ML24219A397
| ML24219A397 | |
| Person / Time | |
|---|---|
| Issue date: | 08/06/2024 |
| From: | Technical Specifications Task Force |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| Download: ML24219A397 (1) | |
Text
Technical Specifications Task Force A Joint Owners Group Activity TSTF 1
Proposed Approach to Address Ultimate Heat Sink (UHS)
Temperature Limits
Technical Specifications Task Force A Joint Owners Group Activity TSTF Introduction
- The industry and NRC have been discussing how to address rising UHS temperatures for over 20 years.
- Many licensees have experienced UHS temperatures approaching or exceeding the TS UHS temperature limit during short periods of high temperature.
- Modifying the plant design to accommodate temperature peaks that occur only a few days per year is prohibitively expensive.
- The UHS temperature limit is an input to many plant calculations and accident analyses. Revising those calculations and analyses is very resource intensive.
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Technical Specifications Task Force A Joint Owners Group Activity TSTF Regulatory Guidance The UHS design basis calculation method described in Regulatory Guide 1.27, Ultimate Heat Sink for Nuclear Power Plants, is very, very conservative. For example, the method assumes:
- Large-Break LOCA (or most limiting accident)
- Concurrent loss of offsite power
- Failure of an EDG to start on demand (limiting single failure)
- Concurrent seismic event disabling all non-Category 1 structures
- External events such as river blockage, ship collisions, and airplane crashes
- Event occurs at a day and time such that the maximum UHS temperature coincides with maximum historical diurnal temperature
- Maximum assumed heat exchanger tube plugging
- Minimum permitted pump performance The probability of simultaneous occurrence of all of these limiting assumptions is infinitesimally small.
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Technical Specifications Task Force A Joint Owners Group Activity TSTF Proposed Approach In accordance with the Surveillance Frequency Control Program SR 3.7.9.2 ----------------------- NOTE -----------------------------
For a cumulative period of no more than 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> per calendar year, the average water temperature of the UHS is [102.7].
Verify average water temperature of UHS is [90]°F.
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Technical Specifications Task Force A Joint Owners Group Activity TSTF How is this Different from Other Approaches?
- The time-limited higher temperature limit would be a permanent TS change.
- The NRC would have the opportunity to review and approve the licensees choice of time-limited assumption(s), the associated risk impact, and the corresponding UHS temperature limit calculation.
- The time-limited upper limit is presented as an SR limit.
- UHS is operable while using the higher limit.
- No cascading TS requirements or LCO 3.0.6 questions.
- No new allowances needed to perform required testing.
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Technical Specifications Task Force A Joint Owners Group Activity TSTF What are the Next Steps?
- The industry and the NRC should establish the ground rules (acceptable candidates for temporary relaxations from the conservative assumptions, calculational method, use of estimations versus explicit calculations, etc.) before a licensee expends the considerable resources needed to develop a submittal.
- If this approach appears to be viable, the industry and NRC would schedule a more detailed future meeting.
- After establishing a sufficiently detailed approach, a licensee lead submittal could be pursued to pilot the concept.
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Technical Specifications Task Force A Joint Owners Group Activity TSTF 7
Discussion