ML24043A217

Hermes 2 General Audit Questions 5.2-10, 5.2-11, and 5.2-12

ML24043A217
Person / Time
Site:	Hermes  File:Kairos Power icon.png
Issue date:	02/12/2024
From:	Michael Orenak NRC/NRR/DANU/UAL1
To:	Gardner D, Peebles D Kairos Power
References
EPID L-2023-CPS-0000
Download: ML24043A217 (1)

Text

From: Michael Orenak To: Darrell Gardner; Drew Peebles Cc: Matthew Hiser; Pravin Sawant; William Jessup; Candace de Messieres

Subject:

Hermes 2 Audit Questions 5.2-10, 5.2-11, and 5.2-12 Date: Monday, February 12, 2024 4:04:00 PM

Darrell and Drew,

Below is the ninth set of audit questions for the Hermes 2 General Audit.The NRC staff would like to have a discussion regarding these questions when Kairos is ready. Kairos is welcome to post answers to these questions on the Kairos electronic reading room, but no written answers are requested by the NRC staff at this time.

In response to audit question 5.2-3 and during the presentation to the NRC staff on February 1, 2024, Kairos clarified that the intermediate heat transport system (IHTS) rupture disks are classified as safety-related to prevent Flibe-water interactions that could result from an intermediate heat exchanger (IHX) failure caused by a postulated superheater tube failure. Kairos also stated that Flibe-water interaction can lead to loss of functional containment capability of the Flibe. Furthermore, Kairos stated that by preventing IHX failure due to superheater tube rupture, the rupture disks are protecting the reactor vessel from potential overpressure failure; however, it was also mentioned that the reactor vessel would be protected by its own pressure relief system.

PSAR Section RequestRequest/Question Number 5.2, Intermediate 5.2-10 Section 13.1.10.11, IHX Failure Due to Heat Transport Superheater Tube Rupture or Leak, of the System preliminary safety analysis report (PSAR) notes that a superheater tube rupture could lead to overpressurization of the IHTS. The PSAR also notes that a safety-related pressure relief feature (i.e., rupture disks) would be used to preclude failure of the IHX due to the postulated superheater tube failures. Additional information regarding the rupture disks is discussed in PSAR Section 5.2, Intermediate Heat Transport System, of the PSAR.

a. The PSAR discussions are not clear aboutthe safety function of the rupture disks and

their adequacy for the mitigating the effects of a Flibe-water interaction without crediting functions performed by other structures, systems, and components (SSCs) in the IHTS. Provide additional details regarding the progression of a postulated superheater tube rupture or leak event, with a focus on how the intermediate salt vessels, intermediate inert gas system, and rupture disks are used to mitigate the mass and energy release associated with a superheater failure. Provide specific information regarding the potential for damage to the IHX or other components (e.g., Flibe-water interactions in the primary heat transport system) that could be incurred if the rupture disks fail to perform their intended functions.

b. Relatedly, discuss the basis for identifying the rupture disks as safety-related and discuss why all other SSCs in the IHTS have preliminarily been classified as nonsafety related. This discussion should focus on how the rupture disks would prevent or mitigate the consequences of accidents which could result in potential exposures exceeding the limits set forth in Title 10 of the Code of Federal Regulations, Section 100.11, Determination of exclusion area, low population zone, and population center distance.

5.2, Intermediate 5.2-11 During the audit discussion on February 1, 2024, Heat Transport Kairos indicated that Flibe-water interactions could System lead to a loss of functional containment capability of the Flibe when mixed with water.

a. Table 14.1-1 of the Hermes 2 PSAR states that limiting conditions for operation (LCOs) will be proposed to limit the quantity of water and Flibe in the intermediate coolant.

The proposed LCOs imply that there would be permissible amounts of Flibe and water that could exist simultaneously in the IHTS.

Discuss the interaction of Flibe and water within the intermediate coolant under normal IHTS operation.

b. The consequences of Flibe-water interaction from water entrained in the intermediate coolant during an IHX tube failure event are not discussed in the PSAR. Please discuss the mechanisms or processes that would lead to loss of functional containment capability of the Flibe if it mixes with water.

5.2, Intermediate 5.2-12 During the discussion of audit question 5.2-7 on Heat Transport the use of the overpressure protection provisions in System the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC),

Section III, Rules for Construction of Nuclear Facility Components, Division 5, High Temperature Reactors, for the safety-related rupture disks, Kairos clarified that Section VIII, Rules for Construction of Pressure Vessels, of the ASME BPVC will be used for the construction of all components of the IHTS, including the safety-related rupture disks.

a. In NRC Regulatory Guide 1.87, Acceptability of ASME Code Section III, Division 5, 'High Temperature Reactors',

Revision 2, the NRC staff endorsed Division 5 of the ASME BPVC,Section III, for use in the design, construction, testing, and quality assurance of safety-related SSCs for high temperature applications. The NRC staff has not provided a similar endorsement of ASME BPVC, SectionVIII, for use in safety-related applications. Please describe how the use of ASME BPVC,Section VIII, is adequate for the design and construction of the safetyrelated rupture disks.

b. Please clarify Section 5.2 of the PSAR to identify the construction code for the rupture disks, the materials of construction (grade of stainless steel) for the rupture disks, and whether the rupture disks will be included in the in-service testing program.

If you have any questions or need clarifications on the questions before the discussion, please do not hesitate to contact me. This email will be added to ADAMS and will be made public.

Mike