Recent activities in environmental science and health physics

Since mid-2011, many studies related to the Fukushima Daiichi Nuclear Power Plant (F1NPP) accident have been published. These studies were useful for worker and resident dose assessment, environmental distribution analysis of radionuclides released into the atmosphere and ocean, and development of decontamination methods in environmental science and health physics fields. However, only limited information exists on the early accident stage. In the future, reducing uncertainty in the dose and environmental distribution of radionuclides presents a significant challenge to environmental science and health physics. Thus, ongoing, high-precision studies are required. Essential environmental monitoring and radiation protection techniques were also published, such as dose rate variation factor and internal dosimetry methodologies. We introduce some papers published from 2012 to early 2014 in the Journal of Nuclear Science and Technology (JNST) and other journals.

In 2012, environmental radiation monitoring results related to the Fukushima Daiichi Nuclear Power Plant accident were published by Takeyasu et al. in the JNST [1]. The environmental radiation dose, the air concentration of several nuclides (¹³²Te, ¹³¹I, ¹³³I, ¹³⁴Cs, ¹³⁶Cs, and ¹³⁷Cs), and the fallout of ¹³⁴Cs, ¹³⁶Cs, and ¹³⁷Cs were measured at Ibaraki Prefecture monitoring stations (110 km or more from F1NPP) when a large amount of radionuclides was released to the atmosphere on 15, 16, and 21 March 2011. This investigation revealed that the ¹³¹I/¹³⁷Cs concentration ratio in air varied considerably every day.

Hosoda et al. investigated the ¹³¹I/¹³⁷Cs and ¹³²I/¹³¹I activity ratio in environmental samples collected south of F1NPP at Iwaki City, Fukushima Prefecture. They reported the highest ¹³¹I/¹³⁷Cs, ranging from 49 to 70 [2].

From the viewpoint of initial internal dosimetry, Kurihara et al. reported the radioactivity measurement of ¹³¹I in the thyroids of employees involved in the F1NPP accident [3]. They measured radioactivity of iodine and estimated the internal dose in 560 employees using a low-background whole-body counter (WBC) with high-purity germanium semiconductor detectors. The effects of measurement position and the distance between the detector and target tissue to the measurements were also described.

Preliminary measurements of the equivalent dose of ¹³¹I in the thyroids of residents and evacuees were published by Tokonami et al. [4]. The maximum thyroid equivalent dose was 23 mSv for children, and 33 mSv for adults. However, the median thyroid dose was 4.2 mSv for children, and 3.5 mSv for adults. The values were much less than the mean dose in Chernobyl accident evacuees.

As a biodosimetry approach, Suto et al. estimated the F1NPP recovery worker's internal dose by dicentric chromosome assay [5]. The results for 12 individuals indicate that the estimated doses were less than 300 mGy and that the mean was 101 mGy. The values were compared with corresponding personal dosimeter measurements.

To estimate the resident dose using actual evacuation patterns and Fukushima health management survey monitoring data, an external dose estimation system was developed by Akanahe [6]. External dose was assumed for 18 resident evacuation patterns in the deliberate evacuation area. The estimated dose was 1–6 mSv for representative residents from the mandatory evacuation area, and the maximum dose was 19 mSv.

In “The 1st NIRS Symposium on Reconstruction of Early Internal Dose in the TEPCO Fukushima Daiichi Nuclear Power Station Accident” proceedings, comprehensive relevant information, such as external and internal dose estimates, and environmental monitoring data were presented [7]. The 2nd NIRS Symposium was held in January 2013, and the proceedings will soon be published.

Atmospheric and oceanic dispersion of radionuclides released from F1NPP was simulated using a computational code and monitoring data. Estimates of atmospheric ¹³¹I and ¹³⁷Cs source terms were validated and refined by Kobayashi et al. [8] by coupling atmospheric and oceanic dispersion.

Hirao et al. [9] evaluated the uncertainty of calculation, and indicated the effectiveness and usefulness of environmental monitoring data for estimation of the atmospheric release source term for nuclear accidents in coastal areas. They estimated the rate of ¹³¹I and ¹³⁷Cs released from F1NPP using the atmospheric dispersion model and environmental monitoring data from 116 points throughout Kanto and Tohoku areas around Fukushima Prefecture.

An interesting new decontamination technique, the rapid radiocesium measurement method was developed by Yasutaka et al. [10]. Radiocesium in water was collected using nonwoven fabrics impregnated with Prussian blue and subsequently measured in a germanium semiconductor detector. It was possible to achieve a speedy measurement and a high radiocesium recovery rate in a large volume of water for the lower detection limit.

In the 50th anniversary review of the JNST, Urabe et al. summarized a lesson of radiological disaster countermeasure learned from the F1NPP accident, such as the application of reference levels and dose limits, the dose assessment [11].

In addition, other articles related to general monitoring techniques and environmental assessment methods were also published. Measurement of air radon concentration was introduced by Inagaki et al. [12]. Air radon concentration was measured by a PICORAD detector system using high-density polyethylene, contains a charcoal-silica adsorption material and a liquid scintillation counter, at various places in Masutomi spa, where the radon concentration was high. Air radon concentration varied with spring water condition.

To evaluate the potential environmental impact from an unplanned radionuclide release from a nuclear power plant, Sohn and Lee used a commercial simulation program to calculate tritium migration released from heavy water reactors into ground water and the sea [13]. These simulation results would be helpful for a site-specific ground water safety management. Tritium exposure is not critical for a light-water reactor dose assessment. However, it is difficult to recover tritium water. An accidental large amount of tritium release into the sea received much attention in the wake of the F1NPP accident. Thus, tritium transfer in the environment would be important to study for dose assessment from accidental, or unplanned, releases of tritium-contaminated water.

The use of the CR-39, a solid-state nuclear track detector, for measuring alpha emitters is well known. This is a simple and relatively inexpensive approach. The method of radiation measurement using CR-39 developed by Mori will play an important role in the school science education and the human resource development of future nuclear science technologists [14].