Assessment of core cooling capability of emergency core cooling system in LBLOCA condition for TAPS 3 and 4

A Large Break Loss of Coolant Accident (LBLOCA) in Pressurized Heavy Water Reactor (PHWR) occurs when a large diameter pipe ruptures such as the Reactor Inlet Header (RIH), Reactor Outlet Header (ROH) or Pump Suction Line (PSL). Emergency core cooling system (ECCS) is provided as an important safety system to cool the core and thereby limit the release of fission products from the fuel in the event of a postulated Loss-Of-Coolant Accident (LOCA). The design requirement of ECCS is to provide sufficient cooling to the reactor core following a LOCA, so as to limit the release of fission products from the fuel and to ensure coolable geometry of the fuel channels. The ECCS incorporated in Tarapur Atomic Power Station-3 and 4 (TAPS-3 and 4) involves the following two stages; high pressure light water injection by accumulators and long-term recirculation from the suppression pool through ECCS pumps. The computer code ATMIKA, developed in-house in NPCIL, is being extensively used to analyze LOCA in Indian Pressurized Heavy Water Reactors (PHWRs). In LBLOCA, two-phase conditions will be experienced early in the blowdown phase and will result in either persistent flow reversal or persistent low flow condition in part of the reactor core. Reduction in heat removal, occurring with the deterioration of the heat transfer from the fuel pin due to conversion of heat transfer mode from sub cooled forced convection to film boiling, causes deposition of fission/decay energy inside the fuel and the fuel temperature rises. At higher temperatures, expected in the limiting breaks, sheath also starts interacting with steam which being an exothermic reaction, results in more heat generation. To maintain coolable geometry and limit fuel failure it is necessary that the stored heat in the fuel and decay heat should be removed by the discharging hot coolant and injecting cold ECCS water. In this paper, LOCA analysis has been carried out with ECCS available as per design intent. Further studies include partial degradation in ECCS due to failure of opening of header injection valves in any one of the headers in scenarios involving different break locations. As a matter of defence-in-depth, analysis has also been done to examine the effect with only long-term recirculation from suppression pool through ECCS pumps. (authors)