**5.6. Summary of assumptions, codes and standards and methods**

The assumptions accepted for the re-evaluation are summarised in the Table 4. The applicable codes and methods are summarised in the Table 5.

The mixed use of the codes was excluded by careful definition of the evaluation packages. The assumptions, allowable stresses, etc. of the KTA and ASME have been compared.

The operability of active technological components should be qualified by empirical requalification procedures or test. The equipment classes and applied empirical qualification methods for active and certain passive components are summarised in the Table 6.


Seismic Safety Analysis and Upgrading of Operating Nuclear Power Plants 115

GIP-VVER, tests if the item does not fit to

Test if the item does not fit to the database

**Equipment classes Recommended qualification procedure** 

4. Transformers GIP-VVER experience data or tests

Equipment classes Recommended qualification procedure

23. Vertical and Horizontal Tanks Limited analysis, GIP-VVER

The final evaluation of the effectiveness of upgrading measures and justification of the acceptable level of achieved safety in terms of CDF have been made via seismic PSA (Katona & Bareith, 1999; Bareith, 2007; Elter, 2006). The seismic PSA demonstrated that the CDF ensured by the implementation of rather extensive upgrading programme is of order of magnitude 10-4/a. The PSA identified also several week links. For example, the capacity of the joints of the turbine hall structure was found insufficient. Eventual collapse of the

the database

1. Motor Control Centres GIP if applicable

A. The original twenty classes:

2. Low Voltage Switch-gears 3. Medium Voltage Switch-gears

5. Horizontal Pumps 6. Vertical Pumps

11. Cooling Devices 12. Air Compressors 13. Motor Generators 14. Distribution Panels 15. Batteries on Racks

17. Engine Generators 18. Instrument Racks 19. Sensor Racks

B. Additional classes:

25. Ventilation Ducts

**5.7. Seismic PSA** 

Solenoids, Sensors

C. Additional VVER classes:

26. Cable Trays and Conduits 27. Small and Large Bore Cold Pipes **Table 6.** Summary of qualification methods

9. Fans

7. Fluid-Operated Valves 8. Motor-Operated Valves

10. Air Conditioning Devices

16. Battery Chargers and Inverters

20. Control Panels and Cabinets

21. Relays, Switches, Transmitters,

22. Electrical Penetration Assemblies

24. Vertical and Horizontal Heat Exchangers

**Table 4.** Summary of applicable standards and methods


**Table 5.** Summary of applicable standards and methods


**Table 6.** Summary of qualification methods

### **5.7. Seismic PSA**

114 Nuclear Power – Practical Aspects

Passive equipment (tanks, pressure vessels, etc.)

**Load combinations NOL+DBE** 

Damping, ductility Code values or realistic for repeated checking of outliers

In specific case best estimate

Capacity evaluation Design type evaluation KTA, primary system and vital

Margin type evaluation CDFM+ASME

mechanical equipment and pipelines inside the confinement

ASME BPVC Section III, Service level D

ASME BPVC Section III, Service level D

KTA 3205; Subsection according to

area

Simplified evaluation Code based simplified procedures

KTA 3201/3211 Supports ASME BPVC Section III Subsection NF

KTA 3201/3211 Supports ASME BPVC Section III Subsection NF KTA 3205; Essential nozzles ASME BPVC Section III, Service level D KTA 3201/3211

KTA 3201/3211 Supports ASME BPVC Section III Subsection NF KTA 3205;

Classes. Essential nozzles ASME BPVC Section III, Service level D KTA 3201/3211

Floor response spectra Conservative design floor response spectra.

Material strength Minimum values determined by standard

Operability GIP or GIP-VVER, if applicable, otherwise test

**Equipment Item Applicable standards** 

Interactions GIP, GIP-VVER

Interactions GIP, GIP-VVER Pipelines Pipelines ASME BPVC Section III, Service level D

Interactions GIP, GIP-VVER

**Table 4.** Summary of applicable standards and methods

Component body including internal parts

Active equipment Operability GIP-VVER Component body including internal parts

**Table 5.** Summary of applicable standards and methods

Structural models Graded approach to the modelling: best estimate if applicable

The final evaluation of the effectiveness of upgrading measures and justification of the acceptable level of achieved safety in terms of CDF have been made via seismic PSA (Katona & Bareith, 1999; Bareith, 2007; Elter, 2006). The seismic PSA demonstrated that the CDF ensured by the implementation of rather extensive upgrading programme is of order of magnitude 10-4/a. The PSA identified also several week links. For example, the capacity of the joints of the turbine hall structure was found insufficient. Eventual collapse of the 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.

Seismic Safety Analysis and Upgrading of Operating Nuclear Power Plants 117

**S PSA at Paks NPP** 

*Nonlinear soil for GM; Analysis of liquefaction;* 

*Event tree/ fault tree* 

*Detailed information available, from the previous works* 

*Based on the extensive previous works* 

*Detailed information available + fragility development based on the results of the performed analyses; Containment + Liquefaction* 

*Screen and limited fragility development* 

*Certain additional needs for upgrades identified* 

*uncertainty evaluated* 

*modelling* 

*Hazard curve; UHRS;* 

The seismic recording systems composed from tri-axial accelerometers that are installed at critical locations of structures and main components, provide information for the post-event

The comparison of the seismic re-evaluation methods "as usual" and the methods applied at

**NPP** 

*1.208* 

*active* 

*exceedance, site specific GMRS (UHRS), nonlinear soil, DRS as per Reg. Guide* 

*All Safety related (+ interacting)* 

*Conservative structural response, (Gmin, Gmax, Gav), conservative FRS, median FRS in limited cases, Class 3 outside of containment* 

*Walk-down and screening per margins considerations and GIP, GIP-VVER, only the bounding spectrum criterion was accepted.* 

*As per new design for Class 1 and 2 SCs, realistic damping and ductility for Class 3; Testing, GIP, GIP-VVER and replacement for* 

*Qualification per screening Test or replacement* 

*Replacements, upgrading per design requirements* 

CDF *Design basis reconstituted Weak links, CDF and its* 

**Seismic Margin Seismic PSA DB reconstitution at Paks** 

evaluation of the plant condition.

Paks NPP is shown in the Table 7.

Scope Success +

Structural response

Evaluation, qualification

Relay qualification backup path

screening per margins criteria, experiencebased

Analysis of selected SCs (CDFM)

Screening; qualification of outliers

needed

HCLPF

Modifications Upgrades if

Results Plant level

Median structural response, frequency shifting

Screening Walk-down and

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

Input RLE Hazard curve *PSHA: median 10-4/a non-*

tree

Event tree/ fault

Walk-down and screening per fragility estimations

Selected fragility calculations. Median capacities+ log standard deviations

Screening + limited fragility

Risk informed upgrades

shear modulus; CDFM – Code Deterministic Failure Margin (see Section 3.2.2).

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

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

Probabilistic structural response

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 5.2.2 are focused on the liquefaction hazard.

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 effort of the plant and plenty of walk-downs.
