MOBILE SURVEY OF ENVIRONMENTAL GAMMA-RADIATION AND FALLOUT LEVELS IN FINLAND AFTER THE CHERNOBYL ACCIDENT

Results from a survey of environmental gamma radiation levels in Finland after the Chernobyl accident in 1986 are presented. The measurements were made in 1986-87 by means of sensitive Geiger counters and a gamma spectrometer placed in cars. The results show the level of external radiation caused by the caesium fall-out on the first of October 1987. The fall-out patterns of 137Cs, as well as of 95Zr and 103Ru, are also presented. In the centre of Southern Finland there are wide areas with exposure levels exceeding 0.03 μSv.h-1, areas exceeding 0.10 μSvh-1 being very rare. The surface area weighted mean dose rate for the 461 municipalities in Finland was 0.027 μSv.h-1 (range 0-0.19 μSv.h-1). The population weighted mean dose rate was 0.037 μSv.h-1. The corresponding estimated surface activity of 137Cs was 10.7 kBq.m-2. The passage of the Chernobyl plume over Finland in 1986 led to various fall-out patterns for different radionuclides. The deposition of the non-volatile nuclides., 95=Zr and 141Ce, is closely related to the passage of the hot particle dust formed at the initial explosion in the reactor at 01.23 LT on 26 April. This cloud passed over Finland between the morning and the night of 27 April. The deposition of volatile fission products such as 131I, 132Te, 134Cs and 137Cs in Finland was caused by releases from the burning reactor after the initial explosion. The radioactive plume spread over Southern and Central Finland between Sunday 27 April and Tuesday 29 April. On 30 April, and finally on 1 May, a cold northerly airstream spread into the whole of Finland purifying the atmosphere. The volatile nuclides were mainly deposited by intermittent rain on 28-30 April. The deposition pattern of 103Ru is a combination of the fall-out patterns due to the initial explosion and the reactor burn, as well as the wet deposition occurring on 10-12 May caused by the releases from the burning reactor in early May.