STUDY OF THERMO-MECHANICAL DEFORMATION OF SCALED DOWN FUEL CHANNEL UNDER HEATUP
The reactor core 220 MWe Indian Pressurized Heavy Water Reactor (PHWR) consists of 306 fuel channels. Each channel consists of three main components, Pressure Tube (PT), Calandria Tube (CT) and Fuel Bundles (FB). Heavy water coolant carries heat generated by FB to steam generator, by circulation through primary heat transport system. Postulated Initiating Event like Large Break Loss of Coolant Accident (LBLOCA) along loss of Emergency Core Cooling System (ECCS) and failure in moderator cooling circuit may lead to Severe Core Damage Accident (SCDA) for PHWR. Such postulated accident proceeds with low-pressure inside coolant channels. Continuous boil-off of moderator with channel decay-heat will lead to exposure of fuel channels in upper section of the core. The exposure leads to heat-up of the channels. This induces the deformation of channels under its self-weight and fuel bundle weight. For single fuel channel, deformation can be envisaged as the first PT sags and contact the CT and subsequently both deform as one unit. If this postulated accident continues then fuel channels will disassemble and collapse at Calandria bottom forming a debris bed. It is planned to study the deformations aspects through experiments with a scaled-down (1:3) fuel channel. An analysis of the scaled-down model is carried out to generate insights on the deformation pattern.
A multi-physics coupled-field finite element model, with ANSYSv16.2, is used to study the thermo-mechanical deformation of a scaled-down channel. The study also includes the effect of bundle and its weight on stiffness of the coolant channel under this low-pressure event. Coupled-field element is used for analysis because of non-linear dependency between thermal and structural fields. The paper describes the analysis methodology, thermo-mechanical deformation of PT and CT, and the influence of fuel bundles on stress-levels generated at the bundle junctions.