Radioactivity in surface air and precipitation in Japan after the Chernobyl accident

Abstract Radioactive plumes from the Chernobyl reactor accident first passed over Japan on 3 May 1986. Measurements of 103 Ru, 131 I and 137 Cs in rainfall and airborne dust collected at Chiba near Tokyo show that, in fact, at least two or more kinds of plume arrived during May. Their altitudes were calculated to be about 1500 m in early May and 6300 m in late May. Radionuclides detected in 33 precipitation samples collected by the network of prefectural radiation monitoring stations from 1 to 22 May were 7 Be, 89 Sr, 95 Sr, 95 Zr, 95 Nb, 103 Ru, 106 Ru, 110m Ag, 125 Sb, 129m Te, 131 I, 132 Te, 132 I, 134 Cs, 136 Cs, 137 Cs, 140 Ba, 140 La, 141 Ce and 144 Ce, the measurements being made using germanium detectors and low-background GM counters after radiochemical separation. The radiation was characterized by higher levels of the volatile nuclides, such as 103 Ru (in the form of RuO 2 ), 132 Te, 131 I and 137 Cs, than fallout levels in nuclear weapons testing, and by activity ratios of 0·48 and 14 for, respectively, 134 Cs/ 137 Cs and 89 Sr/ 90 Sr, as on 26 April. The fallout activity was higher in northwestern Japan, the average depositions of 90 Sr and 137 Cs in Japan from 1 May (or 30 April) to 22 May being 1·4 Bq m −2 and 95 Bq m −2 , inventories which are 14 and 550 times higher than the pre-Chernobyl values.