Civil Engineering Reference
In-Depth Information
Lessons Learned
The main reasons for the Chernobyl reactor disaster were the
wrong design of the RBMK reactors with a positive coefficient of coolant temper-
ature, and also of the control/shutdown systems containing zones of graphite.
Moreover, shutting down the automatic protection system and other safety devices
by the operators in the course of the test was inadmissible and banned, respectively.
Western light water reactors may be built and operated only with a sufficiently
high negative coefficient of coolant temperature.
9.3 The Reactor Accident of Fukushima, Japan
On March 11, 2011, an earthquake of the intensity M
9 (Richter scale) hit the
northeastern coast of Japan east of the city of Sendai (Fig.
9.7
). This intensity
corresponds to intensity XI on the European MSK/EMS-98 seismic intensity scale
[
12
-
21
].
Roughly 1 h later, a tsunami wave hit the coast, flooding the Fukushima nuclear
power plant up to a water level of 14 m. The earthquake was number four on the list
of the severest earthquakes so far registered worldwide; its intensity had not been
foreseen by Japanese seismologists when the reactor was designed [
15
]. Tsunami
waves of much greater heights (up to 38 m) had impacted the Japanese coast in the
past in the course of earthquakes of lower intensity (Richter scale) (Table
7.14
,
Sect.
7.4
). However, the Fukushima-Daiichi nuclear power plant had been designed
only against tsunami waves up to 5.7 m high (Fig.
9.8
)[
12
].
Four nuclear power plants with an aggregate 14 BWRs in the environment of the
city of Sendai were hit by the earthquake (with a number of subsequent seismic
events) and the tsunami wave on March 11, 2011. These were (Fig.
9.7
) the nuclear
power plants of Onagawa with three BWRs, Fukushima-Daiichi with six BWRs,
Fukushima-Daini with four BWRs, and the Tokai Research Center with one BWR
and one research reactor. At that time, only three BWRs were in operation in the
Fukushima-Daiichi plant, while the fourth BWR was down (with all fuel elements
removed and located in the spent fuel pool), and BWRs 5 and 6 had been shut down
for inspection and repair. The eleven BWRs in operation
were duly shut down
automatically by the earthquake instrumentation and changed into the resid-
ual heat removal mode.
The seismic waves hitting the Fukushima-Daiichi nuclear power plant from the
epicenter caused a horizontal acceleration of 507 cm/s
2
of the most highly loaded
reactor of the three units (unit 1). This plant had been designed to 449 cm/s
2
.
Nevertheless, the BWR was duly shut down. The reactor cooling system and the
reactor core were not damaged. In units 2 and 3, horizontal acceleration had not
exceeded the design basis levels (Fig.
9.9
)[
12
,
13
,
16
,
18
].
The operating crew immediately started accident management measures in each
of the three reactors of the Fukushima plant. First, core cooling was maintained in
each of the three reactors by means of the battery power available and a small steam
turbine pump system fed by steam from the reactor pressure vessel.
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