Land contamination activity data interpretation from Fukushima Daiichi accident

•Radiochemical analysis of activity sources from land contamination.•Calculation of activity release from unit 3.•Analysis of origin of land contamination from units 1–3. Radioactive aerosols were released during the accident from the units 1–3 of the Fukushima Daiichi nuclear power station. Release paths were due to leakage from the containment through the reactor building into the environment, due to containment venting and due to the explosions of the reactor buildings of the units 1, 3 and 4. The release due to containment venting can normally be estimated by dose measurements in the release stacks but for the other release paths there is no direct dose measurement available. Dose measurements away from the reactors only showed the release of fission products, but necessarily cannot identify the source of the radioactivity. Using the ratio of the activity measurements of the Cesium nuclides Cs-134g and Cs-137 allows the comparison with the ratios examined in the spent fuel pools and also the turbine halls of the three destroyed reactor cores. From the analysis of more than 1600 activity measurements in the area north-west from the Fukushima Daiichi power plant it could be proved, that mostly the units 1 and 3 are responsible for the land contamination in this area. The local contamination measured at the Fukushima Daiichi site is mainly caused by aerosol release from unit 2 and the sea water contamination origins from all three of the degraded reactor cores.