and events, discuss how the preliminary design accounts for the operating characteristics that are unique to the Hermes 2 intermediate heat transport system.
Kairos indicated during the audit that the rupture disks will be of a diaphragm design. Kairos stated they may implement design features such as large lines and clean gas purge that will influence the environment in terms of temperature and chemistry. Kairos confirmed in RCI 3 what, at a minimum, the final design of the IHTS and rupture disks will address for the OL application.
5.2-7 Specify whether Kairos intends to follow the overpressure protection provisions for rupture disks in the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel (BPV) Code,Section III, Division 5, High Temperature Reactors.
Kairos plans to design the rupture disks to ASME BPV Code,Section VIII, Rules for Construction of Pressure Vessels, consistent with the rest of the IHTS that is designed to the same code. Kairos provided a justification for the use of ASME BPV Code,Section VIII, instead of the ASME BPV Code,Section III, Division 5 (ML24138A214), as part of their May 16, 2024, submittal.
5.2-8 Discuss how the rupture disk will be qualified to operate in its environment (i.e., hydrogen fluoride (HF) exposure and potential salt vapor).
Kairos confirmed in RCI 3 that they will provide potential qualification testing with the OL license application to justify that the rupture disks will reliably perform their safety function.
5.2-9 State whether a build-up of salt vapor or corrosion products (e.g., from HF) could prevent the rupture disk from properly functioning or discuss how the rupture disk is being designed to prevent a loss of functionality under these conditions.
Kairos confirmed in RCI 3 that the final design of the rupture disks will consider the potential for salt vapor deposition to impede rupture disk function.
25 5.2-10 Section 13.1.10.11, IHX Failure Due to Superheater Tube Rupture or Leak, of the preliminary safety analysis report (PSAR) notes that a superheater tube rupture could lead to over-pressurization of the IHTS. The PSAR also notes that a safety-related pressure relief feature (i.e., rupture disks) would be used to preclude failure of the IHX due to the postulated superheater tube failures. Additional information regarding the rupture disks is discussed in PSAR section 5.2, Intermediate Heat Transport System, of the PSAR.
- a. The PSAR discussions are not clear about the safety function of the rupture disks and their adequacy for the mitigating the effects of a Flibe-water interaction without crediting functions performed by other structures, systems, and components (SSCs) in the IHTS. Provide additional details regarding the progression of a postulated superheater tube rupture or leak event, with a focus on how the intermediate salt vessels, intermediate inert gas system, and rupture disks are used to mitigate the mass and energy release associated with a superheater failure. Provide specific information regarding the potential for damage to the IHX or other components (e.g., Flibe-water interactions in the primary heat transport system) that could be incurred if the rupture disks fail to perform their intended functions.
- b. Relatedly, discuss the basis for identifying the rupture disks as safety-related and discuss why all other SSCs in the IHTS have preliminarily been classified as non-safety related. This discussion should focus on how the rupture disks would prevent or mitigate the consequences of accidents which could result in potential exposures exceeding the limits set forth in Title 10 of the Code of Federal Regulations, section 100.11, Determination of exclusion area, low population zone, and population center distance.
a) Kairos clarified the safety function of the rupture disks in Revision 1 to the PSAR, stating in PSAR section 5.2.1.2 that, [t]he rupture disks preclude a gross failure of the IHX that could occur as a result of a postulated superheater tube leak or rupture event by relieving pressure in the IHTS and providing a relief path for the steam (see Section 13.1.10). While specific details on the adequacy of the rupture disks and the progression of a postulated superheater tube rupture or leak event will not be available until the OL application, Kairos confirmed in RCI 1 (ML24103A241) that a superheater tube rupture event will be analyzed and in RCI 3 that the rupture disks will reliably perform their safety function to prevent a gross failure of the IHX.
b) Kairos stated that the rupture disks are designed to mitigate the effects of a superheater tube rupture to preclude a gross failure of the IHX by relieving pressure in the IHTS and providing a relief path for the steam entering the IHTS. The final design details will be important in evaluating the IHX safety classification; therefore, Kairos confirmed in RCI 1that the final design for Hermes 2 will demonstrate that the IHX tubes are not safety-related or that their failure is not credible.
26 5.2-11 During the audit discussion on February 1, 2024, Kairos indicated that Flibe-water interactions could lead to a loss of functional containment capability of the Flibe when mixed with water.
- a. Table 14.1-1 [Proposed Variables and Conditions for Technical Specifications], of the Hermes 2 PSAR states that limiting conditions for operation (LCOs) will be proposed to limit the quantity of water and Flibe in the intermediate coolant. The proposed LCOs imply that there would be permissible amounts of Flibe and water that could exist simultaneously in the IHTS. Discuss the interaction of Flibe and water within the intermediate coolant under normal IHTS operation.
- b. The consequences of Flibe-water interaction from water entrained in the intermediate coolant during an IHX tube failure event are not discussed in the PSAR. Please discuss the mechanisms or processes that would lead to loss of functional containment capability of the Flibe if it mixes with water.
a) Kairos stated that during normal operation of the IHTS, there would not be significant Flibe or water in the IHTS, but small amounts could enter through minor leaks in the heat exchangers (IHX and superheater). The intent of the proposed LCOs is that significant leaks would be identified and actions taken to mitigate or stop the leaks.
The water would first interact with BeNaF, resulting in beryllium oxide and HF which are corrosive chemicals. The LCO would also limit corrosion in the IHTS by limiting the Flibe and water ingress.
b) Kairos stated that significant Flibe-water interaction is considered a precluded event; therefore, significant Flibe-water interactions were not analyzed. Kairos considers Flibe to be a secondary barrier and TRISO as the primary containment barrier. The material at risk for release (MAR) from any Flibe that exists in the IHTS (based on a LCO) is included in the estimate of MAR in the IHTS.
Kairos stated that the release fraction for the salt spill model based on jet breakup or splashing is 1x10-5. This salt spill release fraction is assumed for a Flibe leak from an IHX break into the IHTS. Kairos characterized the application of this release fraction as non-physical and conservative because the Flibe from the PHTS would be entering the liquid BeNaF in the IHTS instead of jetting or splashing into an empty space as assumed in the NRC-approved mechanistic source term (MST) methodology topical report KP-TR-012-A, KP-FHR Mechanistic Source Term Methodology, (ML22136A291). The MAR release
27 fraction from the IHTS is in accordance with the MST methodology, which is conservative because it does not account for interactions within the BeNaF.
The staff will evaluate the detailed modeling of transport and release from the IHTS analyses submitted as part of the OL application.
5.2-12 During the discussion of audit question 5.2-7 on the use of the overpressure protection provisions in the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel (BPV) Code,Section III, Rules for Construction of Nuclear Facility Components, Division 5, High Temperature Reactors, for the safety-related rupture disks, Kairos clarified that Section VIII, Rules for Construction of Pressure Vessels, of the ASME BPVC will be used for the construction of all components of the IHTS, including the safety-related rupture disks.
- a. In NRC Regulatory Guide 1.87, Acceptability of ASME Code Section III, Division 5, 'High Temperature Reactors', Revision 2, the NRC staff endorsed Division 5 of the ASME BPV Code,Section III, for use in the design, construction, testing, and quality assurance of safety-related SSCs for high temperature applications.
The NRC staff has not provided a similar endorsement of ASME BPV Code,Section VIII, for use in safety-related applications. Please describe how the use of ASME BPV Code,Section VIII, is adequate for the design and construction of the safety-related rupture disks.
- b. Please clarify Section 5.2 of the PSAR to identify the construction code for the rupture disks, the materials of construction (grade of stainless steel) for the rupture disks, and whether the rupture disks will be included in the in-service testing program.
a) Kairos provided additional information on the docket regarding the use of ASME BPV Code,Section VIII, on May 16, 2024 (ML24138A214).
b) Kairos updated section 5.2, Intermediate heat Transport System, in Revision 1 of the PSAR to discuss the construction code and materials for the rupture disks. Regarding in-service testing, section 5.2.4 of the PSAR states that descriptions of any tests and inspections of the IHTS will be provided with the application for the OL.
28 5.2-13 Preliminary safety analysis report (PSAR) section 1.2.3, Design Features and Design Bases, provides the following definition for safety-related structures, systems, and components (SSCs):
Those SSCs that are relied upon to remain functional during normal operating conditions and during and following design basis events to assure:
The integrity of the portions of the reactor coolant boundary relied upon to maintain coolant level above the active core (see below);
The capability to shut down the reactor and maintain it in a safe shutdown condition; or The capability to prevent or mitigate the consequences of accidents which could result in potential exposures exceeding the limits set forth in 10 CFR 100.11.
PSAR table 3.6-1, Structures, Systems, and Components, states that the intermediate heat exchanger (IHX) is non-safety related. Given that the IHX tubes are relied upon to separate Flibe in the primary heat transport system from water/steam in the intermediate heat transport system (IHTS) following a postulated superheater tube leak or rupture event, discuss how the IHX was evaluated against the three criteria that make up the definition of safety-related SSCs. In particular, discuss whether the IHX is relied upon to assure the capability to prevent or mitigate the consequences of accidents following design basis events such as the superheater tube leak or rupture event.
Kairos stated that the IHX tubes are not considered safety-related because they are not relied upon to prevent or mitigate postulated events. The IHX tube break events are considered a separate event and are bounded by the salt spill event. Kairos stated that in a superheater tube leak or rupture, IHX is not safety-related because the rupture disks are the components relied upon to relieve the pressure; the rupture disks preclude all downstream effects of higher pressure.
Since specific design details to confirm that the IHX complies with the Hermes 2 definition of safety-related SSCs will not be available until the OL stage, Kairos confirmed in RCI 1 that the final design for Hermes 2 will demonstrate that the IHX tubes will not need to be classified as a safety-related SSC or that their failure is not credible.
29 5.2-14 PSAR table 3.6-1 states that the IHX and superheater are seismically designed to local building codes and are non-safety related. Given that none of the components of the IHTS, other than the rupture disks, are designed to withstand a design basis earthquake (DBE), discuss the effects of a DBE on the IHTS SSCs and any potential Flibe water-interactions that could result from failures caused by a DBE.
Kairos stated that PSAR sections 13.1.6, Radioactive Release from a Subsystem or Component, and 13.1.9, Internal and External Hazard Events, discuss seismic events. The IHTS (including the safety-related rupture disks) and PGS are not relied upon to maintain their structural integrity during a DBE; therefore, failure of those systems is postulated to occur in these events and the MAR in the IHTS and PGS is assumed to release at a release fraction of 1.
Kairos doesnt view selective SSC failures in a DBE to be credible.
Kairos stated that the PHTS, IHTS, and PGS are mostly in separate rooms, so Flibe-water interaction is unlikely, and minimal at worst, during a DBE.
5.2-15 In a postulated superheater tube leak or rupture event, discuss when the intermediate salt pumps would be secured in the event progression. State whether the pump trips are accomplished through automatic means by an instrumentation and control system (e.g., reactor protection system) or by manual operator action. If the pumps are tripped automatically, state what conditions are used to initiate the pump trips (e.g., sensed pressure at the rupture disk). For manual operator action, state what indications the operator would be expected to acknowledge prior to initiating the pump trips.
Kairos stated they do not have a safety concern if the intermediate salt pumps continued to run during a superheater tube rupture. The rupture disks are relied upon to relieve pressure and any additional energy added by pump operation would be relieved through the rupture disks.
Kairos also stated that in a superheater tube rupture, the intermediate salt pumps would only trip from a reactor protection system (RPS) trip due to increased PHTS temperature. This PHTS temperature rise would be caused by a rise in IHTS temperature from the loss of the PGS heat sink due to the superheater tube rupture.
30 Decay Heat Removal System Questions (PSAR Section 6.3)
Question Number Question Resolution 6.3-1 PSAR section 6.3, Decay Heat Removal System, states that the DHRS is credited in PSAR chapter 13, Accident Analysis, for decay heat removal during postulated events that assume the PHTS is unavailable. Please state whether the DHRS is also credited during postulated events that assume the IHTS is unavailable.
Kairos confirmed that the DHRS removes heat from the reactor vessel during postulated events that assume the normal heat rejection system is unavailable, as stated in PSAR section 6.1, Summary Description. No additional clarification was needed.
Instrumentation and Controls Questions (PSAR Chapter 7)
Question Number Question Resolution 7.1-1 NUREG-1537, Part 2, section 7.6, Control Console and Display Instruments, contains acceptance criteria that relate to human factors-related considerations, such as the readability of indications, the usability of controls, and the observability of alarms by the operator. Due to the added complexity of the proposed Hermes 2 facility using a dual-unit control room, please describe which standard(s) Kairos plans on using in the design process to ensure that these criteria from NUREG-1537 will be met in the finished design. It is reasonable for a test reactor PSAR to contain a high-level commitment to apply human factors engineering within the design of the control room to the extent necessary to ensure that these criteria will be met by the design submitted at the OL stage.
Kairos modified the Hermes 2 PSAR by letter dated January 3, 2024 (ML24003A766), to state that human factor engineering principles will be considered in the main control room (MCR) design.
7.1-2 Table 7.2-3, Plant Control System Interlocks and Inhibits, of the Hermes 2 PSAR lists high radiation detection in the pebble handling area as an input for plant control system (PCS) interlocks or inhibits; however, the sensor input is not reflected in figure 7.1-1, Instrumentation and Controls System Architecture. Please discuss if this sensor input should be on figure 7.1-1. Additionally, please specify where these radiation monitor(s) would be located and any impact on the reactor when the setpoint for this interlock/inhibit is reached (on radiation) since the pebble flow is stopped.
Kairos modified the Hermes 2 PSAR by letter dated January 3, 2024 (ML24003A766), to include the radiation monitors to figure 7.1-1. The radiation monitoring will occur in the pebble handling area. During the audit, Kairos clarified that when pebble flow stops, the reactor will have a controlled shutdown because fresh fuel isnt being added, so power level drops slowly over a long period of time.
31 7.1-3 Figure 7.1-1 indicates that the power generation system (PGS) instrumentation and control architecture is made up of six subsystems. However, there is limited information provided in the remainder of the PSAR regarding the air cooler control system, water supply and treatment control system, and auxiliary steam control system (e.g., not included in table 3.6-1, Structures, Systems, and Components, or section 9.9, Power Generation System, of the PSAR). Please clarify the relationships between these systems and the PGS and discuss the safety significance (if any) of these systems relative to plant operations (e.g., safety classification).
Kairos modified the Hermes 2 PSAR by letters dated January 3, 2024 (ML24003A766), and January 26, 2024 (ML24026A300). Kairos updated figure 7.1-1 by removing the auxiliary steam control system and renaming the Remote Handling System to Remote Maintenance and Inspection System. In addition, Kairos updated section 7.2.1.5, Power Generation Control System, to clarify the five subsystems after removal of the auxiliary steam control system.
32 7.1-4 The separation of functions and interactions between the reactivity control system (RCS), reactivity shutdown system (RSS), and the reactivity control and shutdown system (RCSS) is unclear throughout the PSAR:
a) RCS: RCS is defined as the reactivity control system in figure 7.1-1. However, the RCS is also defined as the reactor control system in section 7.2.1, Description, and in the master list of acronyms in the table of contents.
b) RSS: In figure 7.1-1 and section 5.2.4, Testing and Inspection, the RSS is defined as reactivity shutdown system, but in section 13.1.10.1, Recriticality or Reactor Shutdown System Failure, it is defined as the reactor shutdown system. RSS is not listed in the master list of acronyms in the table of contents. Additionally, the RSS is listed on figure 7.1-1 and discussed in chapter 7, Instrumentation and Control Systems, as a component of the non-safety related PCS. However, the RSS is described in section 13.1.10.1 as the safety-related portion of the RCSS and is relied upon to shut down the reactor.
section 4.2.2, Reactivity Control and Shutdown System, also discusses a safety-related portion of the RCSS but does not list the RSS.
c) RCSS: The RCSS appears to be a system that combines the RCS with the RSS functions, as discussed in chapter 3, Design of Structures, Systems, and Components, section 7.3.1, Description, and many other locations in PSAR. The relationships and interactions between the RCS, RSS, and the RCSS with regards to instrumentation and controls is unclear considering that the only system that is discussed in sufficient detail in chapter 7 is the RCS in section 7.2.1.1, Reactor Control System. Please clarify the relationships and interactions between RCS, RSS, and RCSS.
Kairos modified the Hermes 2 PSAR by letter dated January 3, 2024 (ML24003A766), to clarify the acronym discrepancies. The RCS is the mechanical non-shutdown control rods. The reactivity shutdown system (RSS) is the mechanical shutdown control rod system. The RCSS is the whole mechanical control rod system, encompassing both the RCS and RSS.
33 7.1-5 Figure 7.1-1 has multiple systems and control systems. Many of those systems are not listed in table 3.6-1 (e.g., the intermediate loop thermal management system). The NRC staff would like to understand Kaiross criteria for having inclusion of systems listed on in table 3.6-1.
Kairos modified the Hermes 2 PSAR by letter dated January 3, 2024 (ML24003A766), to update figure 7.1-1 to clarify that the PCS incorporated many of the control systems which were not listed in table 3.6-1.
7.2-1 In Hermes 1 PSAR section 7.2.1.3, the Primary Heat Transfer Control System (PHTCS) is described as supporting the control of the heat rejection system. In the Hermes 2 PSAR, the PHTCS is no longer listed as supporting the control of the heat rejection subsystem (HRS). Please discuss what control system provides this functionality.
Kairos modified the Hermes 2 PSAR by letter dated January 3, 2024 (ML24003A766), to state in section 7.2.1.3 that the PHTCS controls the HRS.
7.2-2 For chapter 7 of the Hermes 2 PSAR, please discuss the plant control system (PCS) initiated runbacks of the reactivity control and shutdown system (RCSS), primary salt pump (PSP),
intermediate salt pump (ISP), and feedwater pumps.
Additionally, describe the impacts to the reactor protection system from the runback of these SSCs.
Kairos stated during the audit that PCS initiated runbacks would take significant time to propagate back to the reactor system due to the high heat capacity of the coolant and that the details on the runbacks will be provided in the OL application. In addition, Kairos stated that the runbacks will not have a notable effect on the Hermes 2 safety analysis.
7.2-3 Section 7.2.1.5, Power Generation Control System, of the Hermes 2 PSAR lists the five capabilities of the Power Generation Control System. Table 3.6-1 and section 9.9 of the Hermes 2 PSAR describe the Power Generation System as having three subsystems. The matching of these capabilities to the subsystems is not clear. The NRC staff would like to understand the descriptions of this system and subsystems and have a discussion regarding how they align and support each other to provide a consistent understanding of the functions performed.
Kairos modified the Hermes 2 PSAR by letter dated January 3, 2024 (ML24003A766), to update figure 7.1-1. In addition, the document, Hermes Instrumentation, Controls & Electrical Description and Architecture, was provided on the Kairos ERR to provide additional information on the power generation control system.
34 7.4-1 Anhydrous hydrogen fluoride is used for tritium management (see PSAR section 9.1.3, Tritium Management System).
section 2.7, Storage, Treatment, and Transportation of Radioactive and Nonradioactive Materials, of the Hermes 2 Environmental Report states that anhydrous hydrogen fluoride would be stored in the IHTS tritium management system.
Section 4.8.1, Nonradiological Impacts, of the Hermes 2 Environmental Report states that quantities of hydrogen fluoride would be maintained at less than the 29 CFR 1910.119 threshold quantity of 1,000 pounds. No specific discussion is provided in PSAR section 2.2.3.3, Evaluation of Airport Hazards and Helicopter Operations, regarding the potential hazards of anhydrous hydrogen fluoride toxicity, its ability to affect main control room personnel, or protections against it.
Please discuss how protection against the toxicity of anhydrous hydrogen fluoride is addressed with respect to control room habitability, including considerations for control room and ventilation system design.
Kairos stated that information regarding potential hazards of anhydrous hydrogen fluoride toxicity, its ability to affect MCR personnel, or protections against it will be provided in the Hermes 2 OL application. Kairos indicated that an isolation strategy for the MCR will be used to protect the operators from chemical threats, however, the MCR ventilation is not safety-related.
Chemistry Control, Inert Gas and Tritium Management System Questions (PSAR Section 9.1)
Question Number Question Resolution 9.1-1 PSAR section 9.1.3, Tritium Management System, references the previously approved topical report KP-TR-012, KP-FHR Mechanistic Source Term Methodology (Agencywide Document Access and Management System Accession No. ML22088A228) to determine speciation of tritium between the salt and gas space. In section 5.3.1, Tritium Speciation, of the referenced topical report, Kairos describes how the methodology relies on the redox potential and solubility of tritium in a Flibe salt. Discuss whether Kairos evaluated this portion of the methodology to determine whether use of a different (i.e.,
BeNaF) salt would impact tritium speciation in the IHTS.
Kairos stated during the audit that the MHA release fraction is 1, meaning that all tritium in the reactor is assumed to be released in the MHA analysis. The Hermes 2 design includes tritium isotopic exchange in the IHTS to control the tritium and tritium speciation. Given the conservative tritium release assumption, tritium speciation in the IHTS does not have an effect on the safety analysis.
35 9.1-2 PSAR figure 5.1-1, Heat Transport System and Intermediate Heat Transport System Process Flow Diagram, shows the heat rejection radiator (HRR) and the IHX as components of the reactor coolant system. PSAR section 9.1.4.1.3, PHTS Fill/Drain Tank, states that, the PHTS fill/drain tank is sized to hold the PHTS and the IHX reactor coolant inventory, but does not mention the reactor coolant inventory for the HRR (which is a component of the HRS). State whether the Hermes 2 PHTS fill/drain tank is sized to also include the reactor coolant inventory of the HRS.
Kairos stated during the audit that the PHTS fill/drain tank is sized to hold the coolant inventory of the PHTS and its subsystems which include the IHX and the HRR.
Auxiliary Systems (PSAR Chapter 9)
Question Number Question Resolution 9.3-1 Table 3.6-1, Structures, Systems, and Components, of the Hermes 2 preliminary safety analysis report (PSAR) identifies safety-related full core offload and spent fuel storage rack and spent fuel air cooled storage rack structures, systems, and components (SSCs) that are associated with section 9.3, Pebble Handling and Storage System. However, the remainder of the PSAR, including section 9.3, does not contain any discussion of these SSCs.
- 1. Please clarify the role of these SSCs in the pebble handling and storage system.
- 2. Identify and discuss the materials of construction and codes and standards to be used to design and construct these safety-related SSCs.
Kairos updated section 9.3.1.8, Pebble Storage, in Revision 1 of the PSAR to clarify the roles of the SSCs in the pebble handling and storage system and to identify and discuss the material of construction and codes for those SSCs.
36 9.6-1 By letter dated October 27, 2023 (ML23304A144), Kairos stated that its responses to Hermes audit questions were applicable to Hermes 2, with the exception of Hermes general audit questions 1.3.9-1 and 9.6 2. Regarding Hermes general audit question 9.6-2, Kairos stated that this question does not apply to Hermes 2 because the Hermes 2 design includes a secondary coolant loop, and therefore text regarding tritium in secondary coolant is included in the Hermes 2 PSAR. Hermes 2 PSAR section 5.2 and section 9.9, Power Generation System, discuss the expected presence of tritium in the IHTS and PGS, respectively, and also discuss how tritium in these systems will be controlled. However, Hermes 2 PSAR section 9.6, Possession and Use of Byproduct, Source, and Special Nuclear Material, only appears to discuss tritium in the primary coolant system, fuel pebbles, pebble handling and storage system, and tritium management system. Clarify and discuss whether PSAR section 9.6 should be updated to address the presence of tritium in additional systems that are included in Hermes 2.
Kairos updated section 9.6.3, Byproduct Material, in Revision 1 of the PSAR to include the IHTS and PGS to the systems that may contain tritium.
9.8-1 Section 9.8.4.2, Design Bases, of the Hermes 2 PSAR states that, consistent with PDC 4, the crane and rigging are designed to protect against the dynamic effects potentially created by the failure of the crane and rigging equipment. However, the discussion in section 9.8.4.3, "System Evaluation," emphasizes administrative controls instead of discussing design features and states that "administrative controls and interlocks prevent the crane and rigging from moving heavy loads over safety-related SSCs except when the reactor is shut down..."
Please clarify the extent to which the design of the crane and rigging protect against dynamic effects of their failure. Also, please clarify protection of SSCs performing safety-related functions during reactor shutdown from the dynamic effects of crane or rigging failure.
Kairos updated section 9.8.4, Cranes and Rigging, in Revision 1 of the PSAR to state that the safety-related SSCs are protected from dynamic effects of crane and rigging failure.
During the audit discussion, Kairos described the safety-related inner shell of the reactor building has a maintenance deck which is designed to withstand the drop of any items to protect the reactor cell, including the transfer cask. The holes in the maintenance deck are designed to closely fit the size of the components requiring replacement, prohibiting larger items from falling through the deck and onto safety-related SSCs.
37 Radiation Protection Program and Waste Management Questions (PSAR Chapter 11)
Question Number Question Resolution 11.1-1 Please clarify whether activation products are produced in the IHTS salt or cover gas (in addition to those transported from the primary heat transport system).
Kairos stated that there may be a limited amount of activation products produced in the IHTS. The dominant source of MAR in the IHTS is expected to be from transport of activation products produced in the PHTS across the IHX, with tritium as the dominant MAR. Additional information regarding the transport of activation products across the IHX and MAR in the IHTS will be provided during the Hermes 2 OL application review.
38 Accident Analysis Questions (PSAR Chapter 13)
Question Number Question Resolution 13.1-1 Key design changes for the Hermes 2 test reactor facility, as compared to the Hermes 1 test reactor facility, include the use of an intermediate heat transport system (IHTS) and a shared power generation system (PGS). However, the PSAR and associated technical report KP-TR-022-P, Hermes 2 Postulated Event Methodology, [ML24144A094] do not discuss any new calculations or analysis results that include modeling of the IHTS (with associated SSCs such as intermediate heat exchanger and superheater), the PGS, or interactions between the two units. If such calculations or related analyses have been performed, please make them available in the electronic reading room for the audit. If Kairos has not performed such calculations or analyses because it has separately determined that the Hermes 1 events remain bounding, please make available for audit any justifications or analyses to support the conclusion that events evaluated for the Hermes 1 test reactor facility remain bounding for the Hermes 2 test reactor facility.
Kairos stated that the limiting events selected in the PSAR are mostly based on engineering judgment, as was done for Hermes 1. Kaiross intent was to capture all the initiating events in the category to provide assurance that they would evaluate them all and confirm the limiting event as part of the OL application when they have final design details and will perform sensitivity studies.
Kairos considered new initiating events and scenarios associated with the IHTS and PGS.
Most are grouped under existing postulated events. Kairos identified that the figures of merit (FOMs) relevant to the initiators would be bounded.
Kairos stated that the IHTS contains tritium, but the methods for analyzing leaks from the IHTS would be similar to how tritium was analyzed for Hermes 1. Leaks from the IHTS are grouped under the salt spill postulated event.
Kairos confirmed the staffs understanding that the quantitative results for the MHA are the same for Hermes 2 as they were for Hermes 1.
The staff sent audit questions 13.1-7, 13.1-8, and 13.1-9 as a follow-up to this question.
39 13.1-1 (double-numbered)
The maximum hypothetical accident (MHA) source term, functional containment, and consequence analysis modeling effectively neglect transport or retention in the IHTS and power generation system (PGS). Discuss how the consequences of the MHA will be shown to be bounding for the events with expected releases from or failure of the IHTS or PGS.
Kairos clarified that the MAR-containing IHTS and PGS are grouped with other systems not qualified to maintain structural integrity during a design basis event. Potential radiological releases from the IHTS and PGS would be bounded by the assumed amount of MAR released in the MHA.
Kairos updated table 13.1-1, Acceptance Criteria for Figures of Merit, in Revision 1 of the PSAR and KP-TR-022, Revision 1, table 3-2, Derived Figures of Merit and Acceptance Criteria for Postulated Events, (ML24144A090) to add the Radiological Release from Subsystem or Component as an applicable event to the Amount of Materials at Risk Released, FOM.
13.1-2 Table 3-2, Derived Figures of Merit and Acceptance Criteria for Postulated Events, of KP-TR-022, Hermes 2 Postulated Event Analysis Methodology, identifies an intermediate heat exchanger tube break as an applicable event, but table 13.1-1, Acceptance Criteria for Figures of Merit, of the Hermes 2 PSAR does not. Clarify the reason for this discrepancy.
During the audit discussions, Kairos stated that IHX break is grouped with the salt spill event in the PSAR.
13.1-3 PSAR section 9.9.1.2, System Evaluation, states that the steam system contains radiological contaminants, including tritium. It is not clear from table 13.1-1 (nor from KP-TR-022) which figures of merit (FOM) would assure that events with releases due to steaming from the main steam power relief valves or safety valves would be shown to be bounded by the MHA.
Kairos updated table 13.1-1 in Revision 1 of the PSAR and KP-TR-022, Revision 1, table 3-2, to identify that the FOM is the MAR in the system.
13.1-4 It is not clear from table 13.1-1 (nor from KP-TR-022) which FOM would assure that events with failure of the IHTS would be shown to be bounded by the MHA. Please discuss whether the FOM for the Flibe salt spill event are applicable to or shown to be bounding for the BeNaF salt in the IHTS (e.g., airborne release fraction of spilled/splashed Flibe).
During the audit, Kairos stated that the only MAR that is modeled for release from the IHTS is gaseous tritium, which is not retained in the salt for subsequent aerosolization. Kairos further stated that Hermes 2 does not need a FOM for salt spill events for BeNaF because the IHTS MAR FOM limits the tritium available for release to ensure MHA doses are bounding.
40 13.1-5 The discussion of the amount of radioactive material at risk for release (MAR) for the MHA in Hermes 2 PSAR sections 13.1.1
[Maximum Hypothetical Accident] and 13.2.1 [Maximum Hypothetical Accident] does not include the IHTS or the PGS.
Clarify how the MHA consequence analysis remains bounding when not modeling the potential MAR in these systems and transport through these systems.
Kairos stated that limits on tritium in the IHTS will be used to ensure that the MHA remains bounding.
13.1-6 Hermes 2 PSAR section 13.1.6 states that there is a design requirement on the MAR available for release from the subsystems and components to remain below the MAR for release assumed in the MHA. Discuss whether these MAR limitations consider the concurrent failure of the relevant systems in both Hermes 2 units, considering that the limiting event for the category is assumed to be a seismically initiated event. Additionally, discuss if these limitations also include consideration of the concurrent failure of the relevant systems in the Hermes 1 test reactor located nearby.
Kairos stated that the postulated event analysis does not consider concurrent failures in both units in the Hermes 2 facility, nor in the adjacent Hermes 1, because accident dose acceptance criteria are defined for an individual reactor due to the independence of the reactors in compliance with the requirements in 10 CFR 100.11(b)(1).
41 13.1-7 The language in PSAR chapter 13 and topical report KP-TR-022, Hermes 2 Postulated Event Methodology, has several definitive statements regarding the bounding nature of certain limiting events. However, the staff understands based on previous discussions (e.g., November 2023 audit) with Kairos that the limiting nature of these events will be validated as part of developing the Hermes 2 final safety analysis report (FSAR) (see example below; emphasis added).
PSAR Section 13.1.2, Insertion of Excess Reactivity Staffs Understanding Based on Previous Discussions The limiting insertion of reactivity event is initiated by a control system error or an operator error that causes a continuous withdrawal of the highest worth control element at maximum control element drive speed This postulated insertion of excess reactivity bounds other insertion of reactivity events, including:
The limiting insertion of reactivity event is assumed to be initiated by a control system error Kairos will confirm in the application for an operating license that this postulated insertion of excess reactivity bounds other possible insertion of reactivity events, including:
Clarify whether Kairos intends to update PSAR chapter 13 and KP-TR-022 to reflect the preliminary nature of the Hermes 2 safety analyses.
Kairos stated during the audit that the limiting events will be analyzed or confirmed for the OL application. Kairos further stated that no PSAR changes will be made. The staff will include an item in Appendix A.2, Additional Items for an Operating License Application, of the Hermes 2 CP safety evaluation that states Kairos must confirm the limiting event, key conservative analysis assumptions, and initial conditions through sensitivity calculations or other appropriate analysis as part of the OL application.
42 13.1-8 The language in PSAR sections 13.1.2, Insertion of Excess Reactivity, through 13.1.9, Internal and External Hazard Events, does not appear consistent with the audit discussion held on November 6, 2023. Specifically, the PSAR definitively identifies the limiting event in each section (category). The absence of any changes to language in chapter 13 of the Hermes 2 PSAR regarding limiting events, as compared to chapter 13 of the Hermes 1 PSAR, suggests that the events evaluated for Hermes 1 facility are bounding for the Hermes 2 facility. However, Kairos stated during the audit discussion that calculations are not available to completely justify the assumed limiting event for each category. Rather, the limiting events in the Hermes 2 construction permit application were selected primarily using engineering judgment. Kaiross stated intent was to capture all the events in each category to provide assurance that all events would be checked and the limiting event confirmed at FSAR stage when final design details are available and sensitivity studies can be performed.
Paragraph 10 CFR 50.34(a)(4) requires, in part, that construction permit applicants include [a] preliminary analysis and evaluation of the design and performance of [SSCs] 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 [SSCs], provided for the prevention of accidents and the mitigation of the consequences of accidents Please clarify what analyses and evaluations have been performed in support of the Hermes 2 construction permit application, considering the addition of the IHTS and PGS, and provide a justification for how these analyses meet the relevant regulatory requirements. Further, discuss the analyses that Kairos expects to perform in support of a future Hermes 2 operating license application, considering the information in PSAR chapter 13 and KP-TR-022 regarding Kairos updated section 13.1, Initiating Events and Scenarios, in Revision 1 of the PSAR to clarify that the postulated event methodology for the Hermes 1 design is applicable to Hermes 2, as both the designs employ the same functional containment strategy and safety features. The PSAR update also clarified that the additional safety features in the Hermes 2 design (the rupture disks and ISP trip) and new LCOs (MARs in the IHTS and PGS, and Flibe and water limits in the IHTS) ensure that events initiated from new Hermes 2 systems (IHTS and PGS) are either prevented by design or can be grouped under existing Hermes 1 event categories. Kairos further stated in the PSAR update that the Hermes 2 design specific events do not introduce new phenomena that require evaluation of additional FOMs to assure MHA remains bounding.
43 limiting events and the November 6, 2023, audit discussion.
13.1-9 PSAR section 13.1.2 states that the consequences of increased heat removal events are bounded by a postulated insertion of excess reactivity event; this is also discussed in KP-TR-022 section 3.2.2.3 [Increase in Heat Removal].
However, limited information has been provided to-date regarding the potential consequences associated with a Flibe freezing event. With the introduction of the IHTS and PGS, the staff expects that the potential for overcooling due to increase of heat removal events may be higher in the Hermes 2 design than the Hermes 1 design. For example, new initiating events such as steam line break or spurious opening of a turbine bypass valve or steam safety valve may lead to overcooling and approach the freezing temperature of Flibe in the reactor vessel. Discuss whether the consequences of a Flibe freezing event are also bounded by the limiting reactivity insertion event.
Kairos clarified that the freezing in the downcomer is handled in the Hermes 2 design the same way it was for the Hermes 1 design (i.e., Flibe temperature in the reactor vessel must remain above the Flibe freezing temperature to satisfy Kaiross definition of a safe state in PSAR chapter 13) and the analysis will be submitted as part of the OL application. Kairos stated that the limiting long-term overcooling event is the loss of circulation found in section 3.2.2.4, Loss of Forced Circulation, of technical report KP-TR-022 due to the loss of electrical power. Kairos stated that the overcooling in Hermes 2 design is going to be limited by the RPS tripping the ISP.
Kairos further clarified that the PSP and ISP trips would minimize heat transfer in the IHX, limiting the cooldown due to events initiated in the PGS and IHTS. Kairos stated that a PSP overspeed would bound the cooldown caused by events initiated in the PGS or IHTS. Kairos also noted that the large thermal inertia of the IHTS and PHTS salt volumes result in a mild temperature response to the event and stated that a steam isolation valve trip would not be needed to mitigate this type of event.
44 13.1-10 PSAR figure 4.6-1, Coolant Flow Path, shows that there are flow paths for circulation of Flibe through the graphite bottom core support channels. Discuss whether a partial blockage of core channels is considered in the safety analysis or whether this event is bounded by other postulated events. Additionally, discuss if a partial core blockage or flow degradation could lead to over-heating of portions of the core.
Kairos stated that PSAR section 4.3.3, System Evaluation, identifies that the failure of flow path structures is beyond the design bases. In addition, Kairos stated that the Hermes 1 request for additional information (RAI) 339 response provided clarification on flow blockage (ML22243A250) and that this RAI response is applicable to Hermes 2 (ML23300A141). In this response, Kairos identified the basis for PDC 32 that states that...the potential for flow blockages/restriction from failed internals (such as graphite reflector blocks) is addressed as part of compliance with PDC 35, 36, and 37, including inspections if appropriate. Therefore, final design compliance with PDCs 32, 35, 36, and 37 addresses the impact of partial blockage of core flow channels.
During the audit, Kairos described the phenomena expected in the core bypass channel and stated that further information on core phenomena will be provided in the OL application.
13.1-11 In the PSAR section 13.1 and in KP-TR-022-NP, intermediate salt (BeNaF) freezing is not explicitly listed as an event grouped into postulated event categories or as a prevented event.
a) State the consequences of intermediate salt freezing.
b) State whether Kairos considered intermediate salt freezing as either an initiating event or as part of an event sequence.
c) State if a technical specification limiting condition for operation will be proposed to prevent intermediate salt freezing.
Kairos stated that BeNaF freezing would lead to a loss of heat sink. Since loss of heat sink is already analyzed through the loss of forced circulation events in the PSAR, the NRC staff did not need further clarification.
45 13.2-1 Section 13.2.1.1, [Methodology and Inputs,] of the Hermes 2 PSAR states that the quantity of retained tritium is conservatively bound within the graphite and structures over 10 years of operation. With the understanding that the Hermes 2 units would be licensed for 11 years of operating life (10 effective full power years) as described in the Hermes 2 Environmental Report, discuss whether this assumption is bounding for the tritium retention for the expected full licensed operating life and conditions.
Kairos stated that their tritium methodology report, Tritium Inventory and Release Calculation Methodology for the Maximum Hypothetical Accident and Postulated Events, provides a conservative value of tritium accumulation in the graphite and structures. This report stated that the tritium retention maximizes around year 4 and Kairos separately concluded that the additional 7 years of Hermes 2 operational lifetime have no significant impact on tritium retention.
The staff assessed the information in the tritium methodology report, and the Kairos internal memo, Applicability of the Maximum Hypothetical Accident to Hermes 2. These documents support the information in the PSAR and discussion during the audit that the quantity of retained tritium in the reactor graphite and structures is bounded for Hermes 2.
46 Accident Methodology Questions from Topical Report (KP-TR-022, Hermes 2 Postulated Event Methodology)
Question Number Question Resolution TR-1 KP-TR-022 section 3.2.2.7, Intermediate Heat Exchanger Tube Break.
a) KP-TR-022 section 3.2.2.7 postulates that a complete break of one IHX tube is the limiting event. Discuss whether any other break scenarios were considered that could be more limiting (e.g., partial breaks of multiple tubes due to a common-mode degradation mechanism).
b) KP-TR-022 section 3.2.2.7 states, A conservative amount of Flibe is assumed to flow into the intermediate loop to mix with the intermediate salt. Provide the basis for why the assumed Flibe flow rate is conservative.
c) State which reactor protection systems signal(s) would lead to a reactor trip for an IHX tube break event.
d) Discuss whether a break in the IHX tube(s) would lead to over-pressurization of the IHTS and safety-related rupture disk burst. If so, discuss whether the consequences of this event would still be bounded by the salt spill event.
Kairos provided the following responses during the audit:
a) A single IHX tube break is currently expected to be the limiting scenario and breaks in more than one tube were not analyzed. Kairos did not perform extensive sensitivities at the current design stage. To address common-mode failures, Kairos will monitor long-term indicators of IHX tube degradation.
b) This amount is assumed to be limited by the amount of a pipe break accident. In addition, the volume of Flibe in the IHTS is limited since the IHTS is a closed-loop system.
c) The level sensor trip due to loss of Flibe would be the safety-related RPS trip.
d) The rupture disks are focused on a superheater tube break and not on an IHX break. The superheater tubes operate at a much higher pressure than the IHTS. IHX tube breaks are not likely to lead to over-pressurization. The salt spill event will still bound the IHX break event.
47 TR-2 Please state whether the IHTS, IHX, and HRS are included in the plant KP-SAM model. If so, discuss how they are modeled.
Kairos stated that the IHX is modeled as a pressure boundary heat exchanger component.
The IHTS is modeled as a secondary side boundary condition. The HRS is not modeled at this time, but it will be modeled for the OL application for events for which it is relevant.
The safety analysis approach at this point is to demonstrate using FOM that the postulated events are bounded by the MHA.
The rest of the postulated event analysis will be provided in the OL, and final modeling choices will be dispositioned at that time.
TR-3 Given the addition of the IHTS and PGS relative to Hermes 1 design, discuss whether Kairos will consider initial IHTS and PGS conditions as input parameters with sensitivities similar to the parameters listed in KP-TR-022, table 4-4, Input Parameters Considered for Postulated Events.
Kairos stated the currently identified initial conditions would see the impact due to IHTS and PGS conditions. Table 4-4 provides the most important items to the FOM. Additional parameters will be evaluated at OL stage.
48 Technical Specifications (PSAR Chapter 14)
Question Number Question Resolution 14.1-1 In table 14.1-1, Proposed Variables and Conditions for Technical Specifications, one of the Limiting Conditions for Operation (LCOs) under section 3.3 is proposed to limit the quantities of MAR in the primary heat transport system cover gas, the IHTS, and the PGS. Clarify whether this LCO would include MAR quantities for the IHTS cover gas and the coolant, and if there would be separate values for the IHTS cover gas and the coolant, or if it is a combined value for the entire system boundary. Also, it is not clear if the quantities of MAR for the PGS are activity concentrations in the water and steam.
Kairos stated that neither the quantities of MAR in the IHTS, or which separate value(s) will be in an LCO(s), have been determined at this time. Kairos clarified that the quantities of MAR for the PGS will be distinguished between steam and water.
Kairos stated that further information will be provided during the Hermes 2 OL application review.
14.1-2 Section 13.1.10.11 of the PSAR states that superheater leaks are expected to be limited by technical specification. Please confirm that the LCO presented in table 14.1-1 under section 3.3 [t]he quantity of water in the intermediate coolant shall be maintained below an upper bound limit is specified to limit superheater leaks.
Kairos stated that the LCO is to limit the amount of water in the IHTS to identify a superheater leak as a superheater is the only way water can ingress into the IHTS.
49 6.0 EXIT BRIEFING The staff conducted an audit closeout meeting on June 6, 2024. At the closeout meeting, the staff reiterated the purpose of the audit and discussed their activities.
7.0 ADDITIONAL INFORMATION RESULTING FROM AUDIT Three RCIs (ML24135A257 and ML24135A257) were generated as a result of this audit. Kairos submitted two documents describing the Hermes 1 docketed information that is applicable to Hermes 2 (ML23300A141 and ML23300A144). In addition, Kairos provided supplemental information and voluntarily updated portions of the Hermes 2 PSAR (ML24138A211, ML24136A253, ML24073A317, ML24072A297, ML24039A189, ML24026A299, ML24025C685, ML24011A290, ML24009A247, ML24003A765, ML23356A078, ML23355A161, ML23355A192, and ML23349A147). PSAR updates were captured in aggregate though PSAR Revision 1, which included an updated version of KP-TR-022 (ML24144A090).
8.0 OPEN ITEMS AND PROPOSED CLOSURE PATHS Not applicable. There were no deviations from the audit plan.
50
SUBJECT:
SUMMARY
REPORT FOR THE REGULATORY GENERAL AUDIT OF KAIROS POWER LLC HERMES 2 CONSTRUCTION PERMIT PRELIMINARY SAFETY ANALYSIS REPORT DATED: JULY 11, 2024 ADAMS Accession No. ML24193A214
- concurred by email OFFICE PM: NRR/DANU/UAL1 LA: NRR/DANU/UAL1 BC: NRR/DEX/EXHB NAME MOrenak DGreene (SLent for)
BHayes*
DATE 6/17/2024 6/24/2024 7/09/2024 OFFICE BC: NRR/DEX/EICB BC: NRR/DEX/ESEB BC: NRR/DANU/UTB2 NAME FSacko ITseng CdeMessieres*
DATE 7/03/2024 7/08/2024 07/10/2024 OFFICE BC: NRR/DANU/UAL1 PM: NRR/DANU/UAL1 NAME JBorromeo*
MOrenak DATE 7/09/2024 7/11/2024