Behavior of bundles with Cr coated claddings under BDBA conditions at the DEGREE facility

To improve accident management measures for commercial LWR cores where ATF technology is implemented, integral tests with single rod and 3x3 bundle of three types of fuel cladding materials (Zircaloy 4, Opt. ZIRLO, and E110) coated with Cr or double- or multi-layer Cr/CrN were carried out under beyond design basis accident conditions using an induction heating furnace at the CRIEPI's DEGREE facility. Test rods pressurized to 6.0 or 0.1 MPa were heated up to the temperature range from 1350 ˚C to 1600 ˚C under steam atmosphere without water quenching. In addition to online measurements of cladding surface temperature and fuel rod internal pressure, post-test analysis was conducted to reveal interdiffusion layer structure formed at the interface between coating and Zr alloy substrate containing Cr, Zr, oxygen, and hydrogen, radial profile of micro-Vickers hardness, and ballooning and bursting behavior. Comparisons of accident behavior between different coating or cladding materials and between single rod and bundle tests are also discussed.