Fracture behaviour of zirconium alloy pressure tubes for Canadian nuclear power reactors

Abstract

The fracture behaviour of zirconium alloy pressure tubes was measured using internally pressurised specimens containing artificial flaws and three-point bend specimens loaded in impact. Four materials were tested: cold-worked Zircaloy-2 and Zr-2·5 wt% Nb, heat treated Zr-2·5 wt% Nb and a recently-developed complex alloy called XL, also cold-worked. Critical crack lengths pertinent to reactor operating conditions exceed fifteen times tube wall thickness and are large enough to claim ‘leak-before-break’ as a realistic safety criterion. Fracture modes during crack extension are discussed; these are influenced by temperature and hydrogen but are always ductile at reactor operating temperatures ( > 513 K ). Relevant aspects of delayed hydrogen cracking are discussed: effects are highly localised at stress concentrations and will not affect conclusions about tube fracture behaviour.