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

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

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

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

removal after the design base earthquake shall be ensured unlimited in time, contrary to the 72 hours requirement applicable in usual margin-type assessment.

Seismic Safety Analysis and Upgrading of Operating Nuclear Power Plants 103

**Figure 1.** Structure and tasks of seismic safety programme at Paks NPP

The real objective has been clearly understood after performing the first Periodic Safety Reviews in 1999, since the compliance with the just issued Nuclear Safety Regulations (Governmental Decree No 108/1997) requested to be achieved and demonstrated. It was recognised that the methodologies mentioned above does not provide the required for Paks NPP result regarding design base reconstitution, on the other hand they can't be directly applied for VVER plants, certain adaptation was needed for accounting the VVER design features.

The qualification of the Paks NPP have been executed as for the newly defined design basis earthquake by applying procedures and criteria for the new design, combined with the methods and techniques developed for seismic re-evaluation of operating nuclear power plants. The seismic safety programme of the Paks NPP is presented below in Figure 1.

The description of the project as given below clearly indicates the similarities and differences between the programmes as understood in (Gürpinar & Godoy, 1998; Campbell at al, 1998) and programme at Paks NPP (Katona, 2001).

The selection and use of methodologies has been graded in accordance with safety relevance of the system, structure or component.

The Hungarian regulation requires performance of probabilistic safety analyses for internal and external events/initiators. Therefore, after implementing the seismic safety upgrading measures, the achieved level of safety has been quantified via seismic PSA, which provides the value of the CDF and also indicated certain week links to be avoided or accounted.

The implementation of the programme was broken into three phases:

Preparatory phase before 1995: The objective was to prepare the programme in a way that it could be executed within reasonable technical and economical limits (example see in Section 5.4). Learning and trial of the methods were going on simultaneously with the site evaluation. The conservatism had to be handled carefully during the seismic safety assessment. The easy to perform fixes had been designed for preliminary conservative seismic input and implemented. The easy-fix project covers 10184 items for 4 units. The volumes of the works are given in Table 2. Total amount of structural steel used for fixes is equal to 445 tons. Safety related batteries for all four units have been replaced and fixed in the frame of the easy-fix project.

Between 1996-1999: Selection of the methodology, evaluation of as-is seismic capacity and identification of the fixes had been performed.

Design and implementation of fixes 1999-2003: The amount of works is illustrated in Figure 2 and 3 and in Table 3.

The programme was broken down into manageable tasks and projects while the uniformity of the requirements and assumptions between these tasks had been ensured by appropriate quality assurance programme and methodological and criteria documents developed for each task.

**Figure 1.** Structure and tasks of seismic safety programme at Paks NPP

removal after the design base earthquake shall be ensured unlimited in time, contrary to the 72

The real objective has been clearly understood after performing the first Periodic Safety Reviews in 1999, since the compliance with the just issued Nuclear Safety Regulations (Governmental Decree No 108/1997) requested to be achieved and demonstrated. It was recognised that the methodologies mentioned above does not provide the required for Paks NPP result regarding design base reconstitution, on the other hand they can't be directly applied for VVER plants,

The qualification of the Paks NPP have been executed as for the newly defined design basis earthquake by applying procedures and criteria for the new design, combined with the methods and techniques developed for seismic re-evaluation of operating nuclear power plants. The seismic safety programme of the Paks NPP is presented below in Figure 1.

The description of the project as given below clearly indicates the similarities and differences between the programmes as understood in (Gürpinar & Godoy, 1998; Campbell

The selection and use of methodologies has been graded in accordance with safety relevance

The Hungarian regulation requires performance of probabilistic safety analyses for internal and external events/initiators. Therefore, after implementing the seismic safety upgrading measures, the achieved level of safety has been quantified via seismic PSA, which provides the value of the CDF and also indicated certain week links to be avoided or accounted.

Preparatory phase before 1995: The objective was to prepare the programme in a way that it could be executed within reasonable technical and economical limits (example see in Section 5.4). Learning and trial of the methods were going on simultaneously with the site evaluation. The conservatism had to be handled carefully during the seismic safety assessment. The easy to perform fixes had been designed for preliminary conservative seismic input and implemented. The easy-fix project covers 10184 items for 4 units. The volumes of the works are given in Table 2. Total amount of structural steel used for fixes is equal to 445 tons. Safety related batteries for all four units have been replaced and fixed in

Between 1996-1999: Selection of the methodology, evaluation of as-is seismic capacity and

Design and implementation of fixes 1999-2003: The amount of works is illustrated in Figure

The programme was broken down into manageable tasks and projects while the uniformity of the requirements and assumptions between these tasks had been ensured by appropriate quality assurance programme and methodological and criteria documents developed for

hours requirement applicable in usual margin-type assessment.

certain adaptation was needed for accounting the VVER design features.

The implementation of the programme was broken into three phases:

at al, 1998) and programme at Paks NPP (Katona, 2001).

of the system, structure or component.

the frame of the easy-fix project.

2 and 3 and in Table 3.

each task.

identification of the fixes had been performed.


Seismic Safety Analysis and Upgrading of Operating Nuclear Power Plants 105

dampers)

1500 fixes

building, relays qualification

2000-2002 160 t of added steel structures

**Figure 3.** Steel-frame bridge between localisation towers for fixing the frames of the reactor hall

**Qualification and upgrades Time frame Tasks/Volume** 

Electrical and I&C equipment 1993-2002 Qualifications, replacements

and equipment 1997-1999 250 fixes (GERB viscous-

Structure of the turbine and reactor hall 1999-2000 1360 t of steel fixes

Support bridge at localization towers 2000-2001 300 t of steel fixes

circuit and the components 1998-2000 760 fixes

Other classified pipelines and equipment 2001-2002 80 fixes

Measures identified by seismic PSA 2002- Fixing the joints in the turbine

(columns) in cross-wise direction

High energy pipelines of primary circuit

Other classified pipelines of primary

Classified piping and components of secondary circuit, fixes of supporting steel structures in the turbine building

**Table 3.** Tasks and work volumes of the easy-fix project

**Table 2.** Tasks and work volumes of the easy-fix project

**Figure 2.** Viscous-dampers below the steam-generator

**Figure 3.** Steel-frame bridge between localisation towers for fixing the frames of the reactor hall (columns) in cross-wise direction


**Table 3.** Tasks and work volumes of the easy-fix project

**Task number of items fixes** 

mechanical equipment 202 anchorages

electrical equipment 465 anchorages

cable trays 2498 anchorages

brick walls 281 steel frame fixes

total number of easy-fix items 5507

**Table 2.** Tasks and work volumes of the easy-fix project

**Figure 2.** Viscous-dampers below the steam-generator

I&C (cabinets, racks) 2061 anchorages and top bracing
