Civil Engineering Reference
In-Depth Information
Cs-134, Cs-137, Sr-90 etc. were released into the pressure vessel and into the
primary containment. The water in the primary containment with pressure suppres-
sion chamber heated up and its design pressure was soon exceeded (Fig.
9.9
).
Leakage paths within the primary containment vessel led to hydrogen release into
the upper part of the reactor building (Fig.
9.10
). About 1 day after the earthquake
and the impact of the tsunami wave on unit 1 a hydrogen detonation occurred in the
upper part of the reactor building. It destroyed the upper structures of the reactor
building. Four technicians were injured [
20
,
21
].
The records did not show any deliberate attempt by the operation crew to
depressurize the reactor pressure vessel during the accident course. This would
have been necessary to add water by emergency pumps. Only about 2 h after the
hydrogen detonation, when the primary coolant system had depressurized itself, the
operators could begin feeding in fresh water using fire pumps [
21
]. However, the
longer term water level in the reactor pressure vessel did not recover to more than
midplane, regardless of the make-up water quantity being added. This indicates a
low elevation leak in the pressure boundary of the reactor pressure vessel.
The accident developed in an almost similar pattern in units 2 and 3, though with
a larger shift in time. The reactor core isolation cooling system worked longer (for
70 h in unit 2 and 20 h in unit 3). When this emergency cooling system failed in
units 2 and 3 the operators tried to depressurize the reactor pressure vessel in order
to inject water using the fire extinguisher lines. Problems occurred, however, due to
the lack of electricity for the solenoid valves and lack of pressurized nitrogen to
open the safety/relief valves. Therefore, water could not be injected for about 6.5 h
in unit 2 and for about 7 h in unit 3. The core fuel became uncovered in both units.
The fuel heated up, was significantly damaged, hydrogen was produced and volatile
fission products and radioactive noble gases were released into the primary con-
tainment. A longer term water level in the reactor pressure vessel could not be
restored to higher than about midplane in both units 2 and 3, indicating also a low
elevation leak in the pressure boundary of the reactor pressure vessel. The primary
containment pressure increased. Hydrogen was released probably through leakage
paths as the containment vent lines could not be opened. This was due to not high
enough pressure to break a rupture disk. As a consequence a hydrogen detonation
occurred also in unit 3. In unit 2 no hydrogen detonation happened [
21
].
Also in unit 4 the total emergency electricity supply (diesel generators and
batteries) was lost. This lead to an increase of the coolant water temperature in
the fuel storage pool of unit 4. At 6
AM
on March 15 a hydrogen explosion also
occurred in this reactor building (unit 4), severely damaging its upper structures. At
first it was thought to be due to hydrogen production from fuel heat up and coolant
uncovery in the spent fuel pool. Later, photographs indicated that there was no
overheat damage of that fuel in the spent fuel pool. The source of hydrogen was
traced to be a backflow through the standby gas treating system shared as common
piping with unit 3 [
21
].
