Steam Generator Blockage: A Thermal-Hydraulic Approach Based on CATHARE 2

In some French steam generators (SG) with broached tube support plates (TSP), the TSP flow paths have become blocked by deposits. Due to these blocked broached TSP, flow paths decrease the flow area and result in increased void fraction in the tube bundle region and pressure drop across the TSP. As a consequence, it also leads to a decrease of the recirculation ratio which can be observed through the SG level measurement. The major impacts of this phenomenon can be classified in three main categories: the mechanical impact on the TSP due to an increase of the pressure drop, the impact on the safety demonstration itself due to the less secondary side inventory that have to be considered for the thermal-hydraulic studies and finally the impact on the normal transients mainly due to the level oscillation encountered in the steam generator. This paper which focuses on the last item presents the Insitute for Radiological Protection and Nuclear Safety (IRSN) approach to calculate with CATHARE 2 [1] code the response of steam generators experiencing the blockage: both are considered transients with only a “stand alone” steam generator and transients performed with our SImulator for Post-Accident so called “SIPA” [2 to 4] which enables to simulate normal and accident scenarios. Sensitivity calculations on the blockage ratio, on the blockage distribution and on the power level are made to be able to assess this phenomenon. Some conclusions are arrived at the power levels that should be adopted to deal with blockage in order to restore the water level stability.