Regional Basic Professional Training Course in Korea

❙ 933 ❙
16. In‐Plant Accident Management
boils as in a BWR, however they are moderated by graphite. The events of April 26, 1986
started as an experiment to see how long a spinning turbine could provide electrical
power to certain systems in the plant before their diesels started. Unfortunately, the
operator made a mistake, and instead of holding power at about 30%, the power fell to
about 1 %. Since this was too low for the test, he tried to raise power. In order to offset
neutron absorption by liquid water and Xenon, the operator pulled out almost all the
control rods by violating the procedure on the control rods which prohibited running with
less than 30 rods. Besides, he had to take over manual control of the flow of water
returning from the turbine, but he could not succeed in getting the flow correct. The
operator disabled a number of the emergency shutdown signals.
As the test begins, the turbine was disconnected and its energy fed to 4 of the 8 main
pumps. Twenty seconds later the power started rising slowly, then faster, and the power
had risen to perhaps 100 times full power and had destroyed the reactor. The release of the
radioactivity to the environment was exacerbated by lifting of the concrete lid on top of
the reactor due to the steam pressure and breaking all 1660 exit pipes. The pressure and
energy generated were sufficient to hurl the concrete lid, topple the fuel machine, blow the
roof off the confinement building and form a high pressure plume which rose to heights
where the winds transported the radioactive products to the neighboring countries. Most of
the radioactivity deposited in Ukraine, Belarus and Russia, however, radioactive particles
were detected in many neighboring European countries and even in Japan. The heavy land
contamination, which persists to this day occurs in Ukraine and Belarus.
The number of immediate deaths is relatively small (31 persons) for such a disastrous
accident. It is possible that there may be shortening of life for part of the liquidator
population which absorbed substantial doses during the clean‐up process of the site and
environs. The accident was terminated by adding with helicopter almost 5000 tons of
sand, clay and boron bearing material on top of the core region. It is not clear whether