# CFD Application to Hydrogen Risk Analysis  and PAR Qualification

Authors: Jinbiao Xiong; Yanhua Yang; Xu Cheng
Journal: Science and Technology of Nuclear Installations
 (2009)

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## Abstract

A three dimensional computation fluid dynamics (CFD) code, GASFLOW, is applied to analyze the hydrogen risk for Qinshan-II
nuclear power plant (NPP). In this paper, the effect of spray modes on hydrogen risk in the containment during a large
break loss of coolant accident (LBLOCA) is analyzed by selecting three different spray strategies, that is, without spray, with direct spray and with both direct and recirculation spray. A strong effect of spray modes on hydrogen distribution is observed. However, the efficiency of the
passive auto-catalytic recombiners (PAR) is not substantially affected by spray modes. The hydrogen risk is significantly increased by the direct spray, while the recirculation spray has minor effect on it. In order to simulate more precisely the processes involved in the PAR operation, a new PAR model is developed using CFD approach. The validation shows that the results obtained by the model agree well with the experimental results.


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## Body

## 1. Introduction

During  severe accidents, hydrogen can be generated in
water-cooled reactors by metal-steam reaction. Hydrogen
which is released into the containment may form combustible or
even detonable gas mixture in the containment. As one of the
mitigation measures against severe accidents in the Qinshan-II NPP, the containment
spray system
which starts when the containment pressure reaches the threshold value is utilized
to prevent the
containment overpressure. In the viewpoint of
hydrogen risk, the spray operation is concerned due to two aspects. In one
respect, the
condensation introduced by the operation
of the containment spray reduces the steam concentration, which leads to an increase of the hydrogen concentration and adds to the hydrogen combustion or detonation risk. In the other respect, the containment spray brings a gas temperature
difference in the containment which promotes
the gas mixing and leads to a more uniform distribution of the gas concentration. In this paper,
a three-dimensional CFD code GASFLOW [1] is utilized to simulate the gas mixing
and distribution in the LBLOCA and to
evaluate the effect of the containment spray on the
hydrogen risk. The case assuming no activation of the
containment spray is selected as the base case. Two other cases assuming different containment spray operation strategies are
involved in this paper. One of the spray cases
considers only the direct spray (indicated as Case A, hereafter), while the
other simulates the
complete spray operation according to the design of
the containment spray system, which includes the direct and recirculation
spray (indicated as Case B, hereafter). The same source term is used in all the
cases.As one of the major hydrogen mitigation measures, the passive autocatalytic
recombiner (PAR) has been widely used in nuclear
power plants. In most studies presented in open literatures, including lumped parameter code analysis and CFD
code analysis, the PAR is simply simulated by introducing energy and mass source terms obtained from
empirical correlations. According to the state-of-art report on PAR proposed by the PARSOAR project
[2], a theoretical PAR model is recommended for the CFD
analysis codes,
such as GASFLOW, TONUS. In this paper, a PAR model is developed based on the CFD approach,
in order to provide
more insight into the processes inside a PAR.


## 2. Hydrogen Risk Analysis for
Qinshan-II NPP

### 2.1. Containment Geometry and Mitigation Systems

The containment of Qinshan-II NPP is a large dry containment
which consists of a cylindrical part and
a spherical dome, as indicated in Figure1. The height of the containment is about 60 m,
and the diameter is about 38 m [3]. The compartments are mainly located below
the operation deck at the height of 20 m.
The main
components of two primary loops are symmetrically arranged in the containment.
The deck at the height of 4.5 m supports the major heavy components including the steam generators (SGs), the
reactor coolant pumps (RCPs) and the safety injection tanks (SITs). Similar to
most of the pressurized water reactor (PWR), the pressurizer (PZR) is located in a room next to one of the SG towers. The bottom of the PZR
room is at a height of about 11 m.
The PZR relief tank room is located on
the 0 meter floor and right under the PZR room. The
refueling pool is from 6 to 20 m in height. It connects with the reactor cavity in the center of
the containment and reaches the containment wall. There are other small rooms
accommodating the valve, piping, and heat exchangers on the 0 meter floor and
the underground floor. All the above-mentioned rooms are located inside a
cylindrical missile shielding wall which protects the containment from ejected
missiles. The space above the operation deck is much opener. Only the SG towers
and the PZR room extend beyond the deck. moreover,  in the dome a
crane is installed. GASFLOW can generate structural mesh in both Cartesian and
cylindrical coordinates. According to the characteristics of the containment geometry,
the cylindrical coordinate is selected. The mesh size is adjusted according to the location
of the structures in the containment so that the geometry can be described with
a coarser mesh system without increasing the computation cost. In the mesh system
18, 60, and 51 cells are, respectively, arranged in radial, circumferential, and
axial directions. The free volume of the containment is about 50 000 m3. The average cell volume is about 1.4 m3.
The total wall surface area is about 24 000 m2,
most of which is considered as concrete. The components in the primary loop are
treated as adiabatic because the adiabatic layer is utilized on them. In order to mitigate the hydrogen risk during severe
accidents, 22 passive autocatalytic recombiners (PAR) of
Siemens type are installed in the Qinshan-II NPP containment
compartments. Table 1 lists the position and type of the PAR. Each PAR is
simulated with a single mesh cell.Table 1
Arrangement of recombiners.
LocationTypeNo.PZR relief tank
roomFR90/1–15003Surge line roomFR90/1–9601PZR roomFR90/1–9601No. 1 SG
compartmentFR90/1–15003No. 2 SG compartmentFR90/1–15003No.1 RCP roomFR90/1–9601No.1 RCP roomFR90/1–9601SIT roomFR90/1–9601Dome regionFR90/1–15004Annular compartmentFR90/1–15004Figure 1
Vertical
view of Qinshan-II containment geometry.In Qinshan-II NPP, two separated containment spray systems are
installed. According to the design of the spray systems, the
containment can be depressurized with only one of them during the severe accidents. Each system includes two
nozzle rings, as shown in Figure2, on which about
250 nozzles are attached. The mass flow is uniformly distributed to every nozzle. Heat exchangers in the system control the
temperature of spray water. The spray systems are designed to operate in two
modes: direct spray mode and recirculation spray mode. The direct spray starts
while the pressure in the containment reaches 2.36 bar. During the direct
spray, the spray water comes from the refueling tank, and the temperature at
the nozzle outlet is in the range from 20 to 40C°. In this paper, the temperature of spray water is given to be 27C°.
In about 30 minutes, the water in the
refueling tank  will
be used up. Then the spray switches to
the recirculation mode. In the recirculation mode, the  spray water is pumped
from the water sump in the containment. Because the water in the  sump originally comes from
the primary loop or the spray water and is at a high temperature, the temperature of the recirculation spray water is higher and is designed to be from 40 to 120C°. In this paper, the water temperature of
the recirculation spray is given as 77C°.
According to
the design of the system, the spray mass
flow rates in the direct and the recirculation spray modes are 814 and 1050 ton
per hour, respectively [3].Figure 2
Containment geometry model.


### 2.2. Physics Model

In
GASFLOW two approaches are provided for two-phase flow
simulation [4]. In
the case that the spray model is not activated, the homogeneous equilibrium
approach is applied automatically, which assumes the liquid and gas phases are
in both thermodynamic and mechanical equilibrium. Because the containment spray brings strong transient and thermodynamic
non-equilibrium, in order to more exactly simulate the
interaction between the liquid and gas phases during the
spray operation, GASFLOW offers another approach in which
the thermodynamic non-equilibrium between liquid and gas
phases is considered, but the difference
of mechanical behavior between the liquid and gas phases  is still neglected. The GASFLOW spray model has been validated with
TOSQAN experiments and provided satisfactory predictions of the experiment data
[5].The
spray simulation induces a much smaller time step size and increases the
computation time by several times. Due to
heavy time consumption in the spray simulation, turbulence model is not used in
all the cases because the computational cost becomes unacceptable
while using the turbulence model. According to the experience from Karlsruhe Research
Center (FZK), GASFLOW predicts nearly the same flow field and vortex formation
with and without turbulent models in a coarse grid [6], such as the grid used in containment geometry
model. The
heat conduction inside the structures is simulated in a one-dimensional
approach. The PARs installed in the containment were simulated with the
standard model provided by the GASFLOW code.


### 2.3. Accident Scenario

The hydrogen/water source
term for this analysis was obtained by scaling from GKN surge line LBLOCA source
term reported in [6], as indicated in Figure 3. The break is located in the lower part of the SG tower which is next to the pressurizer room. At the beginning, a large amount of saturated water is discharged to the containment. The water discharge
flow rate decays promptly  due to the limited coolant inventory in the primary
loop. Water in the reactor core is heated by the decay heat which rises the
water temperature to superheated condition
after the saturated release period. At about 1400 seconds, hydrogen
generation and release starts. About 270 kg hydrogen is released into the
containment during the first 7000 s.seconds At 5930 seconds, hydrogen release peak rate is
produced due to an enhancement of steam/zirconium reaction after the failure of
the core support. Besides the source term, Figure 3(a) indicates also the two spray periods. The
start time of direct spray is determined according to the pressure variation
obtained in the base case. In about 60 seconds, the pressure in the containment reaches the threshold value and the direct spray
starts.Source term
and spray activation.
(a)
Water release
rate and temperature(b)
Hydrogen release
rate


### 2.4. Thermal Hydraulics

At the beginning of the LBLOCA
accident, along with a heavy discharge of water to the containment, the
pressure and gas temperature inside the containment increase sharply. The hot
steam mixes promptly and intensively with the atmosphere and soon spreads
through the containment, which results in strong condensation on the structure
surface. The condensation helps delaying the containment pressurization in the severe accident. Figures4 and 5 compare the pressure and gas
temperature variation in three cases. In all the
cases, a sharp pressure and temperature increase occurs and the pressure
reaches the maximum at the beginning of the blowdown.
Although different approaches are utilized to deal with the two phases in the
base case and spray cases, large discrepancies of the global behavior are not observed before the spray
activation in the cases. For the base case, the pressure and gas temperature
variation inside the containment is mainly affected by the water injection and the condensation
on the structure. The computation result shows that, with the
current source term, the pressure does not exceed the design value. In Case
A and B, the pressure and the gas temperature inside the
containment show lower peak values and
decrease fast after the activation of the direct
spray until its shutdown. Without the circulation spray, the pressure and gas
temperature rebounds to high level in Case A because the hot steam continues being discharged after the shutdown of the direct
spray. In Case B, the recirculation spray holds the pressure and the gas temperature slightly higher than the lowest value induced by the direct spray.Figure 4
Pressure
variation in three cases.Figure 5
Average
temperature variation.Figure6 presents the bulk
evaporation rate in the containment during all the cases. The direct
containment spray introduces intensive bulk condensation (negative value in
Figure 6). Conversely, the recirculation spray brings bulk evaporation. The
evaporation of the
spray water is beneficial with respect to
hydrogen risk because it increases the steam concentration and
builds up an inertial atmosphere which resists the hydrogen combustion. With respect to
hydrogen risk, the outlet temperature of the recirculation spray should be optimized to enhance the
evaporation of the
spray water. Figure 7 gives the total
condensation rate on the structure surface. During the direct spray phase in the Cases A and B,
the condensation on the structures is at a much lower rate than that in the base case due to the enhancement of
the bulk condensation which reduces both the gas temperature and steam concentration. The condensation on the structures is one of the
major ways in which heat is transferred from the atmosphere to the structures. Hence, a low condensation rate leads to a low heat transfer rate from the gas to the structures, as
indicated in Figure 8. However, during the recirculation spray in Case B, the
condensation on the structures is observed to be at a similar
rate with that in the base case. In Case B, both the bulk evaporation and
the surface condensation are enhanced by the recirculation spray. The
evaporation of spray water takes away a lot of sensible heat from the atmosphere and contributes to stabilize the gas temperature inside the
containment. Comparing both the bulk evaporation rate and the surface
condensation rate in the Cases A and B shows that the recirculation spray
generally increases the steam inventory inside the containment, which reduces
the hydrogen combustion or detonation risk. Hence, it can be concluded that
besides controlling the pressure and gas temperature inside the containment,
the recirculation spray can build a comparatively inertial atmosphere for
hydrogen.Figure 6
Bulk
evaporation rate in all cases.Figure 7
Total
condensation rate on structure surfaces.Figure 8
Energy
absorption by structures.


### 2.5. Flow Field

Figure9 presents the flow
fields during the heavy hydrogen release period in all the cases. In the base case and Case A,
similar flow fields are
observed. Because the released gas is hydrogen-rich and of high temperature and low density, a
buoyancy jet flow forms above the SG tower in which the
break is located. The jet flow  is reflected by the
dome and flows downward into the other side of the containment. A large-scale vortex
can be observed in the upper space. The
condensation and the convection heat transfer on
the structures remove the steam and heat from the atmosphere, the gas near the structures is heavier than the gas in the bulk.
Hence, the downward flow can be observed near the structure
surfaces. During the hydrogen release peak in the base case and Case A, the magnitude of velocity is generally less
than 0.5 m/s. A
chaotic flow field is induced by the recirculation spray, as indicated in Figure 9(c). The flow velocities in Case B are
much higher than that in the other two cases. As mentioned in  Section  2.2,
GASFLOW spray model uses the mechanical equilibrium assumption while dealing
with the two-phase flow. Actually, the heavy liquid phase is more inclined to
drop down than the gas phase. The assumption inevitably leads to an  artificial
flow. In the actual situation, mechanical interaction between the liquid and gas phases
could lead to a flow pattern different from the obtained results.Flow fields
during the hydrogen release peak.
(a)
base caset = 5910 s(b)
Case At = 5910 s(c)
Case Bt = 5910 s


### 2.6. Hydrogen Recombination, Hydrogen and Steam Distribution

In the analyzed scenario, the
hydrogen release can be generally divided into two periods. The first period lasts from 1400 s to 3500 s. In this period, the global hydrogen volume fraction in the containment reaches 3%, but the flammable clouds (at hydrogen
concentration above 4%) rarely appear. Due to the hydrogen-oxygen
recombination, the hydrogen concentration can be reduced to less than 3% before
the second hydrogen
release period starts. During the second period, the hydrogen release is
discontinuous. However, intensive hydrogen release between 5900 and 6000 s
could lead to extremely high local hydrogen concentration. Figure10 presents
snapshots of hydrogen and steam clouds right after the hydrogen release peak. A
clear gas stratification can be observed in both the base case and Case A. Hydrogen—rich clouds are
enveloped by steam-rich clouds during most of the time, which provides an
inertial atmosphere for hydrogen and prevents early hydrogen combustion. The
combustible hydrogen cloud in Case A is of the biggest size among the three
cases. Compared with the base case, the steam concentration is low in the spray
cases. Due to strong mixing induced by the recirculation
spray, hydrogen stratification is not observed in Case B, as shown in Figure  10(c). Hence, the direct spray reduces the
steam volume fraction and increases the hydrogen volume fraction, while the
recirculation spray does not lead to an increase of hydrogen concentration but
prevents the hydrogen concentration stratification.Hydrogen
and steam clouds at the moment when hydrogen release reaches peak value.
(a)
base case at
5960 s(b)
Case A at 5960 s(c)
Case B at 5950 sIn the Siemens PAR
correlations [4], the recombination rate depends on the pressure and inlet
hydrogen and oxygen concentration. Although the pressure is very different in
the analyzed cases, as mentioned in Section 2.4, the total recombination rate
of 22 PARs does not show great difference in all the
cases, as shown in Figure 11. Generally, the evolution of
the recombination rate is in the same trend. Following the hydrogen release
into the containment, the recombiners start up when the inlet hydrogen
concentration reaches the startup threshold (2 vol. %). Along with the hydrogen
accumulation in the containment, the recombination rate ascends. At the end of the first hydrogen release period, the recombination is at
an almost stable rate. During the second period, when the hydrogen release is
discontinuous and at a quite low rate, the recombination rate reduces smoothly
and slowly. The oscillation of recombination rate is observed in this period in Case B because strong flow caused by the spray brings
the strong variation of hydrogen concentration at the inlet of PARs. After 5900 s, the spray cases show a higher hydrogen removal capability than the base case
due to higher global hydrogen concentration. In GASFLOW, the volume flow rate
through the PAR is deduced from the recombination rate obtained from Siemens
correlations. In this case, the recombination rate is affected only by the gas
species concentration and pressure at the inlet of PAR. However, BMC Zx test
results [7] suggest an increase of the volume flow rate through PARs and
recombination rate due to the spray.Figure 11
Total
recombination rate of 22 PARs.In order to indicate the
characteristics of the hydrogen mixture and the hydrogen combustion risk in the
containment, the volume of sigma cloud is involved in this paper. The sigma cloud is a volume of the hydrogen-air-steam
mixture with a combustion expansion ratio higher than the critical value
obtained from experimental data [8]. Flame acceleration could occur in a sigma
cloud. Figure 12 presents the evolution of total sigma volume inside the
containment in three cases. During the analyzed accident, a sigma volume peak can be observed in all the cases.
And the maximum sigma volume in the spray cases is larger than that in the base case, while the maximum sigma volume in the Cases A and B is  not quite different. However, if compared
carefully, it can be found that the sigma volume in Case A shows to be larger
than that induced by Case B at the second peak, and smaller at the third peak. It
implies that the recirculation spray reduces the hydrogen risk during the slow
hydrogen release period, but can slightly increase
the local hydrogen concentration around the hydrogen source and lead to a larger
sigma volume at the moment when the peak hydrogen release rate occurs.Figure 12
Total
volume of sigma clouds.


## 2.1. Containment Geometry and Mitigation Systems

The containment of Qinshan-II NPP is a large dry containment
which consists of a cylindrical part and
a spherical dome, as indicated in Figure1. The height of the containment is about 60 m,
and the diameter is about 38 m [3]. The compartments are mainly located below
the operation deck at the height of 20 m.
The main
components of two primary loops are symmetrically arranged in the containment.
The deck at the height of 4.5 m supports the major heavy components including the steam generators (SGs), the
reactor coolant pumps (RCPs) and the safety injection tanks (SITs). Similar to
most of the pressurized water reactor (PWR), the pressurizer (PZR) is located in a room next to one of the SG towers. The bottom of the PZR
room is at a height of about 11 m.
The PZR relief tank room is located on
the 0 meter floor and right under the PZR room. The
refueling pool is from 6 to 20 m in height. It connects with the reactor cavity in the center of
the containment and reaches the containment wall. There are other small rooms
accommodating the valve, piping, and heat exchangers on the 0 meter floor and
the underground floor. All the above-mentioned rooms are located inside a
cylindrical missile shielding wall which protects the containment from ejected
missiles. The space above the operation deck is much opener. Only the SG towers
and the PZR room extend beyond the deck. moreover,  in the dome a
crane is installed. GASFLOW can generate structural mesh in both Cartesian and
cylindrical coordinates. According to the characteristics of the containment geometry,
the cylindrical coordinate is selected. The mesh size is adjusted according to the location
of the structures in the containment so that the geometry can be described with
a coarser mesh system without increasing the computation cost. In the mesh system
18, 60, and 51 cells are, respectively, arranged in radial, circumferential, and
axial directions. The free volume of the containment is about 50 000 m3. The average cell volume is about 1.4 m3.
The total wall surface area is about 24 000 m2,
most of which is considered as concrete. The components in the primary loop are
treated as adiabatic because the adiabatic layer is utilized on them. In order to mitigate the hydrogen risk during severe
accidents, 22 passive autocatalytic recombiners (PAR) of
Siemens type are installed in the Qinshan-II NPP containment
compartments. Table 1 lists the position and type of the PAR. Each PAR is
simulated with a single mesh cell.Table 1
Arrangement of recombiners.
LocationTypeNo.PZR relief tank
roomFR90/1–15003Surge line roomFR90/1–9601PZR roomFR90/1–9601No. 1 SG
compartmentFR90/1–15003No. 2 SG compartmentFR90/1–15003No.1 RCP roomFR90/1–9601No.1 RCP roomFR90/1–9601SIT roomFR90/1–9601Dome regionFR90/1–15004Annular compartmentFR90/1–15004Figure 1
Vertical
view of Qinshan-II containment geometry.In Qinshan-II NPP, two separated containment spray systems are
installed. According to the design of the spray systems, the
containment can be depressurized with only one of them during the severe accidents. Each system includes two
nozzle rings, as shown in Figure2, on which about
250 nozzles are attached. The mass flow is uniformly distributed to every nozzle. Heat exchangers in the system control the
temperature of spray water. The spray systems are designed to operate in two
modes: direct spray mode and recirculation spray mode. The direct spray starts
while the pressure in the containment reaches 2.36 bar. During the direct
spray, the spray water comes from the refueling tank, and the temperature at
the nozzle outlet is in the range from 20 to 40C°. In this paper, the temperature of spray water is given to be 27C°.
In about 30 minutes, the water in the
refueling tank  will
be used up. Then the spray switches to
the recirculation mode. In the recirculation mode, the  spray water is pumped
from the water sump in the containment. Because the water in the  sump originally comes from
the primary loop or the spray water and is at a high temperature, the temperature of the recirculation spray water is higher and is designed to be from 40 to 120C°. In this paper, the water temperature of
the recirculation spray is given as 77C°.
According to
the design of the system, the spray mass
flow rates in the direct and the recirculation spray modes are 814 and 1050 ton
per hour, respectively [3].Figure 2
Containment geometry model.


## 2.2. Physics Model

In
GASFLOW two approaches are provided for two-phase flow
simulation [4]. In
the case that the spray model is not activated, the homogeneous equilibrium
approach is applied automatically, which assumes the liquid and gas phases are
in both thermodynamic and mechanical equilibrium. Because the containment spray brings strong transient and thermodynamic
non-equilibrium, in order to more exactly simulate the
interaction between the liquid and gas phases during the
spray operation, GASFLOW offers another approach in which
the thermodynamic non-equilibrium between liquid and gas
phases is considered, but the difference
of mechanical behavior between the liquid and gas phases  is still neglected. The GASFLOW spray model has been validated with
TOSQAN experiments and provided satisfactory predictions of the experiment data
[5].The
spray simulation induces a much smaller time step size and increases the
computation time by several times. Due to
heavy time consumption in the spray simulation, turbulence model is not used in
all the cases because the computational cost becomes unacceptable
while using the turbulence model. According to the experience from Karlsruhe Research
Center (FZK), GASFLOW predicts nearly the same flow field and vortex formation
with and without turbulent models in a coarse grid [6], such as the grid used in containment geometry
model. The
heat conduction inside the structures is simulated in a one-dimensional
approach. The PARs installed in the containment were simulated with the
standard model provided by the GASFLOW code.


## 2.3. Accident Scenario

The hydrogen/water source
term for this analysis was obtained by scaling from GKN surge line LBLOCA source
term reported in [6], as indicated in Figure 3. The break is located in the lower part of the SG tower which is next to the pressurizer room. At the beginning, a large amount of saturated water is discharged to the containment. The water discharge
flow rate decays promptly  due to the limited coolant inventory in the primary
loop. Water in the reactor core is heated by the decay heat which rises the
water temperature to superheated condition
after the saturated release period. At about 1400 seconds, hydrogen
generation and release starts. About 270 kg hydrogen is released into the
containment during the first 7000 s.seconds At 5930 seconds, hydrogen release peak rate is
produced due to an enhancement of steam/zirconium reaction after the failure of
the core support. Besides the source term, Figure 3(a) indicates also the two spray periods. The
start time of direct spray is determined according to the pressure variation
obtained in the base case. In about 60 seconds, the pressure in the containment reaches the threshold value and the direct spray
starts.Source term
and spray activation.
(a)
Water release
rate and temperature(b)
Hydrogen release
rate


## 2.4. Thermal Hydraulics

At the beginning of the LBLOCA
accident, along with a heavy discharge of water to the containment, the
pressure and gas temperature inside the containment increase sharply. The hot
steam mixes promptly and intensively with the atmosphere and soon spreads
through the containment, which results in strong condensation on the structure
surface. The condensation helps delaying the containment pressurization in the severe accident. Figures4 and 5 compare the pressure and gas
temperature variation in three cases. In all the
cases, a sharp pressure and temperature increase occurs and the pressure
reaches the maximum at the beginning of the blowdown.
Although different approaches are utilized to deal with the two phases in the
base case and spray cases, large discrepancies of the global behavior are not observed before the spray
activation in the cases. For the base case, the pressure and gas temperature
variation inside the containment is mainly affected by the water injection and the condensation
on the structure. The computation result shows that, with the
current source term, the pressure does not exceed the design value. In Case
A and B, the pressure and the gas temperature inside the
containment show lower peak values and
decrease fast after the activation of the direct
spray until its shutdown. Without the circulation spray, the pressure and gas
temperature rebounds to high level in Case A because the hot steam continues being discharged after the shutdown of the direct
spray. In Case B, the recirculation spray holds the pressure and the gas temperature slightly higher than the lowest value induced by the direct spray.Figure 4
Pressure
variation in three cases.Figure 5
Average
temperature variation.Figure6 presents the bulk
evaporation rate in the containment during all the cases. The direct
containment spray introduces intensive bulk condensation (negative value in
Figure 6). Conversely, the recirculation spray brings bulk evaporation. The
evaporation of the
spray water is beneficial with respect to
hydrogen risk because it increases the steam concentration and
builds up an inertial atmosphere which resists the hydrogen combustion. With respect to
hydrogen risk, the outlet temperature of the recirculation spray should be optimized to enhance the
evaporation of the
spray water. Figure 7 gives the total
condensation rate on the structure surface. During the direct spray phase in the Cases A and B,
the condensation on the structures is at a much lower rate than that in the base case due to the enhancement of
the bulk condensation which reduces both the gas temperature and steam concentration. The condensation on the structures is one of the
major ways in which heat is transferred from the atmosphere to the structures. Hence, a low condensation rate leads to a low heat transfer rate from the gas to the structures, as
indicated in Figure 8. However, during the recirculation spray in Case B, the
condensation on the structures is observed to be at a similar
rate with that in the base case. In Case B, both the bulk evaporation and
the surface condensation are enhanced by the recirculation spray. The
evaporation of spray water takes away a lot of sensible heat from the atmosphere and contributes to stabilize the gas temperature inside the
containment. Comparing both the bulk evaporation rate and the surface
condensation rate in the Cases A and B shows that the recirculation spray
generally increases the steam inventory inside the containment, which reduces
the hydrogen combustion or detonation risk. Hence, it can be concluded that
besides controlling the pressure and gas temperature inside the containment,
the recirculation spray can build a comparatively inertial atmosphere for
hydrogen.Figure 6
Bulk
evaporation rate in all cases.Figure 7
Total
condensation rate on structure surfaces.Figure 8
Energy
absorption by structures.


## 2.5. Flow Field

Figure9 presents the flow
fields during the heavy hydrogen release period in all the cases. In the base case and Case A,
similar flow fields are
observed. Because the released gas is hydrogen-rich and of high temperature and low density, a
buoyancy jet flow forms above the SG tower in which the
break is located. The jet flow  is reflected by the
dome and flows downward into the other side of the containment. A large-scale vortex
can be observed in the upper space. The
condensation and the convection heat transfer on
the structures remove the steam and heat from the atmosphere, the gas near the structures is heavier than the gas in the bulk.
Hence, the downward flow can be observed near the structure
surfaces. During the hydrogen release peak in the base case and Case A, the magnitude of velocity is generally less
than 0.5 m/s. A
chaotic flow field is induced by the recirculation spray, as indicated in Figure 9(c). The flow velocities in Case B are
much higher than that in the other two cases. As mentioned in  Section  2.2,
GASFLOW spray model uses the mechanical equilibrium assumption while dealing
with the two-phase flow. Actually, the heavy liquid phase is more inclined to
drop down than the gas phase. The assumption inevitably leads to an  artificial
flow. In the actual situation, mechanical interaction between the liquid and gas phases
could lead to a flow pattern different from the obtained results.Flow fields
during the hydrogen release peak.
(a)
base caset = 5910 s(b)
Case At = 5910 s(c)
Case Bt = 5910 s


## 2.6. Hydrogen Recombination, Hydrogen and Steam Distribution

In the analyzed scenario, the
hydrogen release can be generally divided into two periods. The first period lasts from 1400 s to 3500 s. In this period, the global hydrogen volume fraction in the containment reaches 3%, but the flammable clouds (at hydrogen
concentration above 4%) rarely appear. Due to the hydrogen-oxygen
recombination, the hydrogen concentration can be reduced to less than 3% before
the second hydrogen
release period starts. During the second period, the hydrogen release is
discontinuous. However, intensive hydrogen release between 5900 and 6000 s
could lead to extremely high local hydrogen concentration. Figure10 presents
snapshots of hydrogen and steam clouds right after the hydrogen release peak. A
clear gas stratification can be observed in both the base case and Case A. Hydrogen—rich clouds are
enveloped by steam-rich clouds during most of the time, which provides an
inertial atmosphere for hydrogen and prevents early hydrogen combustion. The
combustible hydrogen cloud in Case A is of the biggest size among the three
cases. Compared with the base case, the steam concentration is low in the spray
cases. Due to strong mixing induced by the recirculation
spray, hydrogen stratification is not observed in Case B, as shown in Figure  10(c). Hence, the direct spray reduces the
steam volume fraction and increases the hydrogen volume fraction, while the
recirculation spray does not lead to an increase of hydrogen concentration but
prevents the hydrogen concentration stratification.Hydrogen
and steam clouds at the moment when hydrogen release reaches peak value.
(a)
base case at
5960 s(b)
Case A at 5960 s(c)
Case B at 5950 sIn the Siemens PAR
correlations [4], the recombination rate depends on the pressure and inlet
hydrogen and oxygen concentration. Although the pressure is very different in
the analyzed cases, as mentioned in Section 2.4, the total recombination rate
of 22 PARs does not show great difference in all the
cases, as shown in Figure 11. Generally, the evolution of
the recombination rate is in the same trend. Following the hydrogen release
into the containment, the recombiners start up when the inlet hydrogen
concentration reaches the startup threshold (2 vol. %). Along with the hydrogen
accumulation in the containment, the recombination rate ascends. At the end of the first hydrogen release period, the recombination is at
an almost stable rate. During the second period, when the hydrogen release is
discontinuous and at a quite low rate, the recombination rate reduces smoothly
and slowly. The oscillation of recombination rate is observed in this period in Case B because strong flow caused by the spray brings
the strong variation of hydrogen concentration at the inlet of PARs. After 5900 s, the spray cases show a higher hydrogen removal capability than the base case
due to higher global hydrogen concentration. In GASFLOW, the volume flow rate
through the PAR is deduced from the recombination rate obtained from Siemens
correlations. In this case, the recombination rate is affected only by the gas
species concentration and pressure at the inlet of PAR. However, BMC Zx test
results [7] suggest an increase of the volume flow rate through PARs and
recombination rate due to the spray.Figure 11
Total
recombination rate of 22 PARs.In order to indicate the
characteristics of the hydrogen mixture and the hydrogen combustion risk in the
containment, the volume of sigma cloud is involved in this paper. The sigma cloud is a volume of the hydrogen-air-steam
mixture with a combustion expansion ratio higher than the critical value
obtained from experimental data [8]. Flame acceleration could occur in a sigma
cloud. Figure 12 presents the evolution of total sigma volume inside the
containment in three cases. During the analyzed accident, a sigma volume peak can be observed in all the cases.
And the maximum sigma volume in the spray cases is larger than that in the base case, while the maximum sigma volume in the Cases A and B is  not quite different. However, if compared
carefully, it can be found that the sigma volume in Case A shows to be larger
than that induced by Case B at the second peak, and smaller at the third peak. It
implies that the recirculation spray reduces the hydrogen risk during the slow
hydrogen release period, but can slightly increase
the local hydrogen concentration around the hydrogen source and lead to a larger
sigma volume at the moment when the peak hydrogen release rate occurs.Figure 12
Total
volume of sigma clouds.


## 3. Recombiner CFD Model

### 3.1. Passive Autocatalytic Recombiner

A passive autocatalytic
recombiner consists of a vertical channel and a stack
equipped with a catalyst bed in the lower part, as presented in Figure13. In the case of severe accidents, the catalyst is
in contact with the gas mixture of the containment. Hydrogen reacts with oxygen at the
catalyst surface and generates steam, as indicated
in Figure  14. The reaction heat released at the catalyst surface causes a
buoyancy-induced flow which increases the inflow rate
and thereby feeding the catalyst with a larger amount of hydrogen that ensures
high efficiency of recombination. The buoyancy-driven
circulation ensures a continuous gas supply to the PAR [2]. The catalyst sheets can be heated up to
900 K or
even higher; so, considerable amount of heat is also transferred from the
catalyst to the environment by heat radiation.Figure 13
Schematic of a typical PAR  [9].Figure 14
Physics phenomena in the catalyst flow channel  [10].Left in Figure14 shows a typical channel between two catalyst sheets. For
small and medium recombiners of Siemens type, both height and depth are about 15 cm. The width of flow channel is less
than 1 cm. In the PAR, the gas velocity, u, is in the magnitude of 1 m/s. The gas temperature could vary
from 300 K to 700 K. Assuming the gas in the PAR is dry air, the Reynolds
number of the flow between the catalyst sheets is Re  = 2 ud/υ =  400~1250. The flow is considered as a laminar flow in the
channel.


### 3.2. Model Development

A two-dimensional PAR model is
developed to simulate the flow in the channel, the heat transfer between the
catalyst sheet and gas flow, the heat conduction in the catalyst sheet and the
chemical reaction on the catalyst surface. The variation of flow velocity,
temperature and gas concentration in the depth direction is then neglected. The
continuity equation, Navier-Stokes equation,
and energy equation are coupled and solved with the SIMPLE algorithm. The
Boussinesq assumption is applied to consider the buoyancy caused by heatingup.
Since the flow is laminar, no turbulence model is utilized in the present model. For the radiation heat transfer, the
emissivity and absorption ratios of the catalyst sheet are assumed to be one. The view factor can be easily obtained for the parallel and perpendicular plates in a two-dimensional
model, as indicated in Figure15. An environment temperature is
assigned at the inlet and outlet of the channel to calculate the radiation heat
transfer between the catalyst and the environment. The effect of steam in the
heat radiation is currently not considered in this model. In the catalyst plate
two-dimensional heat conduction is simulated. It was observed that the
temperature difference can be neglected in the normal direction of catalyst
plate surface. It can be concluded that one-dimensional heat conduction will be
enough.Figure 15
View factor of parallel and
perpendicular plates.Besides the basic equations,
the concentration   equations (1) are solved for all the gas species except
for nitrogen(1)ρu∂ci∂x+ρv∂ci∂y=ρDi(∂2ci∂x2+∂2ci∂y2)+Sρ,i. A one-step  reaction model developed by  Ikeda et al. [11]  is applied to simulate the chemical
reaction on the catalyst surface(2)H2+12O2→H2Owith (3)R˙=14ϕH2exp(−3580R⋅T).In
(3), the gas temperature in the cell next to the catalyst surface is applied
in order to avoid an extremely high reaction rate. Based on the reaction rate
obtained from (3), the source term for the energy and species equation can be
easily calculated.


### 3.3. Model Validation

The REKO-3 experiment results [9] are utilized to validate the model. REKO-3
experiments are conducted by Forschungszentrum Juelich,
Germany. The test section of the REKO-3 facility consists of
four catalyst sheets forming three flow channels. The facility provides the
measurement of catalyst temperature and gas concentration at different heights.
Experiment results are obtained at different inlet velocities.Figures16 and 17 compare the
numerical results with the experimental data under three inlet velocities. The
hydrogen volume fraction at the inlet is 4% in all cases.
Among all the cases, the model gives the best prediction at lowest inlet
velocity (0.25 m/s). A
clear deviation of the catalyst temperature near the inlet is observed for
other two cases. An increasing catalyst temperature leads to a
significant heat loss from the catalyst to the environment, especially for the
inlet neighborhood where both the temperature and the view factors to the
environment are high. The deviation of the catalyst temperature can be
minimized by optimizing the environment temperature and by setting the exact
emissivity and absorption ratio of the catalyst material. In the cases where the
inlet velocity is 0.5 m/s and 0.8 m/s, an overestimation of recombination
by the model is observed. This could be caused by overestimating the chemical
reaction rate on the catalyst or by over-predicting the mass transfer to the
catalyst. Generally, the model gives satisfactory prediction of the experiment
results.Figure 16
Catalyst
temperature, model prediction versus experiment.Figure 17
Average
hydrogen concentration in the flow channel, model prediction versus experiment.


## 3.1. Passive Autocatalytic Recombiner

A passive autocatalytic
recombiner consists of a vertical channel and a stack
equipped with a catalyst bed in the lower part, as presented in Figure13. In the case of severe accidents, the catalyst is
in contact with the gas mixture of the containment. Hydrogen reacts with oxygen at the
catalyst surface and generates steam, as indicated
in Figure  14. The reaction heat released at the catalyst surface causes a
buoyancy-induced flow which increases the inflow rate
and thereby feeding the catalyst with a larger amount of hydrogen that ensures
high efficiency of recombination. The buoyancy-driven
circulation ensures a continuous gas supply to the PAR [2]. The catalyst sheets can be heated up to
900 K or
even higher; so, considerable amount of heat is also transferred from the
catalyst to the environment by heat radiation.Figure 13
Schematic of a typical PAR  [9].Figure 14
Physics phenomena in the catalyst flow channel  [10].Left in Figure14 shows a typical channel between two catalyst sheets. For
small and medium recombiners of Siemens type, both height and depth are about 15 cm. The width of flow channel is less
than 1 cm. In the PAR, the gas velocity, u, is in the magnitude of 1 m/s. The gas temperature could vary
from 300 K to 700 K. Assuming the gas in the PAR is dry air, the Reynolds
number of the flow between the catalyst sheets is Re  = 2 ud/υ =  400~1250. The flow is considered as a laminar flow in the
channel.


## 3.2. Model Development

A two-dimensional PAR model is
developed to simulate the flow in the channel, the heat transfer between the
catalyst sheet and gas flow, the heat conduction in the catalyst sheet and the
chemical reaction on the catalyst surface. The variation of flow velocity,
temperature and gas concentration in the depth direction is then neglected. The
continuity equation, Navier-Stokes equation,
and energy equation are coupled and solved with the SIMPLE algorithm. The
Boussinesq assumption is applied to consider the buoyancy caused by heatingup.
Since the flow is laminar, no turbulence model is utilized in the present model. For the radiation heat transfer, the
emissivity and absorption ratios of the catalyst sheet are assumed to be one. The view factor can be easily obtained for the parallel and perpendicular plates in a two-dimensional
model, as indicated in Figure15. An environment temperature is
assigned at the inlet and outlet of the channel to calculate the radiation heat
transfer between the catalyst and the environment. The effect of steam in the
heat radiation is currently not considered in this model. In the catalyst plate
two-dimensional heat conduction is simulated. It was observed that the
temperature difference can be neglected in the normal direction of catalyst
plate surface. It can be concluded that one-dimensional heat conduction will be
enough.Figure 15
View factor of parallel and
perpendicular plates.Besides the basic equations,
the concentration   equations (1) are solved for all the gas species except
for nitrogen(1)ρu∂ci∂x+ρv∂ci∂y=ρDi(∂2ci∂x2+∂2ci∂y2)+Sρ,i. A one-step  reaction model developed by  Ikeda et al. [11]  is applied to simulate the chemical
reaction on the catalyst surface(2)H2+12O2→H2Owith (3)R˙=14ϕH2exp(−3580R⋅T).In
(3), the gas temperature in the cell next to the catalyst surface is applied
in order to avoid an extremely high reaction rate. Based on the reaction rate
obtained from (3), the source term for the energy and species equation can be
easily calculated.


## 3.3. Model Validation

The REKO-3 experiment results [9] are utilized to validate the model. REKO-3
experiments are conducted by Forschungszentrum Juelich,
Germany. The test section of the REKO-3 facility consists of
four catalyst sheets forming three flow channels. The facility provides the
measurement of catalyst temperature and gas concentration at different heights.
Experiment results are obtained at different inlet velocities.Figures16 and 17 compare the
numerical results with the experimental data under three inlet velocities. The
hydrogen volume fraction at the inlet is 4% in all cases.
Among all the cases, the model gives the best prediction at lowest inlet
velocity (0.25 m/s). A
clear deviation of the catalyst temperature near the inlet is observed for
other two cases. An increasing catalyst temperature leads to a
significant heat loss from the catalyst to the environment, especially for the
inlet neighborhood where both the temperature and the view factors to the
environment are high. The deviation of the catalyst temperature can be
minimized by optimizing the environment temperature and by setting the exact
emissivity and absorption ratio of the catalyst material. In the cases where the
inlet velocity is 0.5 m/s and 0.8 m/s, an overestimation of recombination
by the model is observed. This could be caused by overestimating the chemical
reaction rate on the catalyst or by over-predicting the mass transfer to the
catalyst. Generally, the model gives satisfactory prediction of the experiment
results.Figure 16
Catalyst
temperature, model prediction versus experiment.Figure 17
Average
hydrogen concentration in the flow channel, model prediction versus experiment.


## 4. Conclusion

The hydrogen analysis with the
CFD code GASFLOW is conducted to investigate the effect of spray modes on
hydrogen risk in the Qinshan-II NPP containment during a large break loss of coolant
accident (LBLOCA). The direct spray sharply depresses the pressure and
temperature in the containment and reduces the heat transfer from the
atmosphere to the structures. However, the direct spray mode (case A) is still
not capable of controling  the pressure and gas temperature during the accident due
to the strong release of hot steam after the shutdown of the direct spray. A
considerable evaporation of the recirculation spray water is observed.
Compared  with Case A, the enhancement of the condensation
on the structures is also observed during the recirculation
spray (case B). Because the evaporation induced by the recirculation spray is
generally stronger than the enhancement of surface condensation, the steam
inventory inside the containment is increased due to the recirculation. During
the hydrogen release peak, a chaotic mixing flow field is produced by the
recirculation spray, while a regular natural convection flow forms in the other
two cases. From the aspect of hydrogen safety, the direct spray increases the
global hydrogen concentration and the maximum sigma volume, but does not prevent
the stratification. The recirculation spray does not increase the global hydrogen concentration inside the containment and
promotes mixing,
but can increase the local hydrogen concentration near the hydrogen release
source. The effects of the containment
spray on the PAR performance are found to be minor.A
CFD recombiner model is developed in order to
provide more detailed insights into the process along the catalyst sheets. The
model is validated with the data from the REKO-3 experiment [9] and gives satisfactory prediction. Further work is
needed to develop a full recombiner model by incorporating the chimney part. For implementation of the
CFD PAR model in containment analysis, appropriate boundary conditions are
needed at the inlet of the PAR.



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*Source: 213981-2009-04-14.xml*