ML25224A200
| ML25224A200 | |
| Person / Time | |
|---|---|
| Site: | Technical Specifications Task Force |
| Issue date: | 08/07/2025 |
| From: | Technical Specifications Task Force |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| Download: ML25224A200 (1) | |
Text
Technical Specifications Task Force A Joint Owners Group Activity TSTF OVERVIEW OF TSTF-605, "REACTOR COOLANT SYSTEM (RCS) WATER INVENTORY CONTROL (WIC)"
Technical Specifications Task Force A Joint Owners Group Activity TSTF Introduction
- TSTF-605 replaces the Pressurized Water Reactor (PWR) standard Technical Specification (STS) 3.5.3, "ECCS - Shutdown," with a new specification titled "Reactor Coolant System (RCS) Water Inventory Control (WIC)."
- The specification remains applicable in Mode 4.
- In Mode 4, the reactor is subcritical, the reactor vessel head closure bolts are fully tensioned, and the RCS average temperature is between [350]°F and
[200] °F.
- In the Westinghouse and Babcock and Wilcox STS, STS 3.5.3 requires one ECCS train, consisting of a high-pressure subsystem and a low-pressure subsystem, to be operable.
- In the Combustion Engineering STS, one high-pressure subsystem is required to be operable.
- For the purposes of today's overview, we will be concentrating on the Westinghouse STS changes.
Technical Specifications Task Force A Joint Owners Group Activity TSTF Reason for the Proposed Traveler
- The current "ECCS - Shutdown" TS lacks an analytical basis.
- The Bases state that the requirements are to respond to a LOCA, MSLB, or SGTR.
- There is no NRC-approved LOCA analysis methodology for Mode 4.
- The Mode 4 TS requirements and bases are inconsistent.
- When a low-pressure ECCS pump is required for ECCS injection, it cannot be used for residual heat removal.
- It is estimated that during peak summer months, the ability to use an additional train of RHR cooling during shutdown could save on the order of 12-24 hours of critical path time.
- The proposed change establishes a consistent safety basis for TS requirements in Mode 4, similar to the approach taken in BWR travelers TSTF-542, "Reactor Pressure Vessel Water Inventory Control," and TSTF-582, "RPV WIC Enhancements."
Technical Specifications Task Force A Joint Owners Group Activity TSTF Overview of Proposed TS Changes
- Rename TS 3.5.3 to "RCS Water Inventory Control."
- If not met, restore within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> (consistent with TSTF-542), or
- Be in Mode 5 within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
- SRs consistent with TSTF-542:
- Verify piping full of water every 31 days or per the Surveillance Frequency Control Program (SFCP).
- Perform subsystem run > 10 minutes every 92 days or per the SFCP.
- Verify manual alignment and initiation from the control room every [18] months or per the SFCP.
- Remove Mode 4 ECCS actuation instrumentation requirements.
Technical Specifications Task Force A Joint Owners Group Activity TSTF Current Safety Basis
- The current ECCS requirements in Mode 4 are based on engineering judgment.
- After issuance of 10 CFR 50.46 in 1974, the NRC began requiring one train of ECCS during the equivalent of Mode 4.
- Neither 10 CFR 50.46 nor Appendix K explicitly require a LOCA analysis in shutdown modes.
- Appendix K requires the assumption of extended operation at 102% of rated thermal power prior to the accident.
- There are no NRC-approved assumptions or methodologies for evaluating a LOCA in Mode 4.
- PWR STS Bases since the 1970's have stated one train of ECCS was required, and no single failure is assumed, due to "the stable reactivity condition of the reactor and the limited core cooling requirements" and "sufficient time exists for manual actuation of the required ECCS to mitigate the consequences of a DBA."
- Industry agrees, but there is no analysis to support these statements.
Technical Specifications Task Force A Joint Owners Group Activity TSTF Credible Events in Mode 4
- TSTF reviewed the PWR events in Chapter 15 of the Standard Review Plan for their applicability in Mode 4.
- For most events, the proposed TS change has no effect (e.g., loss of non-emergency AC power to the station auxiliaries), or the event is not possible or credible in the Mode 4 conditions (e.g., SGTR, MSLB), due to one or more of the following factors:
- Low RCS pressure, less than approximately 350 psig or approximately 15% of the normal operating pressure for the RCS.
- Mode 4 shutdown margin requirements prevent reactor criticality.
- When using RHR for decay heat removal, there is no reliance on reactor coolant pumps, steam generators, feedwater, or steam lines.
- If using an RCP and steam generator for decay heat removal, the TS currently permits them to be stopped for an hour. This demonstrates sufficient time to establish an alternate decay heat removal mechanism.
- The turbine is not in service.
(continued)
Technical Specifications Task Force A Joint Owners Group Activity TSTF Credible Events in Mode 4
- Loss of coolant is the only event that may be credible and could be affected by the proposed change to the ECCS - Shutdown TS.
- The SRP defines a LOCA as "postulated accidents that would result from the loss of reactor coolant, at a rate in excess of the capability of the normal reactor coolant makeup system, from piping breaks in the reactor coolant pressure boundary."
- A fault of significant size of Class 1 RCS piping is not credible at the Mode 4 RCS pressure (< ~15% of normal operating pressure and < ~13% of the Safety Limit maximum pressure).
- Non-Class 1 piping can be isolated.
- It is debatable whether a small, non-isolable pipe break is credible and could exceed the capability of normal makeup.
- However, the TSTF agrees that a loss of RCS inventory could occur in Mode 4 due to inadvertent valve manipulation.
- Determining whether such an event should be considered a LOCA is not necessary when it is clear the capability to respond to an unexpected loss of reactor coolant inventory in Mode 4 is appropriate.
Technical Specifications Task Force A Joint Owners Group Activity TSTF Proposed Safety Basis
- The following safety basis is proposed for Mode 4 for ECCS and supporting equipment:
- The inadvertent opening of a valve results in a loss of reactor coolant from the RCS at a rate in excess of the capability of the normal makeup system.
- Valves are manually aligned from the control room and an ECCS high-pressure or low-pressure pump can be manually started to add RCS inventory, drawing from the refueling water storage tank.
- The required ECCS pump is capable of being powered by an operable emergency diesel generator should offsite power be lost.
- If necessary, the operators may manually swap the ECCS pump water source from the refueling water storage tank to the containment sump.
- The event is terminated by closing or isolating the valve and restoring the RCS inventory.
- No additional single failure or operator error beyond the initiating event is considered.
Technical Specifications Task Force A Joint Owners Group Activity TSTF Schedule
- Currently anticipate we will request a "normal" priority review.