Analysis of pellet-to-cladding gap closure for a metallic micro-cell pellet under normal operating conditions

Abstract

To reduce fission gas release under both accident conditions and normal operation, Korea Atomic Energy Research Institute has developed a metallic micro-cell pellet following the 2011 Fukushima accident. One concern, however, about the metallic micro-cell pellet is that due to the presence of metallic wall that reduces both temperature and gas release, its capability of higher gas retention could result in increased gaseous swelling, leading to earlier pellet-to-cladding gap closure than the conventional UO 2 pellet. To analyze and compare the gap closure in two fuel rods in which two kinds of pellets, UO 2 and metallic micro-cell, are loaded into each of the two identical Zr claddings, we developed a model to estimate gaseous swelling in the two pellets. Then considering mainly different gaseous swelling and thermal expansion by different temperature profiles in the two pellets, we compared the pellet-to-cladding gaps as a function of burnup for the case where they would be irradiated with the same constant linear power of 30 kW/m up to 60 MWd/kgU. Due to both reduced gaseous swelling and lower thermal expansion, gap closure in the fuel rod loaded with the metallic micro-cell pellet is expected to be delayed by 6–16 MWd/kgU compared with the one loaded with the UO 2 pellet, implying pellet-cladding mechanical interaction occurrence would also be postponed by the same degree. We note that the result of this approach is preliminary until confirmed by measured data from metallic micro-cell pellets irradiated up to high burnup. • Higher gas-retention capability in the metallic micro-cell pellet could lead to earlier pellet-to-cladding gap closure than the UO 2 pellet. • Despite the higher gas-retention capability, gaseous swelling in the metallic micro-cell pellet would be reduced by its lower temperature. • Pellet-to-cladding gap closure in the metallic micro-cell pellet could be delayed by 6–16 MWd/kgU compared with the UO 2 pellet.