ML24138A207
| ML24138A207 | |
| Person / Time | |
|---|---|
| Site: | Hermes File:Kairos Power icon.png |
| Issue date: | 05/17/2024 |
| From: | NRC |
| To: | NRC/NRR/DANU |
| References | |
| Download: ML24138A207 (32) | |
Text
From:
Cayetano Santos Sent:
Friday, May 17, 2024 3:48 PM To:
Weidong Wang; Larry Burkhart Cc:
Michael Orenak; Matthew Hiser; Brian Bettes; Josh Borromeo
Subject:
Preliminary Chapter 13 of Hermes 2 SE to ACRS Attachments:
Chapter 13 - Accident Analyses.pdf
- Weidong, Attached is the last preliminary chapter (Chapter 13) from the safety evaluation (SE) for the Hermes 2 construction permit application. This chapter has been reviewed by branch chiefs and received a preliminary review by OGC. However, this chapter is not final because it still needs to be reviewed by division management and receive the final OGC review. Thus, this preliminary chapter could change between now and the approved SE that will be sent to ACRS for formal review. I am sending this chapter in advance so that members can become familiar with the safety evaluation and begin preparing for the formal review.
The other preliminary chapters were provided to you on March 14, April 4, April 22, April 23, May 10, and May 15, and May 17, 2024.
Kairos Power submitted Revision 0 of the preliminary safety analysis report (PSAR) for Hermes 2 in July 2023. All preliminary SE chapters being sent to the ACRS refer to Revision 1 of the PSAR. Although Revision 1 of the PSAR has not been submitted, Kairos stated their intent to do so once all PSAR changes are known (i.e., the end of the technical review). The current list of docketed PSAR changes that will be incorporated into Revision 1 of the PSAR can be found in ADAMS and on the NRC public webpage at https://www.nrc.gov/reactors/non-power/new-facility-licensing/hermes2-kairos/documents.html.
The staff has taken a different approach toward the Hermes 2 SE considering that Hermes 1 and Hermes 2 CP applications have most of the same information. Due to these similarities, the staff leveraged the Hermes 1 SE to the extent possible for Hermes 2, using an incorporation by reference for many sections. The following description, taken from Chapter 1 of the Hermes 2 SE, discusses how the staffs review of Hermes 1 was applied to its review of the Hermes 2:
Use of Docketed Information The staffs review of the Hermes 2 CP application was informed by the Hermes 1 CP application review. The Hermes 2 facility includes many SSCs that are identical to those that would be used in the Hermes 1 facility. Accordingly, large portions of the Hermes 1 PSAR are identical to the Hermes 2 PSAR. In the July 14, 2023, CP application submittal, Kairos highlighted the differences between the Hermes 1 and Hermes 2 PSARs in two ways. First, Kairos used blue font in the Hermes 2 PSAR to identify any modified or new text. Second, Kairos provided a summary of the information deleted from the Hermes 1 PSAR to generate the Hermes 2 PSAR (ML23195A132).
In addition, Kairos identified the docketed information and audit information from Hermes 1 that is applicable to the Hermes 2 CP application in two letters dated October 27, 2023 (ML23300A141 and ML22300A144). This information is considered docketed information for the Hermes 2 CP application and was used to inform the staffs review.
Format and Content of Hermes 2 Safety Evaluation Sections Based on the consistencies between the Hermes 1 and Hermes 2 PSARs described above, the staff leveraged the Hermes 1 SE to the greatest extent possible to support its review of the Hermes 2 CP application. Accordingly, applicable contents of the Hermes 1 SE were incorporated by reference into this SE. To determine which Hermes 1 SE content could be incorporated by reference, the staff reviewed the differences between the Hermes 1 and Hermes 2 PSARs. Where the Hermes 2 PSAR only contained minor deviations (e.g., minimal or no effect on the NRC SE or editorial changes, as compared to the Hermes 1 PSAR), the staffs SE was largely limited to incorporating by reference applicable portions of the Hermes 1 SE. Similarly, where the Hermes 2 PSAR contained a limited number of significant but discrete changes, but was otherwise identical to the Hermes 1 PSAR, the staffs SE was likewise limited to an evaluation of the variances between the two PSARs. In this case, the balance of the staffs SE also incorporated by reference applicable portions of the Hermes 1 SE. Based on this approach, many of the Hermes 2 SE sections are organized using the following structure:
Brief introduction summarizing the Hermes 2 PSAR content with a focus on any changes in comparison to the Hermes 1 PSAR.
Regulatory evaluation section that, in most cases, incorporates by reference the regulations and guidance from the corresponding section of the Hermes 1 SE due to the similarities between the Hermes 1 and Hermes 2 facility designs.
Technical evaluation that:
o Identifies the consistent and modified Hermes 2 PSAR information, as compared to the Hermes 1 PSAR.
o Incorporates by reference, as appropriate, content from the Hermes 1 SE for PSAR information that is consistent between Hermes 1 and Hermes 2.
o Evaluates the new design information and non-editorial changes (i.e.,
minor and/or few significant changes), as compared to the Hermes 1 SE.
The depth of the staff review provided for each change is dependent on the significance of that change.
A full conclusion specific to the Hermes 2 review.
For Hermes 2 PSAR sections that contain entirely new information and/or several significant changes when compared to the Hermes 1 PSAR, the staff performed its evaluation without incorporation by reference from the Hermes 1 SE. One example of a section which reflects such an evaluation by the staff is Section 5.2, Intermediate Heat Transport System, of this SE related to the intermediate salt loops. These systems are not in the design of the Hermes 1 test reactor; therefore, they were not evaluated by the staff in its review of the Hermes 1 CP application. Accordingly, the staff evaluated this system without incorporation by reference of the Hermes 1 SE.
If you have any questions, please contact me, Michael Orenak, or Matt Hiser.
- Tanny Santos
Hearing Identifier:
Kairos_Hermes2_CPDocs_Public Email Number:
19 Mail Envelope Properties (MW4PR09MB901076B9E75527AC373B0544E5EE2)
Subject:
Preliminary Chapter 13 of Hermes 2 SE to ACRS Sent Date:
5/17/2024 3:47:42 PM Received Date:
5/17/2024 3:47:00 PM From:
Cayetano Santos Created By:
Cayetano.Santos@nrc.gov Recipients:
"Michael Orenak" <Michael.Orenak@nrc.gov>
Tracking Status: None "Matthew Hiser" <Matthew.Hiser@nrc.gov>
Tracking Status: None "Brian Bettes" <Brian.Bettes@nrc.gov>
Tracking Status: None "Josh Borromeo" <Joshua.Borromeo@nrc.gov>
Tracking Status: None "Weidong Wang" <Weidong.Wang@nrc.gov>
Tracking Status: None "Larry Burkhart" <Lawrence.Burkhart@nrc.gov>
Tracking Status: None Post Office:
MW4PR09MB9010.namprd09.prod.outlook.com Files Size Date & Time MESSAGE 5835 5/17/2024 3:47:00 PM Chapter 13 - Accident Analyses.pdf 376265 Options Priority:
Normal Return Notification:
No Reply Requested:
No Sensitivity:
Normal Expiration Date:
13 ACCIDENT ANALYSES This chapter of the Hermes 2 construction permit safety evaluation (SE) describes the technical review and evaluation by the U.S. Nuclear Regulatory Commission (NRC) staff (the staff) of the preliminary design of the Kairos Power LLC (Kairos) Hermes 2 facility as presented in Chapter 13, Accident Analysis, of the Hermes 2 preliminary safety analysis report (PSAR),
Revision 1. The staff reviewed Hermes 2 PSAR Chapter 13 against applicable regulatory requirements using regulatory guidance and standards to assess the sufficiency of the preliminary design of the Hermes 2 facility.
Chapter 13 of the Hermes 2 PSAR provides information and analyses results for the potential radiological consequences of an accident at the Hermes 2 facility. Kairos uses the dose consequences of a maximum hypothetical accident (MHA) to bound the potential postulated events (anticipated operational occurrences and design-basis accidents). The evaluation of the safety of a test reactor requires analyses of the plants responses to postulated equipment failures or malfunctions. Such analyses help to determine the limiting conditions for operation (LCOs), limiting safety system settings, and design specifications for safety-related components and systems to protect public health and safety. Kaiross analyses are also performed to demonstrate that the reactor site criteria required by Title 10 of the Code of Federal Regulations (10 CFR) Part 100 are met for each Hermes 2 unit individually. Kairos is not requesting Commission approval of the safety of any design feature or specification in the construction permit (CP) application, as permitted by 10 CFR 50.35(b).
Kairos uses the functional containment approach in which the tri-structural isotropic (TRISO) fuel and a molten salt coolant, a mixture of lithium fluoride (LiF) and beryllium fluoride (BeF2) that is referred to as Flibe, are the barriers credited to retain fission products. The use of the functional containment approach to protect public health and safety was approved by the Commission in the Staff Requirements Memorandum to SECY-18-0096, Functional Containment Performance Criteria for Non-Light-Water-Reactors. A more detailed discussion on the functional containment approach can be found in Section 6.2, Functional Containment, of this SE.
In its review of PSAR Chapter 13, the staff considered the information in technical report KP-TR-017, KP-FHR Core Design and Analysis Methodology, Revision 1, dated September 29, 2022, and technical report KP-TR-022, Hermes 2 Postulated Event Methodology, Revision 1, dated [month] xx, 2024, which are part of the Hermes 2 CP application and referenced in Revision 1 of the PSAR.
For a site with multiple reactor facilities such as Hermes 2, 10 CFR 100.11(b) requires that consideration be given to the degree of independence of the reactors in evaluating the site. If the reactors are independent to the extent that an accident in one reactor would not initiate an accident in another, the size of the exclusion area, low population zone, and population center distance shall be fulfilled with respect to each reactor individually. However, if the reactors are interconnected to the extent that an accident in one reactor could affect the safety of operation of any other, the size of the exclusion area, low population zone, and population center distance shall be based upon the assumption that all interconnected reactors emit their postulated fission product releases simultaneously.
PSAR Section 1.4, Shared Facilities and Equipment, states that the only shared systems between Hermes 2 Unit 1 and Unit 2 are non-safety systems. In addition, PSAR Section 1.4 THIS NRC STAFF DRAFT SE HAS BEEN PREPARED AND IS BEING RELEASED TO SUPPORT INTERACTIONS WITH THE ACRS. THIS DRAFT SE HAS NOT BEEN SUBJECT TO FULL NRC MANAGEMENT AND LEGAL REVIEWS AND APPROVALS, AND ITS CONTENTS SHOULD NOT BE INTERPRETED AS OFFICIAL AGENCY POSITIONS.
states that the onsite infrastructure that is not credited to perform a safety function or needed for safe operation may be shared with other nearby or onsite facilities such as Hermes 1. PSAR Chapter 7, Instrumentation and Control Systems, clarifies that only portions of the non-safety related instrument and controls systems that control shared systems are shared between the units. In PSAR Section 13.1, Initiating Events and Scenarios, Kairos further indicates that certain initiating events may result in a transient in both units; however, Kairos asserts that the progression of events and the response of each unit remain independent. In the staffs evaluation of the MHA and other postulated events presented in this chapter of the SE, the degree of independence between Hermes 2 Unit 1 and Unit 2 is considered to determine compliance with 10 CFR 100.11(b) requirements.
13.1 Initiating Events and Scenarios Section 13.1 of the Hermes 2 PSAR describes the postulated events evaluated by Kairos for the Hermes 2 facility. Several design features in the Hermes 2 design are similar to those in the Hermes 1 design, such as the reactor vessel system, reactor core and nuclear design, thermal-hydraulic design of the reactor, and the primary heat transport system (PHTS).
Furthermore, the use of functional containment to contain the radionuclides and the safety-related systems, such as the decay heat removal system (DHRS), reactor control and shutdown system (RCSS), and reactor protection system (RPS) are identical between Hermes 1 and Hermes 2. In Section 13.1 of the Hermes 2 PSAR, Kairos states that the postulated event description and analysis methodology presented in Chapter 13 of the PSAR and in technical report KP-TR-022 are derived from the Hermes 1 postulated event methodology in technical report KP-TR-018, Hermes Postulated Event Methodology, by identifying new postulated initiating events unique to Hermes 2.
The key differences in the Hermes 2 design as compared to the Hermes 1 design include:
x Intermediate heat transport system (IHTS) and its associated subsystems and components, such as the intermediate heat exchanger (IHX), superheater, intermediate salt pumps (ISPs), and rupture disks x
Power generation system (PGS) and its associated subsystems such as the turbine generator system, steam system, and the condensate and feedwater system x
Two-unit plant configuration with the shared PGS x
New safety features such as the safety-related rupture disks in the IHTS and an RPS trip of the ISPs x
Limits on the allowable amount of material at risk for release (MAR) in the IHTS and PGS x
Limits on the allowable amount of Flibe and water in the IHTS In Section 13.1 of the Hermes 2 PSAR, Kairos states that the new safety features and proposed operational limits ensure that the events initiating from failures in the Hermes 2 design-specific systems (e.g., IHTS and PGS) can either be prevented or be grouped under the existing Hermes 1 event categories. Furthermore, Kairos states that the new Hermes 2 events do not introduce new phenomena that require identification of additional figures of merit (FOMs) to ensure that consequences of postulated events remain bounded by the MHA.
The objectives of the postulated event analysis as listed in NUREG-1537, Guidelines for Preparing and Reviewing Applications for the Licensing of Non-Power Reactors, Part 1, Chapter 13, are:
x Ensure that enough events have been considered to include any accident with significant radiological consequences. Rejection of a potential event should be justified in the discussions.
x Categorize the initiating events and scenarios by type and likelihood of occurrence so that only the limiting cases in each group must be quantitatively analyzed.
x Develop and apply consistent, specific acceptance criteria for the consequences of each postulated event.
Therefore, the scope of technical evaluation presented in this SE is to ensure that:
x Hermes 2 design-specific postulated events are adequately identified and categorized, x
Hermes 2 design-specific postulated events are adequately described, and important mitigation features are identified, x
Adequate FOMs and associated acceptance criteria are identified to ensure that the consequences of the postulated events remain bounded by the MHA, x
MHA remains bounding for Hermes 2 events, and x
Progression of events and the response of each unit remain independent in compliance with 10 CFR 100.11(b) requirements.
In addition to the information in the Hermes 2 PSAR and KP-TR-022, the staffs evaluation was also supported by the general audit (Agencywide Documents Access and Management System (ADAMS) Accession No. MLxxxxxxxxx) and the independent scoping calculations performed by the staff and its contractors using a MELCOR/SCALE computer model.
The event categories identified in PSAR Section 13.1 are consistent with those listed in NUREG-1537, Section 13.1, Accident-Initiating Events and Scenarios, except for the malfunction of an experiment. As discussed in Chapter 10, Experimental Facilities and Utilization, of this SE, Hermes 2 will not include special facilities dedicated to the conduct of reactor experiments or experimental programs, so the experiment malfunction event is not applicable. Events Kairos precluded by design are discussed in Section 13.1.10, Prevented Events, of this SE.
13.1.1 Maximum Hypothetical Accident 13.1.1.1 Introduction Section 13.1.1, Maximum Hypothetical Accident, of the Hermes 2 PSAR provides the MHA event description and assumptions. Section 13.2.1, Maximum Hypothetical Accident, of the Hermes 2 PSAR provides a conservative evaluation of the MHA in order to bound the radionuclide release of all credible accidents. This analysis is done to demonstrate that the 10 CFR 100.11, Determination of exclusion area, low population zone, and population center distance, dose reference values are met at the exclusion area boundary (EAB) and outer boundary of the low population zone (LPZ) in support of the safety analysis requirements in 10 CFR 50.34(a)(1)(i) and the test reactor siting evaluation factors in 10 CFR 100, Subpart A, Evaluation Factors for Stationary Power Reactor Site Applications Before January 10, 1997 and for Testing Reactors, for all credible accidents.
An important component of the analysis of the MHA and its consequences is the identification of the radionuclide MAR due to the accident scenario, which includes the release from in-service TRISO fuel, fuel manufacturing defects, Flibe coolant inventory, gas space inventory, and radionuclides deposited in steel or graphite structures within the reactor and PHTS. Except for the transient fuel releases, the radionuclide retention and transport are determined by following the methodology provided in the mechanistic source term topical report KP-TR-012-NP-A, KP-FHR Mechanistic Source Term Methodology Topical Report, Revision 3, dated March 28, 2022 (ML22136A291). Bounding values for Flibe circulating activity, retained tritium, and activated argon are assumed in KP-TR-012-NP-A. In PSAR Section 14.1, Introduction, Kairos provided regulatory controls based on the bounding values assumed in the MHA analysis. PSAR Chapter 14, Technical Specifications, Table 14.1-1, Proposed Variables and Conditions for Technical Specifications, includes proposed technical specification (TS) LCOs in Section 3.3, Coolant Systems, which will include an upper bounding radionuclide limit in the reactor coolant during normal operation to ensure postulated events do not exceed limits. The proposed LCO will also include an upper bounding limit on quantity of radioactive MAR in the cover gas, IHTS, and PGS to ensure a postulated event does not exceed limits. The specific values for the upper bounding limits will be based upon the finalized event analyses, including the MHA, and are to be provided by Kairos in the Hermes 2 operating license (OL) application.
Retention and transport of radionuclides is predominately a function of the associated fuel, Flibe, and vapor space temperatures, as well as the available surface area and volumes for depositing radionuclides. The MHA uses a bounding fuel, Flibe, and stainless steel 316 temperature versus time profile. These factors bound the conditions of the other evaluated events.
13.1.1.2 Regulatory Evaluation The staff reviewed Section 13.1.1.2, Regulatory Evaluation, of the Hermes 1 SE for applicability to the Hermes 2 SE. Based on the similarities between the Hermes 1 and Hermes 2 facility designs and the consistency of the evaluated MHA between Hermes 1 and Hermes 2, the staff finds that the regulations and guidance listed in Section 13.1.1.2 of the Hermes 1 SE are applicable to Hermes 2. Therefore, this section incorporates by reference Section 13.1.1.2 of the Hermes 1 SE.
13.1.1.3 Technical Evaluation The staff reviewed Section 13.1.1 of the Hermes 2 PSAR and compared it to the equivalent section in the Hermes 1 PSAR (Section 13.1.1 Maximum Hypothetical Accident). The staff found that Section 13.1.1 of the Hermes 2 PSAR contains information consistent with that in the Hermes 1 PSAR, except for minor changes, which are evaluated below. The staff also verified that the MHA remains identical to Hermes 1. Based on these consistencies, this section incorporates by reference Section 13.1.1.3 of the Hermes 1 SE.
13.1.1.3.1 Minor Changes Compared to Hermes 1 PSAR The minor changes in Hermes 2 PSAR Section 13.1.1, as compared to the information in Hermes 1 PSAR Section 13.1.1, include the following:
x The addition of the IHTS and PGS x
Use of KP-TR-022 instead of KP-TR-018
In Hermes 2 PSAR Section 13.1.1, the MHA, in effect, considers the MAR in the two new systems of the Hermes 2 facility, the IHTS and PGS. The staff reviewed Hermes 2 PSAR and KP-TR-022, including information supporting the PSAR reviewed during the general audit (MLxxxxxxxxx), and determined that the Hermes 2 conservative analysis assumptions in the MHA remain identical to those in Hermes 1. Specifically, the staff verified that no additional radionuclide generation occurs in the IHTS and PGS, the MAR in the IHTS and PGS originates in the PHTS, the Hermes 2 functional containment does not include the IHTS or PGS, and Hermes 2 PSAR Table 13.1-1, Acceptance Criteria for Figures of Merit, includes FOMs and acceptance criteria to ensure that the consequences of events that include releases from the IHTS and PGS are bounded by the consequences of the MHA. The staff observed that, in effect, the conservative radionuclide release modeling in the MHA analysis neglects any radionuclide retention in the IHTS and PGS. Based on the above, the staff finds that the addition of the IHTS and PGS to the MHA is acceptable.
Additionally, the staff finds the use of KP-TR-022 instead of KP-TR-018 to be acceptable because KP-TR-022 is an updated version of KP-TR-018 that contains additional information specific to Hermes 2.
On the basis of its review, the staff finds that the level of detail provided in the Hermes 2 MHA demonstrates an adequate basis for a preliminary design and satisfies the applicable guidance and acceptance criteria of NUREG-1537, Part 2, Chapter 13, Accident Analyses, allowing the staff to make the following relevant findings:
x Kairos developed and analyzed an MHA, which is an accident that would release fission products and would have consequences greater than any credible accident. The MHA scenario is not credible, and the combination of bounding conditions analyzed is beyond what is assumed for design-basis accidents. The MHA serves as a bounding accident analysis for the Hermes 2 facility.
x Because the assumptions of the scenario are bounding, the doses calculated for the MHA will likely not be exceeded by any accident considered credible.
13.1.1.4 Conclusion Based on the staff findings above, and as incorporated by reference from the Hermes 1 SE, the staff concludes that the preliminary analysis of the MHA, as described in Hermes 2 PSAR Section 13.1.1 is sufficient and meets the applicable regulatory requirements and guidance identified in this section for the issuance of CPs in accordance with 10 CFR 50.35, Issuance of construction permits, and 10 CFR 50.40, Common standards. Further technical or design information required to complete the safety analysis may reasonably be left for later consideration. The staff will confirm that the final design conforms to the design basis during the evaluation of the final safety analysis report (FSAR) as part of the OL application review.
13.1.2 Insertion of Excess Reactivity 13.1.2.1 Introduction Section 13.1.2, Insertion of Excess Reactivity, of the Hermes 2 PSAR and KP-TR-022 discuss the insertion of excess reactivity postulated event. KP-TR-022 Appendix A, Section A.1, Insertion of Excess Reactivity, provides an example analysis of the insertion of excess
reactivity postulated event. The reactivity insertion transient involves a change in core reactivity that adds heat to the primary system. The narrative for the assumed limiting event is given in KP-TR-022 Section 3.2.2.2, Insertion of Excess Reactivity, and in PSAR Section 13.1.2. The PSAR states that this assumed limiting event bounds other insertion of reactivity events, including events listed in PSAR Section 13.1.2. Kairos identified this event as one of the bounding fuel performance cases, as stated in KP-TR-022 Section 4.5.2.2, Transient Analysis Methods. In KP-TR-022 Section 3.2.2.3, Increase in Heat Removal, Kairos also states that the increase in heat removal events are bounded by the insertion of excess reactivity event.
Therefore, the increase in heat removal events are grouped under the insertion of excess reactivity event category in Section 13.1.2 of the PSAR.
PSAR Section 13.1.10.7, Insertion of Excess Reactivity Beyond Rate Assumed in Postulated Events, describes why rapid reactivity insertions beyond that assumed in PSAR Section 13.1.2 are not considered. The staff evaluated this in Section 13.1.10.3, Technical Evaluation, of this SE.
13.1.2.2 Regulatory Evaluation The staff reviewed Section 13.1.2.2, Regulatory Evaluation, of the Hermes 1 SE for applicability to the Hermes 2 SE. Based on the similarities between the Hermes 1 and Hermes 2 facility designs and the consistency of the postulated insertion of excess reactivity events between Hermes 1 and Hermes 2, the staff finds that the regulations and guidance listed in Section 13.1.2.2 of the Hermes 1 SE are applicable to Hermes 2. Therefore, this section incorporates by reference Section 13.1.2.2 of the Hermes 1 SE.
13.1.2.3 Technical Evaluation The staff reviewed Section 13.1.2 of the Hermes 2 PSAR and compared it to the equivalent section in the Hermes 1 PSAR (Section 13.1.2, Insertion of Excess Reactivity). The staff found that Section 13.1.2 of the Hermes 2 PSAR contains information consistent with that in the Hermes 1 PSAR, except for a few significant changes, which are evaluated below in Section 13.1.2.3.1. The staff found that the following portions of Section 13.1.2 in the Hermes 2 PSAR contain information consistent with the Hermes 1 PSAR (e.g., minor or editorial changes only):
x Section 13.1.2.1, Initial Conditions Assumptions x
Section 13.1.2.2, Structures, Systems, and Components Mitigation Assumptions x
Section 13.1.2.3, Transient Assumptions Since the Hermes 2 system design and functionality largely remain identical, apart from the differences evaluated below, Section 13.1.2 of the Hermes 2 PSAR contains information consistent with Section 13.1.2 of the Hermes 1 PSAR. Based on these consistencies, this section incorporates by reference Section 13.1.2.3 of the Hermes 1 SE.
13.1.2.3.1 Significant Changes Compared to the Hermes 1 PSAR Significant changes contained in Section 13.1.2 of the Hermes 2 PSAR, as compared to Section 13.1.2 of the Hermes 1 PSAR, include information regarding the following:
x The following events were added to the increase in heat removal group of events in the insertion of excess reactivity postulated event category:
o ISP overspeed o spurious opening of a turbine bypass valve or steam safety valve o superheater shell leak o steam line break o spurious actuation of PHTS normal decay heat removal rejection radiator These changes are identified in Section 13.1.2 of the Hermes 2 PSAR.
The staff evaluated the sufficiency of this additional preliminary information on the Hermes 2 preliminary analysis of the insertion of excess reactivity postulated event using the guidance and acceptance criteria from NUREG-1537, Parts 1 and 2, Chapter 13.
The newly identified increase in heat removal events adequately account for the potential initiators from the newly added IHTS and PGS systems. Insertion of excess reactivity can be caused by overcooling due to the combination of decreasing PHTS temperature and negative coolant and void reactivity coefficients. Thus, the staff finds the that the Hermes 2 insertion of excess reactivity postulated events and increase in heat removal events are adequately identified and classified.
The Hermes 2 PSAR identifies a continuous withdrawal of a control element as the limiting insertion of excess reactivity event for the Hermes 2 design. The staff completed scoping calculations for a superheater heat exchanger tube break event and a main steam line break event to further inform the review of the newly identified insertion of excess reactivity postulated events for Hermes 2. These scoping calculations showed that neither of these events surpass the identified limiting insertion of excess reactivity postulated event in terms of severity and that the MHA remains bounding. Furthermore, the scoping calculations also show that the large thermal inertia of the IHTS and PHTS salt result in an overall mild temperature response in the reactor core. As identified in Appendix A of this SE, 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.
Regarding event progression and mitigation, the Hermes 1 safety systems that mitigate postulated events are also present in the Hermes 2 design. Specifically, the Hermes 1 and Hermes 2 reactor trip signal for the insertion of excess reactivity postulated events is initiated by a high flux protection signal. The Hermes 2 RPS has new safety-related trip signals for the ISP to trip concurrently with the primary salt pump (PSP) to limit overcooling. The PSP and ISP trips effectively isolate the PHTS from the IHTS and PGS due to a significant reduction in heat transfer, leading to a subsequent event progression similar to those for Hermes 1 events.
The staff observed that some insertion of excess reactivity postulated events (e.g., opening of turbine bypass valve) may result in initiation of postulated events in both units; however, systems needed to mitigate these events (i.e., RPS, RCSS, DHRS, reactor vessel) are independent and not shared between units. Additionally, the large thermal inertia of the intermediate loops minimizes the possibility of transients in one unit affecting the other unit and maintains unit independence.
Kairos has not identified any new FOMs for the Hermes 2 insertion of excess reactivity postulated events. As discussed above in SE Section 13.1.1.3, the Hermes 2 design and
functionality remain identical to Hermes 1 with respect to the conservative analysis assumptions in the MHA, no additional radionuclide generation occurs in the Hermes 2 systems not included in the Hermes 1 (i.e., IHTS and PGS), the Hermes 2 functional containment does not include the IHTS and PGS, and Hermes 2 PSAR Table 13.1-1 includes FOMs and acceptance criteria to ensure that the consequences of events that include releases from the IHTS and PGS are bounded by the consequences of the MHA. Therefore, the staff finds that the FOMs identified for the Hermes 2 insertion of excess reactivity postulated event are acceptable.
On the basis of its review, the staff finds that:
x The Hermes 2 PSAR adequately identifies and categorizes insertion of excess reactivity and increased heat removal events that are unique to the Hermes 2 design.
x The Hermes 2 PSAR provides adequate description of design-specific insertion of excess reactivity and increased heat removal events, including the event mitigation features.
x The Hermes 2 PSAR identifies adequate FOMs and associated acceptance criteria to ensure that the consequences of the insertion of excess reactivity and increased heat removal postulated events remain bounded by the MHA.
x The MHA remains bounding for the insertion of excess reactivity and increased heat removal events specific to Hermes 2.
x The Hermes 2 reactors are independent to the extent that postulated insertion of excess reactivity and increased heat removal events in one unit will not initiate an accident in the other unit, in compliance with 10 CFR 100.11(b) requirements.
13.1.2.4 Conclusion Based on the staff findings above, and as incorporated by reference from the Hermes 1 SE, the staff concludes that the preliminary analysis of the insertion of excess reactivity postulated events, as described in PSAR Section 13.1.2, is sufficient and meets the applicable guidance and regulatory requirements identified in this section for the issuance of CPs in accordance with 10 CFR 50.35 and 10 CFR 50.40. Further information as may be required to complete the review of the Hermes 2 insertion of excess reactivity postulated event can reasonably be left for later consideration at the OL stage since this information is not necessary for the review of a CP application.
13.1.3 Salt Spills 13.1.3.1 Introduction Section 13.1.3, Salt Spills, of the Hermes 2 PSAR describes the postulated limiting salt spill event. A salt spill is a loss of coolant event in which a pipe break or other leak causes Flibe and associated radionuclides to be released into the reactor building. The PSAR states that the loss of salt inventory is detected by the RPS based on low reactor coolant inventory, and the reactor is shut down by inserting the shutdown elements. The decay heat removal safety function is accomplished by parasitic losses or the operation of the DHRS and parasitic heat losses.
Design features such as the safety-related trip function for the PSP and anti-siphon features limit the salt inventory loss. Radionuclides are mobilized by aerosol generation from the break, pool splash, evaporation from the Flibe free surface, and air ingress oxidizing the non-wetted graphite surfaces. Leaks from the IHTS are also included in the salt spill event category.
13.1.3.2 Regulatory Evaluation The staff reviewed Section 13.1.3.2, Regulatory Evaluation, of the Hermes 1 SE for applicability to the Hermes 2 SE. Based on the similarities between the Hermes 1 and Hermes 2 facility designs and the consistency of the evaluation of postulated salt spill events between Hermes 1 and Hermes 2, the staff finds that the regulations and guidance listed in Section 13.1.3.2 of the Hermes 1 SE are applicable to Hermes 2. Therefore, this section incorporates by reference Section 13.1.3.2 of the Hermes 1 SE.
13.1.3.3 Technical Evaluation The staff reviewed Section 13.1.3 of the Hermes 2 PSAR and compared it to the equivalent section in the Hermes 1 PSAR (Section 13.1.3, Salt Spills). The staff found that Section 13.1.3 of the Hermes 2 PSAR contains information consistent with that in the Hermes 1 PSAR, except for a few minor and significant changes, which are evaluated below in Sections 13.1.3.3.1 and 13.1.3.3.2, respectively. The staff found that the following portions of Section 13.1.3 in the Hermes 2 PSAR contain information consistent with the Hermes 1 PSAR (e.g., minor or editorial changes only):
x 13.1.3.1, Initial Conditions Assumptions x
13.1.3.2, Structures Systems and Components Mitigation Assumptions x
13.1.3.3, Transient Assumptions Since the Hermes 2 system design and functionality largely remain identical, apart from the differences evaluated below, Section 13.1.3 of the Hermes 2 PSAR contains information consistent with Section 13.1.3 of the Hermes 1 PSAR. Based on these consistencies, this section incorporates by reference Section 13.1.3.3 of the Hermes 1 SE.
13.1.3.3.1 Minor Change Compared to the Hermes 1 PSAR The minor change in Hermes 2 PSAR Section 13.1.3, as compared to the information in Hermes 1 PSAR Section 13.1.3, includes the following:
x Added text to clarify that the RPS trips the heat rejection blower to limit the amount of air ingress following a postulated heat rejection radiator (HRR) tube break during low power operations.
In Hermes 2 PSAR Section 13.1.3, Kairos states that in the limiting salt spill postulated event scenario, the RPS trips the heat rejection blower to limit the amount of air ingress following postulated HRR tube breaks during low power operations. The added text during low power operations is not a substantive change from the Hermes 1 PSAR description of the scenario for the limiting salt spill postulated event. As described in Hermes 2 PSAR Section 5.1.1.6, Heat Rejection Subsystem, for the Hermes 2 reactor, the HRR is only in operation during low power operations when the PGS is not in operation. Therefore, the limiting salt spill postulated event scenario remains the same with respect to limiting air ingress following postulated HRR tube breaks. Based on the above, the staff finds that the clarification regarding the RPS trip of the heat rejection blower is acceptable.
13.1.3.3.2 Significant Changes Compared to the Hermes 1 PSAR Significant changes contained in Section 13.1.3 of the Hermes 2 PSAR, as compared to Section 13.1.3 of the Hermes 1 PSAR, include information regarding the following:
x Two new Hermes 2-specific events were added to the other salt spill group of events bounded by the limiting salt spill postulated event:
o leaks from the intermediate heat transport system that contains a non-Flibe coolant, which may contain a non-zero amount of radionuclides o intermediate heat exchanger tube break or leak These changes are identified in Section 13.1.3 of the Hermes 2 PSAR.
7KHVWDIIHYDOXDWHGWKHVXIILFLHQF\\RIWKLVDGGLWLRQDOSUHOLPLQDU\\LQIRUPDWLRQRQWKH+HUPHV
preliminary analysis of the limiting salt spill postulated event using the guidance and acceptance criteria from NUREG-1537, Parts 1 and 2, Chapter 13.
The additional salt spill events specific to the Hermes 2 design adequately account for events that initiate in the newly added IHTS and include the additional radionuclide release pathways through and from the IHTS, as discussed in KP-TR-022, Sections 3.2.2.1, Salt Spills, 3.2.2.7, Intermediate Heat Exchanger Tube Break, and 4.5.1, Salt Spills. Thus, the staff finds the that the Hermes 2 salt spill postulated events are adequately identified and classified.
As described in Hermes 2 PSAR Section 13.1.3, the assumed limiting event for this category is a hypothetical double-ended guillotine break in the PHTS piping. As identified in Appendix A of this SE, Kairos must confirm the limiting event, as well as key conservative analysis assumptions and initial conditions, as part of the OL application through sensitivity calculations or other appropriate analysis.
The event progression and mitigation approach for the additional salt spill events specific to Hermes 2 are similar to those for the limiting salt spill postulated event described in the PSAR, while considering the potential additional transport and release pathways through the IHTS.
Furthermore, important phenomena such as jet breakup and aerosolization for the BeNaF intermediate salt are similar to the phenomena for Flibe primary salt in the limiting salt spill postulated event and are modeled in the same way.
The same safety systems that play an accident mitigation role in the limiting salt spill postulated event for Hermes 1 are also available for Hermes 2. As described in Hermes 2 PSAR Section 13.1.3.2, there is no change in the role of the RPS, DHRS, pebble handling and storage system (PHSS) trip, and RCSS in shutting down the reactor and removing decay heat during the limiting salt spill postulated event from that described in the Hermes 1 PSAR, even with consideration of the added IHTS and PGS in Hermes 2. In addition, as described in Hermes 2 PSAR Section 13.1 and KP-TR-022 Section 3.2.2.7, the Hermes 2 RPS will concurrently trip both the PSP and ISP to minimize heat transfer in the IHX, therefore limiting the PHTS cooldown due to events initiated in the IHTS. Consistent with the Hermes 1 PSAR, the Hermes 2 PSAR Section 13.1.3 states that the anti-siphon design feature in the PHTS is credited to limit the amount of Flibe available to spill out of the break in the limiting salt spill postulated event. The staff observed that the same design feature will limit the amount of Flibe lost in the IHX tube break salt spill.
With respect to the salt spill event radionuclide transport and release phenomena, the aerosolization of salt from an IHX tube break or leak event would be limited as compared to aerosolization from the PHTS salt spill event due to the difference in thermal-hydraulic conditions in the downstream IHTS volume. Additional radionuclide generation does not occur in the Hermes 2 IHTS, and the primary source of MAR in the IHTS is operational leakage from the PHTS. Therefore, the limiting salt spill postulated event is bounding for other postulated salt spill events with respect to the MAR. Furthermore, as described Hermes 2 PSAR Table 13.1-1 and Table 14.1-1, Hermes 2 will have a limit on the quantities of MAR in the IHTS, which will significantly limit the releases and any dose consequences of a break from the IHTS and ensure that a postulated event does not exceed the MHA dose.
As described in Hermes 2 PSAR Section 13.1.3 and Table 13.1-1, no new FOMs and associated acceptance criteria to ensure that the MHA remains bounding for salt spill events are identified for the salt spill postulated events specific to Hermes 2. No additional radionuclide generation occurs in the Hermes 2 IHTS. In addition, the MHA preliminary analysis is not affected by the Hermes 2 design differences because the radionuclide release modeling in the MHA analysis assumes a release directly from the PHTS, which effectively neglects any radionuclide retention in the IHTS and PGS. Therefore, the currently identified FOMs for Hermes 1 are also acceptable for Hermes 2 events in this category.
On the basis of its review, as described above, the staff finds that:
x The Hermes 2 PSAR adequately identifies and categorizes the salt spill events that are unique to the Hermes 2 design.
x The Hermes 2 PSAR adequately describes Hermes 2 design-specific salt spill events, including the event mitigation features.
x The Hermes 2 PSAR identifies adequate FOMs and associated acceptance criteria to ensure that the consequences of the salt spill events remain bounded by the MHA.
x The MHA remains bounding for the Hermes 2-specific salt spill events.
x The Hermes 2 reactors are independent to the extent that a postulated salt spill event in one unit will not initiate and accident in the other unit, in compliance with 10 CFR 100.11(b) requirements.
13.1.3.4 Conclusion Based on the staff findings above, and as incorporated by reference from the Hermes 1 SE, the staff concludes that the preliminary analysis of the postulated salt spill event, as described in PSAR Chapter 13, is sufficient and meets the applicable guidance and regulatory requirements identified in this section for the issuance of CPs in accordance with 10 CFR 50.35 and 10 CFR 50.40. Further technical or design information required to complete the safety analysis may reasonably be left for later consideration. The staff will confirm that the final design conforms to the design basis during the evaluation of the FSAR as part of the OL application review.
13.1.4 Loss of Forced Circulation 13.1.4.1 Introduction Section 13.1.4, Loss of Forced Circulation, of the Hermes 2 PSAR and KP-TR-022-NP Sections 3.2.2.4 Loss of Forced Circulation, and 4.5.3, Loss of Forced Circulation, discuss
the loss of forced circulation postulated event. KP-TR-022-NP Section A.4, Loss of Forced Circulation, provides example analyses of the loss of forced circulation postulated event. Two bounding transient scenarios, overheating and overcooling, are analyzed for the loss of forced circulation postulated event. The postulated overheating event intends to bound the consequence of overheating due to a loss of forced circulation or loss of normal heat sink, and the postulated overcooling event intends to bound the consequence of freezing in the downcomer due to a loss of forced circulation. Kairos states in PSAR Section 13.1.4 that the assumed limiting loss of forced circulation event bounds other loss of forced circulation events, including events listed in PSAR Section 13.1.4.
13.1.4.2 Regulatory Evaluation The staff reviewed Section 13.1.4.2, Regulatory Evaluation, of the Hermes 1 SE for applicability to the Hermes 2 SE. Based on the similarities between the Hermes 1 and Hermes 2 facility designs and the consistency of the postulated loss of forced circulation event between Hermes 1 and Hermes 2, the staff finds that the regulations and guidance listed in Section 13.1.4.2 of the Hermes 1 SE are applicable to Hermes 2. Therefore, this section incorporates by reference Section 13.1.4.2 of the Hermes 1 SE.
13.1.4.3 Technical Evaluation The staff reviewed Section 13.1.4 of the Hermes 2 PSAR and compared it to the equivalent section in the Hermes 1 PSAR (Section 13.1.4, Loss of Forced Circulation). The staff found that Section 13.1.4 of the Hermes 2 PSAR contains information consistent with that in the Hermes 1 PSAR, except for a few significant changes, which are evaluated below in SE Section 13.1.4.3.1. The staff found that the following portions of Section 13.1.4 in the Hermes 2 PSAR contain information consistent with the Hermes 1 PSAR (e.g., minor or editorial changes only):
x Section 13.1.4.1, Initial Conditions Assumptions x
Section 13.1.4.2, Structures Systems and Components Mitigation Assumptions x
Section 13.1.4.3, Transient Assumptions Since the Hermes 2 system design and functionality largely remain identical to Hermes 1, apart from the differences evaluated below, Section 13.1.4 of the Hermes 2 PSAR contains information consistent with Section 13.1.4 of the Hermes 1 PSAR. Based on these consistencies, this section incorporates by reference Section 13.1.4.3 of the Hermes 1 SE.
13.1.4.3.1 Significant Change Compared to the Hermes 1 PSAR The significant change contained in Section 13.1.4 of the Hermes 2 PSAR, as compared to Section 13.1.4 of the Hermes 1 PSAR, includes information regarding the following:
x The following events were added to the loss of normal heat sink group of events in the loss of forced circulation postulated event category:
o Turbine trip o ISP failure o Superheater tube rupture x
A paragraph was added to further discuss a postulated superheater tube rupture.
These changes are identified in Section 13.1.4 of the Hermes 2 PSAR.
7KHVWDIIHYDOXDWHGWKHVXIILFLHQF\\RIWKLVDGGLWLRQDOSUHOLPLQDU\\LQIRUPDWLRQRQWKH+HUPHV
preliminary analysis of the loss of forced circulation postulated event using the guidance and acceptance criteria from NUREG-1537, Parts 1 and 2, Chapter 13.
The newly identified events adequately account for the potential loss of normal heat sink initiators in the newly added IHTS or PGS. Loss of forced circulation and loss of normal heat sink events result in a reduction in heat removal from the reactor. Compared to other loss of normal heat sink events in the Hermes 1 and Hermes 2 PSARs, a loss of heat sink due to initiating events in the IHTS or PGS would have a similar impact on the reactor thermal-hydraulic and neutronic response; however, they would likely evolve more slowly than an initiating event in the PHTS because of the separation of the PHTS and IHTS and the large thermal inertia of those systems. For these reasons, the staff finds that the Hermes 2 loss of forced circulation postulated events are adequately identified and classified.
The Hermes 2 PSAR identifies a PSP seizure as the limiting loss of forced circulation event for the Hermes 2 design. The staff performed scoping calculations for a loss of forced circulation event for Hermes 2. These scoping calculations showed that the MHA bounds a loss of forced circulation event. As identified in Appendix A of this SE, Kairos must confirm the loss of forced circulation limiting event, as well as key conservative analysis assumptions and initial conditions, as part of the OL application through sensitivity calculations or other appropriate analysis.
The same safety systems that mitigate loss of forced circulation events for the Hermes 1 design are also available for Hermes 2. These include the RPS, which detects the off-normal condition and initiates a reactor trip and PSP trip; the DHRS, which is credited for decay heat removal when the normal heat removal pathway is unavailable; and the RCSS, which provides the negative reactivity to shut down the reactor when a trip is demanded.
For the Hermes 2 design, the RPS will also initiate an ISP trip. The PSP and ISP trips effectively isolate the PHTS from the IHTS and PGS due to a significant reduction in heat transfer, leading to a subsequent event progression similar to those for Hermes 1. In addition, Hermes 2 relies upon the design of the IHTS to mitigate a postulated superheater tube rupture event and prevent gross failure of the IHX, as further evaluated in Section 13.1.10.3.2 of this SE.
The staff observed that events initiating in the PGS (e.g., turbine trip) may result in postulated events in both units. However, systems needed to mitigate the events are independent and not shared between units, so the event progression in one unit would not affect the progression in the other unit.
Kairos has not identified any new FOMs for the Hermes 2-specific loss of forced circulation events. As discussed in SE Section 13.1.1.3, the Hermes 2 design and functionality remain identical to Hermes 1 with respect to the conservative analysis assumptions in the MHA; no additional radionuclide generation occurs in the Hermes 2 systems not included in Hermes 1 (i.e., IHTS and PGS); the Hermes 2 functional containment does not include the IHTS and PGS; and Hermes 2 PSAR Table 13.1-1 includes FOMs and acceptance criteria to ensure that the consequences of events that include releases from the IHTS and PGS are bounded by the consequences of the MHA. Therefore, the staff finds that the FOMs identified for the Hermes 2 loss of forced circulation postulated events are acceptable.
On the basis of its review, the staff finds that the level of detail provided on the preliminary analyses of the loss of forced circulation postulated events satisfies the applicable acceptance criteria of NUREG-1537, Part 2, Chapter 13 and that:
x The Hermes 2 PSAR adequately identifies and categorizes loss of forced circulation and loss of heat sink events that are unique to the Hermes 2 design.
x The Hermes 2 PSAR adequately describes Hermes 2 design-specific loss of forced circulation and loss of heat sink events, including the event mitigation features.
x The Hermes 2 PSAR identifies adequate FOMs and associated acceptance criteria to ensure that the consequences of the loss of forced circulation and loss of heat sink events remain bounded by the MHA.
x The MHA remains bounding for the Hermes 2-specific loss of forced circulation and loss of heat sink events.
x The Hermes 2 reactors are independent to the extent that a postulated loss of forced circulation or loss of heat sink event in one unit will not initiate an accident in the other unit, in compliance with 10 CFR 100.11(b) requirements.
13.1.4.4 Conclusion Based on the staff findings above, and as incorporated by reference from the Hermes 1 SE, the staff concludes that the preliminary analyses of the loss of forced circulation postulated events, as described in PSAR Section 13.1.4, are sufficient and meet the applicable regulatory requirements and guidance identified in this section for the issuance of CPs in accordance with 10 CFR 50.35 and 10 CFR 50.40. Further technical or design information required to complete the safety analysis may reasonably be left for later consideration. The staff will confirm that the final design conforms to the design basis during the evaluation of the FSAR as part of the OL application review.
13.1.5 Mishandling or Malfunction of Pebble Handling and Storage System 13.1.5.1 Introduction Section 13.1.5, Mishandling or Malfunction of the Pebble Handling and Storage System, of the Hermes 2 PSAR describes the preliminary evaluation of the category of postulated events involving a mishandling or malfunction of the PHSS. As assumed by Kairos, the limiting postulated event for this category of events is a break in a fuel transfer line during extraction, resulting in a spill of pebbles onto the floor of the room.
13.1.5.2 Regulatory Evaluation The staff reviewed Section 13.1.5.2, Regulatory Evaluation, of the Hermes 1 SE for applicability to the Hermes 2 SE. Based on the similarities between the Hermes 1 and Hermes 2 facility designs and the consistency of the postulated events involving a mishandling or malfunction of the PHSS between Hermes 1 and Hermes 2, the staff finds that the regulations and guidance listed in Section 13.1.5.2 of the Hermes 1 SE are applicable to Hermes 2.
Therefore, this section incorporates by reference Section 13.1.5.2 of the Hermes 1 SE.
13.1.5.3 Technical Evaluation The staff reviewed Section 13.1.5 of the Hermes 2 PSAR and compared it to the equivalent section in the Hermes 1 PSAR (Section 13.1.5, Mishandling or Malfunction of Pebble Handling
and Storage System). The staff found that Section 13.1.5 of the Hermes 2 PSAR contains information consistent with that in the Hermes 1 PSAR. The staff also verified that the Hermes 2 PHSS design and functionality remain identical to Hermes 1. Based on these consistencies, this section incorporates by reference Section 13.1.5.3 of the Hermes 1 SE.
On the basis of its review, the staff finds that the level of detail provided on the postulated PHSS event is consistent with the applicable guidance and acceptance criteria in NUREG-1537, Parts 1 and 2, Chapter 13, Accident Analysis, and demonstrates an adequate design basis for a preliminary design.
13.1.5.4 Conclusion Based on the staff findings above, and as incorporated by reference from the Hermes 1 SE, the staff concludes that the preliminary analysis of the postulated PHSS event, as described in PSAR Chapter 13, is sufficient and meets the applicable regulatory requirements and guidance identified in this section for the issuance of CPs in accordance with 10 CFR 50.35 and 10 CFR 50.40. Further technical or design information required to complete the safety analysis may reasonably be left for later consideration during the OL application. The staff will confirm that the final design conforms to the design basis during the evaluation of the FSAR as part of the OL application review.
13.1.6 Radioactive Release from a Subsystem or Component 13.1.6.1 Introduction Section 13.1.6, Radioactive Release from a Subsystem or Component, of the Hermes 2 PSAR describes the preliminary evaluation of the category of postulated events with radioactive release from a subsystem or component due to the failure of a system or component that contains radioactive material. As assumed by Kairos, the limiting event for this category of events is a seismic event that results in the failure of all systems containing radioactive material that are not qualified to maintain structural integrity in a design-basis earthquake for a single unit (including shared systems).
13.1.6.2 Regulatory Evaluation The staff reviewed Section 13.1.6.2, Regulatory Evaluation, of the Hermes 1 SE for applicability to the Hermes 2 SE. Based on the similarities between the Hermes 1 and Hermes 2 facility designs and the consistency of the preliminary evaluation of postulated radioactive release events from a subsystem or component between Hermes 1 and Hermes 2, the staff finds that the regulations and guidance listed in Section 13.1.6.2 of the Hermes 1 SE are applicable to Hermes 2. Therefore, this section incorporates by reference Section 13.1.6.2 of the Hermes 1 SE.
13.1.6.3 Technical Evaluation As shown in PSAR Table 13.1-1, the only FOM to ensure that the radioactive release from a subsystem or component category of events is bounded by the MHA radiological consequence analysis is the amount of radioactive material contained in the subsystems and components. As stated in Section 13.1.6 of the PSAR, Kairos is applying a design requirement on the amount of MAR in the systems expected to accumulate radionuclides during operation. Kairos identifies systems where the accumulation of radionuclides is expected to occur during the plant
operating lifetime and could be released by a single initiating event such as a design-basis earthquake. The systems identified by Kairos in PSAR Chapter 9, Auxiliary Systems, that contain radionuclides are the tritium management system, inert gas system, chemistry control system (including filters), inventory management system, IHTS, and PGS. The staff finds that subsystem or component MAR design limits can be set such that the MHA release remains bounding, but the combined release from all subsystems and components not designed to withstand the limiting external or internal event will need to be evaluated as part of the OL application when specific MAR values are available.
The staff finds that the level of detail provided on radioactive release from a subsystem or component is consistent with the applicable guidance and acceptance criteria in NUREG-1537, Parts 1 and 2, Chapter 13. Specifically, NUREG-1537, Part 2, Chapter 13, subheading External Events, states that:
[f]or events that cause facility damage [], the damage is within the bounds discussed for other accidents in this chapter. Therefore, exposure to the workers and the public is within acceptable limits and external events do not pose an unacceptable risk to the health and safety of the public.
Limiting the stored MAR in subsystems or components below an amount that ensures the MHA remains bounding satisfies the NUREG-1537 guidance.
13.1.6.4 Conclusion Based on the staff findings above, the staff concludes that the preliminary analyses of the radioactive releases from a subsystem or component are sufficient and meet the applicable regulatory requirements and guidance identified in this section for the issuance of CPs in accordance with 10 CFR 50.35 and 10 CFR 50.40. The combined release from all subsystems and components not designed to withstand the limiting external or internal event will need to be evaluated as part of the OL application when specific MAR subsystem or component values are available.
13.1.7 (not used) 13.1.8 General Challenges to Normal Operation 13.1.8.1 Introduction Section 13.1.8, General Challenges to Normal Operation, of the Hermes 2 PSAR states that this category of events includes challenges to normal operation that require or cause an automatic or manual shutdown of the plant but are not covered by another event category.
These events could be caused by control system anomalies, operator actions, or malfunctions of equipment or instrumentation.
13.1.8.2 Regulatory Evaluation The staff reviewed Section 13.1.7.2, Regulatory Evaluation, of the Hermes 1 SE for applicability to the Hermes 2 SE. Based on the similarities between the Hermes 1 and Hermes 2 facility designs and the consistency of the postulated challenges to normal operation between Hermes 1 and Hermes 2, the staff finds that the regulations and guidance listed in
Section 13.1.7.2 of the Hermes 1 SE are applicable to Hermes 2. Therefore, this section incorporates by reference Section 13.1.7.2 of the Hermes 1 SE.
13.1.8.3 Technical Evaluation The staff reviewed Section 13.1.8 of the Hermes 2 PSAR and compared it to the equivalent section in the Hermes 1 PSAR (Section 13.1.8, General Challenges to Normal Operation). The staff found that Section 13.1.8 of the Hermes 2 PSAR contains information consistent with that in the Hermes 1 PSAR, except for a minor change, which is evaluated below. The staff also verified that the Hermes 2 general challenges to normal operations remain largely identical to Hermes 1. Based on these consistencies, this section incorporates by reference Section 13.1.7.3 of the Hermes 1 SE.
13.1.8.3.1 Minor Change Compared to the Hermes 1 PSAR The minor change in Hermes 2 PSAR Section 13.1.8, as compared to the information in Hermes 1 PSAR Section 13.1.8, includes the following:
x heat rejection subsystem was replaced with intermediate heat transport system in the list of plant systems that could challenge normal operations The IHTS for Hermes 2 is the analog of the heat rejection subsystem for Hermes 1; the PHTS transfers heat to these respective systems during normal operation.
While the Hermes 2 design includes new systems compared to Hermes 1, the systems do not introduce new phenomena or FOMs. Therefore, the staffs conclusion that events in the challenges to normal operation event category are expected to be less limiting than the bounding events in other event categories (e.g., insertion of excess reactivity, loss of forced circulation) is unchanged. Based on the above, the staff finds that replacing the heat rejection subsystem with the intermediate heat transport system in Section 13.1.8 to be acceptable.
On the basis of its review, the staff finds that the level of detail provided regarding the Hermes 2 general challenges to normal operation demonstrates an adequate basis for a preliminary design and satisfies the applicable acceptance criteria of NUREG-1537, Part 2, Chapter 13.
13.1.8.4 Conclusion Based on the staff findings above, and as incorporated by reference from the Hermes 1 SE, the staff concludes that the preliminary analyses of general challenges to normal operation are sufficient and meet the applicable regulatory requirements identified in this section for the issuance of CPs in accordance with 10 CFR 50.35 and 10 CFR 50.40. Final determination that the consequences of Chapter 13 postulated events bound those of general challenges to normal operation will be evaluated when a detailed design is provided as part of the OL application.
13.1.9 Internal and External Hazard Events 13.1.9.1 Introduction Section 13.1.9, Internal and External Hazard Events, of the Hermes 2 PSAR describes the types of internal and external hazard events considered in the Hermes 2 design bases and the designs ability to cope with these events.
13.1.9.2 Regulatory Evaluation The staff reviewed Section 13.1.8.2, Regulatory Evaluation, of the Hermes 1 SE for applicability to the Hermes 2 SE. Based on the similarities between the Hermes 1 and Hermes 2 facility designs and the consistency of the evaluation of internal and external hazard events between Hermes 1 and Hermes 2, the staff finds that the regulations and guidance listed in Section 13.1.8.2 of the Hermes 1 SE are applicable to Hermes 2. Therefore, this section incorporates by reference Section 13.1.8.2 of the Hermes 1 SE.
13.1.9.3 Technical Evaluation The staff reviewed Section 13.1.9 of the Hermes 2 PSAR and compared it to the equivalent section in the Hermes 1 PSAR (Section 13.1.9, Internal and External Hazard Events). The staff found that Section 13.1.9 of the Hermes 2 PSAR contains information consistent with that in the Hermes 1 PSAR, except for a few significant changes, which are evaluated below in Section 13.1.9.3.1. The staff found that the portions of Section 13.1.9 in the Hermes 2 PSAR that discuss the following subjects contain information consistent with the Hermes 1 PSAR:
x The internal fire and internal water flood events listed as internal hazard events in the design basis x
The list of all external hazard events in the design basis x
The general discussion of protection of engineered safety features x
The discussion of seismic events Since the Hermes 2 system design, functionality, and analyses largely remain identical, apart from the differences evaluated below, Section 13.1.9 of the Hermes 2 PSAR contains information consistent with Section 13.1.9 of the Hermes 1 PSAR. Based on these consistencies, this section incorporates by reference Section 13.1.8.3 of the Hermes 1 SE.
13.1.9.3.1 Significant Change Compared to the Hermes 1 PSAR A significant change contained in Section 13.1.9 of the Hermes 2 PSAR, as compared to Section 13.1.9 of the Hermes 1 PSAR, includes information regarding the following:
x Identification of two additional hazard events unique to Hermes 2: turbine missiles and high-energy breaks in the PGS The staffs evaluation of a turbine missile generated due to a postulated failure of a turbine generator is presented in Section 9.9.2, Turbine Generator System, of this SE, and the staffs evaluation of the high-energy steam line break is presented in Section 9.9.1, Steam System, of this SE. As described in those SE sections, the staff finds that the preliminary design of the facility is consistent with PDC 4 for safety-related SSCs to be appropriately protected against the dynamic effects of turbine generator and steam system malfunctions and accidents. In
addition, the staff finds that the preliminary design satisfies the applicable acceptance criteria of NUREG-1537, Part 2, Section 9.7, Other Auxiliary Systems, in that potential malfunctions should not result in reactor accidents, uncontrolled release of radioactivity, or interfere with or prevent safe shutdown of the reactor.
The other consequences of a turbine failure event are bounded by the loss of forced circulation event as evaluated in Section 13.1.4.3 of this SE. The staffs evaluation of increased heat removal due to a steam line break is presented in Section 13.1.2.3 of this SE.
The staff finds the inclusion of turbine missiles and high-energy breaks in the PGS as internal hazard events in PSAR Section 13.1.9 acceptable because they account for design differences between Hermes 1 and Hermes 2, and Kairos has adequately addressed the events elsewhere in the PSAR.
13.1.9.4 Conclusion Based on the staff findings above, and as incorporated by reference from the Hermes 1 SE, the staff concludes that the preliminary analyses of the internal and external hazard events are sufficient and meet the applicable regulatory requirements and guidance identified in this section for the issuance of CPs in accordance with 10 CFR 50.35 and 10 CFR 50.40. Further technical or design information required to complete the safety analysis may reasonably be left for later consideration during the OL application.
13.1.10 Prevented Events 13.1.10.1 Introduction PSAR Section 13.1.10 describes events that were not analyzed in the PSAR as they are precluded by design.
13.1.10.2 Regulatory Evaluation The staff reviewed Section 13.1.9.2, Regulatory Evaluation, of the Hermes 1 SE for applicability to the Hermes 2 SE. Based on the similarities between the Hermes 1 and Hermes 2 facility designs and the consistency of the evaluation of prevented events between Hermes 1 and Hermes 2, the staff finds that the regulations and guidance listed in Section 13.1.9.2 of the Hermes 1 SE are applicable to Hermes 2. Therefore, this section incorporates by reference Section 13.1.9.2 of the Hermes 1 SE.
13.1.10.3 Technical Evaluation The staff reviewed Section 13.1.10 of the Hermes 2 PSAR and compared it to the equivalent section in the Hermes 1 PSAR (Section 13.1.10, Prevented Events). The staff found that Section 13.1.10 of the Hermes 2 PSAR contains information consistent with that in the Hermes 1 PSAR, except for a few minor and significant changes, which are evaluated below in Sections 13.1.10.3.1 and 13.1.10.3.2, respectively. The staff found that the following portions of Section 13.1.10 in the Hermes 2 PSAR contain information consistent with the Hermes 1 PSAR (e.g.,
minor or editorial changes only):
x Section 13.1.10.1, Recriticality or Reactor Shutdown System Failure x
Section 13.1.10.2, Degraded Heat Removal or Uncooled Events
x Section 13.1.10.3, Flibe Spill Beyond Maximum Volume Assumed in Postulated Salt Spills x
Section 13.1.10.4, In-Service TRISO Failure Rates and Burnups Above Assumptions in Postulated Events x
Section 13.1.10.5, Significant Air Ingress Into the PHTS x
Section 13.1.10.6, DHRS Reactor Cavity Flooding x
Section 13.1.10.7, Insertion of Excess Reactivity Beyond Rate Assumed in Postulated Events x
Section 13.1.10.8, Criticality Occurrence External to Reactor Core x
Section 13.1.10.9, Excessive Radionuclide Release from Flibe Since the Hermes 2 system design, functionality, and analyses largely remain identical, apart from the differences evaluated below, Section 13.1.10 of the Hermes 2 PSAR contains information consistent with Section 13.1.10 of the Hermes 1 PSAR. Based on these consistencies, this section incorporates by reference Section 13.1.9 of the Hermes 1 SE.
13.1.10.3.1 Minor Changes Compared to the Hermes 1 PSAR The minor changes in Hermes 2 PSAR Section 13.1.10, as compared to the information in Hermes 1 PSAR Section 13.1.10, include the following:
x Section 13.1.10.3 adds the ISP as a component tripped by the RPS.
x Section 13.1.10.7 adds the ISP as a component susceptible to pump overspeed that could increase core cooling.
Hermes 2 PSAR Section 7.3, Reactor Protection System, adds a safety-related ISP trip resulting from RPS actuation. Because the change to Section 13.1.10.3 is consistent with the change to the RPS functions described in PSAR Section 7.3, the staff finds that the addition of the ISP as a component tripped by the RPS to Section 13.1.10.3 to be acceptable.
The addition of the ISP to the Hermes 2 design introduces the possibility of an ISP overspeed condition leading to overcooling and insertion of excess reactivity. However, the ISP will operate within defined limits that ensure the consequences of an ISP overspeed are bounded by the insertion of excess reactivity events analyzed in Section 13.1.2 of the Hermes 2 PSAR.
Therefore, the staff finds that adding the ISP as a component susceptible to pump overspeed is acceptable. The staff will confirm that the consequences of an ISP overspeed are bounded as part of the OL application review.
13.1.10.3.2 Significant Changes Compared to the Hermes 1 PSAR Significant changes contained in Section 13.1.10 of the Hermes 2 PSAR, as compared to Section 13.1.10 of the Hermes 1 PSAR, include information regarding the following:
x New potential internal or external events involving the IHTS or PGS that could prevent safety-related SSCs from performing their safety function x
Gross failure of the IHX due to a PGS superheater tube rupture or leak These changes are identified in Hermes 2 PSAR Sections 13.1.10.10, Internal or External Events Interfering with SSCs, and 13.1.10.11, IHX Gross Failure Due to Superheater Tube Rupture or Leak, respectively.
The staff evaluated the sufficiency of this additional preliminary information on the Hermes 2 prevented events using the guidance and acceptance criteria from NUREG-1537, Parts 1 and 2, Chapter 13.
Internal or External Events Interfering with SSCs Hermes 2 PSAR Section 13.1.10.10 adds commitments to locate components containing Flibe and BeNaF in areas that use design features such as steel liners to prevent Flibe-concrete and BeNaF-concrete interactions that could adversely affect safety functions of SSCs. Hot Flibe or BeNaF could potentially degrade concrete and mix with the degradation products, which has a currently unknown impact on the chemical form and radionuclide retention properties of the salt.
The use of steel liners in areas of potential salt spills would minimize or prevent this phenomenon. Therefore, the staff finds the commitments to be acceptable.
Hermes 2 PSAR Section 13.1.10.10 also adds information about potential impacts of the PGS.
It states that the turbine will be oriented such that turbine blade missiles would not prevent safety-related SSCs from fulfilling their safety functions. Furthermore, as discussed in Hermes 2 PSAR Section 9.9.2, Turbine Generator System, the turbine generator is located in the turbine building away from the safety-related SSCs and is designed with a control feature that monitors the turbine shaft speed for overspeed protection. These design features are expected to minimize the potential for turbine missile generation and its impact on the safety-related SSCs.
In addition, as discussed in Hermes 2 PSAR Section 9.9.1 and Section 9.9.3, Condensate and Feedwater System, PGS piping would be located to ensure that postulated failures of the piping would not prevent safety-related SSCs from fulfilling their safety functions. The staff finds these changes acceptable because they demonstrate Kaiross consideration of how the non-safety-related PGS could potentially affect safety-related SSCs and preclusion of such impacts by design.
The staff will confirm the design features and commitments described in Hermes PSAR Section 13.1.10.10 as part of its review of the OL application.
IHX Gross Failure Due to Superheater Tube Rupture or Leak Hermes 2 PSAR Section 13.1.10.11 states that a superheater tube rupture could over-pressurize the IHTS. A safety-related feature relieves the pressure to prevent significant Flibe-water interaction in the PHTS that could result from a gross failure of the IHX due to over--pressurization.
Hermes 2 PSAR Figure 5.1-1, Primary Heat Transport System and Intermediate Heat Transport System Process Flow Diagram, shows how BeNaF from the IHTS flows through the tube side of the superheater and transfers heat to the steam in the shell side of the superheater.
As shown in Hermes 2 PSAR Table 9.9-1, Key Design Power Generation System Parameters, the superheater steam pressure is 14 MPa, whereas the IHTS design pressure is near ambient pressure, according to Hermes 2 PSAR Table 5.2-1, Key Design Parameters of the Intermediate Heat Transport System.
Therefore, a breach of a superheater tube would result in a large driving force propelling steam from the PGS into the IHTS, causing an overpressure condition. Absent a means to mitigate the overpressure, the IHTS could fail, with failures potentially occurring in the IHX. In this situation, steam or water could mix with the intermediate coolant and a small allowable amount of primary
coolant (Flibe) that may already be present in the IHTS (see Hermes 2 PSAR Table 14.1-1, Proposed Variables and Conditions for Technical Specifications, for more information) or a greater amount of Flibe if the IHX has failed due to the overpressure. The impact of water/steam mixing with Flibe on the radionuclide retention of Flibe has not been experimentally investigated and no reliable analytical models exist to predict the behavior. Therefore, Kaiross approach is to preclude the scenario by design.
Hermes 2 PSAR Section 5.2.1.2, Intermediate Inert Gas Subsystem, describes the safety-related rupture disks that are designed to preclude a gross failure of the IHX resulting from a superheater tube break or rupture. Hermes 2 PSAR Section 5.2.3, System Evaluation, states that the rupture disks are sized to ensure that pressures in the intermediate salt vessels (ISVs), the IHX, and connecting piping do not exceed allowable pressure limits and that the rupture disk and ISV design provide a relief path for steam to prevent it from reaching the IHX.
Hermes 2 PSAR Section 13.1.4, Loss of Forced Circulation, states that the orientation of the ISV inlet and outlet prevents the steam from the superheater tube rupture from entering the piping connecting the ISVs to the IHX. The thermal-hydraulic conditions in the IHTS (i.e., high temperature and low pressure relative to steam in PGS) will prevent steam or water from accumulating in the IHTS, further reducing the potential for steam or water to reach the IHX.
In the absence of any explicit modeling or experimental data, the staff finds that there is large uncertainty in the progression of events following a postulated superheater tube break or rupture. It appears that the IHX tubes are relied upon to prevent Flibe-water interaction during this event. However, as discussed in SE Section 5.2, Intermediate Heat Transport System, the IHTS (including the IHX tubes) is considered non-safety related by Kairos except for one component, the rupture disks. Consequently, as discussed in Section 3.6.3.2 of this SE, the staff was unable to confirm that the IHX complies with the Hermes 2 definition of safety-related structures, systems, and components (SSCs). In response to request for confirmation of information (RCI) number 1 (ML24103A241), Kairos confirmed 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, if the IHX tubes are relied upon to remain functional during and after a postulated event, Kairos will demonstrate that their failure is not credible considering all relevant factors.
Given the descriptions in the Hermes 2 PSAR, the staff considers it plausible for Kairos to produce a final IHTS design, including IHX and rupture disks, that meets its design bases. The staff will confirm the adequacy of the rupture disks and overall IHTS design to preclude significant Flibe-water interaction resulting from a superheater tube break or rupture as part of its review of the OL application.
On the basis of its review, the staff finds that the level of detail provided in PSAR Section 13.1.10 is consistent with the applicable guidance and acceptance criteria in NUREG-1537, Parts 1 and 2, Chapter 13, Accident Analysis, and demonstrates an adequate design basis for a preliminary design. Specifically, PSAR Section 13.1.10 provides sufficient justification for why the prevented events are excluded from the design basis. However, the staff will confirm that these events should be excluded during review of the final design submitted as part of an OL application.
13.1.10.4 Conclusion Based on the staff findings above, and as incorporated by reference from the Hermes 1 SE, the staff concludes that the information in PSAR Section 13.1.10 is sufficient and meets the applicable regulatory requirements and guidance identified in this section for the issuance of
CPs in accordance with 10 CFR 50.35 and 10 CFR 50.40. Further technical or design information required to complete the safety analysis may reasonably be left for later consideration during the OL application.
13.2 Accident Analysis and Determination of Consequences Section 13.2 of the Hermes 2 PSAR provides Kaiross analysis of the radiological consequences of accidents, focusing on the MHA.
13.2.1 Maximum Hypothetical Accident 13.2.1.1 Introduction Section 13.2.1, Maximum Hypothetical Accident, of the Hermes 2 PSAR describes the evaluation of the radiological consequences of the MHA. The MHA analysis is intended to be bounding for the postulated events described in PSAR Chapter 13 and is performed to demonstrate that the 10 CFR 100.11 dose reference values are met at the EAB and outer boundary of the LPZ in support of the safety analysis requirements in 10 CFR 50.34(a)(1)(i) and the siting evaluation factors in 10 CFR 100, Subpart A.
13.2.1.2 Regulatory Evaluation The staff reviewed Section 13.2.1.2, Regulatory Evaluation, of the Hermes 1 SE for applicability to the Hermes 2 SE. Based on the similarities between the Hermes 1 and Hermes 2 facility designs and the consistency of the proposed MHA between Hermes 1 and Hermes 2, the staff finds that the regulations and guidance listed in Section 13.2.1.2 of the Hermes 1 SE are applicable to Hermes 2. Therefore, this section incorporates by reference Section 13.2.1.2 of the Hermes 1 SE.
13.2.1.3 Technical Evaluation The staff reviewed Section 13.2.1 of the Hermes 2 PSAR and compared it to the equivalent section in the Hermes 1 PSAR (Section 13.2.1, Maximum Hypothetical Accident). The staff found that Section 13.2.1 of the Hermes 2 PSAR contains information consistent with that in the Hermes 1 PSAR, except for a minor change, which is evaluated below. The staff also verified that the Hermes 2 MHA remains identical to Hermes 1. Based on these consistencies, this section incorporates by reference Section 13.2.1.3 of the Hermes 1 SE.
13.2.1.3.1 Minor Change Compared to the Hermes 1 PSAR The minor change in Hermes 2 PSAR Section 13.2.1, as compared to the information in Hermes 1 PSAR Section 13.2.1, includes the following:
x The MHA is applicable to each Hermes 2 unit separately.
In Hermes 2 PSAR Section 13.2.1, Kairos did not identify changes to the MHA compared to Hermes 1 PSAR Section 13.2.1; however, accidents for each unit need to be addressed. As discussed in the introduction to Hermes 2 PSAR Chapter 13, the two units are independent to the extent that a postulated accident in one unit does not affect the safety of operation of the other unit. Therefore, in accordance with the requirements in 10 CFR 100.11(b), the MHA and postulated events are evaluated against the siting criteria for each unit separately. The staff
reviewed the description of the Hermes 2 MHA analysis methods, inputs, and assumptions and determined they are same as in the Hermes 1 PSAR, and similarly finds that Kaiross MHA analysis assumptions and methods are consistent with the approved methodology in topical report KP-TR-012-NP-A.
On the basis of its review, the staff finds that the level of detail provided regarding the Hermes 2 evaluation of the radiological consequences of the MHA demonstrates an adequate basis for a preliminary design and satisfies the applicable acceptance criteria in NUREG-1537, Parts 1 and 2, Chapter 13 to ensure that the radiation dose requirements of 10 CFR 100.11 are met.
13.2.1.4 Conclusion Based on the staff findings above, and as incorporated by reference from the Hermes 1 SE, the staff concludes that the preliminary analysis of the radiological consequences of the MHA, as described in PSAR Chapter 13 and confirmed during the staffs audit of supporting documents, is sufficient and meets the applicable regulatory requirements identified in this section for the issuance of CPs in accordance with 10 CFR 50.35 and 10 CFR 50.40. Additionally, the staff concludes that the discussion of design bases and preliminary design for control room radiological habitability, as described in Hermes 2 PSAR Sections 3.1, Introduction, and 7.4, Main Control Room and Remote Onsite Shutdown Panel, is sufficient and meets the applicable regulatory requirements and guidance identified in this section for the issuance of CPs in accordance with 10 CFR 50.35 and 10 CFR 50.40. Additionally, the staff concludes that the discussion of design bases and preliminary design for control room radiological habitability, as described in Hermes 2 PSAR Sections 3.1, Introduction, and 7.4, Main Control Room and Remote Onsite Shutdown Panel, is sufficient and meets the applicable regulatory requirements and guidance identified in this section for the issuance of CPs in accordance with 10 CFR 50.35 and 10 CFR 50.40. Further technical or design information required to complete the safety analysis may reasonably be left for later consideration. The staff will confirm that the final design conforms to the design basis during the evaluation of the FSAR as part of the OL application review.
13.2.2 Postulated Event Methodology 13.2.2.1 Introduction Section 13.2.2, Postulated Event Methodology and Sample Results, of the Hermes 2 PSAR describes Kaiross use of KP-TR-022-NP to derive the FOMs and associated acceptance criteria that ensure the radiological consequences of the MHA bound those of the postulated events described in PSAR Chapter 13. PSAR Table 13.1-1, Acceptance Criteria for Figures of Merit, provides the FOMs and acceptance criteria for each postulated event category. For each postulated event, the results of the transient analysis are to be compared to each relevant FOM and associated acceptance criterion derived from the MHA analysis.
The referenced technical report, KP-TR-022-NP, describes the postulated events; transient analysis methods, including evaluation models; and the methodology to be used to ensure that the final design features of the Hermes 2 facility are sufficient to mitigate the effects of the postulated events and keep the potential consequences of the events bounded by the MHA.
The technical report summarizes the MHA only to provide context for the derivation of the FOMs for the postulated events.
13.2.2.2 Regulatory Evaluation The staff reviewed Section 13.2.2.2, Regulatory Evaluation, of the Hermes 1 SE for applicability to the Hermes 2 SE. Based on the similarities between the Hermes 1 and Hermes 2 facility designs and the consistency of the postulated event methodology between Hermes 1 and Hermes 2, the staff finds that the regulations and guidance listed in Section 13.2.2.2 of the Hermes 1 SE are applicable to Hermes 2. Therefore, this section incorporates by reference Section 13.2.2.2 of the Hermes 1 SE.
13.2.2.3 Technical Evaluation The staff reviewed Section 13.2.2 of the Hermes 2 PSAR and compared it to the equivalent section in the Hermes 1 PSAR (Section 13.2.2, Postulated Event Methodology and Sample Results). The staff found that Section 13.2.2 of the Hermes 2 PSAR contains information consistent with that in the Hermes 1 PSAR, except for minor changes, which are evaluated below. The staff also verified that the Hermes 2 FOMs and the associated acceptance criteria remain identical to Hermes 1. Based on these consistencies, this section incorporates by reference Section 13.2.2.3 of the Hermes 1 SE.
13.2.2.3.1 Minor Changes Compared to the Hermes 1 PSAR The minor changes in Hermes 2 PSAR Section 13.2.2, as compared to the information in Hermes 1 PSAR Section 13.2.2, include the following:
x PSAR Section 13.3, References, cites technical report KP-TR-022. Technical report KP-TR-018 was cited for Hermes 1.
x PSAR Table 13.1-1 was modified to account for MAR for events that include releases from the IHTS and PGS.
In Hermes 2 PSAR Section 13.3, Kairos cites technical report KP-TR-022 instead of technical report KP-TR-018 that was cited in Section 13.3 of the Hermes 1 PSAR. KP-TR-022 updates the information previously provided in KP-TR-018 with additions to the postulated event methodology to account for the Hermes 2 design, including the added systems and the two units of the facility. To account for these design differences, KP-TR-022 includes an additional postulated event category for the IHX tube break and places other IHTS and PGS failure events within the categories that already existed in KP-TR-018. As provided in Table 3-2, Derived Figures of Merit and Acceptance Criteria for Postulated Events, of KP-TR-022, the methodology did not identify new derived FOMs or acceptance criteria and included the new IHX tube break and releases from the IHTS and PGS as applicable events for relevant FOMs.
The staff used KP-TR-022 in its review of PSAR Chapter 13. Through comparison of the information in the Hermes 2 PSAR and KP-TR-022 Section 1.1, as verified in the general audit (MLxxxxxxxxx), the staff finds that the key design features of the Hermes 2 facility are consistent with those in KP-TR-022. Therefore, the postulated event methodology, including the transient analysis methods in KP-TR-022, is applicable to Hermes 2, and the citation of KP-TR-022 instead of KP-TR-018 is acceptable.
In addition, Hermes 2 PSAR Table 13.1-1 was modified to account for MAR for events that include releases from the IHTS and PGS. This modification appropriately accounts for the additional locations of MAR and the pathways for their release in the Hermes 2 design compared to the Hermes 1 design. The changes to Hermes 2 PSAR Table 13.1-1 are consistent
with the changes to Table 3-2 of KP-TR-022. Based on the above, the staff finds that the changes to Hermes 2 PSAR Table 13.1-1 are acceptable.
On the basis of its review, the staff finds that the level of detail provided regarding the Hermes 2 postulated event evaluation methodology demonstrates an adequate basis for a preliminary design and satisfies the applicable acceptance criteria of NUREG-1537, Parts 1 and 2, Chapter 13, to ensure that final design features are sufficient to mitigate the effects of the postulated events and keep the potential consequences of the events bounded by the MHA.
13.2.2.4 Conclusion Based on the staff findings above, and as incorporated by reference from the Hermes 1 SE, the staff concludes that the methodology used to show that the radiological consequences of the postulated events are bounded by the MHA analysis is sufficient and that the methodology meets the applicable regulatory requirements and guidance identified in this section for the issuance of CPs in accordance with 10 CFR 50.35 and 10 CFR 50.40. Further technical or design information required to complete the safety analysis may reasonably be left for later consideration. The staff will confirm that the final design conforms to the design basis during the evaluation of the FSAR as part of the OL application review.
13.3 Summary and Conclusions on Accident Analyses The staff evaluated the information regarding the Hermes 2 accident analysis described in PSAR Chapter 13 and finds that the accident analysis of the preliminary design, including the PDC, design bases, information relative to materials of construction, general arrangement, and approximate dimensions, as well as the preliminary analysis and evaluation of the design and performance of SSCs of the facility: (1) provides reasonable assurance that the final design will conform to the design basis, (2) meets all applicable regulatory requirements, and (3) meets all applicable acceptance criteria discussed in NUREG-1537. Based on these findings, the staff concludes the following regarding the issuance of CPs in accordance with 10 CFR 50.35 and 10 CFR 50.40:
x Kairos described the proposed design of the systems supporting the accident analysis, including, but not limited to, the principal architectural and engineering criteria for the design, and has identified the major features or components incorporated therein for the protection of the health and safety of the public.
x Such further technical or design information as may be required to complete the safety analysis, and which can reasonably be left for later consideration, will be supplied in the FSAR as part of the OL application.
x There is reasonable assurance that, taking into consideration the site criteria contained in 10 CFR Part 100, the proposed facility can be constructed and operated at the proposed location without undue risk to the health and safety of the public.
x There is reasonable assurance that: (i) the construction of the facility will not endanger the health and safety of the public, and (ii) construction activities can be conducted in compliance with the Commissions regulations.
x The issuance of a permit for the construction of the facility would not be inimical to the common defense and security or to the health and safety of the public.
13.4 References Argonne National Lab, Development of a Reference Model for Molten-Salt-Cooled Pebble-Bed Reactor Using SAM, ANL/NSE-20/31. September 2020.
Electric Power Research Institute (EPRI). Uranium Oxycarbide (UCO) Tristructural (TRISO)-
Coated Particle Fuel Performance. Topical Report EPRI-AR-1(NP)-A. EPRI Report
- 3002019978. 2020. ML20336A052.
Fauske & Associates, Inc., Industry Degraded Core Rulemaking (IDCOR) Program Report, Technical Report 11.7: FAI Aerosol Correlation, March 1985, ML20214T484.
Kairos Power LLC, KP-TR-012-NP-A, KP-FHR Mechanistic Source Term Methodology Topical Report, Revision 3, March 2022. ML22136A291 (redacted version).
. KP-TR-011-NP-A, Fuel Qualification Methodology for the Kairos Power Fluoride Salt-Cooled High Temperature Reactor (KP-FHR), Revision 2, June 2022, ML23089A398 (redacted version).
. Response to NRC Request for Additional Information 348 and 339, August 31, 2022, ML22243A247.
. Transmittal of Response to NRC Question on Salt Spill from Chapters 4 and 13 Audit on Preliminary Safety Analysis Report, August 31, 2022, ML22243A254.
. Submittal of the Preliminary Safety Analysis Report for the Kairos Power Fluoride Salt-Cooled, High Temperature Non-Power Reactor (Hermes), Revision 1, September 29, 2022, Pkg. ML22272A593. (Includes KP-TR-017, KP-FRN Core Design and Analysis Methodology, Revision 1 (redacted version), as Enclosure 4.)
. Submittal of the Preliminary Safety Analysis Report for the Kairos Power Fluoride Salt-Cooled, High Temperature Non-Power Reactor (Hermes), Revision 2, February 24, 2023, Pkg.
ML23055A672. (Includes KP-TR-018, Postulated Event Analysis Methodology, Revision 2 (redacted version), as Enclosure 3.)
. Submittal of the Preliminary Safety Analysis Report for the Kairos Power Fluoride Salt-Cooled, High Temperature Non-Power Reactor (Hermes), Revision 3, May 31, 2023, Pkg.
. Submittal of the Construction Permit Application for the Hermes 2 Kairos Power Fluoride Salt-Cooled, High Temperature Non-Power Reactor, Revision 1, [Month] xx, 2024, Pkg.
ML24xxxxxxx. (Includes KP-TR-022, Hermes 2 Postulated Event Analysis Methodology, Revision 1 (redacted version), as Enclosure X.)
Ougouag, A., Ortensi, J., Hiruta, H., Analysis of an Earthquake-Initiated-Transient in a PBR, 2009 International Conference on Advances in Mathematics, Computation Methods, and Reactor Physics.
U.S. Nuclear Regulatory Commission (NRC). NUREG-1537, Part 1, Guidelines for Preparing and Reviewing Applications for the Licensing of Non-Power Reactors, Format and Content, February 1996, ML042430055.
. NUREG-1537, Part 2, Guidelines for Preparing and Reviewing Applications for the Licensing of Non-Power Reactors, Standard Review Plan and Acceptance Criteria, February 1996, ML042430048.
. Regulatory Guide 1.183, Alternative Radiological Source Terms for Evaluating Design Basis Accidents at Nuclear Power Reactors, July 2000, ML003716792.
. NUREG/CR-7220, SNAP/RADTRAD 4.0: Description of Models and Methods, June 2016, ML16160A019.
. SECY-18-0096, Functional Containment Performance Criteria for Non-Light-Water-Reactors, September 28, 2018, ML18114A546.
. NRC Final RAI 348 on Shutdown Rod Reliability and Defense in Depth. August 2, 2022, ML22227A180.
. Summary Report for the Regulatory Audit of Kairos Power LLC Hermes Construction Permit Preliminary Safety Analysis Report Chapters 4 and 13 (Accident Analysis), June 2023, ML23160A287.
X. Zhou, et. al., Oxidation Behavior of Matrix Graphite and Its Effect on Compressive Strength, Science and Technology of Nuclear Installations, August 2017.