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

Equipment selection methodology of seismic probability safety assessment for nuclear power plants VGB PowerTech 7 l 2021
Tab. 5. Sensitivity analysis result for various equipment exclusion.
Component Description Am βu βr HCLPF ΔCDF(%)
SEIS-EP-BATT Seismic failure of station batteries 1.02 0.19 0.42 0.35 -2.25%
SEIS-EE-DAY-TK Seismic failure of EDG fuel oil day tanks 1.08 0.45 0.24 0.33 -2.22%
SEIS-EP-XFRMR Seismic failure of 4kv-480v transformers 0.83 0.24 0.25 0.37 -0.20%
SEIS-BC-BCHX Seismic failure of Bearing Cooling HXs 0.77 0.25 0.22 0.35 -0.96%
Although an attempt was made to review
more cases of PSHA and seismic core damage
frequency (SCDF), the published data
are limited and insufficient to infer an accurate
correlation; however, the trend can
be confirmed using published data. The
values that represent the correlation between
site SPHA and SCDF are the probability
of exceeding 1.0 g and the probability
of exceeding the safety shutdown earthquake
(SSE). First, the probability of seismic
acceleration exceeding 1.0 g corresponds
to a very large earthquake magnitude,
so most power plants assume that
direct core damage occurs. However, as a
result of reviewing the case of power plants
based on this value, it is difficult to represent
the characteristics of seismic hazards
based on Pr(a>1.0 g), since the core damage
frequency for earthquakes exceeding
Second, we examine the amount of change
in CDF for equipment with similar HCLPF.
Even with similar HCLPF value, the A m
value may show large difference according
to the β value, and the effect may be very
different depending on the seismic acceleration
intervals determined. In addition,
the accident sequence varies depending on
the plant characteristics, so the target
equipment contributes to plant safety
which can have a very different effect on
the entire CDF. Therefore, screening based
on single HCLPF criteria will not be able to
understand all the effects without performing
various sensitivity analyses and
may have an optimistic effect on the overall
results. Ta b l e 5 shows the analysis of the
effect on CDF that is reduced when four
pieces of equipment with similar HCLPF
(battery; emergency diesel generator fuel
oil day tank; 4 kV-480 V transformer; and
bearing cooling heat exchanger) are excluded
from the model.
As a result, it can be seen that in the case of
the battery, the reduction rate of CDF is
very large, corresponding to 2.25 %, but in
the case of the transformer, it is very small,
corresponding to 0.2 %. Therefore, it can
be seen that if a single HCLPF criterion is
applied and screened-out, the effect on the
entire CDF is different for each piece of
equipment, and thus further analysis is required.
Tab. 6. Comparison of PSHA and Seismic CDF for reference plant [6, 7].
Site name Pr (a>SSE) Pr (a>1.0g) Seismic CDF Seismic CDF Proportion
(Over 1.0g)
Seabrook 4.62E-04 3.80E-06 2.99E-05 13 %
Krsko 4.00E-04 1.40E-06 5.96E-05 2 %
Limerick 2.29E-04 1.79E-06 5.72E-06 31 %
IP2 2.12E-04 1.75E-06 8.40E-06 21 %
Millstone 3 1.82E-04 1.47E-06 8.85E-06 17 %
Surry 1.32E-04 6.00E-07 2.37E-05 3 %
Average 2.61E-04 1.79E-06 1.99E-05 14 %
0.00010
0.00008
1.0 g accounts for a range of approximately
2 % - 14 %.
Next, we reviewed the probability of exceeding
SSE. Since SSE is the reference
value for seismic design of safety-related
SSC, it can be assumed that in the event of
an earthquake below SSE, most equipment
can maintain its function, which has a significant
impact on plant safety before and
after the seismic acceleration corresponding
to this value. When plotting the correlation
between the probability of occur-
SCDF
4. Equipment selection
methodology
As discussed in the previous section, equipment
selection for SPSA is not being carried
out using a rational approach, such as
by single screening criteria or conservatively
considering all equipment. In this
paper, we propose a methodology for an
equipment selection methodology based
on all three analysis steps of SPSA.
SCDF (/yr)
0.00006
0.00004
0.00002
0.00000
4.1 Determination of site seismic hazard
region using PSHA
PSHA is the step of evaluating the site-specific
seismic probability that is the input to
the SPSA. Basically, the probability of an
earthquake is the most important step because
it directly affects the probability of a
seismic-induced initiating event. In this paper,
we reviewed the correlation between
various SPSA cases and PSHA of each site.
Ta b l e 6 shows the specific values of
the site-specific seismic hazard curve and
CDF values for each nuclear power plant
[6, 7].
0.0000 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 0.0008
Pr(a>SSE) (/yr)
Fig. 1. Relationship of Seismic Core Damage Frequency (SCDF) and probability for SSE.
Tab. 7. Categorized site region for PSHA.
Category Region name Pr (a>SSE) Recurrence period (yr)
Region A Very high risk region Over 4.00E-04/yr Under 2,500 year
Region B High risk region 2.00E-04/yr ~ 4.00E-04/yr 2,500 year ~ 5,000 year
Region C Medium risk region 1.00E-04/yr ~ 2.00E-04/yr 5,000 year ~ 10,000 year
Region D Low risk region Under 1.00E-04/yr Over 10,000 year
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