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In Indian Pressurised Heavy Water Reactors (IPHWRs), the coolant channel assembly consists of two concentric tubes i.e. pressure tube and calandria tube. The fuel bundles are surrounded by pressure tube, an annulus insulating gap and a concentric calandria tube.
Pressure tube housing the natural uranium fuel bundles is rolled at both the ends to stainless steel end fittings.Material of these pressure tubes is autoclaved and 20% cold worked Zr-2.5Nb alloy and of calandria tube is Zr-2. Close coiled helical springs spacers are provided around hot pressure tube to separate it from cold calandria tube and carbon dioxide annulus gas is filled between two to reduce heat loss to moderator.The coolant channel assembly, being one of the most important components, needs detailed analysis under all operating conditions as well as during postulated conditions of accidents for its thermo-mechanical behaviour. One of the postulated accident scenarios for heavy water moderated pressure tube type of reactors i.e. PHWRs is Loss of Coolant Accident (LOCA) coincident with Loss of Emergency Core Cooling System (LOECCS). In this case, even though the reactor is tripped, the decay heat may not be removed adequately due to low or no flow condition. Since the emergency core cooling system is presumed to be not available, the cooling of the fuel pins and the coolant channel assembly depends on the moderator cooling system, which is assumed to be available. Moderator cooling system is a separate system in PHWRs.
In this postulated accident scenario, a structural integrity evaluation needs to be carried out to assess the modes of deformation of pressure tube – calandria tube assembly in a tube type nuclear reactor. The loading of pressure and temperature causes the pressure tube to sag or balloon and come in contact with the outer cooler calandria tube. The resulting heat transfer could cool and thus control the deformation of the pressure tube thus introducing inter-dependency between thermal and mechanical contact behaviour. Depending on the pressure in pressure tube and fuel bundle weight, the tube will balloon or sag and come in contact with calandria tube. The break size of the LOCA will decide the pressure in the PT which further decides the type of deformation of PT. In case of small break LOCA the pressure in PT is high enough to cause ballooning of PT while in case of large break LOCA pressure tube is more prone to sagging under fuel bundle weight.
The objective of this work is to provide insight into thermo-mechanical behaviour of coolant channel assembly in case of accident by modelling two coaxial thin tubes to simulate pressure tube and calandria tube. Sagging and ballooning pattern of PT is to be analyzed when the tubes are subjected to high temperature transient under bending and internal pressure loading.
Focus of the present work would be to understand the sagging and ballooning phenomena of a thin metallic tube under internal pressure and temperature transient and developing an efficient numerical model to predict the same. In case of two coaxial thin tubes, it becomes rather important to understand the role of thermal contact conductance at the interface for effective transfer of heat. In an assembly of two co-axial thin tubes, ballooning/sagging of inner tube due to internal pressure/bending and its subsequent contact with the outer thin walled tube is of specific interest. The analysis involves material, geometric and contact nonlinearities and temperature dependent material properties. Upon excessive deformation, the inner tube develops contact with the outer tube and dissipates away the heat. It is thus necessary to determine the contact pressure distribution in the contact zone and from this, estimate the thermal contact conductance using available correlations.
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