UF6 RELEASE CALCULATIONS AND RADIOLOGICAL AND ENVIRONMENTAL IMPACTS OF A UF6 CONTAINER SUBJECT TO A LONG DURATION FIRE

Assuming that a 12 tonne-UF6 cylinder without thermal protection is subject to a long duration fire without intervention of the emergency response team, we have used the TENERIFE, PEECHEUR experimental results and the DIBONA model to evaluate the quantity of UF6 that might be released and the associated chemical, radiological and environmental impacts. The PEECHEUR experimental program, relative to the mechanical behaviour of a 48Y container subject to fire, has provided the bursting pressure and the size of the resulting break in container steel wall. The DIBONA model, based on results of the TENERIFE experimental program relative to the thermodynamical behaviour of a UF6 container in fire, has been used to evaluate the state of an unprotected 48Y container subject to an engulfing 800°C fire when the bursting pressure is reached. Considering, at bursting time, the initial temperature distribution in the container and the UF6 solid, liquid and gas phases repartition, the quantity of UF6 gas that might be dispersed in the atmosphere has been calculated. Three steps are considered, the release of compressed UF6 gas at bursting, then the evaporation of UF6 liquid inside container, then the sublimation of the remaining UF6 solid. Flow rates decrease from 300 kg/s down to 0,5 kg/s. 48Y containers containing 12 500 kg of natural UF6 and 30B containers containing 2300 kg of enriched UF6 are considered. By assuming instantaneous hydrolysis of the released UF6 from the container, atmospheric dispersion of HF and UO2F2 is calculated for several atmospheric conditions. The chemical toxic consequences of exposition to UF6 and UO2F2, depending on the distance from the release point, are calculated. The radiological consequences of exposition to uranium is only considered for UO2F2 enriched up to 5% of U235. For all configurations, it appears that: - chemical risk due to HF is higher than the one due to UO2F2, - concerning UO2F2, radiological risk is less important than chemical risk for low enriched, not reprocessed uranium. Finally, the maximum distance from the release point where irreversible effects for population health are reached is about 5 kilometers (3 miles).