atw Vol. 63 (2018) | Issue 2 ı February
tests have been performed with only
DVI-2 (farthest from the broken cold
leg), only DVI-4 (closest to the broken
cold leg), and DVI-2&4 with both
injection nozzles activated. Table 1
provides the experimental conditions
for the 15 tests.
The bypass fractions of the MIDAS
experiment for the test conditions are
presented in the Figure 3. The test
results show that the ECC bypass
fraction is highly dependent on the
injection nozzle location with respect
to the broken leg as well as the injected
steam flow rate.
Injecting through the nozzle closet
to the broken leg (DVI-4 tests)
show that the direct bypass fraction
increases drastically for a steam flow
rate above 0.7 kg/s. This is expected
since at a higher steam flow rate, the
relative speed between the two fluid
streams becomes higher resulting in a
higher shear effect.
On the other hand, injecting
through the nozzle farthest to the
broken leg (DVI-2 test) dramatically
decreases the bypass fraction, and
accordingly most of the injected ECC
water penetrates into the lower downcomer.
This is primarily due to the
lower interfacial interaction between
the two streams. As a result of the
spatial separation, the ECCS stream
becomes more inertially driven.
With both nozzles activated
( DVI-2&4 tests), the bypass ratio
increases with steam flow rate but
at a much slower rate as compared
to that of DVI-4 tests. This may be
attributed to lower interfacial-interaction
between the injected steam and
ECCS stream for the combined case.
Test
No.
Steam
in (kg/s)
ECCS Injection
Nozzle
KM100 1.7924 DVI-2&4
KM101 1.6149 DVI-2&4
KM102 1.3753 DVI-2&4
KM103 1.1711 DVI-2&4
KM104 0.0493 DVI-2&4
KM105 0.9378 DVI-2&4
KM106 0.8592 DVI-2&4
KM107 0.8096 DVI-2&4
KM108 0.7540 DVI-2&4
KM109 1.8086 DVI-2
KM110 1.0555 DVI-4
KM111 0.8995 DVI-4
KM112 0.7991 DVI-4
KM113 0.7360 DVI-4
3 MIDAS Modeling
for the SPACE Code
A SPACE model of the MIDAS facility
is developed with three different
nodalization schemes as shown in
Figure 4 to Figure 6. The downcomer
is modeled as an annulus component
with 4, 6, and 12 circumferential
channels. A nodalization sensitivity
analysis for the ECC bypass phenomenon
was performed using the SPACE
code version 3.0.
For the KREM which has best
estimate LOCA methodology using
RELAP5 code, the downcomer was
represented with 6 channels [4]. The
comparison with MIDAS test results as
a part of the code validation showed
that RELAP5 code over-predicts the
bypass fraction for low steam flow
cases while predicts reasonably for
higher steam flow cases.
The intact cold legs (CL-1, CL-2,
and CL-3) are connected to the
annulus component using a normal
junction with branch components. A
time-dependent volume and a
time-dependent junction were used to
admit the steam flow rate through
each cold leg. The broken cold leg
(CL-4) is connected to the annulus
component using a normal junction
with a branch component.
The DVI nozzle (DVI-4) closest to
the broken leg is connected to the
same hydraulic channel as the break
(CL-4) whereas the DVI nozzle
(DVI-2) farthest from the break shares
the same hydraulic channel as the
intact cold leg (CL-1) as shown in
Figure 4 to Figure 6. The drain valve
was modeled using a trip valve
component which would open if the
water level of the lower downcomer
becomes higher than the set point.
The hot legs, (HL-1 and HL-2)
which are located between CL-1 and
CL-2, and between CL-3 and CL-4,
respectively, are modeled as blunt
bodies that penetrate the downcomer.
The flow areas were calculated by
using the gap width, perimeter, as
well as other geometric parameters at
this section to estimate the equivalent
thermal hydraulic diameter.
The direct ECCS bypass fraction
is calculated based on the flow rates
of ECCS injection, steam injection,
and drain flow rate at the lower downcomer
as follows:
Bypass fraction =
M Water_out
M SI_in +M Condensate
| | Fig. 3.
ECC Bypass Fraction of MIDAS Tests.
M Steam_in is the steam injection mass
flow rate, and M Condensate is the
condensate mass flow rate calculated
as follows:
M Condensate = M Steam_in – M Steam_out
4 Results and Discussion
The model predictions of the bypass
fraction for all three nodalization
cases (4, 6 and 12 channels) were
compared to the experimental data.
The sample standard deviation of the
differences between measured values
and predicted values, RMSE (Root
Mean Square Error), are presented in
Table 2.
For the case with DVI-2 injection
only (KM109), the RMSEs are
relatively small and acceptable for all
three cases with 0.056 for 4 channels
as a representative case. For the
injection through DVI-4 only (KM110
~ KM114), the code over-predicts the
bypass fraction. This is more distinct
at lower steam flow and for finer
nodalization (e.g. 12 channels). For
the cases with injection through both
ENVIRONMENT AND SAFETY 91
KM114 0.6879 DVI-4
| | Tab. 1.
Experimental Conditions of MIDAS Tests [7].
where, M SI_in is the total ECCS injection
mass flow rate, M Water_out is the
discharged liquid mass flow rate,
| | Fig. 4.
MIDAS Nodalization Scheme with 4 Channels.
Environment and Safety
Sensitivity Analysis of MIDAS Tests Using SPACE Code: Effect of Nodalization ı Shin Eom, Seung-Jong Oh and Aya Diab
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