**7. Operability of nuclear power plants during and after the events**

The operability of the plant is defined by the weakest link, that is, by those non-safety-related SSCs designed/qualified to withstand low-magnitude effects of natural phenomena according to building code or conventional industrial standards (e.g., EUROCODE 8 for earthquakes). These magnitudes usually correspond to the 100 years of return period events. In case of earthquakes, the limit of continuous operation is the operation-based earthquake with approximately 100 years of return period or 475 years as per EUROCODE 8.

If the return period, T, of the lower magnitude hazard effects is 100 years, the probability of exceeding the corresponding magnitude of hazard for the entire operational lifetime (60 years) is *PE*<sup>60</sup> ≥0.4528. This is the probability of the shutdown and related economic losses caused by exceedance of magnitude of the operational level.

The relevant hazards limiting the operation for the inland freshwater-cooled plants like the Paks NPP in Hungary are the low flow rate in river and high water temperature. Controlling parameter for freshwater cooled plants is defined in terms of river water temperature measured at some distance from the hot cooling water outflow. In these cases, for safety and environmental protection, the power generation is reduced, or the reactors are shutting down. The hydrometeorological extremes became more frequent and more severe due to climate changes. Assume that the return period of event with the given limiting magnitude will be 50 years as an average over the 60 years of operation instead of 100 years. In this case, the probability of economic losses will be *PE* <sup>60</sup> ≥0.7024. Over the timespan of 60 years, the worsening of hydrometeorological conditions and increase of the magnitude and frequency of extremes will affect the economy of the nuclear power production, especially those plants with freshwater cooling. Even if the values given above for the exceedance probability of magnitude of hazards for continuous operation might be not precise, the tendency is clearly showing the growing probability for economic losses due to climate change for any thermal power plants.
