In Pressurised Heavy Water Reactors, the fuel bundles are located inside coolant channels. Each coolant channel assembly consists of a pressure tube, surrounded by a concentric calandria tube. Calandria tube is joined at its ends to calandria side tube sheets through sandwich type rolled joints. During reactor operation, there is a gradual increase in the sag of the coolant channel assembly due to irradiation creep and growth. As the life of the pressure tube is limited due to degradation issues, there is a need to replace it multiple times during the licensing period of the reactor. However, excessive sag of the calandria tube makes the installation of a new straight pressure tube very difficult, necessitating replacement of the calandria tube also. The method adopted for removal of calandria tube is induction heating and cooling of the rolled joint followed by application of pulling load on the calandria tube. The rolled joint detachment tool being developed for this has three modules namely, induction heating module, axial load module and radial load module. The radial load is applied on the inside surface of the calandria tube for gripping it firmly during its removal. However, higher radial load on the calandria tube can lead to its puncturing due to its low wall thickness and yield strength whereas, lower radial load can cause slippage between the radial gripper and the calandria tube surface. Hence, in order to meet the objective of application of maximum radial load without damaging the calandria tube, a 3D finite element model of the calandria tube has been prepared using commercially available finite element software, COMSOL. A large number of analyses have been carried out to estimate the stress pattern in the calandria tube before finalising the radial gripping force with different loading schemes. This paper describes the methodology of detachment of sandwich rolled joint in brief, finite element simulations of the calandria tube under different loading schemes, results from the analyses, optimisation of the gripper design, observations and conclusions.

Keywords: PHWR, Zirconium alloys, Calandria tube, Rolled joint detachment, Finite element, Yield stress.