HIGHLY RADIOACTIVE RUTHENIUM PARTICLES RELEASED FROM THE CHERNOBYL ACCIDENT - PARTICLE CHARACTERISTICS AND RADIOLOGICAL HAZARD

After the Chernobyl accident highly radioactive Ru particles, more tha n 100 kBq in activity and more than 10 mu m in diameter, were found hu ndreds of kilometres away from the plant. Since particle sampling and analyses have not necessarily been adequate, an approach based on inve ntory calculations is used for estimating their radiological hazard. E lemental composition of the particles suggests that short-lived nuclid es may essentially contribute to the risks, although usually only Ru-1 03 and Ru-106 were detected. Because large Ru particles are not of inh alable size, skin doses are calculated for various particle compositio ns, sizes and decay times. Calculations support the assumption that Ru particles are metallic precipitates. The composition may have a notab le impact on skin doses which are not necessary the largest for partic les originating from fuel of high burnup. Even an individual particle may be a severe radiological hazard. A dose of 50 mGy.cm(-2) for the b asal cell layer may be exceeded in one hour provided that an Ru partic le larger than 8 mu m in diameter is deposited on the skin.