SCANUK. A collaborative programme to develop new zirconium cladding alloys

The primary aim of the programme was to develop alloys with better performance than Zircaloy-2 under BWR/TWR conditions and specifically with improved resistance to short-term high temperature transients. Secondly, the alloys were to be capable of full-term reactor service over a wider temperature range than usual (up to 4500C). For the first objective a Zr-1 wt % Nb alloy was selected and for the second, alloys were composed of small amounts of chromium and/or molybdenum added to a base composition of 1/2 or 1 wt % niobium in zirconium. The test programme obtained on the physical metallurgy, mechanical properties and corrosion resistance of the alloys both before and after irradiation is described and the results are reviewed. Although the requirement for cladding to operate at elevated temperatures is not longer of prime importance, the development work has demonstrated that with some further optimisation some of the alloys might present a viable alternative to Zircaloy-2 for in-reactor operation at approximately 3000C in oxygenated coolants. With regard especially to nodular oxidation resistance these alloys, based on modest additions of niobium to zirconium, tend to be better than Zircaloy-2 but their performance does not consistently approach that of Zr-2 1/2% Nb. (author)