ML24319A148

Meeting Handouts - Proposed Approach to Address UHS Temperature Limits

ML24319A148
Person / Time
Site:	Technical Specifications Task Force
Issue date:	11/14/2024
From:	Technical Specifications Task Force
To:	Office of Nuclear Reactor Regulation
References
Download: ML24319A148 (1)

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Technical Specifications Task Force A Joint Owners Group Activity TSTF 1

Proposed Approach to Address Ultimate Heat Sink (UHS)

Temperature Limits November 14, 2024

Technical Specifications Task Force A Joint Owners Group Activity TSTF Introduction

• The industry and NRC have been discussing how to address increasing UHS temperatures for over 20 years.

• Many licensees have experienced UHS temperatures approaching or exceeding the TS UHS temperature limit for short periods of time.

- The NRC has approved many one-time TS changes to permit higher UHS temperatures.

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Technical Specifications Task Force A Joint Owners Group Activity TSTF Introduction

• An informal survey of the industry indicated that the plants experiencing high UHS temperatures are typically challenged for a cumulative 3 days or less per year, or < 1% of a year.

• Modifying the plant design, such as installing larger heat exchangers, larger pumps, larger cooling ponds, revising the analysis of record with reduced margins, etc., to accommodate temperature peaks that occur so infrequently is prohibitively expensive.

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Technical Specifications Task Force A Joint Owners Group Activity TSTF Introduction

• The UHS temperature limit is an input to many plant calculations and accident analyses.

• The UHS design varies by plant (rivers, ocean, cooling pond, cooling tower, etc.).

• The list of supported systems is also plant specific, but includes the Service Water System, Component Cooling Water System, Containment Spray Coolers, Containment Air Coolers, ECCS heat exchangers, ECCS pump seal cooling, room coolers, Emergency Diesel Generators, etc. A significant change to the UHS temperature can affect a cascading set of calculations.

• As a result, revising those calculations and analyses is very expensive.

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Technical Specifications Task Force A Joint Owners Group Activity TSTF Regulatory Guidance The UHS design basis calculation method is described in Regulatory Guide 1.27, Ultimate Heat Sink for Nuclear Power Plants.

The RG 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 the 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 Industry Proposal The industry proposal is to use a slightly more realistic set of assumptions for a limited period of time per year, while still meeting the majority of the design basis calculation assumptions.

- Providing a time-limited exception to some of the design basis criteria is similar to the basis of TS Required Actions for SSCs.

- As discussed in Generic Letter 80-30, TS Actions and Completion Times provide a limited period during which the single failure criterion is not met. As stated in the Bases for many Actions, the Completion Time takes into account the low probability of a DBA occurring during this period.

- We are proposing a similar limited time exception for some of the UHS design basis assumptions.

The current proposal is that time limit would be a generic cumulative 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> per calendar year, which is less than 1% of a year, based on an industry survey of the time that the UHS temperature limit is at risk of being exceeded.

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Technical Specifications Task Force A Joint Owners Group Activity TSTF Industry Proposal

• How would this be applied?

• The licensee would review their plant-specific UHS calculations and limitations, and identify one or more assumptions that, if relaxed from the current very conservative assumptions, would support a temporary higher limit.

- For example (depending on the plant), non-Category 1 cooling ponds are not assumed to fail, or a LOOP and a EDG failure to start is not assumed, etc.

• The licensee would submit a License Amendment Request (LAR) and would describe the time-limited changes to the NRC for review and approval.

• As discussed previously, these calculations are very expensive, so agreement on an acceptable approach prior to performing the analysis and preparing the LAR is necessary.

- We are not requesting NRC prior approval, but agreement on an acceptable approach.

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Technical Specifications Task Force A Joint Owners Group Activity TSTF Industry Proposal Obviously, some UHS calculational assumptions would have more risk impact than others, based on the likelihood of the event and of the assumed conditions.

- E.g., the likelihood of a LOOP is higher than the likelihood of a DBE seismic event.

The general concept is that not applying one or more conservative assumptions for a very limited period of time per year would be shown to result in an acceptably small increase in risk.

We are not proposing a risk-informed submittal, but a deterministic submittal with risk insights.

One approach could be similar to the unfavorable exposure time concept used for ATWS analyses for Westinghouse NSSS plants. The time during a year when the assumptions of the affected accident analyses are not met is combined with the affected event trees to determine the probability of occurrence of the event. This probability is verified to be less than the assumed frequency of the event in the regulatory basis.

Another approach is to consider that 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> is less than 1% of a calendar year and assuming the CDF from the limiting events is increased during that period. The licensee would verify the annual CDF remains below the Commissions' safety goal.

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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 [92.8].

Verify average water temperature of UHS is [90]°F.

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Technical Specifications Task Force A Joint Owners Group Activity TSTF Proposed Approach 10 The proposed approach would allow the higher temperature limit as a time-limited SR acceptance criteria.

Why this approach?

- Actions only apply when an LCO is not met (i.e., the system is inoperable).

- UHS temperature greater than the SR limit would render the UHS inoperable.

- The UHS is supports many other systems. Under the definition of operability, an inoperable UHS also makes all trains of the supported systems inoperable.

• LCO 3.0.6 permits not taking the Actions of the supported systems in some circumstances, but the supported systems are still inoperable.

• LCO 3.0.6 is not applicable if redundant trains of the supported system are inoperable.

- For example, the UHS supports both trains of CCW. An inoperable UHS would render both trains of CCW inoperable and require an immediate plant shutdown under LCO 3.0.3.

Another alternative would be an LCO Note, but the UHS temperature limit isn't in the LCO, so a Note wouldn't be an "operator friendly" presentation.

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 assumptions and the corresponding UHS temperature limit calculations.

• The time-limited upper limit is presented as an SR limit.

- UHS is operable at 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 How is this Different from Other Approaches?

• At the August 2024 meeting, the NRC asked why the industry wasn't following the Farley LAR approach.

• There are significant differences between the Farley LAR and the UHS temperature proposal.

- The Farley LAR applied to containment temperature.

• Failure to meet the containment temperature limit does not render TS systems inoperable.

There is no "support/supported" effect on other TS.

- The Farley LAR applied the temporary higher limit using a TS action.

• While in the TS Action, the containment temperature LCO is not met.

• This approach would not work with UHS temperature because of the large number of TS systems that would be inoperable when the TS UHS temperature limit is not met.

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Technical Specifications Task Force A Joint Owners Group Activity TSTF What are the Next Steps?

• If this approach is acceptable, the industry and NRC should schedule a more detailed future meeting with the affected NRC Technical Branches.

• The industry and the NRC can establish the approach (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.

• After establishing a sufficiently detailed approach, a licensee lead plant submittal would be submitted to pilot the concept.

• The TSTF will support the lead plant LAR, but we don't anticipate preparing a TSTF traveler due to the plant specific differences.

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