atw Vol. 63 (2018) | Issue 2 ı February
ENVIRONMENT AND SAFETY 92
| | Fig. 5.
MIDAS Nodalization Scheme with 6 Channels.
| | Fig. 6.
MIDAS Nodalization Scheme with 12 Channels.
Steam Flow Rate
| | Tab. 2.
RMSE Calculated Results of Bypass Fraction with Measured Data.
DVI-2 and DVI-4, the steam flow
rate seems to govern the prediction
accuracy. In case of high steam flow
rate tests (≥ 1.1 kg/s), the SPACE
code predicted the bypass fraction
well regardless of the number of
channels chosen. For the low steam
flow rate tests (≤ 1.1 kg/s), the RMSE
is ≥ 0.16 as shown in Table 2. More
detailed examination is presented
4.1 Results of High Steam Flow
The results for the high steam flow
rate tests (KM100 ~ KM103, and
Number of Channels
4 6 12
KM109 1.8086 0.056 0.078 0.005
KM100 ~ 103 ≥ 1.1 0.017 0.019 0.017
KM104 ~ 108
0.161 0.211 0.287
KM110 ~ 114 0.252 0.334 0.462
| | Fig. 7.
Comparison of the Measured and Calculated ECC Bypass Fraction for the
High Steam Flow Cases.
KM109) are presented in Figure 7. For
the high steam flow rate tests, the
SPACE code predicts the bypass
fraction relatively well for all
The liquid flow pattern for the
KM100 test (highest steam flow rate
test) of each nodalization case are
presented in Figure 8 to Figure 10.
The liquid flow pattern for the all
nodalization cases are quite similar.
The direct bypass phenomena occurs
in the upper region of the downcomer
as the ECCS flow joins the high
velocity steam from the intact cold leg
and is swept away through the broken
cold leg. In the case of tests with a
high steam flow rate, the result of
the 4 channels nodalization is similar
to that of 6 and 12 channels. Hence,
the 4 channels representation is considered
a reasonable approximation.
4.2 Results of Low Steam Flow
The results for the low steam flow rate
tests (KM104 ~108 and KM110 ~114)
are presented in Figure 11. Contrary
to the high steam flow rate cases, for
the low steam flow rate tests, the
SPACE code over-predicts the bypass
fraction for the all nodalization cases.
The liquid and vapor flow patterns
of the 6 channels case for the lowest
steam flow rate test (KM114) are
presented in Figure 12 and Figure 13,
respectively. Most of the liquid
injected from the DVI nozzle is swept
with the steam flow through the
break. The test indicated some downward
liquid flow at this steam flow
In the SPACE code, the interfacial
friction model is dependent on the
flow regime of the control volume.
Thus, for quantitative agreement with
the MIDAS experimental measurements,
the estimation of the flow
regime has to be properly predicted to
accurately estimate the bypass flow in
the upper downcomer. The SPACE
code selects the annular mist flow
regime based on the volume average
conditions, which explains the deviation
between the code prediction and
MIDAS tests in the case of low steam
4.3 Results of Condensation
It is worthy to note that for all the
studied cases, the code under-predicts
the condensation fraction as shown in
the Figure 14. The RMSE based on
calculated condensation fraction with
the measured condensation fraction
data are presented in Table 3. The
under-prediction tendency is more
distinct for finer nodalization (e.g. 12
channels) as depicted in Table 3. This
may clearly be tied to the heat transfer
correlation which in turn depends on
the flow regime. Due to mass conservation,
the lower condensation rate
leads to over-estimation of the bypass
Environment and Safety
Sensitivity Analysis of MIDAS Tests Using SPACE Code: Effect of Nodalization ı Shin Eom, Seung-Jong Oh and Aya Diab