Transition Risk Method and Example

Paragraph (a)(4) of the Maintenance Rule (re 10CFR§50.65) states that before performing maintenance activities, the licensees shall assess and manage the increase in risk that may result from the maintenance activities. The rule is explicitly applicable to all operating modes. Currently, most plants use a qualitative tool for assessing and controlling the risk of the various plant conditions during an outage. Fewer plants have any means of performing a quantitative or qualitative assessment of the associated risks of transitioning the plant in each configuration from power to “cold shutdown.” Typically, only the end-state of shutdown is considered. The transition-period includes short-duration configurations when the available set of equipment is not what it was during power operations, e.g., having only one main feedwater train in-service. Given the concern that the NRC may require quantitative risk assessments of plant transitions and plant configurations during shutdown operations, Omaha Public Power District (OPPD) pro-actively authorized Westinghouse Engineering Services to develop a method for assessing risk associated with a transition from all power to shutdown and back to full power. An outage schedule is highly plant specific, with plant-to-plant and outage-to-outage variations in system configurations, and maintenance practices. Accordingly, the duration of the transition largely depends on equipment maintenance activities driving the decision to shutdown and repair. The time spent in various parts of the transition is a strong determinant in the associated risk of the transition. A transition takes the plant through a series of Plant Operational States (POSs). The features important to the characterization of each of the POSs include decay-heat level, plant activities involved, available equipment, as well as RCS temperature and pressure. The risk of the entire transition comes from calculating a figure-of-merit of each POS which can be loosely thought of as a per-hour core-damage frequency (CDF). This number gets multiplied by the associated duration of the POS. The sum is the transition risk. The effective CDF associated with the transition comes from dividing the POS-specific CDF sum by the total transition time, and converting that result to an annual frequency. Our paper describes decomposing OPPD operating procedures into periods for which we quantified sequences. In particular, the method considers the dominant shutdown failure modes: loss of shutdown cooling, loss of inventory, and loss of offsite power (including both plant centered and grid-related events). The transition example presented herein covers a simple shutdown and restart stemming from an indeterminate-quality problem. That is, all equipment is functional and available to the plant operators.