Zirconium and its alloys are extensively used in nuclear applications due to the combination of a low neutron-absorption cross section, good mechanical properties under high stress and temperature conditions, low hydrogen/deuterium uptake and corrosion resistance. The main uses are for some structural components (particu­larly the calandria in CANada Deuterium Uranium (CANDU)-type reactors) and fuel cladding. It is anticipated to be used in the primary containment-vessel of high temperature D2O inside the core of the fourth generation supercritical-water-cooled reactor (SCWR). Its behaviour during manufacturing and in service as well as under accident scenarios is therefore of great importance and a topic of extensive research.

Pure zirconium has a hexagonal closed-packed (hcp) crystal structure up to 866 °C and transforms to a body-centred cubic (bcc) crystal structure at higher temperatures. Their complex deformation-mechanisms have been investigated with neutron diffraction where different crystallographic planes have different elastic constants and post-yield behaviour, leading to strain partitioning. The main alloying elements for nuclear applications are niobium and tin, the latter forming together with other minor elements the so-called Zircaloys (*Zircaloy is a trademark of Westinghouse Electric Company, Pittsburgh, PA.).

Neutron diffraction has been applied to zirconium and its alloys to characterize welds, as an in situ mechanical test technique to characterize deformation modes to allow predictive modelling of deformation, to investigate the development of tex­ture under temperature and stress, and to characterize the phase transformations including texture-variant selection during the hcp/bcc phase transformation. The following sections describe some of these experiments.

Zircaloys are also prone to brittle-hydride formation, particularly in welds. Hydrides can also form in parent plate if unfavourable textures are present due to a particular manufacturing route. Thus, many investigations have been performed on hydrides, such as imaging their location (radiography and prompt gamma), and assessing their susceptibility to hydride formation (residual stress and texture). Hydrogen accumulation or “pick-up” can also occur in the Zircaloy cladding of nuclear fuel and can cause embrittlement. In this case, the hydrogen content can be spatially visualized and quantified by neutron radiography. Blistering of Zircaloy fuel-cladding has also been investigated using neutron radiography, as X-rays are not effective in practice due to the high background, which includes gamma radi­ation from decay products.