Civil Engineering Reference
In-Depth Information
Table 6.5
Internal and external initiating events [
9
]
Frequency per
year
Initiating event
10
7
Plant
internal
Fire
1.7
10
7
External flooding
<
10
6
Plant
external
Earthquake with loss of main heat sink and main feedwater
supply
3
10
7
Airplane crash in reactor containment
<
10
7
per
year. The frequency for core melt due to plant external events (earthquake and
airplane crash) was analyzed in an earlier study [
9
] to be about 3
The frequency for core melt due to internal fire and flooding is 1.7
10
6
per year.
6.6.2 Severe Accident Management Measures
(Safety Level 4)
The detailed analyses of accident sequences in the German Risk Study Phase B [
9
]
showed that it is possible, even after the failure of safety systems, to control
accident sequences by introducing so-called severe accident management mea-
sures. These can then prevent core meltdown.
In a first step the following accident management measures were investigated in
the German Risk Study Phase B [
9
]
- depressurization of the secondary coolant system and feed in of water by mobile
pumps
- depressurization of the primary coolant system by opening the pressurizer valves
to the pressure level such that the high pressure emergency and other emergency
coolant systems, e.g. mobile pumps can feed in water.
A number of accident sequences initiated by loss of coolant or loss of off-site
(auxiliary) power supply are seen to have as their main cause in the failure of
cooling water supply to the steam generators. This failure of steam generator supply
would ultimately lead to core meltdown.
Accident management measures go beyond the automatic safety measures
provided for by the reactor protection system [
21
,
22
] allowing interventions by
the operating personnel. Thus, for instance, the steam generators, in the late
accident phase of running dry can be fed with water from the feed water tank or,
by means of mobile pumps, from water reservoirs inside and outside the reactor.
The decay heat (afterheat) can be removed by blowing off the steam through the
main steam blowdown station (secondary feed and bleed procedure) (Fig.
5.11
).
The opening of the pressurizer valves as an accident management measure in
older PWR designs, e.g. KWU-PWR (see above) lowers the pressure in the primary
system within about half an hour. This allows
