Shinji Tokonami

Thyroid doses for evacuees from the Fukushima nuclear accident

A primary health concern among residents and evacuees in affected areas immediately after a nuclear accident is the internal exposure of the thyroid to radioiodine, particularly I-131, and subsequent thyroid cancer risk. In Japan, the natural disasters of the earthquake and tsunami in March 2011 destroyed an important function of the Fukushima Daiichi Nuclear Power Plant (F1-NPP) and a large amount of radioactive material was released to the environment. Here we report for the first time extensive measurements of the exposure to I-131 revealing I-131 activity in the thyroid of 46 out of the 62 residents and evacuees measured. The median thyroid equivalent dose was estimated to be 4.2 mSv and 3.5 mSv for children and adults, respectively, much smaller than the mean thyroid dose in the Chernobyl accident (490 mSv in evacuees). Maximum thyroid doses for children and adults were 23 mSv and 33 mSv, respectively.

Up-to-date radon-thoron discriminative detector for a large scale survey

An up-to-date radon-thoron discriminative detector has been developed for conducting a large scale survey. Comparing with our previous detector, some functional problems have been solved. The lowest and highest detection limits of the detector were estimated to be around 5 and 1000Bqm−3 for radon, and 15 and 1000Bqm−3 for thoron, respectively, with a 6month exposure and several theoretical assumptions. Small indoor survey were carried out in Japan and in Hungary using the present detector. The presence of thoron was demonstrated in any indoor surveys by the two results. Since any measurements without discrimination of radon isotopes will result in different risk estimates from actual situations, a special attention should be paid to thoron and its concentration should be accordingly measured as well as the radon concentration.

Radon and thoron exposures for cave residents in Shanxi and Shaanxi provinces.

Abstract Tokonami, S., Sun, Q., Akiba, S., Zhuo, W., Furukawa, M., Ishikawa, T., Hou, C., Zhang, S., Narazaki, Y., Ohji, B., Yonehara, H. and Yamada, Y. Radon and Thoron Exposures for Cave Residents in Shanxi and Shaanxi Provinces. Radiat. Res. 162, 390–396 (2004). Measurements of natural radiation were carried out in cave dwellings distributed in the Chinese loess plateau. Those dwellings are located in Shanxi and Shaanxi provinces. Radon and thoron gas concentrations were measured using a passive integrating radon-thoron discriminative detector. Concentrations of thoron decay products were estimated from measurements of their deposition rates. A detector was placed at the center of each dwelling for 6 months and replaced with a fresh one for another 6 months. Measurements were conducted in 202 dwellings from August 2001 through August 2002. A short-term measurement was conducted during the observation period. In addition, γ-ray dose rates were measured both indoors and outdoors with an electronic pocket dosimeter. Radioactivities in soil were determined by γ-ray spectrometry with a pure germanium detector. Among 193 dwellings, indoor radon concentrations ranged from 19 to 195 Bq m−3 with a geometric mean (GM) of 57 Bq m−3, indoor thoron concentrations ranged from 10 to 865 Bq m−3 with a GM of 153 Bq m−3, and indoor equilibrium equivalent thoron concentrations ranged from 0.3 to 4.9 Bq m−3 with a GM of 1.6 Bq m−3. Arithmetic means of the γ-ray dose rates were estimated to be 140 nGy h−1 indoors and 110 nGy h−1 outdoors. The present study revealed that the presence of thoron is not negligible for accurate radon measurements and thus that special attention should be paid to thoron and its decay products for dose assessment in such an environment. More systematic studies are necessary for a better understanding of thoron and its decay products.

A simple passive monitor for integrating measurements of indoor thoron concentrations

A simple passive monitor was developed for integrating measurements of indoor thoron concentrations. The monitor was remodeled from a commercially available radon monitor with allyl diglycol carbonate (CR-39) detector. By adding four holes (φ=12 mm) and covering them with high permeability of filter paper, the air exchange rate of the monitor was largely enhanced. The technical characteristics of both the radon and thoron monitors were examined through calibration experiments. A high conversion factor of 1.32±0.14 tracks cm−2⋅(kBq m−3 h−1)−1 and the low lower detection limit for thoron measurements provide the essential conditions for measuring thoron more precisely and sensitively. Furthermore, the main physical advantages of the monitor are its simple construction, light weight, and compactness as well as its low cost, which are preferable for large-scale and long-term indoor surveys. Simultaneous measurements of both indoor thoron and radon are indispensable for accurate assessments of public exposure ...

The time variation of dose rate artificially increased by the Fukushima nuclear crisis

A car-borne survey for dose rate in air was carried out in March and April 2011 along an expressway passing northwest of the Fukushima Dai-ichi Nuclear Power Station which released radionuclides starting after the Great East Japan Earthquake on March 11, 2011, and in an area closer to the Fukushima NPS which is known to have been strongly affected. Dose rates along the expressway, i.e. relatively far from the power station were higher after than before March 11, in some places by several orders of magnitude, implying that there were some additional releases from Fukushima NPS. The maximum dose rate in air within the high level contamination area was 36 μGy h−1, and the estimated maximum cumulative external dose for evacuees who came from Namie Town to evacuation sites (e.g. Fukushima, Koriyama and Nihonmatsu Cities) was 68 mSv. The evacuation is justified from the viewpoint of radiation protection.

Radon–Thoron Discriminative Measurements in Gansu Province, China, and Their Implication for Dose Estimates

Indoor radon measurements were carried out in cave dwellings of the Chinese loess plateau in Gansu province, where previously the Laboratory of Industrial Hygiene (LIH), China, and the U.S. National Cancer Institute (NCI) had conducted an international collaborative epidemiological study. The LIH–NCI study showed an increased lung cancer risk due to high residential radon levels, and estimated the excess odds ratio at 100 Bq/m3 to be 0.19 (Wang et al., 2002). The present study used two types of newly developed passive monitors: One is a discriminative monitor for radon and thoron; the other is a selective monitor for thoron decay products. The arithmetic mean concentrations of indoor radon and thoron were 91 and 351 Bq/m3, respectively. As reported by our previous study in Shanxi and Shaanxi provinces (Tokonami et al., 2004), the presence of high thoron concentration was confirmed and thoron was predominant over radon in the cave dwellings. However, the mean equilibrium equivalent thoron concentration (EETC) was found to be much lower than expected when assuming the equilibrium factor of 0.1 provided by the UNSCEAR (2000) report. The effective dose by radon and thoron decay products was estimated to be 3.08 mSv/yr. It was significantly lower than the dose of 8.22 mSv/yr estimated from the measurements that did not take into consideration any discrimination between radon and thoron. Excess relative risk of lung cancer per sievert may be much higher than the risk estimated from the LIH–NCI study, considering that discriminative measurements were not used in their study.

WHY IS 220RN (THORON) MEASUREMENT IMPORTANT

New scientific findings based on the latest epidemiological analyses for lung cancer risk due to radon have been demonstrated. The residential radon concentration is mainly measured by passive radon detectors. Although the passive radon detector is usually designed to detect radon efficiently and exclusively, several types of them can detect thoron together with radon. In this case, these detector readings may include both radon and thoron signals. If the readings are overestimated, the lung cancer risk will be given as a biased estimate when epidemiological studies are carried out. In our experience, there seem to be no correlation among radon, thoron and thoron progeny concentrations. Therefore, one parameter cannot be estimated by the other. This study presents the importance of thoron measurement throughout results we have obtained in field and in laboratory so far.

Simple discriminative measurement technique for radon and thoron concentrations with a single scintillation cell

A simple, discriminative measurement technique for radon and thoron concentrations is discussed. In this technique, a single scintillation cell is used for both radon and thoron measurements. It consists of two measurements that use the difference of the half life between the two isotopes. Alpha counting efficiencies for their associated radionuclides were estimated by a Monte Carlo calculation. When evaluating the conversion factor for concentration on two types of scintillation cells, both agreed well with experimental values. Optimum measurement conditions on the timetable are also discussed. This technique can provide two concentrations promptly. Although it is not highly sensitive, it is applicable to performance tests for radon/thoron monitors and simultaneous exhalation rate measurements for both radon and thoron.

Contribution from thoron on the response of passive radon detectors

In order to evaluate the reliability of measured values of radon concentration, a thoron sensitivity test for passive radon detectors was carried out. To do this test, a thoron chamber system was first set up. The system consists of four parts: an exposure chamber, a gas generator, an environmental monitor, and a measuring device. Five types of radon detectors were examined using the chamber system. After connecting the exposure chamber with the gas generator through an external pump, thoron gas was circulated through the system. The detectors were exposed to thoron-rich air for several days. The mean ratio between thoron and radon concentrations throughout the exposure period was 10:1. Some of the detectors provided values different from the actual radon concentration. Although the presence of thoron can be negligible in most cases, it is necessary to check the thoron contribution to the detector response with the proposed or similar test before practical use.

Preliminary results of simultaneous radon and thoron tests in Ottawa

Ottawa is the capital city of Canada. In the previous cross Canada radon survey, Ottawa was not included. There is great interest to know radon level as well as thoron concentration in Ottawa homes. Therefore, radon/thoron discrimination detectors developed at the National Institute of Radiological Sciences in Japan were deployed in 93 houses for a period of 3 months. As expected, thoron is present in Ottawa homes. Radon concentrations ranged from 8 to 1525 Bq m(-3) while thoron concentrations varied from 5 to 924 Bq m(-3). The arithmetic mean of radon and thoron concentrations were found to be 110 +/- 168 and 56 +/- 123 Bq m(-3), respectively.