**5.9. Summary of the methods applied at Paks NPP**

116 Nuclear Power – Practical Aspects

5.2.2 are focused on the liquefaction hazard.

effort of the plant and plenty of walk-downs.

**5.8. Seismic instrumentation and seismic EOPs** 

turbine building may cause steam and feed-water header ruptures that result in total loss of main and auxiliary feed-water and disables closed loop heat removal through the secondary side. Repeated analysis for the case after implementing the additional measures resulted into CDF value of magnitude of 10-5/a, which is acceptable per Hungarian regulations.

The seismic PSA indicated also that the building settlement of the buildings due to the soil liquefaction jeopardizes the communications (pipes for diesel generator cooling and cables coming from the diesel generators) between the buildings. In the lower acceleration ranges the soil liquefaction that cause settlement of the main building plays dominant role in the occurrence probability of total loss of electric power supply. The studies indicated in Section

The methodology of the seismic PSA applied at Paks NPP complies with the best international practice, see IPEEE NUREG-1407 (NRC, 1991) and (IAEA, 1993). The SPSA was developed on the basis of extensive PSA experience and existing PSA models for Paks NPP and information from newly performed response and strength analyses and qualification

In case of an earthquake, the reactor is shutdown either by reactor protection system due to malfunctions, or manually by the operator based on the criteria of CAV and response spectrum criteria for OBE exceedance. The OBE-exceedance criteria is set CAV=0,16gs and response spectrum in the amplified range less than 0,2g. The seismic instrumentation and the pre-earthquake preparedness and post-earthquake actions are defined via adaptation of the IAEA NS-G-1.6, US NRC Regulatory Guide and 1.12, 1.166 and 1.167 respectively. Selection of the OBE exceedance criteria at Paks NPP was based on the analysis of the

It has to be noted that the implementation of the concept and methodology for OBEexceedance was not a simple copy-paste; it has been adapted to the conditions of Paks NPP. At Paks NPP, if the ggset measured at the base mat, the non-upgraded part of certain systems will be automatically isolated from the upgraded one by quick-closing valves. These system's parts do not have function during and after an earthquake and the separation will not disturb the operation either. In the same time, there is also a signal for



frequency of expected events, probability and consequences of spurious signals.

control room. If an earthquake happens, there are two possible cases:

defined by EOPs and trained on the simulator.

testing and walk-downs.

The comparison of the seismic re-evaluation methods "as usual" and the methods applied at Paks NPP is shown in the Table 7.


Abbreviations used in the Table 7: GMRS, UHRS, DRS – ground motion, uniform hazard and design base response spectra respectively, FRS – floor response spectra; *Gmin, Gmax, Gav* – maximum, minimum, and average values of the soil shear modulus; CDFM – Code Deterministic Failure Margin (see Section 3.2.2).

**Table 7.** The seismic re-evaluation methods as usual and the methods applied at Paks NPP
