EXPERIMENTAL INVESTIGATION OF SINGLE PHWR DEBRI

Abstract

In PHWR, pressure tube (PT) and calandria tube (CT) are fitted to the calandria vessel with the help of rolled joints. Rolled joints may fail due to reduced strength at high temperature, the pressure tube (PT) and calandria tube (CT) assembly drops down to the bottom of calandria vessel and settles at the bottom of calandria vessel. In this situation, the pressure tube (PT) touches the calandria tube (CT). When this situation occurs in nuclear reactor, the fallen CT, PT and fuel bundles are called debris bed. Due to decay effect, the fuel bundle could generate heat upto 2% of the rated capacity. This heat is dissipated to the moderator water. As a result, vapour may be generated inside the PT. Vapour locking may create hot spots on PT and fuel bundle, which may lead to meltdown of the fuel bundle assembly. Hence, there is a need to investigate the heat transfer phenomenon in a debris bed. From experimental investigation, it is observed that the temperature of the fuel rod or PT or CT at any moment is not greater than 98°C. It means whatever amount of heat is given to the fuel rod by bus-bar, water absorbs that heat and evaporates. It will continue to evaporate until the whole water in the tank dries out. As long as moderator water is present in nuclear reactor, overheating of fuel rod (FR), PT and CT may not take place. The present dissertation described the experimental and numerical investigation carried out at IIT Roorkee for debris bed heat up experiment. To simulate the debris bed, experimental set-up was designed and fabricated to study the behavior of heat transfer due to the formation of debris bed in Indian PHWR. The experiments were carried out at different heating rates (4, 6, and 8 KW). The circumferential temperature variation in the Fuel rod, PT and CT were measured with the help of thermocouple. For, the present study commercial available FVM package FLUENT (Version 6.2 and GAMBIT 2.3) is used for computational analysis. The experimental and numerical analysis predicts good results which can be applied for the safety of Indian PHWR.