Civil Engineering Reference
In-Depth Information
F
m,1
»
f
m
F
m,2
»
f
m
×
p
4
F
m,3
»
f
m
×
p
3
F
m,4
»
f
m
×
p
3
×
p
4
F
m,5
»
f
m
×
p
2
F
m,6
»
f
m
×
p
2
×
p
3
F
m,7
»
f
m
×
p
1
Fig. 6.2
Simplified event tree for a loss-of coolant accident in a water cooled reactor [
3
]
Table 6.1
Frequencies per year of accident initiating events for a 1.3 GW(e) KWU-PWR at
Neckarwestheim (Germany) [
8
]
Frequency of initiating event per year
(f
m
)
Initiating event
10
2
Loss of electrical auxiliary power supply
2.5
10
2
Loss of main heat sink and loss of main feed water
supply
3.8
Small leak (25-80 cm
2
) in main primary coolant pipe
10
4
1.5
Small leak at pressure vessel (1-6 cm
2
)
10
3
2.5
If power is available, the next possible event will be a potential failure of the
ECCS, which must be assigned the probability of p
2
. The availability of the ECCS
is again characterized by (1
p
2
).
If fission products are released in the course of an accident, the fission product
removal system mitigates the radioactivity release into the containment. The failure
probability of this system is characterized by p
3
, its availability (1
p
3
).
The final barrier against the release of radioactivity is the leak tightness (integ-
rity) of the outer containment. The probability of this containment function failing
is called p
4
the availability of that function, (1
p
4
). If the containment integrity is
preserved, releases of radioactivity can only be slight, but if the containment leaks,
radioactivity can escape into the environment, depending on the size of the leak.
From the results of this simplified event tree of Fig.
6.2
it can be seen that
radioactivity releases can vary between very small and very large releases,
depending on the level at which the safety systems fail.
