Have state-of-the-art seismic analyses been performed for the Krsko nuclear power plant site (e.g. probabilistic seismic hazard analyses, seismic margin analyses, seismic PSA)? Did these analyses result in any upgrading? If such analyses are planned, what is the schedule for their completion?

State of the art seismic analyses have been performed in the scope of Individual plant Examination for external Events. In this scope site specific Seismic Hazard Analysis as well as Seismic PSA (level 1 and level 2) were performed. Analyses were completed in 1995. Team of technical experts through IAEA Technical Assistance programme has also reviewed them. In the analyses identified weak points have been upgraded (for example: MCR ceiling was replaced, some of the electrical distribution panels were additionally welded to the in concrete imbedded steel structure, component cooling surge tank support was strenghtened, supports of several valves installed on the pipes penetrating the containment were modified, analysis of relay chattering was performed and appropriate procedure for corrective actions was established.

How does the seismic hazard curve used in the Krsko probabilistic safety assessment compare with the seismic hazard information developed by the Geophysical Survey of Slovenia?

In fact seismic hazard curves used in NPP Krsko PSA analysis have been developed by domestic organisations including Geophysical Survey of Slovenia. Work was co-ordinated by University of Ljubljana Department of Civil Engineering Institute for Structural and Earthquake Engineering.

What seismic studies related to the Krsko nuclear power plant (including those performed with PHARE funding) have been carried out since the International Commission of Independent Safety Analysis (ICISA) report was issued in 1993, and to what extent was the ICISA seismic evaluation programme proposal included in these studies? What are the results of these studies?

Before ICISA was formed, SNSA issued in early 1991 a Decree to the NPP Krsko requesting to perform PSA level 1 and 2 for internal and external events (including seismic). All recommendations from 1993 ICISA report were addressed in the seismic PSA study. After completion of PSHA analysis an extensive geological, seismological and geophysical programme have been proposed by SNSA and in first phase carried in period 1993 - 1996. International expert panel (through IAEA Technical Assistance Programme) has externally reviewed results. It was recommended that additional geophysical investigation should be conducted in Krsko valley. These additional investigations covering 40 km of seismic reflection profiles and sponsored by PHARE Programme are in progress and are scheduled to be completed next year. By this seismic issue will be closed.

17.1. External events taken into account

Could Slovenia give some information on the methodology used to define the level of the external events taken into account in the design?

The plant is designed regarding the effects of natural phenomena in accordance with Criterion 2 of Appendix A to 10 CFR 50 General Design Criteria for Nuclear Power Plants.

External flood: Maximum probability method has been applied to define 10.000 year flood level

Seismic design: Regarding seismic design NPP Krsko is designed for 0.3g SSE (Safe Shutdown Earthquake) and 0.15 OBE (Operational Basis Earthquake). Its definition included maximum acceleration value, response spectrum and the time histories of acceleration/maximum acceleration value. For the design spectrum, the one recommended by the USAEC Regulatory Guide 1.60 was adopted. For the time histories of acceleration, artificial time histories corresponding to the design spectrum were used. The above values have been determined based on site geological and seismological investigation.

Wind loadings: The fastest kilometre speed applied was 140 km/h at 10.0 meters above grade.

Missile protection: Objectives governing the design input were:

• The integrity of the Containment System must be maintained.

• The capacity for safe shutdown of the reactor and maintenance of core cooling capability must be ensured.

• A missile accident which is not caused by a loss-of-coolant accident does not initiate a loss-of-coolant accident.

17.2. Radiological impact of the plant

Could Slovenia indicate if the regulations prescribe dose limits for design basis accidents?

Dose limits ffor design basis accidents applicable for NPP Krsko are limits from 10CFR100 250 mSv total radiation dose and 3 Sv to the thyroid within two hours at the border of the exclusion area - 500 m for NPP Krsko. These limits were approved in the FSAR on the basis of the recognition of US regulations if domestic do not exist.

17.3. Siting revaluation

Under Article 14, the report indicates that the analysis of the impact of site seismic activity on nuclear safety is ongoing. Could Slovenia give more information on this safety review?

NPP Krsko performed seismic PSA analysis in the scope of IPEEE (Individual Plant examination for External Events). During IAEA technical review of the NPP Krsko PSA study it was recommended that additional geophysical and geological investigation of the vicinity of NPP Krsko should be conducted to reduce uncertainty related to seismic inputs. 

(See also answer to Austrian question on Article 17).

Is there a surveillance programme for assessing the validity of site parameters like land use and population, meteorological, hydro-geological or seismic information?

Site related factors described in FSAR i.e. (meteorology, hydrology, seismology, population density, nearby industrial facilities) are followed by different monitoring programmes:

• Meteorological monitoring

• Seismological monitoring

• Hydrological monitoring

Population density, nearby industrial facilities and transportation routs and activities are inputs for the Emergency plaan update.

Exclusion Area of 500 m and Low Population Zone of 1500 m are subject to restrictions in land use, prescribed in the Site Permit.

Please describe the programme for re-evaluating site-related factors and which are the rules and criteria applied?

Generally, site-related factors used as the input to the original design are not allowed to change in adverse direction.

Site related factors described in FSAR i.e. (meteorology, hydrology, seismology, population density, nearby industrial facilities) are followed by:

• Different monitoring programmes,

• Plant and regulatory body review and approval of changes in population density and in industrial facilities.

Site related factors were also re-evaluated as a part of external events PSA study.