atw 2018-02

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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