**4. Pre-earthquake preparedness and post-earthquake actions**

### **4.1. Operating basis earthquake (OBE) exceedance**

98 Nuclear Power – Practical Aspects

1. to check the as-is conditions, i.e. a. the as-is lay-out conditions, b. the adequacy of the anchorages,

c. to check the compliance with the conditions in the re-qualification database, 2. to identify those interactions, which can potentially affect the performance of the seismic safety related structures, systems and components during the occurrence of an

Examples for checking the interaction items during the walk-down are listed below:

safety-related equipment or cause loss of function of such equipment,

etc. may overturn, slide and impact adjacent safety-related equipment,









The plant walk-down is also required for the assessment of the severe earthquake vulnerabilities and design of accident management and mitigation measures, including the

Design of the upgrading have to be performed according to the design codes and standards

The seismic upgrading are design modifications requiring proper configuration

All methods presented above for the re-evaluation and re-qualification of the operating plants require specific knowledge and experience and decisively based on the expert

(HVAC) ducts may deflect and impact adjacent safety-related equipment,

earthquake and can render this equipment inoperable,

actuation of the fire extinguish system may happen,

overturn, or otherwise impact safety-related equipment,

loss of function of the safety related systems,

identification of the on-site and off-site logistical obstacles.

and for the design basis earthquake as defined by current licensing basis.

**3.6. Design of upgrading** 

management and regulatory approvals.

**3.7. Role of the peer-reviews** 

rooms, may fall and impact adjacent safety-related equipment,

3. to check the feasibility of upgrading measures.

Operating basis earthquake level is understood as a limit for the continuation of the safe operation. If the plant is designed for two levels of earthquake, i.e. OBE and SSE, the limit of safe operation should be set equal to the OBE PGA measured at free-field, or to the response acceleration level at an appropriate location of the structure, e.g. at containment basement, calculated for the OBE. If the acceleration is crossing the set level the reactor protection system is actuated automatically. An automatic seismic trip system could be designed in accordance with the concept of the reactor protection system design with regard to the instrumentation, redundancy and the logic of the generation of actuating command. The system design should eliminate as much as possible the spurious trips. There are different concepts for selection of the trigger level. A "high level" trip could be set based on some percent of the SSE (usually chosen as greater than 60% of the SSE level) and could be designed to minimize spurious trips due to after-shock and low acceleration earthquakes. A "low level" trip would be set to activate on the compressional waves (P waves) when this first arrival caused displacement or acceleration greater than the calculated maximum allowable P wave for an OBE. The decision on the OBE exceedance per acceleration level crossing could be considered as traditional. Considerations have been made regarding advisability of the automated reactor shutdown in case of small earthquakes (Cummings, 1976, IAEA, 1995). There are plants and sites in low and moderate seismic activity regions where an automatic PGA or acceleration level triggered shutdown can be caused by practically harmless ground motions. There are plants that are practically not designed for two levels of earthquakes just upgraded to comply with the SSE related requirements. For these cases the U.S. NRC Regulatory Guide and 1.12, 1.166 and 1.167, and the IAEA as well as the NRC documents on the "Advisability of seismic scram" provide guidance; see the also the (IAEA, 1995).

At the plant in the moderate seismicity regions the operational limit related to the OBE exceedance is formulated in terms of cumulative absolute velocity and spectral amplitude of the acceleration and velocity response spectra measured at the free field; see (NRC, 1997a, 1997b and 2000; EPRI, 1988; 1989, ANS 2002). According to U.S. NRC Regulatory Guide 1.166 the OBE exceedance criteria are as follows:

"The OBE response spectrum is exceeded if any one of the three components (two horizontal and one vertical) of the 5 percent of critical damping response spectra generated using the free-field ground motion is larger than:


The CAV is defined for each component of the free-field ground motion as

$$
\mathcal{CAV} = \int\_0^T |a(t)| \, dt,\tag{8}
$$

Seismic Safety Analysis and Upgrading of Operating Nuclear Power Plants 101

**4.3. Development of emergency operational procedures** 

**5.1. Basic principles and outline of the programme** 

documents (IAEA, 2009b).

Activities that have to be executed during earthquake should be defined and adequate emergency operational procedures for accident prevention should be developed for the case of earthquake. The documents (NRC, 1997a, 1997b and 2000; EPRI, 1988; 1989, ANS 2002)

The development of earthquake related severe accident management guidelines can be performed on the basis of severe accident oriented studies (Section 3.4) and IAEA

The case of Paks NPP is significantly different from the cases of other nuclear power plants regarding the initial basis and objective of their seismic safety programmes. Ab'ovo, Paks NPP has not been designed and qualified for the earthquake loads. The reason was twofold: the site seismicity was underestimated and the design basis was set to the MSK-64 intesity 5 that was equal to the intensity of the historically credible earthquake plus one intensity ball. In mid eighties the safety deficiency had been recognised and a programme for the definition of the site seismic hazard had been launched, which had been extended to a comprehensive site evaluation programme, including geological, geophysical, seismological and geotechnical investigations as for design basis regarding the scope and the methodology. The probabilistic seismic hazard assessment had been completed in 1995 and the design basis earthquake had been defined on the 10-4/a non-exceedance level. The Hungarian Regulatory Authority had approved the new design basis, and requested to

**5. Implementation example – Seismic safety programme at Paks NPP** 

launch a programme for ensuring the compliance with newly defined design basis.

margin method, Seismic Evaluation Procedures of the DoE (see Section 3.2 above).

It was already recognised at the very beginning of the seismic safety programme that a consequent and full scope re-design in line with design codes and standards and subsequent upgrading might be impossible at Paks NPP. Therefore, acknowledging the international practice and IAEA recommendations, the Hungarian authorities allowed the use of methodologies for seismic re-evaluation and re-qualification of operating NPPs, less conservative than the design procedures. Admittedly, in early phase of the implementation of the seismic safety programme of Paks NPP, there was a bloodless hope that the issues at Paks NPP could be managed via application of SQUG/GIP, EPRI deterministic seismic

Contrary to the relative alleviations regarding selection of the re-qualification methodologies, the scope of the seismic safety evaluation and upgrades was set by the regulation as for redesign, covering not only the seismic safety classified SSCs (including interacting items), but the whole scope of safety classified SSCs with three times full redundancy with application of the single failure criterion has been accounted instead of considering a success path and a backup only, etc. Also the process requirements were set as for new design, e.g. the heat

and the (IAEA, 2011) provide guidelines for the development of the procedure.

where a(t) is a component of the ground acceleration, T is the duration of the strong motion.

There are certain rules for the numerical calculation of the CAV: (1) the absolute acceleration (g units) time-history is divided into 1-second intervals, (2) each 1-second interval that has at least 1 exceedance of 0.025g is integrated over time, (3) all the integrated values are summed together to arrive at the CAV.

The CAV check is exceeded if any CAV calculation is greater than 0.16 g-seconds.

If the response spectrum check and the CAV check were exceeded, the OBE was exceeded and plant shutdown is required.

## **4.2. Seismic instrumentation**

The seismic instrumentation has two important roles:


The instrumentation providing response records for the evaluation post-event condition of the plant should be installed at most important/significant location of the structures and main components.

The instrumentation for the judgement on the OBE exceedance has to be designed and fitted to the concept of limitation of the operation in case of earthquake. The instrumentation and voting logic for automatic scram should have the same structure, redundancy etc. as the reactor protection system.

For example the Soviet designed SIAZ (System of Industrial Antisesmic Protection) system initiating automatic reactor scram consists of nine tri-axial accelerometers in three independent systems with independent electric power supply and two sets of them. Contrary to this the tri-axial accelerometers for evaluation of OBE exceedance via CAV and response spectrum criteria should be installed at protected free-field locations.

Regarding design and installation of seismic instrumentation see (NRC, 1997c) and (IAEA, 2003b).
