Cesium Isotope Composition in Fukushima Daiichi Units After Boiling Water Reactor Accident

The three radioactive isotopes of ¹³⁴Cs, ¹³⁶Cs, and ¹³⁷Cs related to the boiling water reactor (BWR) accident at the units of the Fukushima Daiichi nuclear power plant (NPP) have been measured in samples obtained from NPPs around the area and from inside the Fukushima Daiichi buildings. Numerical calculations with sensitivity analyses were carried out to estimate the cesium (Cs) isotope composition in the BWR core, and the origins of the Cs in the samples were clarified based on numerical calculations. Most of the measured Cs radioactivity data suggest that Cs was released from the homogenized state among fuel bundles with different irradiation histories in the core. The origins of the large ¹³⁴Cs/¹³⁶Cs ratios in the Unit 2 spent fuel pool (SFP) suggest two possibilities. One possibility is the existence of a partial release process from the fuel bundles located in the peripheral core region, and the other is damage to the fuel placed in the SFP.