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Unplanned Scrams with Complications (USwC)

Brief Description

The Unplanned Scrams with Complications (USwC) is a new performance indicator (PI) that NRC staff and the Industry Reactor Oversight Working Group have jointly proposed. This new indicator is proposed to replace the Reactor Oversight Process's current Unplanned Scram with Loss of Normal Heat Removal which is in the Initiating Events Cornerstone. The indicator and basis document are currently in draft and are being reviewed for comments by staff and the industry.

The Unplanned Scrams with Loss of Normal Heat Removal (LONHR) PI has been somewhat problematic for licensees and the staff to implement since the initiation of the Reactor Oversight Process (ROP). This PI has been the subject of two prior regulatory issue summaries to address challenges and the PI has resulted in many frequently asked questions over the years. Clarification of guidance by the FAQ process has taken place but there are still many challenges with interpretation of the guidance. In an attempt to address the overall challenges with interpreting the PI, the NRC/Industry Working Group have developed a replacement for the LONHR PI. Briefings on the status of PI development have taken place periodically in the monthly ROP public meeting where recommendations were brought forward for input. Some key aspects of the new indicator are:

  • This indicator will monitor that subset of unplanned automatic and manual scrams that require additional operator actions beyond that of the “normal” scram. Such events or conditions typically present more challenges to the operations staff and therefore may be more risk-significant than uncomplicated scrams.

  • The proposed USwC PI could be a leading indicator of risk-significant events if a plant has too many complicated scrams in a short period of time. The proposed USwC PI monitors six conditions. Problems with any of these conditions can complicate the operators’ scram recovery actions.

  • These conditions are:
    1. Reactivity Control
    2. Turbine Trip (PWR)/Pressure Control (BWR)
    3. Power available to Emergency Buses
    4. Need to actuate emergency injection sources
    5. Availability of Main Feedwater
    6. Normal Scram EOP Use

  • Not all of the complicating conditions are the same for PWRs and for BWRs. Therefore, there is a separate flow chart for each of the two reactor types. If any of the conditions in the appropriate flow chart is met, the licensee must report a “hit” for the PI.

  • The new PI monitors performance over the previous 4 quarters rather than 12 quarters for the existing Unplanned Scrams with Loss of Normal Heat Removal. The new PI threshold to cross from green to white is 2 complicated scrams within one year. There are no yellow or red thresholds.

See the draft basis document for a more detailed discussion of the Unplanned Scrams with Complications PI.

 

Last revised Tuesday, December 19, 2006