Measures taken in the member countries of the European Communities for anti-seismic design compared to actual US practice

Most countries of the European Communities base their anti-seismic design parameters on specific US earthquake characteristics. There are, however, important discrepancies in the basic data reported on the two continents as well as in their design application. This was one of the topics under discussion within an European working group on methodologies, criteria and standards in nuclear safety. Unlike US practice, in some European countries the maximum earthquake that can be envisaged (corresponding to the Safe Shutdown Earthquake-SEE-in US practice) is defined by adding a margin of safety to the maximum probable earthquake (corresponding to the Operating Basis Earthquake-OBE-in US for which statistical data exist). - Differences exist also in the design parameters to be taken into account in the different European countries especially in the evaluation of the maximum acceleration and on the relationship of the acceleration vs. earthquake intensity. For design purposes, in US as well as in European countries, the assumption is made that seismic waves basically approximate a sustained simple harmonic motion. Under this assumption the Neumann correlation which gives the relationship between the modified Mercalli intensity, the wave period and the ground acceleration is applied. While in the US a whole spectrum of wave periods (from 0.33 to 6.0 sec) -in function of the type of foundation (soil, bed-rock) and the distance of the epicenter- are considered, the European countries base their investigations on shorter wave periods (approximately 0.3 sec). - Mention is made of the existing differences in the relationship of horizontal to vertical acceleration levels. These differences in the evaluation of the earthquake characteristics influence the design to protect the power plants against seismic effects especially as far as stress and strain limits for structures and components within the elastic range and in the excess of yield are concerned