Civil Engineering Reference
In-Depth Information
Table 6.4
Expected frequencies for core melt down for a 1.3 GW(e) KWU-PWR at
Neckarwestheim (Germany) [
8
]
Frequency per
year
Initiating event
Medium to large leak (80-200 cm
2
) in a primary coolant
pipe
10
8
Loss of coolant
9.0
Small leak in primary coolant pipe (25-80 cm
2
)
10
7
1.6
Very small leak in primary coolant pipe (2-25 cm
2
)
10
6
1.3
Leaks at pressurizer (40 cm
2
)
10
7
4.0
Very small leak at reactor pressure vessel (1-6 cm
2
)
10
7
1.8
1.4
10
6
Operational
transients
Loss of auxiliary power supply
2.2
10
6
Loss of main feedwater supply without loss of main heat
sink
1.8
10
6
Loss of main heat sink without loss of main feedwater
supply
3.0
10
7
Loss of main feedwater supply and loss of main heat sink
1.3
10
7
Break of steam line out of outer containment
2.9
10
7
Break of feedwater line out of outer containment
8.2
10
6
Sum total
6.6 Results of Reactor Safety Studies
6.6.1 Results of Event Tree and Fault Tree Analyses
The methods and analyses described above were applied to both PWRs and BWRs,
e.g. in the US Reactor Safety Study [
1
] and in the German Risk Studies [
2
,
8
,
9
] for
PWRs and for BWRs [
7
]. The results of recent risk studies for modern PWR- and
(BWRs) will be reported as examples below.
Tables
6.1
,
6.4
and
6.5
list the main findings of the event tree and fault tree
analyses in a German risk study for a PWR operating at Neckarwestheim [
8
]. In
analyzing all possible initiating events, mainly four groups of accident sequences
were identified to result in core damage for the PWR:
- loss-of-coolant accidents initiated by a leak or break in the reactor coolant
system,
- operational transients leading to an imbalance between the heat generated in the
core and the heat removed from the core.
- reactor plant internal initiating events like fire
- reactor plant external events e.g. earthquakes, airplane crash and flooding.
The data in Table
6.4
as well as Table
6.5
result from detailed studies of the
above accident sequences.
The sum total of the frequency per year of all conceivable loss of coolant
accidents and operational transients is 8.2
10
6
per year.
