Civil Engineering Reference
In-Depth Information
Fig. 9.1
Simplified schematic diagram of unit 2 [
1
]
It had a reactor power of 880 MW(e) and only two cooling circuits, A and B. The
sequence of accidents began with technical problems in the feed water loop for the
steam generator. This caused a turbine trip and triggered the startup of the emer-
gency feed water systems. However, the valves in the emergency feed water system
were closed by mistake. As the steam generators did not get enough feed water and,
for this reason, were not able to remove enough heat, also the temperatures and
pressures in the primary cooling circuit started to rise. When an excessively high
primary pressure had been reached, the pilot-operated relief valve (PORV) of the
pressurizer opened, and the steam was released into the pressure relief tank in the
containment. The “primary pressure too high” signal caused the reactor to be shut
down. The reactor power dropped to the residual heat level. As a consequence, also
the primary pressure dropped. Apart from the mistake mentioned above the events
vent on in accordance with the measures planned for that incident condition.
However, the PORV did not shut again at the lower pressure, but remained open.
The operators, though, were given the opposite information by the instruments in
the control room, namely the indication that the PORV had closed again. The open
PORV continued to discharge more steam, and the primary pressure continued to
drop. When the primary pressure reached the level at which the high-pressure
emergency cooling pumps start to feed, these pumps were activated automatically
in order to compensate for the loss of primary coolant. The pressure relief tank was
overfilled by the steam released, the water spilled over and collected in the sump of
the reactor building. From here it was automatically pumped into storage tanks for
radioactive water in the auxiliary systems building (not shown in Fig.
9.1
).
The operating crew, who had become confused by the wrong readings of the
instruments and did not know precisely the status of the plant, now throttled the
