Duplex claddings for high burn-up applications

To reduce the fuel-cycle costs the nuclear industry strives to extend the discharge bum-up of the fuel and to prolong the operating cycles. In PWRs this trend is accompanied by new operating regimes, including higher B and Li levels in the primary coolant. Under these operating conditions, corrosion, hydriding and irradiation induced growth of Zr-based materials are important factors for the performance of the fuel. To address this concern ABB has developed a number of Zr-based alloys that enable operation to elevated burn-ups under aggressive operating conditions. This paper focuses on the Duplex claddings that have an inner component of Zircaloy-4 and an outer layer of Zr with small additions of alloying elements. ABB has developed several types of Duplex claddings. These claddings have been irradiated to assembly average burn-ups above 50 MWd/kgU in high duty plants. The corrosion resistance is excellent and the Duplex claddings enable considerably higher discharge burn-ups than what is achievable with low-Sn Zircaloy-4. During the development work ABB discovered that hydrogen picked up by the cladding during the oxidation of the outer surface can be accumulated as hydrides at the interface between the inner and outer components. This accumulation is explained by the observation that the two components after the final annealing can have different types of microstructure. Based on this knowledge ABB has developed Duplex claddings that have no tendency for preferential hydrogen precipitation at the interface between the inner and outer components.