The objective Mitigating Systems cornerstone is to monitor the availability, reliability, and capability of systems that mitigate the effects of initiating events to prevent core damage. Licensees reduce the likelihood of reactor accidents by maintaining the availability and reliability of mitigating systems. Mitigating systems include those systems associated with safety injection, decay heat removal, and their support systems, such as emergency AC power. This cornerstone includes mitigating systems that respond to both operating and shutdown events. There are five indicators in this cornerstone:
- Inspection Procedures
- As listed above, there are twenty-three attachments to the inspection procedure for the reactor safety strategic performance area, which includes the initiating events cornerstone, mitigating systems cornerstone, and the barrier integrity cornerstone.
- Performance Indicators
- There are currently five performance indicators in the mitigating systems cornerstone:
- Safety System Functional Failures
- The number of events or conditions that alone prevented, or could have prevented, the fulfillment of the safety function of structures or systems in the previous four quarters.
- Emergency AC Power Systems
- The sum of the unavailability of the emergency AC power plus the unreliability for the emergency AC power system during the previous twelve quarters.
- High Pressure Injection Systems
- The sum of the unavailability of the high pressure injection system plus the unreliability for the high pressure injection system for the during the previous twelve quarters.
- Heat Removal Systems
- The sum of the unavailability of the heat removal system plus the unreliability for the heat removal system during the previous twelve quarters.
- Residual Heat Removal Systems
- The sum of the unavailability of the residual heat removal system plus the unreliability for the residual heat removal system during the previous twelve quarters.
- Cooling Water Systems
- The sum of the unavailability of cooling water systems plus the unreliability for the cooling water systems during the previous twelve quarters.
The NRC and the nuclear industry have jointly implemented a replacement to the Safety System Unavailability Performance Indicators, called the Mitigating System Performance Index (MSPI). Data collection for the MSPI began in April 2006 with the first licensee submittal occurring in July 2006 (to include data up through 2Q2006).
The Mitigating Systems screening questions are categorized into four sub-sections titled Mitigating Systems, Structures, Components (SSCs) and Functionality (except Reactivity Control Systems), External Event Mitigation Systems (Seismic/Fire/Flood/Severe Weather Protection Degraded), Reactivity Control Systems, and Fire Brigade. Below is additional guidance to support answering the screening questions for each sub-section:
Mitigating SSCs and Functionality
(except Reactivity Control Systems) –
For the purposes of this subsection, the SSCs (and their associated functions) of concern are those that provide a risk significant or risk relevant mitigating function in response to an initiating event. Normally those SSCs that are in the risk model provide a risk significant or risk relevant function; however that is not always the case (e.g., some SSCs are not modeled explicitly). There are several ways to determine whether an SSC provides a risk significant or risk relevant mitigating function and below are some sources of information to support this determination:
- 1) Plant Risk Information eBook (PRIB) (Table 6) – Table lists systems/functions that are included in the SPAR model. It also provides specific success criteria given a particular initiating event. See PRIB definition in section 6.0 “Detailed Risk Evaluation”.
- 2) PRIB (Table 7) – Table lists the components included in the SPAR model with their associated risk importance measures.
- 3) SDP Workspace – The SDP workspace contains risk significant and risk relevant SSCs derived from the specific SPAR model.
- 4) UFSAR – Although the systems/function described in the UFSAR might be different than the systems/function modeled in the SPAR, the licensed design bases for systems/functions can provide useful information in determining safety significance.
- 5) Licensee Risk Insights – If provided, risk insights from the licensee risk model (e.g., importance measures, dominant sequences, delta CDF calculations, etc) and risk/safety significant SSCs from their maintenance rule program can be a good source of risk information.
External Event Mitigation Systems
(Seismic/Fire/Flood/Severe Weather Protection Degraded) – No additional guidance
Reactivity Control Systems
Reactor Protection System (RPS) – The main focus of the screening question is to screen findings that result in a minor functional degradation of RPS (e.g., one automatic trip from one instrument) but there are several redundant trips that provide the same function (e.g., three other automatic functional trips). If there is a significant functional degradation to RPS, a detailed risk evaluation is warranted. The determination of what a “significant” or “minor” functional degradation of RPS should be based on reasonable technical judgment of the inspectors, SRA, and management.
No additional guidance