ML24138A156
| ML24138A156 | |
| Person / Time | |
|---|---|
| Site: | 05000611, 05000612 |
| Issue date: | 05/17/2024 |
| From: | NRC |
| To: | NRC/NRR/DANU |
| References | |
| Download: ML24138A156 (16) | |
Text
From: Cayetano Santos Sent: Friday, May 17, 2024 9:48 AM To: Weidong Wang; Larry Burkhart Cc: Michael Orenak; Matthew Hiser; Brian Bettes; Josh Borromeo
Subject:
Preliminary Chapter 5 of Hermes 2 SE to ACRS Attachments: Chapter 05 - Reactor Coolant System.pdf
- Weidong,
Attached is another preliminary chapter (Chapter 5) 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.
Sixteen preliminary chapters were provided to you on March 14, April 4, April 22, April 23, May 10, and May 15, 2024. I will send the remaining preliminary chapter (13) to you after it receives the preliminary OGC review.
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: 18
Mail Envelope Properties (MW4PR09MB9010F15F8F3EE05ADE652D68E5EE2)
Subject:
Preliminary Chapter 5 of Hermes 2 SE to ACRS Sent Date: 5/17/2024 9:47:32 AM Received Date: 5/17/2024 9:47:00 AM 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 5923 5/17/2024 9:47:00 AM Chapter 05 - Reactor Coolant System.pdf 218471
Options Priority: Normal Return Notification: No Reply Requested: No Sensitivity: Normal Expiration Date:
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.
For the Hermes 2 test reactor facility, the reactor coolant system consists of the primary heat transport system (PHTS) that circulates coolant through the reactor core and an intermediate heat transport system (IHTS) that transfers the heat from the PHTS to the power generation system (PGS).
This chapter of the Kairos Power LLC (Kairos) Hermes 2 test reactor facility construction permit (CP) safety evaluation (SE) describes the U.S. Nuclear Regulatory Commission (NRC) staffs (the staffs) technical review and evaluation of the preliminary information regarding the Hermes 2 PHTS and IHTS. This information is presented in Chapter 5, Heat Transport Systems, of the Hermes 2 preliminary safety analysis report (PSAR), Revision 1. The staff reviewed PSAR Chapter 5 against applicable regulatory requirements using regulatory guidance and standards to assess the sufficiency of the preliminary information Kairos provided regarding the Hermes 2 facility PHTS and IHTS for the issuance of CPs in accordance with Title 10, Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing of Production and Utilization Facilities. As part of this review, the staff evaluated information on the Hermes 2 PHTS and IHTS, with special attention to design and operating characteristics, unusual or novel design features, and principal safety considerations. The staff evaluated the preliminary designs of the Hermes 2 PHTS and IHTS to ensure the design criteria, design bases, and information relative to construction are sufficient to provide reasonable assurance that the final designs will conform to the design basis. In addition, the staff reviewed Kaiross identification and justification for the selection of those variables, conditions, or other items which are determined to be probable subjects of technical specifications (TS) for the facility, with special attention given to those items which may significantly influence the final design.
5.1 Primary Heat Transport System
5.1.1 Introduction
Section 5.1, Primary Heat Transport System, of the Hermes 2 PSAR describes the PHTS, which transfers heat from the reactor core by circulating reactor coolant between the reactor core and the IHTS during normal operations. The PHTS includes a primary salt pump (PSP), an intermediate heat exchanger (IHX), a heat rejection subsystem (HRS), and associated piping.
The HRS includes a heat rejection radiator (HRR), heat rejection blower, and the associated ducting. The PHTS also includes thermal management features to maintain the reactor coolant in the liquid phase when the reactor core is not generating heat and the capability to drain external piping, the IHX, and the HRR to allow cooldown, inspection, and maintenance. The PHTS performs non-safety related functions as described in PSAR Section 5.1.1, Description.
The PHTS interfaces with the IHTS as described in Section 5.2, Intermediate Heat Transport System. The PHTS also interfaces with various other systems, including the reactor thermal management system, inert gas system (IGS), tritium management system (TMS), and inventory management system as described in PSAR Chapter 9, Auxiliary Systems.
5.1.2 Regulatory Evaluation
The staff reviewed Section 5.1.2, 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 PHTS design between Hermes 1 and Hermes 2, the staff finds that the regulations and guidance listed in Section 5.1.2 of the Hermes 1 SE are applicable to Hermes 2. Therefore, this section incorporates by reference Section 5.1.2 of the Hermes 1 SE.
5.1.3 Technical Evaluation
The staff reviewed Section 5.1 of the Hermes 2 PSAR and compared it to the equivalent section in the Hermes 1 PSAR (Section 5.1, Primary Heat Transport System). The staff found that Section 5.1 of the Hermes 2 PSAR contains information consistent with that in the Hermes 1 PSAR, except for several significant changes, which are evaluated below in SE Section 5.1.3.1.
The staff found that the following portions of Section 5.1 in the Hermes 2 PSAR contain information consistent with the Hermes 1 PSAR (e.g., minor or editorial changes only):
x Sections 5.1.1.1, Reactor Coolant, and 5.1.1.2, Primary Salt Pump x Sections 5.1.1.4, Primary Loop Piping, and 5.1.1.5, Primary Loop Thermal Management x Section 5.1.2, Design Basis x Section 5.1.4, Testing and Inspection
Since the Hermes 2 PHTS design and functionality largely remain identical, apart from the differences evaluated below, Section 5.1 of the Hermes 2 PSAR contains information consistent with Section 5.1 of the Hermes 1 PSAR. Based on these consistencies, this section incorporates by reference Section 5.1 of the Hermes 1 SE.
5.1.3.1 Significant Changes to the Hermes 1 PSAR
Significant changes contained in Section 5.1 of the Hermes 2 PSAR, as compared to Section 5.1 of the Hermes 1 PSAR, include information regarding the following:
x In Section 5.1.1, the IHX is discussed as the component that transfers heat from the Flibe in PHTS to an intermediate molten slat (BeNaF) in IHTS. The IHX design is described in Section 5.1.1.3, Intermediate Heat Exchanger.
x In Section 5.1.1.6, Heat Rejection Subsystem, the discussion is updated for the changes in the HRR operation and the related changes in the tritium management.
x Section 5.1.3, System Evaluation, discusses a postulated IHX tube failure.
x Section 5.1.3 states that tritium in the reactor coolant will normally permeate through the IHX and enter the IHTS.
The staff evaluated the sufficiency of this additional preliminary information regarding the Hermes 2 PHTS using the guidance and acceptance criteria from 10 CFR 50.34, 10 CFR 50.35, 10 CFR 50.40, and NUREG-1537, Part 2, Section 5.2, Primary Coolant System.
The design of Hermes 2 introduces an IHTS containing a different salt (BeNaF) for the intermediate coolant compared to the reactor coolant salt (Flibe). The IHX serves as the boundary between the reactor coolant and the intermediate coolant. In Table 3.6-1 of the Hermes 2 PSAR, the IHX, including the IHX tubes, are classified as non-safety related. 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).
Kairos confirmed in the response to request for confirmation of information (RCI) number 1 (ML24103A241) 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. Based on the information identified in RCI 1 to be provided in the OL application, the staff will assess the safety classification of the IHX as part of the OL application review.
The NRC staff reviewed the potential impact of an intermediate salt ingress into the reactor coolant because this event may impact the thermophysical and radionuclide retention properties of the reactor coolant. During the general audit (report found at MLxxxxxxxxx), the staff and Kairos discussed the consequences of BeNaF ingress into the reactor coolant. Kairos stated that any potential ingress scenarios would either introduce impurities within the allowable specification found in KP-TR-005-P-A, or that Kairos would provide justification for exceeding the purity specification at the time of the OL application. This approach was confirmed by Kairos in the response to RCI 2 (ML24103A241). Additionally, PSAR Section 5.1.1 states that the reactor coolant in the IHX is maintained at a higher pressure than the IHTS during normal operations. Therefore, during an IHX tube leak, the reactor coolant is driven into the IHTS and will maintain its specified chemistry within the PHTS. The staff finds the preliminary information related to potential ingress of the intermediate salt acceptable because either potential ingress scenarios will remain bounded by the reactor coolant specification, or Kairos will submit a justification for deviating from the reactor coolant specification. Additionally, the staff finds the preliminary information related to potential ingress of the intermediate salt is consistent with PDC 10, 16, and 60 because the reactor coolant will remain within its specifications for the thermophysical and radionuclide retention properties, or Kairos will justify why the exceedance of these specifications does not affect its performance. Further information related to potential ingress of the intermediate salt can reasonably be left for later consideration at the OL stage.
The NRC staff also reviewed the potential impact of an intermediate salt ingress into the reactor coolant on its compatibility with the structural materials. As discussed in Section 4.3, Reactor Vessel System, of this SE, Kairos will perform material compatibility testing with a postulated BeNaF ingress. The proposed limiting condition for operation (LCO) on reactor coolant chemistry proposed in Table 14.1-1, Proposed Variables and Conditions for Technical Specifications, of the Hermes 2 PSAR also helps to ensure that the reactor coolant will remain within its specification during a IHX tube break event as the LCO would require facility actions to correct the reactor coolant chemistry if the intermediate salt infiltration exceeds the specified limit. The proposed material compatibility testing is consistent with PDCs 14 and 31 because the effect of intermediate salt ingress on corrosion of safety-related components will be assessed.
The proposed testing is further evaluated in Section 4.3 of this SE. The preliminary information provided by Kairos in PSAR Section 4.3 is consistent with PDC 70 because material compatibility testing will determine the reactor coolant purity necessary to mitigate chemical attack on safety-related metallic materials in the PHTS. Additionally, the chemistry control system (CCS) will be able to monitor and adjust Flibe purity as described in PSAR Section 9.1.1 and evaluated in Chapter 9 of this SE. This function of the CCS is consistent with NUREG-1537, Part 2, Section 5.2, because there are means to maintain coolant chemistry and quality to limit corrosion of the fuel components, control rod cladding, vessel material, and other essential components in the primary system.
PSAR Section 5.1.3 states that a postulated IHX tube failure could cause the reactor coolant to leak into intermediate coolant since the reactor coolant is maintained at a higher pressure than the intermediate coolant. PSAR Section 5.1.3 states that the loss of reactor coolant inventory would be detected by the inventory management system discussed in PSAR Section 9.1.4 and by the detection capability in the IHTS. Additionally, Section 3.2.2.7, Intermediate Heat Exchanger Tube Break, of the technical report KP-TR-022-NP, Postulated Event Analysis Methodology, states that the quantity of reactor coolant assumed to flow into the IHTS is the same or bounded by the volume of reactor coolant spilled during a postulated pipe break. PSAR Section 5.1.3 also states that the compatibility of the reactor coolant to intermediate coolant interaction will be demonstrated as part of an OL application as it is noted as one of the planned research activities in Hermes 2 PSAR Section 1.3.9, Research and Development. The staff also observed that the anti-siphon design features on the hot and cold legs should maintain adequate reactor coolant inventory and mitigate any breaks outside the vessel. Accordingly, the staff finds that the preliminary design of the Hermes 2 PHTS is consistent with PDC 33 to maintain reactor coolant inventory to protect against small breaks in the safety related elements of the reactor coolant boundary. The staff also finds that this preliminary information for the PHTS design is consistent with the relevant guidance in NUREG-1537, Part 2, Section 5.2, that the primary coolant system (of a forced-convection coolant flow) should be designed to prevent coolant loss.
PSAR Section 5.1.1.6 states that during normal plant operations, the tritium permeating through the HRR is captured by a subsystem of the TMS that is discussed in PSAR Section 9.1.3.
During startup and normal shutdown conditions, tritium permeation losses through the HRR are released through the HRS as a gaseous effluent. Also, tritium in the reactor coolant will permeate through the IHX heat transfer surfaces and enter the intermediate coolant; tritium management in the IHTS is evaluated in Section 5.2.3 of this SE. As stated in PSAR Section 11.1.5, Radiation Exposure Control and Dosimetry, facility effluents are monitored for radioactivity during normal operations and postulated events, and facility SSCs are designed to limit uncontrolled gaseous effluent releases to work areas or the environment, consistent with the goal of maintaining radiation exposures as low as reasonably achievable. Accordingly, the staff finds that the preliminary design of the Hermes 2 PHTS is consistent with PDC 60 to support the control of radioactive materials during normal reactor operation.
The NRC staff reviewed the potential impact of feedback effect from the IHTS loop on the reactor power oscillations to evaluate compliance with PDC 12. The staff finds that the Hermes 2 design features, such as the small core height and diameter and long neutron diffusion length, limit the flow- and inlet temperature-induced power oscillations. Furthermore, the staff also finds that high coolant thermal inertia, the TS LCO that limits air in the reactor coolant, and the ability of instrumentation and control system to detect and suppress temperature and mass flow oscillations are important for suppression and/or prevention of power oscillations. Based on these inherent features and the proposed TS LCO, the staff finds that the preliminary design of the Hermes 2 PHTS is consistent with PDC 12.
On the basis of its review, the staff finds that the primary coolant system design will be able to accomplish the design functions of fuel integrity and sufficient heat removal, coolant loss prevention, conversion to passive natural-convection flow, limited corrosion of essential components, and sufficient radiation shielding for limiting personnel exposures. The staff finds that the preliminary information provided for the PHTS is adequate at this stage of the design and is consistent with PDCs 2, 10, 12, 16, 33, 60, and 70 and with the acceptance criteria of NUREG-1537, Part 2, Section 5.2.
5.1.4 Conclusion
Based on its findings above, the staff concludes the information in Hermes 2 PSAR Section 5.1, as supplemented by the responses to RCI 1 and RCI 2 (ML24103A241), 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 Hermes 2 PHTS can reasonably be left for later consideration at the OL stage since the information is not necessary for the review of a CP application.
5.2 Intermediate Heat Transport System
5.2.1 Introduction
Section 5.2 of the Hermes 2 PSAR describes the IHTS. The IHTS includes two intermediate salt pumps (ISPs), an intermediate heat exchanger (IHX), two intermediate salt vessels (ISVs), a superheater, and the associated piping. It transfers heat from the PHTS to the PGS by circulating intermediate coolant between the IHX and the superheater during normal operations.
The IHTS contains several subsystems including intermediate coolant inventory management subsystem, intermediate inert gas subsystem, intermediate coolant chemistry control subsystem, and intermediate loop auxiliary heating subsystem. The IHTS performs non-safety related functions as described in PSAR Section 5.2.1, Description. The IHTS interfaces with the PHTS and PGS as described in Sections 5.1 and 9.9, Power Generation System, respectively. The IHTS also interfaces with the tritium management system as described in Section 9.3.1, Tritium Management System.
5.2.2 Regulatory Evaluation
The applicable regulatory requirements for the evaluation of the Hermes 2 non-power test reactor IHTS design criteria are as follows:
x 10 CFR 50.34, Contents of applications; technical information, paragraph (a),
Preliminary safety analysis report.
o 10 CFR 50.34(a)(3)(ii) requires The design bases and the relation of the design bases to the principal design criteria.
o 10 CFR 50.34(a)(3)(iii) requires Information relative to materials of construction, general arrangement, and approximate dimensions, sufficient to provide reasonable assurance that the final design will conform to the design bases with adequate margin for safety.
o 10 CFR 50.34(a)(4) which requires A preliminary analysis and evaluation of the design and performance of structures, systems, and components [SSCs] of the facility x 10 CFR 50.35, Issuance of construction permits.
x 10 CFR 50.40, Common standards.
x 10 CFR 20.1406, Minimization of contamination.
The applicable guidance for the evaluation of Hermes 2 IHTS is as follows:
x NUREG-1537, Guidelines for Preparing and Reviewing Applications for the Licensing of Non-Power Reactors, Part 1, Format and Content, and Part 2, Standard Review Plan and Acceptance Criteria, Chapter 5, Reactor Coolant Systems. Based on the role of the IHTS in the Hermes 2 design, the staff evaluated the system using the applicable acceptance criteria in Section 5.3, Secondary Coolant System for a non-light water reactor.
5.2.3 Technical Evaluation
PSAR Section 3.1.1, Design Criteria, describes the principal design criteria (PDC) that are applicable to the Hermes 2 reactor. These PDC were reviewed and approved by the staff in KP-TR-003-NP-A, Revision 1, Principal Design Criteria for the Kairos Power Fluoride Salt-Cooled, High Temperature Reactor. PSAR Section 5.2.2, Design Basis, identified the design bases for the IHTS. The PSAR states that the following PDCs are applicable to the IHTS:
x PDC 2, Design bases for protection against natural phenomena, which requires safety related SSCs be designed to withstand the effects of natural phenomena.
x PDC 60, Control of releases of radioactive materials to the environment, which requires the plant design to control the release of radioactive materials, including during postulated events.
x PDC 64, Monitoring radioactivity releases, which requires means to monitor for radioactivity that may be released during operations, AOOs, and postulated accidents.
x PDC 73, Reactor coolant system interfaces, which requires two passive barriers between the reactor coolant and any chemically incompatible fluid, or one passive barrier between the reactor coolant and a chemically compatible fluid provided postulated leakage doesnt result in SR SSCs failing to perform their safety functions or result in exceeding specified acceptable system radionuclide release design limits.
PSAR Section 5.2.3, System Evaluation, relates the design bases to the design criteria and identifies how the IHTS satisfies the PDC applicable to the design of the IHTS. In the following paragraphs, the staff addresses each PDC by summarizing the information presented in the PSAR and explaining the staff evaluation of the adequacy of the preliminary information in the Kairos PSAR relative to NUREG-1537, Part 2, Section 5.3, acceptance criteria.
5.2.3.1 PDC 2, Design bases for protection against natural phenomena
PSAR Section 5.2.3 states that the design of non-safety related IHTS SSCs is such that a failure of IHTS SSCs would not affect the performance of safety-related SSCs due to a design basis earthquake (DBE). The PSAR also states that the safety-related SSCs will be protected by either seismically mounting the relevant IHTS components, assuring sufficient physical separation from the IHTS components, or by the erection of protective barriers between the IHTS components and the safety-related SSCs to preclude any adverse interactions. Further, the PSAR states that the portions of the IHTS that cross the isolation moat around the safety-related reactor building are designed to accommodate differential displacement due to a DBE as discussed in PSAR Section 3.5. PSAR Section 3.5 states that this design feature minimizes the stresses on the components that cross the isolation moat and reduces the likelihood of their failure that would adversely affect the ability of safety-related SSCs during the DBE. Furthermore, as evaluated in Section 13.1.4 of this SE, a postulated loss of normal heat sink due to failure of an IHTS component (e.g., ISP failure) would not lead to inadequate heat removal because the safety-related decay heat removal system (DHRS) or parasitic heat loss provides sufficient residual heat removal.
A novel safety-related component in the Hermes 2 design is the rupture disks of the intermediate inert gas system. PSAR Section 5.2.1.2 states that these rupture disks preclude a gross failure of the IHX that could occur as a result of a postulated superheater tube leak or rupture event by relieving pressure in the IHTS and providing a steam relief path. These disks are in the gas space of the ISVs and, accordingly, are connected to the non-safety related IHTS.
PSAR Table 3.6-1 shows that the rupture disks are seismically designed to the local building code despite being considered safety-related. The staff finds this acceptable because as stated in Note 6 of Table 3.6-1, the IHTS SSCs are not relied upon to maintain their structural integrity during and following a DBE that may result in the loss of the IHTS pressure boundary. The loss of the pressure boundary during a DBE removes the need for the safety function of the IHTS rupture disks. Therefore, the IHTS rupture disks are not required to perform their safety function during a seismic event and the staff finds the preliminary seismic design of the safety-related rupture disks is consistent with PDC 2.
Because failures in the IHTS would not affect the ability of safety-related SSCs to perform their safety function, the staff finds that the preliminary design of the IHTS is consistent with PDC 2.
The staff also finds that the preliminary design of the IHTS is consistent with the guidance provided in NUREG-1537, Part 2, Section 5.3, that requires staff to ensure that the malfunction in secondary coolant system will not lead to reactor damage, fuel failure, or uncontrolled release of radioactivity to the environment.
5.2.3.2 PDC 60, Control of releases of radioactive materials to the environment
PSAR Section 5.2.3 states that tritium will be present in the IHTS as it will diffuse through the IHX during normal operations. Kairos stated that anhydrous hydrogen fluoride will be used to convert tritium to the gaseous phase that will result in the tritium being removed from the IHTS by the TMS via the intermediate IGS.
The staff evaluated the ability of the IHTS to control tritium during normal operations against PDC 60. The staff finds the preliminary information provided in the PSAR consistent with PDC 60 because the IHTS has ability to control speciation of tritium to guide it to the tritium management system (TMS) for capture and removal. The radiation monitoring in the IHTS cover gas space will also provide assurance that tritium is being removed from the IHTS salt and transferred to the TMS via the intermediate IGS. The staff evaluation of the TMS and its ability to process and remove tritium is found in Section 9.1.3 of this SE. Additionally, as described in PSAR Chapter 14, there are proposed technical specifications to monitor the activity in the IHTS. This provides additional assurance that measures to control tritium in the IHTS will be in place to demonstrate that the final design of the IHTS will be consistent with PDC 60.
5.2.3.3 PDC 64, Monitoring radioactivity releases
The PSAR states that radiation monitoring is provided in the ISV cover gas space to monitor the radioactive material releases that might occur in the IHTS as result of an IHTS SSC failure. The NRC staff finds this preliminary information consistent with PDC 64, which requires a means to monitor for radioactivity that may be released during operations, AOOs, or postulated accidents Further information can reasonably be left for later consideration at the OL stage.
5.2.3.4 PDC 73, Reactor coolant system interfaces
PSAR Section 5.2.3 states that the Flibe primary reactor coolant is separated from water by two passive barriers, the IHX and the superheater. In addition, the Section 5.2.3 states that the PHTS is maintained at a higher pressure than the IHTS, so a leak or failure in the IHX could cause the IHTS coolant to be contaminated with Flibe. The PSAR also states that the reactor coolant and intermediate coolant are chemically compatible. In Section 1.3.9, Research and Development, of the Hermes 2 PSAR Kairos states that completing compatibility evaluations of the intermediate coolant and reactor coolant chemical interaction is a research and development item to confirm the adequacy of the design. Consistent with similar findings in Sections 3.1 and 5.1 of this SE, the staff finds that Kaiross plan to address coolant compatibility in the OL application can be reasonably left for later consideration. Therefore, the staff finds that the preliminary information for the IHTS design is consistent with PDC 73 for coolant compatibility and is acceptable for a preliminary design. The staff will confirm at the OL stage that the final design for the IHTS meets PDC 73 for coolant compatibility by demonstrating compatibility between the primary and intermediate coolants and that the intermediate coolant does not have a safety significant impact on the primary system.
An important postulated event for the IHTS design is a postulated superheater tube leak or rupture. The PSAR states that the IHTS includes safety-related rupture disks to mitigate the effects of a postulated superheater tube leak or rupture by relieving pressure in the IHTS and providing a relief path for the steam to prevent a gross failure of the IHX. PSAR Section 13.1.10.11 states that the rupture disks prevent significant Flibe-water interaction in the PHTS that could result from a gross failure of the IHX due to steam over-pressurization of the IHTS during a postulated superheater tube leak or rupture event. The staff finding related to the safety classification of the IHX, including the IHX tubes that are the boundary between the PHTS and IHTS, can be found in Sections 3.6.3.2 and 5.1.3 of this SE.
For the safety-related rupture disks, Section 5.2.1.2, Intermediate Inert Gas Subsystem, of the Hermes 2 PSAR states that the rupture disks are located in the gas space above the ISVs. The rupture disks will be made of 316H stainless steel with a maximum temperature during normal operations less than the maximum IHTS hot leg temperature (580 - 615 °C). The rupture disks will be designed in accordance with the 2017 version of American Society of Mechanical Engineers Boiler and Pressure Vessel Code,Section VIII, Rules for Construction of Pressure Vessels, Division 2, Alternative Rules, (Section VIII, Division 2). The PSAR states that Section VIII, Division 2, is more appropriate for the design and construction of the rupture disks than American Society of Mechanical Engineers Boiler and Pressure Vessel Code,Section III, Rules for Construction of Nuclear Facility Components, Division 5, High Temperature Reactors, (Section III, Division 5) due to the absence of both a high irradiation environment and the need to maintain a safety-related pressure boundary. In addition, the rupture disks provide pressure relief for the IHTS, which is being designed to Section VIII, Division 2, as well.
The staff observed that all other safety-related SSCs in Hermes 2 will be designed in accordance with Section III, Division 5, which is endorsed by NRC staff for use in RG 1.87, Acceptability of ASME Code Section III, Division 5, High Temperature Reactors', Revision 2.
The staff engaged in discussions with Kairos in the Hermes 2 general audit (MLxxxxxxxxx) to understand the key differences between the use of Section VIII, Division 2 compared to Section III, Division 5, that may impact rupture disk reliability and ability to perform its safety function.
The staff identified that 316H stainless steel is an approved material for high temperature applications within Section III, Division 5. Kairos provided supplemental information related to the design of the rupture disks in a submittal dated May xx, 2024 (MLxxxxxxxxx) and confirmed their final design considerations in the response to RCI 3 (ML24135A382).
In the supplemental information, Kairos compared Section VIII, Division 2, and Section III, Division 5, provisions with respect to rupture disk design and observed the similarities between Section III, Division 5, and Section VIII, Division 2, for the rupture disk design. Further, in the response to RCI 3, Kairos confirmed that the final design of the IHTS and the rupture disks will justify that the overpressure protection safety function will be performed reliably with consideration for the system design, location of the rupture disks, operating environment, material aging or degradation due to environmental effects, potential salt vapor deposition impeding rupture disk function, and other design considerations such as redundancy and independence. The staff observes that the supplemental information on the similarities between Section VIII, Division 2, and Section III, Division 5, provisions with respect to rupture disk design and the additional information confirmed through RCI 3 provide a reasonable basis for developing the final design of the safety-related rupture disks for a non-power reactor. In addition, Table 14.1-1 of the Hermes 2 PSAR identifies a probable TS LCO for IHTS pressure relief device operability, which provides additional assurance that provisions to identify conditions that may challenge the rupture disks ability to perform their safety function will be implemented. Based on the information provided in the PSAR (including the use of 316H for the rupture disk material), the supplemental information, the RCI response, the proposed TS LCO, and Hermes 2s status as a non-power reactor, the staff finds the preliminary information related to the design of the safety-related IHTS rupture disks to perform the safety function to mitigate the effects of a postulated superheater tube leak or tube rupture event and prevent water-Flibe interaction is acceptable and further information can reasonably be left for later consideration at the OL stage.
Based on the findings above related to the coolant compatibility and the design of the rupture disks, the staff finds that the preliminary design information for the IHTS is consistent with PDC
- 73. Further information on the IHTS design related to PDC 73 can reasonably be left for later consideration at the OL stage.
5.2.3.5 10 CFR 20.1406, Minimization of contamination:
PSAR Section 5.2.3 states that the IHTS piping will be designed to the American Society of Mechanical Engineers (ASME) Standard B31.3, Process Piping, and the superheater and ISVs will be designed to ASME Boiler and Pressure Vessel Code,Section VIII. The intermediate coolant can become contaminated with tritium or other radioactive materials from a postulated leak from the PHTS into the IHTS through the IHX. The staff evaluation of the TMS, which is designed to capture and remove the tritium from the IHTS, is provided in Section 9.1.3 of this SE. As described in Section 5.2.3.2 of this SE, the IHTS is also equipped with the capability to monitor the ISV cover gas environment for radioactive material releases from breaks and leaks in the piping system or via pressure relief equipment. In addition, Table 14.1-1 of the Hermes 2 PSAR identifies a TS LCO that limits the quantities of MAR in the ISV cover gas space.
Based on the identification of the quality standards used to fabricate the IHTS, the radiation monitoring features of the IHTS, and the TMS to remove tritium from the IHTS, the staff finds the preliminary design of the IHTS is consistent with 10 CFR 20.1406. Further information on how the IHTS minimizes contamination and meets 10 CFR 20.1406 can reasonably be left for later consideration in the OL application.
5.2.3.6 Additional NUREG-1537 Criteria
As described in PSAR Section 5.2.1, the IHTS performs non-safety related functions to remove heat from the PHTS during normal operations. There are no safety-related heat removal functions specified for the IHTS in Section 5.2.1. Additionally, as described in PSAR Chapters 6 and 13, the DHRS is the only safety-related means to remove heat from the reactor vessel.
The NRC staff evaluated the preliminary IHTS design against applicable criteria in NUREG-1537, Part 2, Section 5.3. These criteria require a secondary coolant system to have the ability to remove heat from the primary coolant system, as needed, to maintain fuel integrity and to support any conditions analyzed in PSAR Chapters 6 or 13. The staff finds the preliminary information related to the IHTS design is consistent with the guidance from NUREG-1537, Part 2, Section 5.3, because the Hermes 2 design does not rely on the IHTS for any safety-related heat removal. Further information regarding the need for heat removal capability by the IHTS can reasonably be left for later consideration at the OL stage and the staff will evaluate the final design to ensure that increases or decreases in heat removal rates from the IHTS are bound by Chapter 13 analyses and will not impact fuel integrity.
PSAR Section 5.2.1.4, Intermediate Coolant Chemistry Control Subsystem, describes the chemistry control system for the IHTS. Additionally, PSAR Chapter 14 indicates a proposed TS LCO that would limit the quantity of water in the IHTS to limit corrosion. The NRC staff evaluated the preliminary design of the IHTS against applicable criteria in NUREG-1537, Part 2, Section 5.3, which require a secondary coolant system to provide any necessary chemistry control to limit corrosion or degradation of the heat exchanger. The NRC staff finds the preliminary information consistent with this guidance because the IHTS has a chemistry control system and there is a proposed TS LCO for the allowed amount of water in the IHTS to limit corrosion.
As described in PSAR Section 5.2.1.5, the ISPs provide motive force for the circulation of intermediate salt in the IHTS. Hermes 2 PSAR Section 7.3, Reactor Protection System, adds a safety-related ISP trip resulting from RPS actuation. To limit overcooling during a postulated event, the ISP trips concurrently with the PSP and interlocks prevent the starting of the ISP with a secured PSP. As evaluated in Sections 13.1.2 and 13.1.4 of this SE, the ISP trip plays critical role in mitigating the events initiated in the IHTS and PGS by significantly reducing the heat transfer between the PHTS, IHTS, and PGS. The ISP trip that reduces heat transfer between the PHTS, IHTS, and PGS provides additional assurance that the consequences of postulated events initiated from the IHTS and PGS remain within the bounds of the MHA. Therefore, the staff finds that the preliminary design of the IHTS is consistent with the guidance in NUREG-1537, Part 2, Section 5.3, that states that secondary coolant system should include necessary instrumentation and control functions and be designed to respond as necessary to postulated events.
5.2.4 Conclusion
Based on its findings above, the staff concludes the information in Hermes 2 PSAR Section 5.2, as supplemented by the submittal dated May xx, 2024 (MLXXXXXXX) and response to RCI 3 (ML24135A382), 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 Hermes 2 IHTS can reasonably be left for later consideration at the OL stage since the information is not necessary for the review of a CP application.
5.3 Summary and Conclusions on the Reactor Coolant System
The staff evaluated the information on the Hermes 2 reactor coolant system as described in PSAR Chapter 5, as supplemented, and finds that the preliminary information on, and design criteria of, the reactor coolant system, including the PDC, design bases, and information relating to materials of construction, general arrangement, and approximate dimensions: (1) provide reasonable assurance that the final design will conform to the design bases, (2) meet all applicable regulatory requirements, and (3) meet the applicable acceptance criteria in NUREG-1537, Part 2. Based on these findings, the staff makes the following conclusions regarding issuance of CPs in accordance with 10 CFR 50.35 and 10 CFR 50.40:
x Kairos has described the proposed design of the reactor coolant system, including, but not limited to, the principal 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 of the reactor coolant system, and which can reasonably be left for later consideration, will be provided in the FSAR.
x Safety features or components which require research and development have been described by Kairos and a research and development program (see SE Section 1.1.5 that reviews PSAR Section 1.3.9) will be conducted that is reasonably designed to resolve any safety questions associated with such features or components.
x There is reasonable assurance that safety questions will be satisfactorily resolved at or before the latest date stated in the application for completion of construction of the proposed facility.
x There is reasonable assurance: (i) that the construction of the facility will not endanger the health and safety of the public, and (ii) that construction activities can be conducted in compliance with the Commissions regulations.
x The issuance of a permit for the construction of the Hermes 2 facility would not be inimical to the common defense and security or to the health and safety of the public.
5.4 References
American Society of Mechanical Engineers (ASME) B31.3, Process Piping. ASME: Two Park Avenue, New York, NY. June 2021
.Section VIII. BPVC Section VIII-Rules for Construction of Pressure Vessels Division 1.
ASME: Two Park Avenue, New York, NY. 2021
Kairos Power LLC. 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. ML23151A743.
. KP-TR-003-NP-A, "Principal Design Criteria for the Kairos Power Fluoride Salt-Cooled, High Temperature Reactor," Revision 1, June 2020, ML20167A174.
. Enclosure 2: Response to NRC Request for Additional Information 350, September 2022, ML22251A400 (redacted version).
. Reactor Coolant for the Kairos Power Fluoride Salt-Cooled High Temperature Reactor Topical Report. KP-TR-005-NP-A, Revision 1. July 2020. ML20219A591. (redacted version).
. Mechanistic Source Term Methodology Topical Report. KP-TR-012-NP-A, Revision 3.
March 2022. ML22136A2918 (redacted version).
. Metallic Materials Qualification for the Kairos Power Fluoride Salt-Cooled High-Temperature Reactor. KP-TR-013-NP, Revision 4. September 2022. ML22263A456. (redacted version).
. Graphite Material Qualification for the Kairos Power Fluoride Salt-Cooled High-Temperature Reactor. KP-TR-014-NP, Revision 4. September 2022. ML22259A142 (redacted version).
. Kairos Power, LLC, Response to NRC Request for Confirmation of Information for the Review of the Hermes 2 PSAR Chapter 5, April 12, 2024, ADAMS Pkg. ML24103A243.
. Kairos Power, LLC, Response to NRC Request for Confirmation of Information for the Review of the Hermes 2 PSAR Chapter 5, May 14, 2024, ADAMS Pkg. ML24135A382.
[Placeholder for Kairos supplement on rupture disks and Section VIII]
. Submittal of the Preliminary Safety Analysis Report for the Kairos Power Fluoride Salt-Cooled, High Temperature Non-Power Reactor (Hermes 2), Revision 1, [month] xx, 2024, ADAMS Pkg. MLxxxxxxxxx.
The U. S. Nuclear Regulatory Commission (NRC). NUREG-1537, Guidelines for Preparing and Reviewing Applications for the Licensing of Non-Power Reactors, Part 1, Format and Content, and Part 2, Standard Review Plan and Acceptance Criteria. NRC: Washington, D.C. February 1996. ADAMS Accession Nos. ML042430055 and ML042430048.
. 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, ADAMS No. ML042430048.
. Safety Evaluation for the Kairos Power LLC Construction Permit Application for the Hermes Non-Power Test Reactor, June 16, 2023, ADAMS Pkg. ML23158A265.
. Request for Confirmation of Information for Hermes 2 Preliminary Safety Analysis Report, April 12, 2024, ADAMS Pkg. ML24103A173.
. Summary Report for the Regulatory Audit of Kairos Power LLC Hermes 2 Construction Permit Preliminary Safety Analysis Report - General Audit, [month] xx, 2024, ADAMS No.
MLxxxxxxxxx.