On April 25, 1986, the nuclear reactor Unit 4 (RBMK) at Chernobyl, Ukraine,
exploded. Besides molecular species, the fallout contained particles of
relatively high specific activity (hot particles) with a wide range of
chemical compositions. The composition of a hot particle bears information
about its genesis. Particle sizes ranged from a few to 100s of micrometers.
Data on a hot particle, found in Berlin, Germany, is presented and discussed
in context with earlier measurements on other particles to understand their
genesis. The chemical composition was determined by electron probe micro
analysis. Our particles are either reactor fuel (one) or fission product
alloys (nine). The alloys were formed during normal reactor operation.
Strongly varying concentrations of Fe and Ni suggest that at least some of
our particles reacted with molten structural material of the reactor. The
particles were mobilized by fuel oxidation or fuel dust generation during
the accident. The fission product composition can only be explained if we
assume that the alloys remained in the solid state in the course of the
accident. Some particles may have been ejected during the explosion, others
later while the reactor was burning. Activities (103Ru and
106Ru, originally up to 160,000 Bq) of our ten year old
particles were re-measured but were no longer detectable. No long-lived
γ-emitters were found. The 99Tc activity was calculated and found
to only lBq. The γ -spectrum of the fuel particle still shows
137Cs (1 Bq) and 60Co (<1 Bq).