EVALUATION OF CR-COATED ZR-ALLOY CLADDINGS DURING A LOSS OF COOLANT ACCIDENT SCENARIO IN LIGHT WATER REACTORS

Abstract

Zirconium-alloys (Zr-alloys) have been used successfully as the materials for fuel cladding (tubes that contain the uranium-dioxide pellets) in light water reactors (LWR) on account of their good hydrothermal corrosion resistance and strength, and high neutron transparency. While Zr-alloys have performed well under normal operating conditions, loss of coolant accidents (LOCA) where temperatures can exceed 1000 °C can lead to profuse oxidation of the cladding in reaction with steam. The oxidation reaction can consume a significant fraction of the cladding wall thickness and generat e hydrogen. A near-term solution to address this challenge is to coat the outer surface of the Zr-alloy cladding with an oxidation-resistant material such as chromium (Cr) that can provide additional coping time in the event of a LOCA. While there are numerous avenues for research on Cr-coated Zr-alloy accident tolerant fuel (ATF) cladding, this study focuses on two aspects: post-quench ductility and the ballooning and burst behavior of cladding. Two Cr-coating deposition methods are used in this research: cold-spray (CS) deposition and physical vapor deposition (PVD). In the cold spray process, powder particles of the coating material are propelled at supersonic velocities on to the surface of a substrate where upon impact they plastically deform to form a coating. CS deposition was performed using the 4000-30 KINETIK commercial cold spray system at the University of Wisconsin-Madison (UW), using two different carrier gases, helium (CS-He) and nitrogen (CS-N2). Helium imparts higher particle velocities and greater plastic deformation during impact. PVD process involves deposition of atoms of the coating material sputtered from a target using energetic argon ions. An advanced version of PVD, HiPIMS (High Power Impulse Magnetron Sputtering) which provides for superior coating microstructure and greater interfacial adhesion, was used. The PVD coating was performed in collaboration with an external company. Reflood tests were performed in a specialized single rod test facility designed and built at the UW. Here, a 16” cladding section was heated to temperatures in the range of 600 °C to 1200 °C with a thermal radiation furnace followed by flowing pre-heated RO (reverse osmosis) water through a quartz tube and around the cladding. Following the tests, ring-shaped samples were sectioned from the cladding and subjected to ring compression tests (RCT), where load-displacement plots are also generated as the ring sample is vertically deformed in the radial direction to maximum displacement of 2mm. Cross-sectional scanning electron microscopy (SEM) in conjunction with energy dispersive spectroscopy (EDS) was performed after the reflood tests to examine changes in the microstructure, thickness of stoichiometric outer oxide layer, and diffusion depth of oxygen into the Zr-alloy cladding which can embrittle the cladding. Microhardness tests were also performed along the cross-section of the entire wall thickness and were found to replicate the oxygen diffusion profile. The beneficial effect of Cr coating became evident at higher reflood test temperatures. At 1200°C, the CS-He Cr coating showed a significantly thinner oxide layer thickness, and dramatically reduced oxygen ingress into the cladding. This resulted in better post-quench ductility of CS Cr-coated cladding. The presence of the strongly

adhered cold spray Cr-coating also mechanically reinforced cladding section as evidenced by the higher maximum load for untested claddings during RCT compared to the uncoated Zr-alloy. PVD Cr-coated Zr-alloy mitigated oxygen diffusion into the cladding but to a lesser extent than the cold spray Cr coating, due to diffusion likely having occurred along the intercolumnar boundaries of the PVD Cr coating. Similarly, the level of improvement in mechanical reinforcement provided by the PVD Cr coating was notably lower than that provided by the cold spray Cr coating. Cladding ballooning and burst tests were performed at initial internal pressures of 6, 8, and 10 MPa by introducing argon gas into the cladding. Argon gas was flown along the outer surface of the cladding to minimize oxidation during tests. All Cr-coated claddings exhibited an increase in burst temperature relative to uncoated Zr-alloy, with CS (He) providing for the highest burst temperature, but a relatively brittle coating failure. Burst dimensions between each coating type largely varied with no discern able pattern. CS (N2) exhibited a repeatable, anomalous reduction in burst size at 8 MPa which could be a result of micro-fracturing along the interparticle boundaries which relieved stress. Post-test characterization of ruptured cladding also involved image analysis to measure the burst area dimensions and to measure the diametric strain profile axially, and to measure the post-test wall thickness of the ruptured cross-section. While uncoated and PVD Cr-coated cladding experienced plastic deformation along the entire axial length of the cladding similarly, CS (He) and CS (N2) constrained the Zr-alloy substrate with CS (He) constraining all deformation to only within 4 cm of the burst site. The presence of a Cr-coating mitigates the severity of oxygen embrittlement, ballooning, and burst size compared to uncoated Zr-alloy.