New technologies for manufacturing of seamless calandria tubes and pressure tubes for PHWRs

Full text: Pressurised Heavy Water Reactors (PHWR) form the mainstay of the First stage of Indian Nuclear Power Programme. The PHWRs are based on coolant channel design. Pressure tubes and Calandria tubes are the two major lifetime components of the fuel channels. The pressure tubes are made of Zr-2.5Nb and are thick walled. They act as containment for the pressurized coolant and the fuel bundles. These pressure tubes are subjected to extremely harsh conditions of temperature, pressure and neutron irradiation during service. Pressure tubes are conventionally manufactured by extrusion and drawing route. NFC has developed a fabrication methodology for pressure tubes based on pilgering process route. The calandria tubes are made of Zirconium alloy and are extremely thin walled (D/t ratio ∼95). They act as a partition between the hot pressure tube and the cold moderator. Calandria tubes are conventionally produced by the 'Welded route', which involves extrusion of rectangular slabs followed by a series of hot and cold rolling passes, intermediate annealing, press forming of sheets into 'O'-shape and finally closing the gap by TIG welding. The important advantages of seamless tubes over the conventionally fabricated welded tubes are substantially improved material recovery, lower cycle time, superior grain size, absence of weld, superior and more uniform texture, lesser ovality and skew over the full length, low eccentricity with expanded ends and above all negligible residual stresses. This paper deals with the manufacturing route followed at Nuclear Fuel Complex adopting unique fabrication processes and the consequent improvements in the quality of these critical core structural components