Civil Engineering Reference
In-Depth Information
water in the primary coolant system. The primary coolant system heated up and
exceeded its design pressure. The core became uncovered, the zirconium claddings
of the coolant system chemically reacted with water and formed hydrogen. The core
melted down. The pressure in the pressure vessel was relieved into the primary
containment because core melt penetrated the lower bottom wall. The pressure in
the primary containment led to release of hydrogen and fission product gases into
the upper reactor building, where a hydrogen explosion occurred destroying the
upper building structures.
In units 2 and 3 the accident developed in a similar pattern, though with a larger
shift in time. However a hydrogen explosion only occurred in unit 3 (BWR) not in
unit 2 (BWR). However, a hydrogen explosion also occurred in unit 4 (BWR) due
to a backflow through the common gas treating system. The hydrogen explosion
destroyed the upper structures of the reactor building. The spent fuel pools of unit
1, 3 and 4 had to be cooled part time by concrete pumping trucks, water cannons or
helicopters dropping water, but no damage occurred to the fuel in the spent fuel
pools. After detailed measurements of the radioactivity released into the environ-
ment the Japanese government evacuated about 200,000 people. Four persons of the
operating crew were killed by the earthquake and the tsunami wave. Some 20 staff
members were injured by the hydrogen explosions. Out of the about 23,000
emergency workers 12 received effective radiation doses up to 700 mSv and
75 workers received
10 mSv.
The contamination of land was measured. About 2,200 persons would not be
allowed to return to a no-entry zone because of too high radiation exposure. The
Fukushima severe reactor accident was classified level 7 on the International
Nuclear Event Scale (INES).
<
200 mSv. The radiation dose of all others was
<
Three major reactor accidents with core meltdown/core disruption occurred at
Three Mile Island (TMI), USA, on March 28, 1979; at Chernobyl, Ukraine, on
April 26, 1986, and at Fukushima, Japan, on March 11, 2011. The three reactor
accidents at Three Miles Island, Chernobyl and Fukushima will be discussed and
described briefly below. Prior to the most severe accident at Chernobyl, Ukraine,
the reactor accident at Windscale (United Kingdom) in a plutonium production
reactor for military purposes in 1957 had been considered the worst nuclear
accident with radioactivity release to the environment. It will not be discussed here.
9.1 The Accident at Three Mile Island
On March 28, 1979, a sequence of accidents occurred in unit 2 of the Three Mile
Island reactor facility in the United States of America which ultimately resulted in
partial meltdown of the reactor core. The pressurized water reactor had been built
by Babcock & Wilcox (Fig.
9.1
)[
1
,
2
].
