Power Generation - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 207

reactor vessel environment. A series of equipment failures and human er-
rors along with the defective instrumentation allowed the reactor core to
be compromised and go into a partial melt. The radioactive water that was
released from the core was confined within the containment building and
very little radiation was released.
At Three Mile Island, the safety devices operated as planned and
prevented any serious injury. This accident resulted in improved proce-
dures, instrumentation, and safety systems being implemented. The Three
Mile Island Unit 2 core has been cleaned up and the radioactive deposit
stored. The Three Mile Island Unit 1 is still operating with a clean record.
Worldwide reactors continued to be built until the accident at Cher-
nobyl occurred. Several features made the Chernobyl accident unique to
a Soviet style reactor. One was the use of graphite as a moderator, which
caught fire. Another was the absence of water to contain radioactivity. But,
the most important may have been an inadequate containment structure.
There were also problems in controlling the stability of the reactor and
the control rods had to be changed frequently in order to keep the reactor
stable.
Before the accident at least 6 safety mechanisms were disconnected
to conduct experiments to increase the output of the reactor. This was the
direct cause of the accident and as the power output surged from 7 to
50% in a few seconds there was a lost of coolant. The heat then melted the
graphite rods used as a moderator.
An experiment to find out how long power was generated as the
reactor unit was shut down was authorized. But, automatic shut-down
mechanisms were blocked that may have come into operation at low ca-
pacity levels. These included the reactor's emergency cooling system and
its low water level safeguard. Extra pumps were also turned on to raise
the amount of steam going to the generator. These pumps were operated
over the allowable level. This became the worse nuclear power plant di-
saster on record when the Chernobyl reactor had a hot gas explosion. In
a Western nuclear power plant, the explosion would have been contained
because Western plants are required to have a containment building with
a solid dome of steel reinforced concrete that contains the reactor. The
Chernobyl plant did not have this containment feature, so the explosion
blew through the roof of the reactor building allowing radiation and reac-
tor core parts to escape into the air.
The design of the Chernobyl plant was deficient in other ways. West-
ern reactors are designed when operating to maintain negative power co-