Comparison between small LOCA scenarios in Eastern and Western type PWRs

In the frame of the use of the Relap5 thermal hydraulic code in the predictions of LOCA transient scenarios in PWRs and considering the recent development of a methodology to evaluate the related uncertainty, the response to a Small Break LOCA of Eastern and Western type PWRs has been analyzed. A four loop/horizontal Steam Generator WWER-1000 (KOZLODUY in Bulgaria) and a two loop/vertical U-tubes Steam Generator Westinghouse (KRSKO in Slovenia) nuclear power plants have been considered in the analysis. The reference transient is a 2% equivalent cold leg break accident, without High Pressure Injection System intervention, as specified in the frame of a ``counterpart test`` activity involving experimental tests on four Integral Test Facilities: LOBI (European Community), SPES (Italy), BETHSY (France) and LSTF (Japan). The code results in the two cases, also taking into account the related uncertainty as evaluated by means of the aforementioned methodology, are discussed in the paper. These show a different behavior of the two plants; in particular, in the case of the WWER-1000 safe conditions are maintained, i.e. no core dryout occurs, without operator interventions, for much more time than in the case of Westinghouse plant. The paper gives some explanation of this fact, consideringmore » the different lay-out of the two considered plants.« less