ABSTRACTVarious types of Chernobyl fuel containing masses named black “lava”, brown
“lava”, porous “ceramic” and “hot” particles that formed during first days
of the accident at the Chernobyl Nuclear Power Plant 4th Unit were studied
by methods of optical and electron microscopy, microprobe and x-ray
diffraction. Data about their chemical, phase and radionuclide composition
are summarized. The products of interaction between fuel, zircaloy and
concrete, produced under experiments in laboratory were examined for
comparison with samples of Chernobyl “lava” and “hot” particles. The
behavior of nuclear fuel in first days of the Chernobyl accident was a
three-stage process. The first stage occurred before the moment of the
Chernobyl explosion and was exceptionally short-lasting, perhaps, less than
a few seconds. It was characterized by reaching a high temperature, ≥2600
°C, in the epicenter of accident and formation of a Zr-U-O melt in a local
part of the core, which is estimated to be not more than 30% of whole core
volume. The second stage lasted for about 6 days since the explosion, during
which there was interaction between uranium products of the destroyed
reactor: UOx, UOx with Zr, Zr-U-O, with the environment and silicate
structural materials of the 4th Unit. The third stage, after 6 days involved
the process of final formation of the radioactive silicate melt or Chernobyl
“lava” at one of the sections of the destroyed 4th Unit. During this stage
the melt's lamination occurred, followed by a break-through of the “lava”
reservoir on the 11 th day of the accident and penetration of the “lava”
into space under the reactor.