Civil Engineering Reference
In-Depth Information
Fig. 9.9
Schematic design of the General Electric BWR-3 boiling water reactor of Fukushima-
Daiichi [
12
,
13
]
power supplies to the nuclear power plant had been destroyed already by the
earthquake. Also the heat exchangers transferring residual heat to the ocean water
outside failed. In addition the direct current batteries which were also located in the
basement of the plant were flooded and lost.
In unit 1 due to the loss of all direct current batteries (after flooding) all
instrumentation required to control the accident became unavailable. The operator
crew had to work in the dark. The high pressure coolant injection system did not
work, because of loss of power from the submerged diesel power generators
(emergency power supply). The isolation condenser system (steam driven pumps)
worked only partially. The primary coolant system with the reactor pressure vessel
was not depressurized, because of lack of electrical power or pressurized nitrogen
and lack of knowledge about the actual state in the various vessels due to the lost
instrumentation. Therefore, low pressure emergency pumps could not feed water
into the primary system for core cooling. The primary coolant system heated up and
soon exceeded its design pressure, the core became uncovered by coolant and the
fuel rods started to melt down. Hydrogen was produced because the fuel rod
claddings (Zirconium) exceeded temperatures of 1,200
C and reacted with steam.
Zr
þ
2H
2
O
!
ZrO
2
þ
2H
2
:
The pressure in the reactor pressure vessel was soon relieved into the primary
containment because core melt penetrated the lower bottom wall by small holes or
by a break of a low elevation pipe of the pressure vessel or by opening of a safety/
relief valve. The radioactive noble gases and the volatile fission products such as
