Analysis for postulated loss of coolant accident for TAPS - 1 and 2

Full text: Over years of operation, Tarapur Atomic Power Station (TAPS) has undergone several modifications viz, the reactor operating at de-rated conditions of 160.0 MWe, the secondary side of secondary steam generators has become part of the primary side due to tube leakage etc. In view of the plant modifications carried out till date, considering the current status of the plant operation and advancing methodology of analytical techniques, a programme of carrying out fresh safety analysis involving various break sizes was worked out for regulatory assessment. The analysis of bounding events was performed using the current best estimate computer code RELAPS MOD3.2, to evaluate and demonstrate the adequacy of existing design of emergency core cooling system including effectiveness of the low pressure core spray system (LPCS) as per the design intent. These simulated enveloping cases were of large LOCA category, which involved the double-ended rupture of recirculation line and steam line respectively. Additionally, the analysis also covers simulation of small and medium rupture in recirculation and steam line. The modeling aspects, inputs and assumptions used and the results explaining thermal-hydraulic phenomena of these events are discussed. In these analyses, the reactor core is modeled using one hot channel, two average channels and a core bypass channel. The thermal-hydraulic processes of the postulated events are generally divided into a blowdown phase and an emergency core-cooling phase initiated by low pressure core spray system. It is seen that during LOCA, the blowdown process provides the initial core cooling, while core re-wetting is achieved during refilling phase. The role of bypass channel in the transient, which governs core re-wetting during the refilling phase, is presented. The core thermal-hydraulics performance and the effectiveness of the core spray cooling are brought out