New risk-informed performance-based inspection program

A new risk-informed and performance-based inspection program will be enacted from April 2020. A part of this program builds on the information obtained from a probabilistic risk assessment (PRA). The significance determination process includes an incremental assessment of the risk incurred due to performance deficiency. The incremental risks of flooding and fire are determined from the results of an internal flooding PRA and internal fire PRA, respectively.

Internal flooding PRA: Consideration of steam propagation.

The incremental risk due to the performance deficiency observed during the inspection of a pipe rupture is determined by a simplified risk analysis, which includes a water and steam propagation analysis modeled in Apros[1].

The internal flooding PRA focused mainly on water propagation. Because many high-energy pipes exist at a nuclear reactor facility, the propagations of water and steam are important for identifying potentially degraded components. Further, steam causes component failures and restricts operator recovery access. The water and steam propagations through 300 compartments were modeled in Apros. To identify the potentially degraded components and the plant response, the 300-compartment model was connected to models of the reactor, the primary coolant system, and the secondary coolant system. Figure 1 shows the temperature in each compartment at 10, 100, and 2000 s after a secondary piping rupture. The water and steam propagation analysis revealed the water level, temperature, and humidity in each compartment.

A simplified risk analysis method for internal flooding was developed. The quantitative screening assessed the maximum incremental risk at which internal flooding is assumed to occur. The incremental core-damage frequency was calculated based on the inputs to the internal flooding PRA such as the flooding frequencies, isolation failure probabilities of the flooding source, and conditional-core damage probabilities.

[1] Falah A., et al., "Progress in dynamic simulation of thermal power plants," Progress in Energy and Combustion Science, March 2017.