Kinetics of deuterium permeation through Zircaloy-4 in the 623-773 K temperature range

Under normal operating conditions in nuclear pressurized water reactors, tritium produced by ternary fission occurring within the uranium fuel may cross the whole cladding in zirconium alloy before being released in primary water during operation, or in containers during transportation and storage. The study aims at identifying and quantifying the rate limiting steps of this permeation process by using deuterium as isotopic tracer for tritium. A dedicated permeation device revealed that, at 773 K, deuterium permeation kinetics from the metal was limited by the surface recombination reaction. In association with gaseous deuterium charging and Thermal Desorption Spectrometry, the apparent activation energy of the deuterium desorption showed that the permeation device induced stress and strain in the specimens. A stress-free Zircaloy-4 exhibited an apparent activation energy around 240 kJ mol^-1 it dropped down to around 140 ± 10 kJ mol^-1 when under stress, in the 623 K -773 K temperature range. (authors)