ML25293A456
| ML25293A456 | |
| Person / Time | |
|---|---|
| Site: | Kemmerer File:TerraPower icon.png |
| Issue date: | 10/20/2025 |
| From: | Reed Anzalone, Matthew Hiser, Bill Lin, Neller A Advisory Committee on Reactor Safeguards, NRC/NRR/DANU |
| To: | |
| References | |
| Download: ML25293A456 (1) | |
Text
NRC Review of Kemmerer Unit 1 Construction Permit Application Control of Heat Removal SSCs Reed Anzalone, Matt Hiser, Bruce Lin, Alec Neller NRR/DANU ACRS Subcommittee Meeting October 22-23, 2025
Topics
- Primary heat transport system (PHT)
- Intermediate heat transport system (IHT)
- SHX design and sodium-salt reaction R&D
- Reactor air cooling system (RAC)
- RAC-related R&D
- Intermediate air cooling system (IAC) 2
Primary Heat Transport System
- Transport heat from reactor core to IHT
- Key components:
- Intermediate Heat Exchanger (IHX)
- Safety significant functions
- DL3-HR1 PSP Coastdown
- DL3-HR2 Natural Circulation of Primary Sodium
- DL3-RR1a IHX Primary System Barrier
- DL3-RR1d Primary Sodium Pump Seal
- DL4-HR1 IAC Passive Mode Operation
- DL4-HR2 PSP Trip Automatic Backup
- DL4-HR6 Manual PSP Trip 3
PSAR Fig. 7.1.3-2
Staff Review of PHT Heat Removal Functions
- PSP coast down to support transition to natural circulation (DL3-HR1)
SR function for DBAs is to assure fuel design limits are maintained NSRST function is to minimize thermal transient experienced by RV and internals Design halving time supports transition to natural circulation as demonstrated by analyses
- Passive flow through the core for heat removal via natural circulation (DL3-HR2)
PSPs designed with flow path to permit passive flow when stopped PHT provides flow path between heat centers sufficient to drive natural circulation flow
- PSP trip functions (DL3-HR5, DL4-HR2, DL4-HR6)
PSP trip on high-high sodium temperature to eliminate heat generation PSP trip automatic backup and manual PSP trip
- Audited LBE analyses confirmed PHT adequately performs functions noted above (demonstrating safety functions and conformance with PDCs 10, 11, 12, 34, 35) based on design parameters to be confirmed at OL Two BDBEs (DHP-LOOP-3 and 4) do not credit DL3-HR1 but result in fuel failures during coastdown. Consequences are acceptably maintained within the F-C target curve.
4
NRC Staff Review of PHT Radionuclide Retention
- Supports DL3-RR1 as part of primary coolant boundary (PDC 14, 15, 30, 31, 32)
- Primary coolant system interfaces (PDC 78)
- III-5 does not explicitly address the design of tube-to-tubesheet welds
- Plan is to use a grooved partial penetration weld and analyze the joint using rules from III-5
- Use of Alloy 800H plate for IHX tube supports
- Extension of 800H from 300,000 to 500,000 hour0 days <br />0 hours <br />0 weeks <br />0 months <br /> allowable stresses can be left for OL
Intermediate Heat Transport System
- Transport heat from primary coolant to Nuclear Island Salt System
- Key components (all NSRST):
- Expansion Tank
- Sodium-Salt Heat Exchangers (SHX)
- Associated piping and valves
- Safety significant functions
- DL2-HR2 ISP trip on low IHT level
- DL4-HR1 IAC passive mode operation
- DL4-HR3 ISP trip automatic backup
- DL4-HR7 Manual ISP trip
- DL4-DID1 Intermediate Leak Guard Piping 6
PSAR Fig. 7.1.4-1
NRC Staff Review of IHT Safety Functions
- Heat removal function (DL4-HR1)
- Supports IAC passive mode operation
- IHT designed to provide natural circulation of intermediate sodium between PHT IHX and IAC AHX (PDC 10, 34)
- ISP trip functions (DL2-HR2, DL3-HR3, DL3-HR12, DL4-HR3, DL4-HR7)
- Supports PDC 35 by reducing heat load needed to be removed by RAC
- ISP trip on high-high primary sodium level to limit intermediate sodium flow to the primary system through IHX tube leakage
- Combines with IHT siphon-break feature to prevent RV overfill
- Intermediate leak guard piping function (DL4-DID1)
- Provide sodium leakage detection and containment for IHT piping located within the HAA, consistent with PDC 73 7
NRC Staff Review of IHT Materials Performance
- Intermediate coolant boundary (PDCs 70, 75, 76, 77)
- Codes and Standards: ASME Section VIII, Division 1 (VIII-1) for vessels and B31.1 for piping
- Materials: 304H or 316H for elevated temperature service; 304 or 316 for low temperature service
- NSRST Special Treatments 8
High temperature considerations Other special treatments For pressure vessels, evaluate using III-5 Class A rules or code case 2843-3 VIII-1 lethal service rules or B31.1 toxic fluid rules For piping, evaluate per III-5, subsection HCB-3634(b) 100% volumetric examination of pressure boundary welds and certified material test reports (CMTRs)
Allowable stresses limited to III-5 extended to 500,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> Delta ferrite limits from III-5 for weld material
SHX Design and Sodium-Salt Reaction R&D
- USO identified an R&D item associated with sodium-salt reaction from an SHX leak
- Prior literature1 on sodium-salt reactions at high temperatures showed an exothermic reaction producing sodium nitrate, sodium oxide, and nitrogen gas, and evidence of enhanced corrosion
- Based on literature, NRC staff noted that a sodium-salt reaction in the SHX may result in local temperature and pressure increases and potential for enhanced corrosion due to reaction products
- USO decided to revisit the SHX design after lab testing completed in late 2024
- The conceptual design of the SHXs is an etched diffusion-bonded heat exchanger (DBHE) made of 316L SS with 16-8-2 weld metal 9
1 D.A. Csejka, et al. The Interaction of Elemental Sodium with Molten NaNO3-KNO3 at 873 K, J. Materials Engineering, Vol. 11, No. 4, 1989.
DBHE Geometry and Fabrication 10 (A) Schematic shows the stacking of pre-etched plates that after the application of high temperature and pressure form a homogeneous diffusion bonded block (B) Optical image shows the cross section of compact heat exchanger (CHX) with 1.6 mm channel diameter (C) Actual diffusion bonded CHX (Illustrative only - not actual design drawings for KU1)
- Diffusion bonding is a solid-state welding process that bonds two surfaces by diffusion under heat and pressure in a vacuum environment
- Performed at elevated temperatures and moderate pressures (700 - 1,400 psi)
From: https://www.osti.gov/servlets/purl/2368568
SHX Design and Sodium-Salt Reaction R&D
- SHX design and R&D activities seek to demonstrate either:
- No credible failures that could result in a sodium-salt reaction or
- Any credible failures will result in a negligible sodium-salt interaction with no credible safety impacts
- SHX R&D activities include:
- Characterizing the sodium-salt reaction
- Reaction rate, heat and gas generation, identifying reaction byproducts,
- Quantifying oxide generated from the reaction and impacts of the reaction byproducts on corrosion
- Investigating the design leak prevention measure impact on heat transfer, and
- Developing and qualifying methods of leak detection
- Additional design information
- SHXs are NSRST and therefore will be included in the final RIM program 11
CP Condition on SHX R&D
- NRC staff plans to include a condition in the KU1 CP requiring updates on the SHX R&D item:
- USO shall submit annual periodic reports to the NRC covering the latest results and future plans for research and development activities associated with the sodium-salt heat exchanger design and sodium-salt reactions
- Further details on the SHX design and R&D will be covered in the closed session 12
Reactor Air Cooling System
- Supports SR heat removal function: DL3-HR4, Inherent - RAC Operation and emergency core cooling (PDC 35)
- RAC continuously transfers heat to the atmosphere via natural circulation at a rate during accident conditions to establish reasonable assurance radionuclide release results in calculated radiological dose under the 10 CFR 50.34 dose criteria at a safe shutdown condition.
- PSAR chapter 3 analyses demonstrate fuel integrity maintained for DBEs, BDBEs, and DBAs that rely on RAC 13
Flow Path
- Heat from the coolant transferred to RV, then to the GV and CCA.
- Ambient air is drawn in through four inlet stacks
- Air in the riser annulus is heated by:
- GV outer surface
- CCA inner surface
- Heated air rises and exits via four outlet stacks 14 PSAR Fig. 7.2.1-1
Protection against Natural Phenomena (PDC 2)
- Environmental considerations
- Primarily located within SR RXB substructure
- Stacks outside the RXB designed to withstand tornadoes and extreme climates
- Designed to ANSI N690-2018, as endorsed by RG 1.243.
- Seismic considerations
- SCS1 seismic classification
- Designed to withstand seismic loads from SSE
- Reviewed seismic interaction considerations of the RXB superstructure with the RAC stacks 15 PSAR Fig. 7.2.1-1
Protection against Natural Phenomena (PDC 2)
- Wind effects
- Historic tests from ANLs NSTF showed wind can cause flow reversal and reduce natural circulation performance
- Audit confirmed RAC maintains performance under adverse wind conditions
- Design features including louver orientation and chimney cowls mitigate wind impact 16 PSAR Fig. 7.2.1-1
Protection against Natural Phenomena (PDC 2)
- Flow blockage considerations
- Inlet louvers minimize precipitation and debris entry
- Drainage under inlet stacks removes accumulated water
- Four inlets and four outlets
- Flowpath accessible for inspection and cleaning
- Remote inspection used for inaccessible areas
- Debris removal supported by remote vacuuming 17 PSAR Fig. 7.2.1-1
RAC Continuous Monitoring
- RAC instrumentation (PDC 13):
- Inlet temperature
- Outlet temperature
- Air flow rate
- Allows for continuous monitoring of heat removal efficacy of the RAC (PDC 37)
RAC Heat Transfer Performance R&D
- USO identified an R&D item to demonstrate adequate RAC heat transfer performance
- Surface roughening of the RV outer surface, GV inner and outer surface, and CCA to increase emissivity
- Refinement of analyses, such as crediting reactor vessel liner overflow
- Selection of a final design solution
- Future work includes sensitivity analyses, methods validation, and establishing a final emissivity value
- R&D activities are expected to be completed before completion of construction activities 19 PSAR Fig. 7.1.3-2
RAC Heat Transfer Performance R&D
- Staff evaluation
- RAC heat transfer performance depends on a number of critical characteristics, including RV surface temperature and emissivity, GV inner and outer surface emissivity and the design of the CCA
- The staff notes that roughening can impact the fatigue performance of the RV and GV
- Separate effects tests (SETs) discussed in NAT-9390-A* are needed to ensure codes used to analyze RAC performance are adequately validated
- Aspects of the RAC design, particularly ducting near the RV and the CCA, must be completed before RAC performance can be demonstrated
- NRC staff determined that the RAC R&D activities are reasonably designed to address safety questions associated with the adequacy of heat removal using RAC
- Further discussion of roughening impact on the fatigue performance of the RV and GV will be covered in the closed session 20
- NAT-9390-A, Design Basis Accident Methodology for In-Vessel Events without Radiological Release, Revision 2 (ML25211A127)
Intermediate Air Cooling System
- Supports NSRST heat removal function: DL4-HR1, IAC Passive Mode Operation and heat removal PDCs (10, 34)
- Relies on natural circulation in the PHT and IHT
- Required for DID
- Audited calculations to verify IACs heat removal capabilities 21
IAC - 3 Modes of Cooling 22
- Active Mode (normal):
- Forced airflow (blowers)
- Blower Mode (off-normal):
- Natural circulation of IHT sodium
- Forced airflow (blowers)
- Passive Mode (emergency)
- Natural circulation of IHT sodium
- Natural convection of air PSAR Fig. 7.2.2-1
NRC Staff Review of IAC Materials Performance
- Intermediate coolant boundary (PDCs 70, 75, 76, 77)
- Designed to ASME Section VIII, Division 2 (VIII-2) with 304H and 16-8-2 weld metal
- NSRST Special Treatments 23 High temperature considerations Other special treatments For pressure vessels, evaluate using III-5 Class A rules or code case 2843-3 100% volumetric examination of pressure boundary welds and certified material test reports (CMTRs)
Allowable stresses limited to III-5 extended to 500,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> Delta ferrite limits from III-5 for weld material
Air Stack Structures and Equipment
- Provides protection to the IAC from natural phenomena (PDC 2)
- Sodium-water considerations (PDC 74)
- ASE located away from water-containing SSCs
- Air intakes and exhausts have weather caps
- Catch pans have detectors to notify operators of water collection 24 PSAR Fig. 1.1-2
Acronyms 25 ACRS - Advisory Committee on Reactor Safeguards AHX - Sodium-Air Heat Exchanger ANL - Argonne National Laboratory ANSI - American National Standards Institute AOO - Anticipated Operational Occurrence ASE - Air Stack Structures and Equipment ASME - American Society of Mechanical Engineers BPVC - Boiler and Pressure Vessel Code BDBE - Beyond Design Basis Event CCA - Collector Cylinder Assembly CFR - Code of Federal Regulations CHX - Compact Heat Exchanger CP - Construction Permit DANU - Division of Advanced Reactors and Non-power Production and Utilization Facilities DBA - Design Basis Accident DBHE - Diffusion-Bonded Heat Exchanger DBE - Design Basis Event PSAR - Preliminary Safety Analysis Report PSP - Primary Sodium Pump R&D - Research and Development RAC - Reactor Air Cooling System RG - Regulatory Guide RIM - Reliability and Integrity Management RV - Reactor Vessel RXB - Reactor Building SET - Separate Effects Test SFR - Sodium Fast Reactor SHX - Sodium-Salt Heat Exchanger SR - Safety Related SSC - Structures, Systems, and Components SSE - Safe Shutdown Earthquake USO - US SFR Owner DID - Defense In Depth GV - Guard Vessel IAC - Intermediate Air Cooling System IHT - Intermediate Heat Transport System IHX - Intermediate Heat Exchanger ISP - Intermediate Sodium Pump KU1 - Kemmerer Unit 1 LBE - Licensing Basis Event NRC - Nuclear Regulatory Commission NRR - Office of Nuclear Reactor Regulation NSRST - Non-safety-related with Special Treatment NSTF - Natural Convection Shutdown Heat Removal Test Facility NST - No Special Treatment OL - Operating License PAM - Post-Accident Monitoring PDC - Principal Design Criteria PHT - Primary Heat Transport System