Civil Engineering Reference
In-Depth Information
Chapter 9
The Severe Reactor Accidents of Three Mile
Island, Chernobyl, and Fukushima
Abstract
Three major severe accidents with core meltdown/core disruption
occurred at Three Mile Island (USA) in 1979, Chernobyl (Ukraine) in 1986 and
Fukushima (Japan) in 2011.
The LWR of Three Mile Island was a two loop PWR with 880 MW(e) output.
The accident started with technical problem in the feedwater loop for the steam
generators. As the steam generators were not able to remove the heat, the pressure
in the primary system increased and the safety valve of the pressurizer opened
thereby releasing steam. The reactor was shut down because of too high pressure.
When the pressure in the primary system dropped the safety valve did not shut again
and remained open. The operators were given the opposite information by the
instruments in the control room. The high pressure emergency core cooling systems
started to feed water in the reactor pressure vessel. But the water in the pressure
vessel rose too high and the operators throttled the emergency cooling systems. As
the primary pumps started to vibrate the operators also shut down both primary
pumps. As a consequence the cooling water in the pressure vessel started to boil.
The zirconium claddings started to chemically react with water: hydrogen was
formed. The reactor core began to melt down. The silver-indium-cadmium control
rods did melt. Part of the molten core collected at the bottom of the pressure vessel.
A hydrogen explosion occurred in the reactor building. Only the radioactive noble
gases and a small part of the fission products iodine and cesium were able to
penetrate the filters of the reactor building. The radioactive exposure of the popu-
lation was therefore very small. Cost for decontamination of the plant and disposal
of the destroyed core were very high. The Three Mile Island accident was classified
a level 5 accident on the International Nuclear Event Scale (INES).
The Chernobyl accident occurred in one of four RBMK1000 reactors at the
Chernobyl site 100 miles north of Kiev. The operators were preparing an experi-
ment in which the energy of rotation of the turbine during shut down should
produce emergency electrical power for the support of the diesel generators.
Unexpectedly the experiment had to be interrupted for some time to comply with
electricity supply which led to the buildup of the fission product Xe-135 (neutron
poison). When the experiment could be continued the power level dropped to about
