atw 2018-03v6


atw Vol. 63 (2018) | Issue 3 ı March

| | Fig. 6.

Event tree of event.

| | Fig. 7.

SD modeling.


| | Tab. 4.

List of event value.

| | Fig. 8.

Causes tree of SD modeling.


LOCA (if then else(random 0 1 () < 0.8, 0, 1))

/ Reactor

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)

Drone Action

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.

3 Results

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.

4 Conclusions

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

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

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