National Report of the Slovak Republic - Radiation and Nuclear Safety , September 1998

5.1.2 Meeting of Criteria at Bohunice and Mochovce Sites

Criteria for the site selection of nuclear installations for the WWER-440/230 units corresponded to the then applicable Soviet standards and approaches, with radiation protection of the public provided for by distance (this also corresponded to the approach in other countries during the 50s). The principle of a three kms wide protection zone with no permanent settlements was applied in selecting the site.

During the time of the site selection, designing and construction of NPP Bohunice, no standards providing for the construction of NPP in a seismic area existed either in Czechoslovakia or in other countries. The Czechoslovak standard "Seismic Loads of Constructions" was used. The standard envisaged earthquakes with a probability of once in 200 years and an intensity of 6.4o MCS at the Jaslovské Bohunice site. No earthquake with an intensity of 8o MCS as recorded in 1906 as the strongest earthquake in the area, with the epicenter at a distance of 17 kms in the area of the Little Carpathian mountains has been considered for the NPP site, mainly because of the profile and the slope of the area.

Re-assessment of sesimic load of the Jaslovské Bohunice site was included into the safety upgrading projects of NPP Bohunice units. The resulting seismic effect was calculated and confirmed, later on, by IAEA missions (see Section 2.1.1) are as follows :

The calculations and analyses supported the necessity to upgrade the seismic resistance of category 1 buildings and structures which have to withstand SSE without any damage. This category includes buildings and structures the failure of which may cause damage to components or systems needed for safe shut-down and post-cooling of the reactor or the failure of which may cause radioactivity leaks. The list of such buildings and structures was approved by IAEA.

Background materials for the selection of the MOCHOVCE site were developed in 09/79, in line of the applicable legislation; the selection was performed in 11/79. The general designer prepared and forwarded to the investor on January 31, 1980, a study of a series of buildings; this study exceeded the scope provided for by Decree No. 163/1973 Coll. on Documentation of Buildings. Study of a series of buildings was developed at a time when basic problems concerning the master plan of the power plant by Soviet designers have not yet been resolved - the issues concerning the master plan could not be discussed since the seismicity level of the NPP site was not known. Seismicity issues could be concluded in 07/80 and a more detailed geological survey was started in 09/80.

The survey gave not very favorable results, and this caused that the units were moved elsewhere to meet the requirement concerning the location of category I premises (with respect to seismic resistance) according to the then applicable Soviet standards (VSN 15-78). The site master plan was completed in 03/81. The commission selected the site in 11/79.

So-called zone proceeding started at ONV Levice on September 18, 1980, and a license together with an annex were issued in 06/81 with respect to the NPP Mochovce site from the viewpoint of seizmicity issues. Subsequently, the designer started developing the "Project" according to Decree No. 163/73 Coll. On the Documentation of Buildings, and the preparatory works at the Mochovce site.

The original project of NPP Mochovce was developed based on the knowledge concerning the seismic hazards of the site collected during the preparatory and design period of the NPP Mochovce construction in the 80s, considering earthquake of MSK grade VI for safe shutdown of the reactor, and horizontal acceleration value of PGA = 0.06 g. Legislative development represented by IAEA guide 50-SG-D15 introduced the smallest acceleration value in horizontal direction of 0.1 g for nuclear power plants.

Based on this, seismic re-assessment of "Selected Buildings and Technological Systems" was performed in the framework of the SE-EMO unit 1 safety upgrade program, and improvement of building structures are gradually implemented.

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