Influence of gaps on seismic behaviour of the primary system of a PWR

This paper deals with the influence of gaps on the dynamic behaviour of the Reactor Coolant System (RCS) of the french PWR when subjected to an earthquake event. One of the basic rule of the french RCS design is that primary components and coolant pipes should not be constrained during the thermal motions of the RCS and of Civil Works. This purpose can be ensured by using, for the RCS lateral support, snubbers which can accomodate without constraints any slow motion. But this solution is not always possible due to mechanical limits, space constraints, or even costs. An other solution, which is used in the french plants for the lateral support of the steam generators, is to use stops with adequate gaps. These gaps, which are adjusted with shims during the hot functional testing, change during lifetime, particularly because of temperature transients. In a first step, the paper gives insight on the reality of these geometrical gaps; the measures of the size of the gaps on site are in good agreement with calculation results. In a second step the paper shows the mechanical analysis process, taking into account the gaps, using a seismic non linear time history analysis of a RCS-building coupled model subjected to base motions (translations and rotations); the time history analysis uses a very performant modal superposition method in a relative coordinated system. (orig./RW)