In-Pile Testing of CrN, TiAlN, and AlCrN Coatings on Zircaloy Cladding in the Halden Reactor

Abstract

There is growing interest among nuclear power utilities to increase power output and to maximize electricity production. In order to deal with these more challenging conditions, new alloys have been developed. Another approach is to apply protective coatings on existing cladding zirconium alloys. Although the coatings described herein were developed in order to improve the corrosion behavior under normal conditions, it is clear that these coatings can also be applied to other materials (iron-chromium-aluminum, molybdenum, silicon carbide) presently being studied for accident-tolerant fuel. In 2011, several commercially available coatings applied by physical vapor deposition were proposed to improve the performance of existing cladding materials. Since then, these coatings have been investigated in out-pile and in-pile experiments at the Halden Reactor Project from 2011 to 2014. Boiling and pressurized water reactor (PWR) in-pile experiments were performed on small cylindrical Inconel 600 and Zircaloy-4 samples with CrN, TiAlN, and ZrO2 coatings. In all cases examined, the CrN coating came out as superior and remained completely intact after the irradiation, as evidenced by SEM analysis. Based on these promising results, an in-pile experiment with coated Zircaloy claddings (containing fuel) was performed in the Halden reactor in PWR conditions in 2014. The experimental rig contained four Zircaloy-4-clad fuel rods—three coated (CrN, TiAlN, and AlCrN) and one uncoated. Due to a mechanical deformation of tubes for cooling water within the rig, the fuel rods were insufficiently cooled, resulting in a higher-than-normal cladding temperature. Nevertheless, an SEM examination showed that the CrN coating largely remained intact apart from several local spots where the coating had cracked or disappeared. Underneath the cracked coating, oxide formation was observed in the Zircaloy-4 cladding. The TiAlN and AlCrN coatings both disappeared after the irradiation.