Nanolaminate Coatings for Improved Nuclear Fuel Cladding Performance

Sumit Bhattacharya, researcher at Northwestern University, shall perform research under the supervision of Prof. David N. Seidman (Northwestern University) and Dr. Michael Pellin (Argonne National Laboratory). As exemplified by the recent Fukishima reactor accident, nuclear materials particularly in the claddings used to protect nuclear reactor fuel are critical both to a reactor’s severe accident tolerance and its operating costs. Zircaloy, the material of choice for claddings in nearly all operating nuclear reactors due to its many outstanding properties in a nuclear environment, fails when driven beyond design limits under severe accident conditions, such as a loss of coolant accident (LOCA) due to degradation of its passivating oxide layer. The research proposed will enhance the passivation properties of the Zircaloy passivation layer with the addition of nanolaminate (NL) layer/layers, which are multiple layer stacks with each layer thickness measured in nanometers, by atomic layer deposition (ALD) to the cladding. ALD is a unique synthesis method that enables the growth of pinhole free films on large and convoluted substrates with nanometer precision. ALD has also demonstrated that NLs can be: (1) grown on a wide variety of materials; (2) that NLs can provide properties well beyond any traditional summation rule; (3) when chosen carefully NLs can be stable to 1200 oC (a key temperature for LOCA testing) even from thicknesses < 10 nm; (4) stacks of alumina/zirconia have shown the lowest water vapor diffusion rates of any known material; (5) NLs can have improved interfacial strain, and; (6) multiple layer stacks each designed to provide a unique individual property can be combined to provide the many needed properties for such a cladding. We applied ALD coatings to standard Zircaloy test tubes (8 mm diam., 2 cm long). The coatings included Al2O3, SiN, SiO2, ZrN, and ZrO2. All these coatings were oxidation tested in both air and water. The water oxidation tests were performed both in high pressure auto clave (Westinghouse) and high temperature steam test facility (Argonne LOCA facility). The air oxidation was performed in open hearth furnace available in Argonne and at Northwestern University. The autoclave test and the water oxidation tests did not come out to be successful. The materials failed in presence of high pressure and steam. In the case of air oxidation the coated materials fared well compared to the uncoated cladding material. The samples with Alumina coating were the most successful ones. We achieved a temperature of 600 oC without any observable weight changes. Above this temperature all the coatings including alumina fails. While performing these tests a very unique feature was observed when we tried to anneal the alumina coated samples in vacuum in hopes reducing any surface stresses developed on the cladding during rolling process of manufacturing. In turn we observed that this annealed samples were giving better oxidation resistance at higher temperatures. Close studies showed that a new phase was formed at the interface between the coating and the cladding. This new phase is a intermetallic compound. This phase acts like a buffer between the two mediums. These new phases which are mainly zirconium aluminides has been used in high temperature applications like rocket nozzles, gas turbine blades as Thermal barrier protection materials. So developing these phases in these cladding materials will be an added advantage to this cause. Right now our focus of research is to study the development of these