VGB POWERTECH 7 (2021) - International Journal for Generation and Storage of Electricity and Heat

VGB PowerTech 7 l 2021 Continuation of table 8
direct core damage. However, this equipment
may be excluded if the frequency of
occurrence is calculated to be below 1.00E-
07/yr, which corresponds to the typical
initial event screening criteria [12]. Ta -
b l e 9 shows equipment that can initiate
seismic event in general light-water reactor
nuclear power plant.
In general, seismic events can occur in seismic-induced
loss of coolant accident, loss
of power, loss of control, loss of ultimate
heat sink, main steam line break, and anticipated
transient without scram.
Next, when the seismic-induced initiating
event occurs, an analysis of the equipment
considered to mitigate the event is performed.
Like all external event analysis,
SPSA basically uses the event tree and fault
tree used in the internal event PSA. Here,
non-safety and non-seismic equipment that
cannot be used conservatively is excluded
from the model. However, although seismic-induced
initiating events are different
from internal events, the primary heat removal,
which is essential for mitigation of
the accident, must be performed by the
same equipment and procedure. Therefore,
in the ASME Standard, which is considered
the standard of the PSA, equipment corresponding
to the standard can be listed
based on FV (Fussell-Vesely) importance
0.005 higher and RAW (Risk Achievement
Worth) value of two or higher, which are
the criteria that can be used to classify significant
basic events. To confirm the application
of importance value, a sensitivity
analysis was performed on the reference
nuclear power plant. In Ta b l e 10 , we
Tab. 10. Sensitivity analysis result for importance analysis of internal events PSA.
CASE
# of basic event for
SPSA model
% of baseline
CDF
Base model 1,798 100.0 %
Only FV important basic event 49 97.0 %
Only RAW important basic event 217 95.9 %
Intersection FV and RAW important basic event 24 95.5 %
Union FV or RAW important basic event 242 97.6 %
show the difference between the results of
the existing SPSA and CDF when only the
equipment that was evaluated as important
in internal events was modeled.
As a result, it is confirmed that the results
of the sensitivity analysis showed no significant
difference between the case of
modeling all equipment and the case of
modeling only equipment that was evaluated
as important in internal events. When
242 items of equipment, 13 % of the total of
1798, were modeled in the SPSA model,
the CDF was 97.6 % of the baseline CDF,
and even when 49 pieces of equipment
classified as important in basic events
based on FV are modeled, a result equivalent
to 97 % of the baseline CDF was obtained.
In addition, even when only 24
pieces of equipment were considered, a
value corresponding to 95.5 % of the CDF
value of the base model was derived, indicating
that the importance of other equipment
other than this was much lower than
expected. Therefore, it is judged that the
equipment selection methodology based
on the values of the internal event FV and
RAW is much more rational than the existing
HCLPF-based method, and is an efficient
method that can reflect the characteristics
of the SPSA model.
4.4 Summary of the proposed
equipment selection methodology
The procedure is shown in F i g u r e 3 , a
flow chart of the method of selecting equipment
for seismic events over the three steps
described above.
First, based on the site seismic hazard curve
obtained as a result of PSHA, the SSE value
is checked, which is the design standard of
the power plant, and probability value exceeding
SSE value is checked. After that,
the region of the site is determined by
checking the SSE reoccurrence period according
to the SSE excess probability value.
Through this value, the group of equipment
that should perform fragility analysis is determined.
Next, based on the results of the
PSA importance of internal events, basic
events with an FV value of 0.005 or more or
RAW value of two or more, are listed and
described in terms of the function of the system.
However, basic events related to non-
Fig. 3. Flowchart for the equipment selection methodology.
67