ML24129A067

5_8_24 Meeting Handouts - Proposed Change to the PWR ECCS Mode 4 Requirements

ML24129A067
Person / Time
Site:	Technical Specifications Task Force
Issue date:	05/08/2024
From:	Technical Specifications Task Force
To:	Office of Nuclear Reactor Regulation
References
Download: ML24129A067 (1)

Text

Technical Specifications Task Force A Joint Owners Group Activity TSTF 1

Proposed Change to the PWR ECCS Mode 4 Requirements

Technical Specifications Task Force A Joint Owners Group Activity TSTF Introduction

• The PWR licensing basis for Mode 4 accident and transient mitigation is unclear.

• The current TS Bases are contradictory regarding the events assumed to occur in Mode 4 and the required mitigating systems.

• The PWR and TSTF are pursuing a project to establish a consistent licensing basis for Mode 4 accidents and transients, similar to the change made for BWRs in TSTF-542, Reactor Pressure Vessel Water Inventory Control, and TSTF-582, RPV WIC Enhancements.

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Technical Specifications Task Force A Joint Owners Group Activity TSTF Mode 4

• Mode 4 (hot shutdown), is defined in the TS as subcritical, all reactor vessel head closure bolts fully tensioned, RCS Temperature > [200]

and < [350]. This temperature range is consistent with an RCS pressure 400 psig (< 20% of the ASME Code design pressure).

• Some TS Bases state that DBAs can occur in Mode 4, while others state they cannot occur.

• SRP Chapter 15 events applicable to a PWR were reviewed to determine which could reasonably occur in Mode 4.

- Most events were not credible because of the low RCS pressure.

- Most of the remaining events are adequately addressed by existing TS that are not being considered for revision.

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Technical Specifications Task Force A Joint Owners Group Activity TSTF Emergency Core Cooling System

• The most operationally impactful Mode 4 TS is TS 3.5.3, ECCS - Shutdown, which requires one train of ECCS to be operable.

- The requirement for a low pressure ECCS subsystem to be operable limits the ability to use both trains of the RHR system for a plant cooldown.

• The Bases for TS 3.5.3 states that the ECCS - Operating Bases apply, but then make contrary statements regarding the plant conditions, automatic systems, single failure, etc.

• A loss of reactor coolant system inventory from a small pipe break or inadvertent opening of a valve in Mode 4 is credible, but a double-guillotine large-break LOCA is not.

- There is no accident analysis for a LOCA initiated in Mode 4.

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Technical Specifications Task Force A Joint Owners Group Activity TSTF Proposed Approach

• Similar to the change made to the BWR TS in TSTF-542, the PWR is pursuing a revision to TS 3.5.3 to rename it Reactor Coolant System Water Inventory Control, and to revise the requirements to ensure equipment is available to respond to a loss of inventory in Mode 4.

- A single ECCS subsystem (high pressure or low pressure) capable of being powered by emergency power.

- A single failure is not assumed (as in the current TS 3.5.3).

- Manual starting of the ECCS pump, and alignment of the water source can be performed from the control room using procedurally controlled actions (similar to some current TS 3.5.3 requirements).

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Technical Specifications Task Force A Joint Owners Group Activity TSTF Proposed Approach

• As was done in TSTF-542, this change is not establishing a new DBA analysis, but will ensure that appropriate equipment is available to respond to credible events.

• As a follow-on activity, the PWROG will examine all of the TS with Mode 4 applicability and propose TS and/or Bases changes as needed to establish a consistent licensing basis.

• This project will start in late 2024 or 2025.

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