Corrosion and deuterium uptake of Zr-based alloys in supercritical water

To increase the thermodynamic efficiency above 40% in nuclear power plants, the use of supercritical water as the heat transport fluid has been suggested. Zircaloy-2, -4, Zr-Cr-Fe, Zr-1Nb and Zr-2.5Nb were tested as prospective fuel cladding materials in 30 MPa D₂O at 500 ^oC. Zircaloy-2 showed the highest rates of corrosion and hydriding. Although Zr-Cr-Fe initially showed a very low corrosion rate, it displayed breakaway corrosion kinetics after 50 h exposure. The best-behaved material both from a corrosion and hydrogen uptake point of view was Zr-2.5Nb. However, the Zr-2.5Nb oxide growth rate was still excessive and beyond the current CANDU design allowance. Similar coupons, coated with Cr, were also tested. The coated layer effectively prevented oxidation of the coupons except on the edges, where the coating was thinner and had some flaws. In addition, the Cr-coated Zr-2.5Nb coupons had the lowest deuterium pickup of all the alloys tested and showed no signs of accelerated or nonuniform corrosion. (author)