THERMAL HYDRAULIC ANALYSIS OF SUSPENDED DEBRIS FORMED DURING SCDA IN PHWR
The 220 MWe Indian Pressurized Heavy Water Reactor (PHWR) has safety features and accident management guidelines to mitigate the Design Basis Accidents (DBAs) and to resist the progression of Beyond Design Basis Accidents (BDBAs). Severe Core Damage Accident (SCDA) falls under BDBA category, in which geometry of the core is considered to be damaged/lost. During postulated scenario of large break Loss of Coolant Accident (LOCA), with failure of all heat sinks like Emergency Core Cooling System (ECCS), the fuel channel temperature likely to rise due to un-availability of coolant inside channels. Postulated moderator cooling system and make up water (through accident management) failure leads to a slow un-covering of top channels of the Calandria. The exposed channels in steam environment are known as suspended debris. With the passage of time, it is expected that exposed channels finally sags and may come in contact with each other. Present investigation aims to study the heat up behaviour of top five exposed channels of the Calandria for 220 MWe PHWR. The analysis considers that channels are stacked on another. It is postulated that decay heat of submerged fuel channels produce steam and acts as coolant for the suspended channels forming debris like configuration. 2D Numerical simulation has been done for one meter section of the channels using ANSYS Fluent software at 1% decay power. Results shows that the maximum temperature of the fuel channels goes up to 998°C. Radiation is found to be the dominant mode of heat transfer and it contributes to 76.1% of total heat transfer. The study concludes that the bundles of the channels are heated upto nearly 1000°C, however steam cooling helps to limit temperature excursion.