Civil Engineering Reference
In-Depth Information
- the high-pressure safety feed pumps to be activated at 11 MPa (KWU-DWR),
- the pressure accumulators to start feeding at 2.5 MPa (KWU-DWR),
- and the residual heat removal systems to be activated at about 1.0 MPa.
In this way, for instance, core meltdown under high pressure can be avoided, or
this accident sequence can be changed into core meltdown at low pressure.
Accident management measures include the actuation of pressure relief valves
or the actuation of smaller pumps of auxiliary circuits by means of batteries. For
this reason the internal battery capacity is increased and reinforced accordingly.
German LWRs, in addition, were equipped after these risk studies [
2
,
8
,
9
] with a
so-called sheltered control room. It is possible to shut down the reactor to decay
heat (afterheat) level from this completely independent control room in case the
main control room can no longer be used, e.g. because of fire or radioactive gases.
Additional emergency instrumentation, e.g. filling level probes in the reactor
pressure vessel or measurements of radioactivity at various points of the cooling
system, allow more precise knowledge to be obtained about the operating status of
the reactor [
9
]. Emergency electric power can be supplied by underground cables
from more distant power plants. Cooling water can be supplied by deep wells
situated on plant site [
22
].
The sum total of the core melt frequency per year applying these severe accident
management measures could thereby be lowered by one order of magnitude [
9
].
6.6.3 Core Melt Frequencies per Reactor Year
for KWU-PWR-1300, AP1000 and EPR
The core melt frequencies and radioactivity release frequencies for different PWR
designs, e.g. KWU-PWR-1300, AP1000 and EPR are given in Table
6.6
. The more
recent PWR-designs have by an order of magnitude lower overall core melt
frequencies per reactor year and so-called large radioactivity release frequencies
per reactor year than the older design of the KWU-PWR-1300. EPR is designed
such that even in case of a core melt accident large radioactivity releases are
6.7 Results of Event Tree and Fault Tree Analyses
for BWRs
Probabilistic risk analyses were carried out also for BWRs. Table
6.7
lists some of
the most important initiating events of accident sequences in the German
