Physical Resources / 129They are then sent for reprocessing. This operation separates fuel that can be
used from the waste, including radioactive products of uranium fission. Fuel is
returned for use.
The remaining waste is converted into cylindrical blocks of a glass-like solid,
packaged, and sent for disposal. This involves secure storage for a number of
years before final disposal. No decision has yet been made on the method to
be used for final disposal, but it is most likely to be below ground in a facility
constructed in a geologically stable environment.
This sequence of operations, from mining to final disposal, constitute the nuclear
fuel cycle.
ChernobylIn the course of the history of civil nuclear power there has been only one
accident in which a significant amount of radioactive material was released
into the environment outside the plant. That accident occurred at about 1.23
a.m. Moscow time on April 26, 1986, in the No. 4 unit of the Chernobyl nuclear
power station. The station is located in the eastern part of the Belorussian-
Ukrainian Woodland about 4 km from the town of Pripyat, where most of the
workers from the station lived, and about 18 km from the town of Chernobyl. It
was reactor of the RMBK-1000 type, using slightly enriched uranium dioxide
fuel encased in cans of zirconium alloy, water as a coolant, and graphite as a
moderator. The coolant water flowed through the channels into which the fuel
rods were inserted. Chernobyl-4 entered service at the end of 1983.
Chernobyl-4 was to have been shut down for routine maintenance on April 25.
As this happened the management planned to carry out experiments to test
the safety equipment. One experiment aimed to determine how long a single
turbogenerator would continue to supply sufficient power for the plant to
continue operating safely once its supply of steam had been cut off and it was
spinning by inertia. This necessitated switching off the reactor emergency
cooling system, a procedure strictly forbidden by the authorities, to prevent it
from automatically restoring the steam supply. The experiment was begun by
the day shift, which included the station managers and specialists, but
conducted mainly by the night shift of less qualified workers.
By about 1.20 a.m. the reactor was running at about 6 per cent of its normal
power and to maintain a sufficient flow of neutrons to sustain the reaction
almost all of the 211 control rods were withdrawn completely. This also broke
a rule on reactor safety. The steam supply was then withdrawn from the turbine.
The head of the shift realized the situation was dangerous and ordered the
control rods to be reinserted. They moved very slowly, however. The number
of neutrons increased suddenly (a condition called ‘prompt criticality’) and within
one second the power surged to several hundred times its normal level. This
caused the first explosion in which some of the fuel cans burst.