An experimental device to study the revaporisation behaviour of fission product deposits under severe accident conditions

The uncontrolled release into the environment of radioactive fission products (FPs) produced in nuclear fuel is the main threat in case of a severe reactor accident. The chemical state of the FPs and their interaction with the surfaces inside the containment and with the atmosphere have great influence on their mobility and thus on the release source term. A revaporisation device was built to provide experimental data on fission product transport and deposition for severe accident codes (e.g. ASTEC). The other aim was to study in detail revaporisation processes associated with different stages of a severe accident involving fuel degradation including melting. The device allows us to study the transport behaviour of gamma-emitting FPs under oxidising and reducing conditions over the temperature range 200-1000 °C at ambient pressure. A lanthanum (III)-bromide (LaBr3) detector is used to measure the amount of revaporised FPs as a function of time and temperature. This paper describes the main features of this device, including a series of upgrades on-going or planned to extend the applicable range of FPs and atmosphere conditions. Test results of FPs deposited on the surface of stainless steel rings from the upper vertical line of the Phébus FP tests are presented. Experiments under steam atmosphere showed that revaporisation of 137Cs starts at 540 °C progressing rapidly until near the total depletion of the deposit, resulting in a total revaporisation of ~92%.