INVESTIGATIONS ON PHWR DISASSEMBLED FUEL CHANNEL DURING SEVERE ACCIDENT CONDITION

Abstract

Several safety measures and accident management guidelines are provided in the nuclear power plants (NPP) for the proper and smooth working of the reactor and to mitigate any accidental scenario and its consequences. Considering severe accidents in a NPP is a vital component in the safety analysis. Under normal operating conditions, pressurized heavy water is used to remove heat generated by the fission reaction in the fuel bundle. In case of Loss of Coolant Accident (LOCA), the coolant flow decreases or is lost altogether in the reactor channel due to breakage in the header or feeder of inlet or outlet of the pipeline. Under postulated LOCA condition, decay heat in the fuel channel is removed by the Emergency Core Cooling System (ECCS). The occurrence of such condition has a probability less than 10−4 per year but its consequences are very high. Further failure of ECCS reduces the convective cooling by coolant resulting in to the temperature excursion of the fuel channel. With time, all the coolant may be converted into steam, and the fuel channel may be fully voided. High temperature along with the internal pressure leads to creep deformation of pressure tube (PT). The deformation of PT takes place by two modes, sagging and ballooning. After deformation, whether by sagging or ballooning, PT establishes contact with calandria tube (CT) that restrict further rise in the temperature of the channel by transferring its heat to the heavy water available outside the CT as moderator, which acts as a heat sink. If the moderator cooling and makeup system fails, the moderator gets heated up and starts boiling. Continuous evaporation causes few top channels uncovered and at high temperature, channels deform and relocate inside the calandria vessel. Several channels burst near the junction of the fuel bundle and attain horizontal as well as inclined position. The steam exposed to fresh zirconium causes an exothermic reaction. This rapidly increases the fuel channel temperature and hence produces hydrogen. When the weight of the suspended debris bed exceeds the load-carrying capacity of the submerged channel, all the channels may get quenched in remaining moderator and settle down at the bottom of the calandria vessel. Decay heat at the time of such accident is 2-5% and further decreases with time.