atw Vol. 63 (2018) | Issue 3 ı March
| | Fig. 6.
Event tree of event.
| | Fig. 7.
| | Tab. 4.
List of event value.
| | Fig. 8.
Causes tree of SD modeling.
LOCA (if then else(random 0 1 () < 0.8, 0, 1))
Piping Integrity if then else(random 0 1 () < 0.3, 0, 1)
Alarm Alert if then else(random 0 1 () < 0.5, 0, 1)
* LOCA * Piping Integrity
Manual Actions if then else(random 0 1 () < 0.4, 0, 1)
* Alarm Alert * Piping Integrity
Reactor SCRAM if then else(random 0 1 () < 0.6, 0, 1)
* Manual Actions *Piping Integrity
Coolant Tank Integrity if then else(random 0 1 () < 0.5, 0, 1)
Flying Integrity if then else(random 0 1 () < 0.3, 0, 1)
Coolant Tank Integrity * Flying Integrity
Emergency Cooling by Operator if then else(random 0 1 () < 0.5, 0, 1)
* Drone Action *Reactor SCRAM
Reactor if then else(random 0 1 () < 0.5, 0, 1)
+ Emergency Cooling by Operator + 0.001
restriction. One of most important
merits in SD is used as the feedback
algorithm in which Reactor is connected
to LOCA. This means the final
event, Reactor, affects to the initial
event, LOCA. There are some cartoon
shapes which could give the operator
the sign of meaning. In the arrow line,
the plus sign means the additive
values of the event. In Table 4, the
values of the event are shown, which
are decided by expert’s judgments. In
the case of Piping Integrity, if the
randomly generated number between
0 and 1 is lower than 0.3, the value is
0.0. Otherwise it is 1.0. So, the
Boolean value is obtained. The others
are similar to this case. In the case of
LOCA and Reactor, the values are
accumulated using the ‘Level’ function
in which the values are summed up by
the designed time step.
The simulation is performed for the
SD modeling. Using passive system of
the free-fall of coolant, the designed
scenarios are quantified. Figure 8 is
the causes tree of SD modeling which
is from the Figure 7. There are results
of the modeling. In Figure 9, there are
the cause tree’s results of SD modeling
as (a) Reactor and (b) LOCA. In
Figure 9 (a), the possibility for LOCA
is shown. The Y-axis has the relative
value where the value is stabilized after
it increases abruptly. In the final
stage as Reactor in Figure 9 (b), the
integrity of the reactor is increased.
The complex algorithm of the SD
modeling is done in the passive
cooling system. The free-fall could be
another kind of the nuclear passive
system which is different from the
conventional passive systems as
gravity and natural circulation. There
are some finding in this study as
• The nuclear passive system is modeled
using the free-fall concept.
• System dynamics (SD) based
algorithm is performed for nuclear
• More realistic safety assessment is
• New kind of nuclear safety analysis
is done successfully
The nuclear passive system by the
free-fall is successfully modeled for
the LOCA accident. Conventional
passive systems of gravity or natural
circulation could be performed when
the piping systems are not damaged.
However, in the Fukushima and
Chernobyl cases, the piping was blown
ENVIRONMENT AND SAFETY 161
Environment and Safety
Applied Reliability Assessment for the Passive Safety Systems of Nuclear Power Plants (NPPs) Using System Dynamics (SD) ı Yun Il Kim and Tae Ho Woo