On the Evolution of a Zirconium Alloy for Use as Pressure Tubes in Indian Pressurized Heavy Water Reactors

 In the early generation of the Indian 220 MWe pressurized heavy water reactor (PHWR220), Zr-2.5Nb pressure tubes (PT) were manufactured from doublemelted (DM) ingots. Later on, quadruple melted (QM) ingots were used to achieve enhanced performance. These pressure tubes were fabricated by hot extrusion followed by double pilgering with intermediate annealing and this fabrication route is designated as an old route (OR). These tubes have performed reasonably well. However, some of these tubes showed higher in-reactor deformation. Subsequently, both alloy chemistry and manufacturing practice were revisited and changes in alloy chemistry and ingot diameter, mode of hot working for breaking the cast structure and hot extrusion of billets with higher extrusion ratio and single pilgering steps have been employed. This route is designated as a new route (NR) and is being used for manufacturing pressure tubes for the current generation of 220MWe pressurized heavy water reactors.

Over the years, changes in Chlorine (Cl), Carbon (C), Phosphorous (P), Iron (Fe) and Hydrogen (H) specification and narrowing down the specification for Niobium (Nb) and Oxygen (O) have been implemented to exploit their beneficial effect on in-reactor deformation and hydrogen pickup. The changes in manufacturing practices had resulted in changes in microstructure and texture. In the old route (OR), pressure tube (PT) microstructure was characterized by the presence of discrete beta-phase precipitates along the interfaces of alpha lamellae while the new route (NR) pressure tube (PT) exhibits more continuous beta film and relatively coarser α lamellae. In terms of crystallographic texture too, the new route (NR) pressure tubes (PTs) had higher FT values (in the order of 0.65) in comparison to old route (OR) pressure tubes (PTs) (FT~0.55 to 0.6).

Because of crystallographic and microstructural anisotropy, the tensile behavior of this material is also anisotropic with the transverse direction exhibiting higher flow stress and lower ductility at and below reactor operating temperatures. The transverse tensile strength of pressure tube (PTs) fabricated from new route (NR) is higher than that fabricated from old route (OR). The fracture toughness of pressure tubes (PT) manufactured from quadruple melted (QM) ingots are significantly higher than that of the pressure tubes (PTs) manufactured from double melted (DM) ingots, which is attributed to the deleterious effect of Chlorine (Cl), Carbon (C), Phosphorous (P) and their complexes. The variation in fracture toughness of pressure tubes (PTs) was evaluated as a function of temperature, hydrogen content and hydride orientation. The hydrided material exhibited a typical S curve showing lower-shelf, transition and upper shelf regimes. Delayed hydride cracking velocity and threshold stress intensity factor were determined as a function of temperature, direction of approach to test temperature and hydride orientation. Threshold stress for hydride reorientation (σth) determined using ex-situ and in-situ methods between 250 and 300oC was observed to decrease with an increase in temperature. Thermal creep behavior was investigated for these tubes at 350, 400 and 450 °C at different stress levels and comparison of the minimum creep rate and the rupture life is presented. This article describes the evolution of the alloy chemistry, microstructural features, texture and mechanical properties and hydride induced embrittlement of the pressure tubes (PTs) used in Indian pressurized heavy water reactor (PHWR) and life extension approaches. An attempt has been made to rationalize the observed properties in terms of alloy chemistry and microstructure.

Keywords: Creep, Delayed hydride cracking, Fracture, Life extension, Microstructure, Pressure tube, Tensile, Zirconium alloy.