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New Indicator: A Closer Look at Safety Systems
The NRC is replacing the original indicator of mitigating system performance—SSU—with one that provides a better indication of performance: the MSPI.

What Are Nuclear Plant Mitigating Systems?
Mitigating systems include those that provide emergency cooling water for the nuclear fuel and their support systems, such as emergency AC power and support system cooling. Their basic function is to keep the temperature and pressure within the reactor’s containment structure—which surrounds the reactor—within safe limits.

Limitations of the Previous Safety System Indicator
When the NRC adopted the current reactor oversight process in 2000, the agency used SSU—a World Association of Nuclear Operators (WANO) indicator—to assess safety system performance. The NRC and the industry recognized several problems with this indicator:

• Although it provided an indication of how long the systems were out of service (including maintenance), it did not fully address the reliability of their performance. The capability of a system to perform its safety function includes two components: system availability and system reliability.
• The indicator attempted to address reliability by measuring “fault exposure” time—the number of hours that a mitigating system is unavailable before being detected. However, there is no efficient, objective way to measure this.
• The WANO and NRC definitions used in the SSU differed, requiring system engineers to calculate multiple indicators to measure the same performance.
• The indicator did not reflect current knowledge about the safety importance of various plant systems, which the NRC and the industry have gained through 40 years of reactor operations and state-of-the-art probabilistic safety studies.
• The SSU considered only the average performance of several sets of equipment. It did not take into account the relative safety importance of individual equipment sets. This approach could mask safety concerns or overemphasize matters of little safety importance.
  

During the past several years, the NRC and the industry worked to develop a more accurate assessment tool—the MSPI.

The MSPI obtains information on safety system performance from WANO and the Institute of Nuclear Power Operations. It factors in industry baseline performance data and safety-importance weighting factors and compares the results to baseline plant equipment performance.