ML23005A225
| ML23005A225 | |
| Person / Time | |
|---|---|
| Issue date: | 10/18/2022 |
| From: | Stephen Bajorek, Antonio Barrett, Syed Haider, Shanlai Lu, Joseph Staudenmeier, Carl Thurston, Peter Yarsky NRC/NRR/DSS/SNRB |
| To: | |
| References | |
| Download: ML23005A225 (1) | |
Text
Long Term Effects and Numerical Simulation of Radiolytic Gas, Non-Condensable Gas and Boron Transport For Small Modular Light Water Reactors By S. Lu, C. Thurston, S. I. Haider, A. Barrett, J. Staudenmeier, P. Yarsky, S. Bajorek International Conference on Topical Issues in Nuclear Installation Safety:
Strengthening Safety of Evolutionary and Innovative Reactor Designs.
October 18-21, 2022
Contents
- Radiolytic Gas Reduce condensation heat transfer Combustion
- Non-condensable Gas Reduce condensation heat transfer
- Boron Transport Precipitation: Core Blockage Dilution: Re-criticality
- Numerical Simulations: System Codes/CFDs October 18-21, 2022 2
Radiolytic Gas Generation
- Radiolytic gas in the form of H2 H2+O2 and O2 are generated in the reactor core due to radiation Heat
- Vapor carrying H2 and O2 Sink condenses in the heat sink region
- Significant H2 and O2 could accumulate over time in certain locations within the primary Vapor+H2+O2 system
- The accumulated H2 and O2 may reduce heat removal Core effectiveness and/or cause sudden combustion, i.e, explosion Condensate Primary System Pressure Boundary October 18-21, 2022 Containment 3
Non-Condensable Gas
- The condensation heat transfer coefficient reduces significantly with the presence of non-condensable gas
- Air, H2 and O2 could accumulate around the heat exchanger to degrade the decay heat removal capacity if not mitigated FIG.1 Condensation Heat Transfer Coefficients vs Non-Condensable Gas Mass Fraction October 18-21, 2022 4
Boron Precipitation FIG 2. Experimental Results Reactor Core Blockage & Fuel Cladding Heat Up October 18-21, 2022 5
Boron Dilution
- A slug of diluted water entering the core of a conventional PWR could lead to substantial power increase, in extreme cases, as high as 10 orders of magnitude
- A PWR SMR may also experience similar type of boron dilution and re-criticality as the result of boron transport in the system
- Proper evaluations need to be performed to assess the possibility.
Hardware designs may be needed to mitigate the potential boron dilution
Boron Volatilization
- When the steam is generated from boric acid solution, a small amount of boric acid is carried away by the vapor
- The volatility increases with higher fluid temperature, interfacial surface area, velocity and vapor/fluid interaction time
- A potential significant long FIG 5. Volatility of boron out of boiling pentaborate solution term transport mechanism (BWR ATWS test)
October 18-21, 2022 7
Numerical Simulation
- System codes, (TRACE, RELAP, etc) have some capabilities to perform non-condensable gas and boron transport analysis based on extensive testings around the world after TMI accident
- Both system codes and CFDs have very limited capabilities to simulate long term and slow evolving transport for LWR SMRs
- Either design specific tests or new code development may be needed to support the LW SMR development October 18-21, 2022 8
Conclusions
- The gradual accumulation of radiolytic gas may degrade condensation and reach the combustion limit, if not controlled
- The presence of non-condensable gas reduces the condensation heat transfer in the reactor system and containment
- The boric acid can precipitate in a PWR core to cause undesirable reactor fuel cladding heat-up. The dilution, however, can cause re-criticality with the subsequent potential power excursions, if not mitigated
- Both numerical analysis tool developments and tests are needed