The prototype steam generating heavy water moderated reactor incorporates Zircaloy-2 pressure tubes 0.2 in thick, 5.14 in internal diameter and 15 ft long to a straightness tolerance of 1/3000, and zirconium-2 1/2% niobium alloy tubes 0.12 in thick. Prior to the construction of this reactor, zirconium alloy tubes of these dimensions had not been made, and this paper describes that programme of work leading to the successful manufacture of such tubes.