SBIR-STTR Award

DOE desires improvements and advances for the fabrication, characterization, and examination of nuclear reactor fuel. Advanced technologies are desired for Advanced Reactors and fuels for sodium- and lead-cooled fast reactors. Specific technologies that improve the safety, reliability, and performance in normal operation as well as in accident conditions are desired. Metal-cooled fast reactors (MFR) typically used metallic nuclear fuels (as opposed to oxide-based fuels for water- based reactors). Almost all nuclear fuel elements swell in operation because of gas (Xe, Kr) reaction products that accumulate in the metal fuel. The swelling is greater in the radial direction vs the axial dimension. Radial swelling can lead to cladding failure. When metal fuel swells during irradiation and contacts the cladding, inter-diffusion of the fuel and the cladding can occur. This inter-diffusion and the associated issues it causes are referred to as fuel- cladding chemical interaction (FCCI). The major issue is reaction between elements in the fuel with elements in the cladding that may lead to relatively low-melting phases, thus compromising the physical integrity of both the cladding and the fuel metal. Keiser and Cole show that one way to eliminate fuel-cladding interaction would be to employ a liner on the inner surface of the cladding tube. Two materials are suggested, V and Zr. Keiser and Cole tested the potential liner materials Zr and V using solid-solid diffusion couples, consisting of liner materials butted against fuel alloys and against cladding materials. At 700?C, minimal interaction was observed between the metallic fuels and either Zr or V. Similarly, limited interaction was observed between the Zr and V and the cladding materials. The neutron cross section of V is 40x larger then Zr so only Zr will be considered in this program. Our proposed approach will be to make an ink or slurry of Zr particles less than 1 micron in size and use it to coat the inside of 316 stainless steel (SS) tubes. We will also use x-ray computer tomography (CT) to characterize the coating and other metrology techniques to characterize adhesion and porosity. Objective 1: Develop and demonstrate a Zr ink for dispensing inside SS tubes. Objective 2: Develop and demonstrate a method of uniformly dispensing Zr ink inside SS tubes. Objective 3: Demonstrate a method for curing the Zr liner layer inside 316 SS tubes. Objective 4: Validate the uniformity of Zr using cross-sections of the tube. Objective 5: Validate Zr thickness uniformity and defect detection using x-ray CT imaging. Objective 6: Develop a detailed plan for Phase II effort. This technology will be used in next generation metal-cooled fast reactors. The public will benefit from cost reduced cost in manufacturing the Zr-coated tubes, in increased life of the fuel assembly and greater up-time and efficiency of the power reactor, thus lower electrical power cost rates.

Advanced technologies are desired for Advanced Reactors and fuels for sodium- and lead-cooled fast reactors. At reactor operating temperatures, inter-diffusion of the fuel, lanthanide fission products and the cladding can occur through the liquid metal bond and increasingly through direct contact resulting from fuel swelling. This inter-diffusion and the associated issues are referred to as “fuel-cladding chemical interaction” (FCCI), that may lead to (1) embrittlement and (2) relatively low-melting phases (eutectics), both of which compromise the physical integrity of the cladding and the fuel metal. As the fuel burnup increases, issues related to lanthanide fission products become more significant. The over-arching goal of this program is to develop a diffusion barrier of lanthanide fission products from the UZr fuel pellets into the cladding material. We developed and tested Zr, Mo, Cr and Nb ink and paste materials (and some combinations of Zr and Mo) and used them to coat coupons and tubes made from 316, 420 and HT9 stainless steel with thickness ranging from 10- 100 microns. The results varied depending on coating material and substrate material. Dense hermitic films of Zr were formed on HT9 with strong adhesion. No evidence of alloying Zr with the SS was observed. Mo formed dense coatings on all substrates but in some cases island formation was observed. Mo alloying with the SS was also suspected in some cases. In Phase II we will demonstrate a coating that will provide a diffusion barrier to FCCI. The goal will not be to block 100% of the lanthanide diffusion, but to demonstrate at significantly lower diffusion with the barrier as opposed diffusion with no barrier present at 600°C tested for 4-6 weeks in a diffusion couple experiment. Molten sodium and lead corrosion studies will also be conducted. This technology will be used in next generation nuclear reactors. The public will benefit from increased life of the fuel assembly and greater up-time and efficiency of the power reactor, lowering electrical power costs.