Initiating Events Coding Guidance

Contents

• Initial Plant Fault Group. 1
• Functional Impact Group. 2

This document presents guidance on the analysis of initiating event frequencies at United States (U.S.) nuclear power plants. The evaluation is based on the operating experience as reported in Licensee Event Reports (LERs). The objectives of the initiating event study are:

• Provide revised, historical frequencies for the occurrence of initiating events in U.S. nuclear power plants;
• Compare these estimates based on operating experience to estimates used in probabilistic risk assessments (PRAs), individual plant examinations (IPEs), and other regulatory issues; and
• Review the operating data from an engineering perspective to determine trends and patterns of plant performance on a plant-type [i.e., pressurized water reactor (PWR) or boiling water reactor (BWR)], plant-specific, and industry-wide basis.
• Each LER is reviewed from a risk and reliability perspective by an engineer with nuclear power plant experience. To be included in this study, an event has to meet all of the following criteria:
• Include an unplanned reactor trip (not a scheduled reactor trip on the daily operations schedule)
• Sequence of events starts when reactor is critical and at or above the point of adding heat
• Occur at a U.S. commercial nuclear power plant (excluding Fort St. Vrain and LaCrosse)
• Be reported by a Licensee Event Report (LER).

Each reactor trip event is reviewed for the following information:

• All occurrences of risk-significant events in the reactor trip sequence that could impact the ability to remove reactor decay heat
• The first event in the sequence of events that causes or leads to the unplanned, automatic or manual reactor trip
• The occurrence of a manual reactor trip.
• The information is collected into two groups or data sets:
• The initial plant fault group -contains the reactor trip event initiator for each reactor trip
• The functional impact group -contains one or more risk-significant events that occur during each reactor trip

Initial Plant Fault Group

The initial plant fault (IPF) is the first event in a sequence of events causing or leading to an unplanned, automatic, or manual reactor trip. The initial plant fault group contains 52 mutually exclusive categories under fourteen headings. Thirteen headings (A through P) include risk-significant categories that could impact the ability to remove decay heat (e.g., loss of offsite power, loss-of-coolant accident, and total loss of condenser heat sink). These thirteen headings and associated categories are identical to all of the risk-significant headings and categories used in the functional impact group. The initial plant fault group also includes an additional heading with 23 categories typically classified as general transients in PRAs (Qxx).

General transients are a compilation of all reactor trip initiators that had no direct impact on mitigating systems ability to remove decay heat.

The events in the initial plant fault group are used in the engineering analysis section of the main report to develop insights from the conditional occurrences of risk-significant events. The events from the initial plant fault and functional impact groups are merged to compare the number of risk-significant events occurring after the reactor trip initiator (i.e., initial plant fault event).

Frequency estimates of initial plant fault categories are best suited for PRA analyses where the occurrence of one or more risk-significant event categories (e.g., total loss of the main feedwater system or steam generator tube rupture) are specifically included in the accident sequence event tree model as a conditional failure. The combination of these conditional functional and/or system successes and failures are depicted along the top heading across the event tree. For this type of event tree model, the frequency of an initial plant fault category (or a group of similar categories) is used as the initiating event frequency for quantification of the event tree. The conditional probability of a risk-significant event category subsequent to the initial plant fault event can be estimated from the data in the appropriate initial plant fault and functional impact categories. However, if a particular event category is not modeled in the event tree as a conditional event, then the frequency estimate of the functional impact category, which includes all occurrences of the event in the frequency estimate, may be more appropriate as the initiating event frequency for event tree quantification.

For each reactor trip event, the sequence of events leading to the reactor trip is examined and the event that occurred first from the list of 48 initial plant fault categories is the initial plant fault. Only one initial plant fault category is selected for each reactor trip. For example, consider the case where a total loss of feedwater flow causes a plant transient that results in a reactor trip and turbine trip, and then the loss of offsite power. The initial plant fault category appropriate for this reactor trip sequence would be Total Loss of Feedwater Flow (category P1), since it happened first. In this example, the total loss of feedwater may be the result of the failure or mis-operation of components in the main feedwater system or associated with another system. However, if the root cause could not be matched to a category from the initial plant fault group, then the next event in the reactor trip sequence that could be matched is selected.

Functional Impact Group

The event categories in the functional impact group include risk-significant events that could impact the ability to remove decay heat. The functional impact (FI) group contains 29 categories under 13 headings. Only those events that could impact the ability to remove decay heat are included in functional impact group. Event categories classified as general transients are excluded in the functional impact group. General transients are a compilation of all reactor trip events that had no direct impact on mitigating systems' ability to remove decay heat.

The headings and categories associated with the functional impact group are the same as the explanations for the Initial Plant Faults in Section Initial Plant Fault Group.

The Function Impact group reflects the IPF (if the IPF is in the A through P headings) automatically. Any additional FIs as found in the LER are then added.

The purpose of the functional impact group is to determine the frequency at which risk-significant events are likely to occur in association with the reactor trip, regardless of their order in the reactor trip sequence. The results presented in the main body of the report are focused on the analysis and evaluation of risk-important event categories from the functional impact group. This focus is chosen by the NRC to support several risk-informed regulatory activities. Frequency estimates of functional impact categories are best suited for PRA analyses where the occurrence of a risk-significant event category (e.g., total loss of the main feedwater system or steam generator tube rupture) is not specifically modeled in the accident sequence event tree as a conditional failure. For this case, the frequency of a functional impact category (or groups of similar categories) is used as the initiating event frequency for quantification of the appropriate event tree.

For each reactor trip, the sequence of events are examined for events occurring any time before and shortly after the reactor trip. Each occurrence of an event from the table of functional impact categories is noted in the database for each reactor trip event. One or more functional impact events may be identified in a single reactor trip event sequence. However, a reactor trip sequence may have no functional impact events, as would be expected for most reactor trips.

For example, consider the case where a total loss of feedwater flow causes a plant transient resulting in a reactor trip and turbine trip, and then the loss of offsite power (due to, for example, the failure to transfer the plant electrical power source from the main generator to the preferred offsite power source). The functional impact categories applicable for this reactor trip sequence are Loss of Offsite Power (category B1) and Total Loss of Feedwater Flow (category P1). The order in which the functional impact events occur is not considered in this study. The turbine trip event is not selected, since the event is not a functional impact category.