Accident Tolerant Barriers for Fuel Rod Cladding in Nuclear Reactors

Zirconium-based alloys are currently used as fuel rod cladding in nuclear power reactors because of their low neutron absorption cross-section and adequate oxidation resistance. However, zirconium (Zr) alloys undergo waterside corrosion by the primary coolant water under normal operating conditions (300-330oC) and at high temperature, especially during loss of coolant accident conditions. During the oxidation reaction, hydrogen generated which can lead to the formation of zirconium hydride precipitates in the Zr alloy cladding - and is detrimental to the mechanical integrity of the fuel rods. The principal objective of this work was to develop a surface-treated layer/coating for pure Zr in order to improve its oxidation resistance (and to determine the feasibility of such surface-treated layers/coatings being used on Zr alloys) - and thereby increase its operational longevity under both normal and abnormal operating conditions. Triode Plasma Nitriding (TPN) diffusion treatments were used to develop diffusion-treated nitride layers intended to enhance load-bearing capacity of bulk Zr while allowing for good adhesion properties to the substrate. Physical Vapour Deposition (PVD) was also used to fabricate magnesium-containing zirconium coatings with a range of magnesium concentrations to study the solubility magnesium and zirconium in a Zr-Mg coating intended to enhance hydrothermal oxidation resistance. Various characterisation techniques were used to investigate the effects of diffusion treatments and sputter deposition on surface morphology, topology and bulk substrate microstructure. The hardness, adhesion and oxidation resistance of the nitrided layers and sputter-deposited coatings were assessed. The hardness of the diffusion-treated surface of pure Zr metal was found to have been significantly improved and a higher load-bearing capacity was achieved after TPN-treatment compared to untreated pure zirconium. This was due to a ‘deep’ hardened case which was achieved at moderately low temperatures at short processing times, allowing significantly higher loads to be applied before failure occurred. TPN treatment of pure zirconium metal was found to have improved the oxidation resistance compared to untreated pure zirconium in aqueous conditions. The Zr-Mg coatings produced by PVD showed that magnesium can remain in substitutional solid solution with zirconium at high magnesium concentration and showed no evidence of phase separation. Hydrothermal oxidation tests of the Zr-Mg coatings revealed the formation of a (metastable) tetragonal zirconia phase throughout the oxide scale which plays an important role in stabilising of the oxide. The evidence presented suggests that both the diffusion treatments and sputter co-deposition process need to be optimised in order to improve oxidation resistance under aqueous conditions and abnormal reactor operating conditions.