# Flow Stagnation under Single and Two-Phase

Authors: José N. Reyes
Journal: Science and Technology of Nuclear Installations
 (2008)

---

## Abstract

Natural circulation experiments were conducted at Oregon State University using the advanced plant experiment (APEX-CE) integral system test facility as configured to simulate a typical2×4 Combustion Engineering nuclear steam supply system. This paper describes the mechanisms by which natural circulation flow was interrupted under single-phase and two-phase natural circulation conditions in APEX-CE.


---

## Body

## 1. Introduction

Natural circulation is an
important means of cooling the nuclear core in the event of a loss of primary
system pumps. This paper describes the mechanisms that can interrupt natural
circulation in loops transporting single- and two-phase fluids. The
investigation of these mechanisms was conducted in the APEX-CE test facility,
at Oregon State University [1–7]. A schematic of
the test facility is shown in Figure 1. APEX-CE was configured to model a 2×4
loop Combustion Engineering PWR. It included the reactor vessel with an
electrically heated rod bundle, a pressurizer, two inverted U-tube steam
generators, four cold legs and reactor coolant pumps, two hot legs, and a
safety injection system. The facility had a length scale ratio of approximately
1 : 4 and a volume ratio of 1 : 274. It was operated at decay powers ranging down
from 6%. Therefore, the tests were conducted after reactor scram with the
reactor coolant pumps tripped in a natural circulation mode of operation.Figure 1
Schematic of the APEX-CE test facility.The motivation for the studies in
APEX-CE was an issue known as pressurized thermal shock (PTS). In the event of
an emergency that results in a significant loss in system pressure or fluid
inventory, cold borated water is typically injected into the primary system via
the cold legs. If the flow rate in the primary loop is significant, the cold
injected fluid will thoroughly mix with the hot water in the primary loop.
However, at very low flow rates, the cold injected fluid will stratify in the
loops and form cold plumes in the downcomer. Should a pre-existing flaw in the
vessel wall or welds exist at a location experiencing prolonged contact with a
cold plume, while at high pressure, there is a potential for the flaw to grow
into a “through-wall” crack. Sections2
and 3 describe mechanisms leading to a
loss of primary loop flow.


## 2. Single-Phase Natural Circulation Stagnation Mechanisms

This section describes the mechanisms that interrupt
single-phase natural circulation. Figure2 presents a simple sketch describing
the single-phase fluid natural circulation flow paths. Each loop in the
Nuclear Steam Supply System shown in Figure 2 contains a hot leg, a steam
generator, a loop seal (i.e., a cross-over leg), a pump, and a cold leg. The
flow areas will vary around the loop. Under certain conditions, involving a
reactor scram, the reactor coolant pumps in the plant may trip causing a coast
down in the loop flow. Subsequent to a reactor scram, reactor decay power
continues to heat the fluid in the vessel, while the steam generators continue
removing heat at an elevation well above the core. The density difference and
elevation difference produce a buoyancy force that drives the fluid around the
loop. This buoyancy driven flow is known as natural circulation. The natural
circulation flow rate is limited by the friction and form losses in the loop
piping and components. The dominant pressure drops in the loop occur in the
core and in the steam generator as a result of form losses, usually expressed
in terms of loss coefficients, K. As shown Figure 2, the primary loop is divided into: a hot
fluid side having an average temperature TH and a cold fluid side
having an average temperature TC.Figure 2
Control volume for
single-phase natural circulation in a two-loop PWR.


### 2.1. Loss
of Heat Sink (Steam Generator Reverse Heat Transfer)

One  mechanism for losing single-phase natural
circulation flow is a loss of heat sink. This can occur as a result of a loss of
main and auxiliary feedwater supplies. This could also occur as a result of a main
steam line break (MSLB) in a single steam generator in a multiloop plant. In
the event of a MSLB, the operators isolate the feedwater to the steam
generators and close the main steam isolation valves. The affected steam
generator, however, will continue to  vent steam and depressurize. The blowdown of
a steam generator may result in a rapid cooling of the primary system fluid. As
a result, the primary loop fluid temperatures may drop below the secondary side
temperatures of the “unaffected” steam generators. The result is a loss of heat
sink in the loops not experiencing the steam line break. Figure3 shows that
stagnation occurs in cold legs no.1 and no.3, connected to the unaffected
Steam Generator no.1 for this test. Figure 4 shows the flow rates for cold leg no.1
and no.3. When primary side temperature exceeds the secondary side temperature,
natural circulation flow is restored.Figure 3
Illustration of steam generator reverse heat
transfer during a main steam line break simulation and recovery
(OSU-CE-0012).Figure 4
Illustration of loss of natural circulation flow during a
main
steam line break simulation and recovery (OSU-CE-0012).


### 2.2. Negatively
Buoyant Regions in Loop  (Loop Seal Cooling)

Another mechanism that can
interrupt single-phase natural circulation flow is loop seal cooling. The
piping that connects the steam generator lower channel head to the reactor
coolant pump is known as the cross-over leg or the reactor pump loop seal as
shown in Figure2. Overcooling transients such as main steam line breaks,
result in a primary side cooldown. If  the primary side pressure drops below the
safety injection  actuation setpoint, cold-borated water will be injected into
the loop. This water is typically injected into the cold legs of a PWR between
the reactor coolant pump and the reactor vessel. Figure 5 is a picture of the
transparent separate effects test loop at Oregon State University
used to
visualize fluid mixing in a side-injection cold leg.Figure 5
Flow visualization of injected coolant
mixing with fluid in a transparent loop seal.The dense injected fluid simulated,
using fluorescent salt-water, falls to the bottom of the cold leg where it
spreads out toward the vessel and the reactor coolant pump loop seal.
Countercurrent flow is established with hot water at the top of the cold leg
pipe flowing toward the injection point. The dense-injected water mixes with
the less dense water in the loop seal creating a negatively buoyant region in
the loop, effectively increasing the resistance to flow in that loop. For
multiloop plants, the flow is preferentially diverted to the adjacent cold leg
through the SG lower provided the same condition does not also exist there.
Figure6 shows the  asymmetric cooling of two loop seals attached to a single
steam generator in APEX-CE. Loop seal no.4
cools faster than loop seal no.2 because the dense fluid back flows over the
lip of RCP no.4 earlier during this particular transient. Figure 7 shows that
loop flow in cold leg no.4 stagnates earlier than the flow in cold leg no.2.Figure 6
Asymmetric loop seal cooling (OSU-CE-0008).Figure 7
Stagnation of a primary loop due to loop seal cooling (OSU-CE-0008).


## 2.1. Loss
of Heat Sink (Steam Generator Reverse Heat Transfer)

One  mechanism for losing single-phase natural
circulation flow is a loss of heat sink. This can occur as a result of a loss of
main and auxiliary feedwater supplies. This could also occur as a result of a main
steam line break (MSLB) in a single steam generator in a multiloop plant. In
the event of a MSLB, the operators isolate the feedwater to the steam
generators and close the main steam isolation valves. The affected steam
generator, however, will continue to  vent steam and depressurize. The blowdown of
a steam generator may result in a rapid cooling of the primary system fluid. As
a result, the primary loop fluid temperatures may drop below the secondary side
temperatures of the “unaffected” steam generators. The result is a loss of heat
sink in the loops not experiencing the steam line break. Figure3 shows that
stagnation occurs in cold legs no.1 and no.3, connected to the unaffected
Steam Generator no.1 for this test. Figure 4 shows the flow rates for cold leg no.1
and no.3. When primary side temperature exceeds the secondary side temperature,
natural circulation flow is restored.Figure 3
Illustration of steam generator reverse heat
transfer during a main steam line break simulation and recovery
(OSU-CE-0012).Figure 4
Illustration of loss of natural circulation flow during a
main
steam line break simulation and recovery (OSU-CE-0012).


## 2.2. Negatively
Buoyant Regions in Loop  (Loop Seal Cooling)

Another mechanism that can
interrupt single-phase natural circulation flow is loop seal cooling. The
piping that connects the steam generator lower channel head to the reactor
coolant pump is known as the cross-over leg or the reactor pump loop seal as
shown in Figure2. Overcooling transients such as main steam line breaks,
result in a primary side cooldown. If  the primary side pressure drops below the
safety injection  actuation setpoint, cold-borated water will be injected into
the loop. This water is typically injected into the cold legs of a PWR between
the reactor coolant pump and the reactor vessel. Figure 5 is a picture of the
transparent separate effects test loop at Oregon State University
used to
visualize fluid mixing in a side-injection cold leg.Figure 5
Flow visualization of injected coolant
mixing with fluid in a transparent loop seal.The dense injected fluid simulated,
using fluorescent salt-water, falls to the bottom of the cold leg where it
spreads out toward the vessel and the reactor coolant pump loop seal.
Countercurrent flow is established with hot water at the top of the cold leg
pipe flowing toward the injection point. The dense-injected water mixes with
the less dense water in the loop seal creating a negatively buoyant region in
the loop, effectively increasing the resistance to flow in that loop. For
multiloop plants, the flow is preferentially diverted to the adjacent cold leg
through the SG lower provided the same condition does not also exist there.
Figure6 shows the  asymmetric cooling of two loop seals attached to a single
steam generator in APEX-CE. Loop seal no.4
cools faster than loop seal no.2 because the dense fluid back flows over the
lip of RCP no.4 earlier during this particular transient. Figure 7 shows that
loop flow in cold leg no.4 stagnates earlier than the flow in cold leg no.2.Figure 6
Asymmetric loop seal cooling (OSU-CE-0008).Figure 7
Stagnation of a primary loop due to loop seal cooling (OSU-CE-0008).


## 3. Two-Phase Natural Circulation Stagnation Mechanisms

During a small break loss of coolant
accident (SBLOCA) in a PWR, steam generator tube draining will result in a
gradual decrease in primary side natural circulation flow until it transitions
to a boiling-condensing mode of operation. Figure8 shows the results of a test
conducted at OSU to investigate this phenomenon. The test, OSU-CE-0002, was a
stepped reduction in inventory test. In essence, it is a quasisteady SBLOCA. It
was conducted at a constant core power of 275 kW and initiated from
steady-state single-phase natural circulation conditions. A break valve on the
reactor vessel was opened to remove primary fluid in stepped intervals. After a
short period, the break valve was closed and the loop was allowed to reach a
new quasisteady state flow rate. The cold leg flow rates were measured at each
interval. These tests were similar to tests performed at the Semiscale test
facility at the Idaho National Engineering Laboratory as shown in Figure 9 [6].Figure 8
Cold leg flow rates as a function of primary side inventory
during a stepped-inventory reduction test (OSU-CE-0002).Figure 9
Cold leg flow rates as a function of primary side inventory during
a stepped-inventory
reduction test (OSU-CE-0002) and semiscale Mod 2A data [6].As liquid mass is removed from the
system, the loop void fraction increases. This resulted in a rise in the loop
flow rates above those observed for single-phase natural circulation as shown
in Figure8. At approximately 70% inventory in APEX-CE, the flow reaches a
maximum value. This corresponds to the maximum two-phase buoyancy driving head
for the test. Eventually, the steam generator tubes begin to drain causing a
decrease in flow rate because the distance between the core and steam generator
thermal centers has decreased and interruption of flow in the longest tubes.Figure10 illustrates the
significant difference in draining time for the longest U-tubes at the top of
the bundle and the shortest U-tubes at the bottom of the bundle. The long tubes
drained much earlier than short tubes. However, some primary loop natural
circulation continued until the short tubes drained. These results suggest that
to obtain faithful simulations of the steam generator tube draining requires
modeling multiple steam generator tubes.Figure 10
Liquid levels in the longest
and shortest tubes of
steam generator no.2 during SLOCA test
(OSU-CE-0008).


## 4. Conclusions

Experiments in the APEX-CE integral
system test facility indicate that natural circulation under single-phase fluid
condition can be interrupted as a result of a loss of heat sink during a main
steam line break (i.e., reverse steam generator heat transfer), or as the
result of the formation of negatively buoyant conditions in the loop seal.
Under two-phase natural circulation conditions, loop stagnation arises as a
result of steam generator tube voiding. The loop flow transitions from
two-phase natural circulation to a boiling-condensing mode of operation. The
APEX-CE tests indicate that the long steam generator tubes drain before the
short tubes suggesting that computer code models include multiple steam
generator tubes. The formation of negatively buoyant conditions in plants with
loop seals can also result in asymmetric loop stagnation under two-phase
natural circulation flow conditions.



---

*Source: 453621-2008-04-30.xml*