Niobium as an alternative cladding material for neutron producing targets: Analysis of erosion response

The current design of neutron producing spallation targets (TS2) at ISIS constitutes a Tungsten (W) core cladded with a thin Tantalum (Ta) sleeve which facilitates circulation of cooling water. These targets are undergoing premature failure due to erosion-corrosion of the Ta sleeve as one of the primary causes. This has drawn attention towards sourcing alternative cladding materials and Niobium (Nb) is potentially seen as a candidate. However, the aqueous slurry erosion response of Nb is unreported and needs scrutiny. In this work, slurry erosion performance of Nb was studied with the help of an impinging jet erosion-corrosion apparatus. Aims included measuring erosion rate of Nb with respect to abrasive particle concentration, slurry velocity, and slurry impingement angle including determining erosion mechanisms. Results revealed peak erosion rate at a slurry impingement angle of 30°, and for velocities of 6 ms-1 having a particle concentration of 7 wt.%. The material removal mechanism involved a dominating ploughing and type-1 micro-cutting action at shallow angles of impact as compared to formation and breakage of platelets due to a ‘plastic deformation-fatigue’ dominated wear mechanism at near normal impact angles. The erosion response of Nb thus could be described as one typically observed for ductile materials. Experiments were also performed to analyse the cavitation erosion resistance of Nb using an ultrasonic vibratory apparatus (sonotrode) in de-ionised water to complement impinging slurry jet studies.