Civil Engineering Reference
In-Depth Information
The results indicate the most highly exposed zone due to Cs-134 and Cs-137
with 6
10
6
Bq/m
2
to be in the northwestern direction from the
Fukushima-Daiichi plant. Other zones show lower soil contamination levels due
to cesium. Evaluations indicated regions in which persons, if they lived there and
consumed only food available locally, would suffer a radiation exposure of 5 mSv/
year and 10 mSv/year or 20 mSv/year, respectively, within the first year after the
accident (This must be compared with the world wide effective annual radiation
dose 2.4 mSv/year) [
19
] and the 8-100 times higher annual natural radiation
exposure in Kerala, India or Brazil (Sect.
4.3
). In the regions more highly exposed
to radioactive cesium, 3,100 persons, if they were to return there, would suffer
50 mSv/year, and 2,200 persons would not be allowed to enter the “no-entry” zone.
They would be exposed there to 100-500 mSv/year [
17
].
The Fukushima accident was classified in top category 7 of the international
event scale of reactor accidents drafted by IAEA.
10
6
to 30
9.3.5 Lessons Learned
Despite the earthquake of Richter scale 9, the reactor units of Fukushima-
Daiichi were shut down automatically as planned
. The emergency power diesel
generators supplied the internal grid as planned for almost 1 h until the tsunami hit.
The disaster occurred because the reactor facility had been designed only
against a tsunami wave of 5.7 m height. The tsunami wave of 14 m height
caused the emergency power diesel generators and direct current batteries to
fail. Unfortunately, they had been installed at the lowest point (the basement of
the turbine hall). The compartments of this turbine building could not be shut
watertight.
Due to the failure of the diesel generators and the loss of the direct current
batteries the reactor pressure vessel could not be depressurized (safety/relief valves
could not be opened). The high pressure water injection system failed. The isolation
condenser system worked only partially.
The efforts of the operators to start emergency core cooling by means of low
pressure fire pumps failed. Water could not be injected because of the high pressure
in the reactor. This caused the water level in the reactor pressure vessel to drop, the
temperature of the fuel claddings to rise, hydrogen to be produced, the fuel elements
to melt down, and fission products to be released into the pressure suppression
chamber. The absence of hydrogen recombiners in the inner containment, and the
non-availability of means for the depressurization of the hydrogen mixed with
steam and fission products to be passed through aerosol filters in the stack of the
plant, led after leaks out of the inner containment to hydrogen explosions and
destruction of the relatively lightweight roof structure. The fuel element storage
pool was uncovered. Radioactive iodine and, above all, radioactive cesium were
released into the atmosphere.
