The Fukushima-Daiichi Accident Computations with ASTEC and How to Match it with the Dose Measurements in the Environment

The ASTEC code is currently used at IRSN to simulate the accidents on the three Fukushima-Daiichi Units for the 3 weeks starting with the Tōhoku earthquake. After the first simulations made during the accidental situations and then within the frame of OECD/BSAF-1 project, new hypotheses are introduced as new data become available that enable to match reasonably well both the qualitative observations and the quantitative measurements available at the moment. The fission products (FP) release from degraded core and their behaviour during their transport up to environment were computed for the 3 units. Taking account of thermal-hydraulics conditions, the releases that led to the main consequences to the environment have been identified to come from Unit 3. Comparisons of direct source term obtained with ASTEC code and inverse or reverse source terms based on environment measurements are done. With default ASTEC modelling it is possible to explain the chronic al release for iodine but not for caesium aerosol form. The FP mass transfer from the wetwell linked to the evaporation rate is needed reproducing Cs chronical release. (author)