Mark Kaltofen and Arnie Gundersen
The March 11, 2011 Great East Japan earthquake and ensuing tsunami resulted in the loss of > 15,889 lives (JNPA). The earthquake and tsunami also caused catastrophic damage to four of the six units at the Fukushima Dai-ichi Station. Radiological materials escaped from the reactor units at the power plant site via airborne plumes of contaminated gases, aerosols and particles, and by contaminated wastewaters (FNAIC, 2012 ; Mikami et al., 2015). Radioactive dust particles consist of particulate matter with high concentrations of radioactive isotopes. These radioactively-hot dust particles can be inhaled or ingested, causing internal radiation exposure.
Airborne dusts can transport radioactive materials as trace contamination in homogenous collections of particles, or as isolated individual particles containing high concentrations of radioisotopes (Adachi et al., 2013 ; Meszaros et al., 2016). Contaminated environmental dusts can accumulate in indoor spaces, potentially causing radiation exposures to humans via inhalation, dermal contact, and ingestion (Lioy, 2002).
Hot particles are small discrete radioactive particles capable of delivering radioactive energy to a target, including to an internal tissue. The U.S. Nuclear Regulatory Commission defines a hot particle as: “a discrete radioactive fragment that is insoluble in water and is less than 1 mm in any dimension.” (NRC, 1990) Proper calculation of an individual’s hot particle exposure is a matter of gathering accurate exposure data for input to the accepted standard model (Evans, 1997). Given the wide variability in hot particle sizes, activities, and occurrence; some individuals may experience a hot particle dose that is higher or lower than the dose calculated by using averaged environmental data. Calculated exposures to members of a group using averaged data may misstate exposure to the subgroup that is exposed to high activity particles. Excess cancer incidences based only on averaged data could wrongly assert causation from averaged doses alone, if higher unrecorded hot particle doses also existed.
The purposes of the study were to identify and collect samples with a high potential to contain radioactively-hot particles for microscopic examination, to determine if local hot spots of contamination existed at the time of the Fukushima Dai-ichi meltdowns, and finally to document whether or not any hot spots persisted five years after the accidents.
The combination of gamma spectroscopy, autoradiography and SEM/EDS analysis was effective in isolating and analyzing hot particles. Many of these particles would have gone unidentified if only one of these techniques has been employed.
Samples have provided evidence that local hot spots of contamination existed at the time of the Fukushima Dai-ichi meltdowns in 2011. Local hot spots still persisted in 2016, five years after the containment failures in 2011.
Radioactively-hot dust and soil particles were routinely detected in samples from Northern Japan in both the 2011 and 2016 sample sets, with autoradiographic and SEM/EDS data showing that isolated particles could have substantially-higher specific activities than the bulk samples from which they were isolated.
Individuals in the contaminated zone, and potentially well outside of the mapped contaminated zone, may receive a dose that is higher than the mean dose calculated from average environmental data, due to inhalation or ingestion of radioactively-hot dust and soil particles. Accurate radiation risk assessments therefore require data for hot particle exposure as well as for exposure to more uniform environmental radioactivity levels.