vgbe energy journal 5 (2022) - International Journal for Generation and Storage of Electricity and Heat

Assessment of loss of shutdown cooling system accident using SPACE Code
equivalent to 5 % of cold leg cross area, and
the opening are located at centerline of the
cold legs. The steady-state results are established
for conducting a null transient calculation.
IV
A
Results and discussion
Initial conditions
In order to confirm the modeling methodology
and input condition, the steady-state
calculation result is compared with experimental
data. The major parameters in
steady-state condition are summarized in
Ta b l e 1 . The core power was 430 kW with
Tab. 1. Steady state calculation results.
Parameters LSTF SPACE
Core power (kW) 430 430.0
Hot-leg temp.(K) 334 334.1
Cold-leg temp.(K) 318 318.0
Primary pressure (MPa) 0.1013 0.1013
Water level at loops (m)
- hot leg void
- cold leg void
Secondary pressure
(MPa)
Mid-loop
Mid-loop
- 0.55
- 0.47
0.1013 0.1013
Secondary fluid temp. (K) 317 317.0
Water level in SG (m) 10 10.2
decay heat at about 20 hours after the reactor
shut down. The water levels of hot and
cold legs maintain at the middle of the loop.
Core power and loop temperature were set to
target values for calculation. Initial conditions
of loop water level represent the same
value with target data. The pressurizer and
SGs relief valves were opened to maintain an
atmospheric pressure. Overall results show
that SPACE code have a reasonable agreement
with target values in steady state analysis.
The steady-state results are established
for conducting a null transient calculation.
B
Transition behavior
The transient calculation was initiated by
decreasing the RHR pump flow rate from the
initial value to zero during 20 seconds with
opening the cold leg valve. The pressurizer
and SGs relief valves were closed with cold
leg opening.
F i g u r e 2 shows the pressure phenomena
of hot and cold legs in intact loop after the
loss of RHR accident. At about 1,500 seconds,
the core liquid started to boil and the
steam migrated toward the hot legs from the
core through core upper plenum. Thus, the
pressure in the hot leg started increasing
rapidly at about 1,600 seconds. At about
2,100 seconds, the pressurization rate reduced
immediately.
The steam flow of guide tubes express the
cause and effect of pressure behavior at this
time as shown in F i g u r e 3 . The guide
tubes were initially submerged under water
in upper plenum. As the water level decreased
below the guide tube bottom opening
due to the boil off, the steam started to
be discharged into upper head with large
volume. The SPACE 3.0 code showed that
the pressurization rate was higher than the
RELAP5/MOD3.2 results. The high pressurization
rate resulted in the accurate simulation
of Loop Seal Clearing (LSC) comparing
with experiment.
Figures 4 and Figure 5 show the differential
pressure behavior of downflow
and upflow sides in the crossover legs.
When the Loop-Seal Clearing(LSC) occurred,
the crossover leg of broken loop was
immediately emptied. The calculation
well predicted the overall LSC behavior.
F i g u r e 4 also shows that the condensate
liquid from the SG U-tube wall started to accumulate
in upper flow direction from about
6,400 seconds. Such a liquid accumulation
of the crossover leg resulted in preventing
the gas flow from the hot leg to the cold
leg. Because of limited steam condensation
of SG U-tube wall, the pressure is re-increasing
gradually in the hot leg as shown in
Figure 2.
160000
140000
LSTF CLO (Hot Leg)
LSTF CLO (Cold Leg)
SPACE 3.0 (Hot Leg)
SPACE 3.0 (Cold Leg)
RELAP5/MOD3.2 (Hot Leg )
RELAP5/MOD3.2 (Cold Leg)
0.10
0.05
SPACE 3.0
RELAP5/MOD3.2
Pressure in Pa
120000
100000
Mass Flow Rate in kg/s
0.00
-0.05
80000
0 2000 4000 6000 8000 10000
Time in s
Fig. 2. Pressure distribution at hot-leg and cold-leg in intact loop.
-0.10
0 2000 4000 6000 8000 10000
Time in s
Fig. 3. Calculated flow rate between guide tube and upper head.
50000
50000
40000
LSTF CLO (Upflow)
LSTF CLO (Down flow)
SPACE 3.0 (Upflow)
SPACE 3.0 (Downflow)
RELAPS/MOD3.2 (Upflow)
RELAPS/MOD3.2 (Downflow)
40000
LSTF CLO (Upflow)
LSTF CLO (Down flow)
SPACE 3.0 (Upflow)
SPACE 3.0 (Downflow)
RELAPS/MOD3.2 (Upflow)
RELAPS/MOD3.2 (Downflow)
Diff. Pressure in Pa
30000
20000
10000
Diff. Pressure in Pa
30000
20000
10000
0
0 2000 4000 6000 8000 10000
Time in s
Fig. 4. Differential pressure at crossover leg in broken loop.
0
0 2000 4000 6000 8000 10000
Time in s
Fig. 5. Differential pressure at crossover leg in intact loop.
48 | vgbe energy journal 5 · 2022