Radon concentrations were measured with a passive radon detector from April 1998 through June 1999 in the Fukuoka Tenjin Underground Shopping Center to assess the dose affecting workers because of radon progeny inhalation. The radon concentration during the period was distributed from a range of 1.9 to 13.6Bq/m³. The arithmetic average concentration was estimated to be 6.9±2.4Bq/m³. The radon level was lower than that in dwellings in Japan and other countries. No spatial distribution of radon concentration was found in that area. From continuous measurement, the radon concentration was found to be high from midnight to noon and low in the afternoon. Little difference was noted between the daily average radon concentration and that during working hours. There was no seasonal variation. The equilibrium factor of 0.21±0.10 was obtained during working hours. The activity-weighted size distribution of radon progeny was evaluated by using the number distribution of ambient aerosols and the classical attachment theory. Consequently, the activity median diameter was 150nm. The unattached fraction of radon progeny was estimated to be 0.025 with an empirical equation. The annual effective dose of workers at the Tenjin center was calculated with the dose conversion factor from the UNSCEAR 1993 report and estimated to be 0.024mSv/y.

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This paper describes the revised index levels of the control of food and water in the Nuclear Safety Commission guidelines, “Off-Site Emergency Planning and Preparedness for Nuclear Power Plants, etc.” Food and water are divided into five categories, and the consumption of each has been conjectured. For this purpose, a nationwide survey for nutrition in Japan by the Welfare Ministry and a survey on the food of infants and children in the coastal area of Ibaraki Prefecture by the National Institute of Radiological Sciences are considered. These categories are (1) drinking water, (2) milk and dairy products, (3) vegetables, (4) grain, and (5) meats, egg, fish, shellfish, and others. The radionuclides groups are then chosen in consideration of their potential importance in regard to food and water contamination. Those chosen were, (1) radio-iodine, (2) radioactive cesium and strontium, (3) uranium, and (4) plutonium and alpha-rayemitting transuranic radionuclides. The intervention dose levels of 5mSv of effective dose and 50mSv of committed equivalent dose to the thyroid for radio-iodine for a period of one year were adopted. The radioactivities of ¹³¹I, ¹³²I, ¹³³I, ¹³⁴I, ¹³⁵I, and ¹³²Te are assumed to be proportional to the contents in nuclear fuel after a cooling time of 0.5 day, and the radioactivity of ¹³¹I is taken as a scale that represents the level of control on the ingestion of food and water. Based on doses to infants, whose exposure is highest, the levels of control are recommended to be 300Bq/kg or more for drinking water and milk and other dairy products, and 2, 000Bq/kg or more for vegetables, except edible roots and potatoes. It is assumed that radio-cesium released in the environment is accompanied by strontium radio-nuclides with a ⁹⁰Sr/¹³⁷Cs radioactivity ratio of 0.1, taking into account the past measurements of fallout. Radio-nuclides are assumed to contain ¹³⁷Cs, ¹³⁴Cs, ⁹⁰Sr, and ⁸⁹Sr with the same mixing ratio as that of the fuel in a nuclear reactor. The sum of radioactivity of ¹³⁴Cs and ¹³⁷Cs is used as a scale that represents the level of control, and the yearly average concentration in food and water is presumed equal to half the concentration of those in the peak term. It has then been recommended that the levels of control for radio-cesium should be 200Bq/kg or more for drinking water and milk and other dairy products, and 500Bq/kg or more for vegetables, grain, meat, eggs, and fish. Moreover, in special cases, measurements on radio-strontium are necessary if a nuclear power reactor operates for less than two years because the ratio of ⁸⁹Sr radioactivity is much higher than in reactors with longer times of operation. For uranium, the index levels are presented in terms of alpha-activity concentration: 20Bq/kg or more for drinking water and milk and other dairy products, and 100Bq/kg or more for vegetables, grain, meat, eggs, and fish. The sum of the radioactivity concentration of ²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, ²⁴²pu, ²⁴¹Am, ²⁴²Cm, ²⁴³Cm, and ²⁴⁴Cm is selected as a scale for representing the levels of control on food and water for plutonium and other transuranic radionuclides. The level of control has been recommended as 1Bq/kg or more for drinking water and milk and other dairy products, and 10Bq/kg or more for vegetables, grain, meat, eggs, and fish. For commercially available food for babies, the recommended level of control is 1Bq/kg or more in a cooked form and served as a meal for plutonium and other transuranic radionuclides.

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“Risk” is an important and significant word that denotes the root of a serious problem symbolizing modern society. The meaning of this word suggests a basic approach to the probable resolution of its difficulties, and its study consequently assures the wholesome future of the world. The concept of risk is neither a simple concept of “danger or hazard” nor of “loss or cost”. But it is the complex concept of behavior or choice of action itself that leads to perilous situations, ruinous consequences, and a damaging sequence of events.

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