Indian Pressurised Heavy Water Reactors (PHWRs) are tube type of reactors. The coolant channel assemblies need detailed analysis under all conditions 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, 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. In this postulated accident scenario, a structural integrity evaluation has been 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 (by weight of fuel bundle) and/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. The amount of heat thus expelled significantly depends on the thermal contact conductance between the two tubes. Deformation of pressure tube creates a heat removal path to the relatively cold moderator. This in turn limits the temperature of fuel for a sufficiently long period and ensures safety of the plant.