# Natural Circulation Characteristics at Low-Pressure Conditions through PANDA Experiments and ATHLET Simulations

Authors: Domenico Paladino; Max Huggenberger; Frank Schäfer
Journal: Science and Technology of Nuclear Installations
 (2008)

---

## Abstract

Natural circulation characteristics at low pressure/low power have been studied by performing experimental investigations and numerical simulations. The PANDA large-scale facility was used to provide valuable, high quality data on natural circulation characteristics as a function of several parameters and for a wide range of operating conditions. The new experimental data allow for testing and improving the capabilities of the thermal-hydraulic computer codes to be used for treating natural circulation loops in a range with increased attention.  This paper presents a synthesis of a part of the results obtained within the EU-Project NACUSP “natural circulation and stability performance of boiling water reactors.” It does so by using the experimental results produced in PANDA and by showing some examples of numerical simulations performed with the thermal-hydraulic code ATHLET.


---

## Body

## 1. Introduction

In the framework of the EU-Project NACUSP, experiments on
thermo-hydraulic characteristics of natural-circulation-cooled boiling water reactors
(BWR) have been performed in four sophisticated thermo-hydraulic test
facilities which complement each other, ranging
from small-scale to large-scale, and from low-pressure/low-power operating
conditions to the nominal operating conditions for BWRs. The ultimate goal of
the project was to improve the economics of operating and future plants through
improved operational flexibility, enhanced availability, and increased
confidence levels on the safety margins regarding the stability issues in BWRs
[1].This paper
presents a synthesis of the results obtained from the experiments
performed in the large-scale PANDA facility [2] and
from the numerical simulations performed with the thermal-hydraulic code ATHLET. The range of parameters covers a large spectrum
of conditions for the low-pressure/low-power range, therefore, being of interest
especially with regard to the start-up procedures for natural-circulation-cooled
BWRs (e.g., for the European simplified boiling water reactor (ESBWR)).


## 2. PANDA Experimental Investigations

### 2.1. PANDA Facility

The multipurpose thermal-hydraulic test facility PANDA is located at Paul Scherrer Institute (PSI), Villigen, Switzerland.
The facility is designed and used for investigating at large-scale system
behavior and phenomena for different light water reactor (LWR) designs. PANDA
has a modular structure, which is based on six cylindrical pressure vessels
with a total volume of 460 m3 and four open pools with a total
capacity of 60 m3 (Figure 1). Four of these pressure vessels are
arranged in two vertical columns. The two lower vessels simulating the wetwell
or suppression chamber are interconnected with two large pipes. The wetwell
vessels support the two vessels, simulating the drywell. They are interconnected
at about midplane elevation with one large pipe. The fifth vessel simulates the
core flooding (GDCS) pool. The sixth vessel is configured to simulate the reactor
pressure vessel (RPV). Its dimension allows for performing natural circulation tests
at roughly 1 : 1 scale in height. The vessel has an inner diameter of 1.23 m and is
19.2 m in height. The RPV includes a heated section, a riser, and a downcomer
(DC). The heated section has a height of 1.3 m and does not reproduce the
geometry of a real core. The power is generated by 115 electrical heater
elements divided into six individually controlled groups. This allows for
conducting tests with any specified power up to 1.5 MW. The heaters are
distributed over the cross-section to obtain a uniform radial distribution over
the whole power range. The riser has a height of 9.5 m (starting at the top of
the heated section). Four rectangular open pools are placed on top of the drywell
vessels and are equipped with heat exchangers/condensers. The facility is well
insulated in order to minimize heat losses: an insulation layer of 300 mm is
applied on the RPV and 100 mm on system lines and pools. The overall height of
the facility is about 25 m. The maximum operating conditions are 10 bar and
200°C.Figure 1
PANDA facility.The basic
instrumentation of the
facility includes about 1000 sensors to measure temperatures, pressures, pressure
differences, water levels, flow rates, gas concentrations, fluid phases, and
electrical heater power. The data acquisition system maximum scanning rate is 5 Hz. The standard operating scanning rate is 0.5 Hz. The facility is remotely
controlled from a graphical-display man-machine interface with process
visualization.


#### 2.1.1. PANDA Facility Configuration for Natural Circulation Tests

For the NACUSP project only
parts of the PANDA facility were used. Figure2 shows the test
configuration consisting of the RPV natural circulation loop and the
condensation/cooling loop. The steam produced in the RPV flows to the condenser
and condenses due to the temperature difference with the secondary side (water
pool at saturation temperature). The pool is open to the atmosphere and water
evaporates from the boiling pool. The condensate flows through the drain line
back to the RPV downcomer. A major
modification to the RPV was related to the core inlet region. A movable ring
was attached to
the lower edge of the shroud (separating the core from the downcomer) to allow for varying the core-inlet
hydraulic resistance. The gap height between the lower edge of the ring and the
bottom of the RPV can be varied in order to obtain the required core-inlet flow
resistance. The PANDA auxiliary systems are used for preconditioning the
facility and for adding water to keep the pool at the required level along the test duration.Figure 2
PANDA configuration for natural circulation
tests.The instrumentation in the RPV
has been significantly improved. In total about 110 sensors are used for the natural
circulation tests. Additional K-type thermocouples inside riser and downcomer have been
installed to measure local fluid and wall temperatures. In the range of 100°C–150°C a
measurement error less than 0.5°C can be expected. For higher temperatures the
error is still less than 1°C. Three ultrasonic flow meters have been added to
measure local fluid flow velocities in the downcomer. The measurement error
(for pipe flow) is typically less than 1% of the reading. Absolute pressure and
heater power are measured as well. Differential pressure sensors in the riser
and downcomer allow for the assessment of void fraction in different vertical sections
of the RPV. A time domain reflectometry (TDR) probe is used to measure the
level swelling due to the void production in the RPV.


### 2.2. PANDA Test Matrix

The test matrix allowed the RPV power and pressure to be varied, as well
as other parameters influencing the natural circulation behavior such as core-inlet
hydraulic resistance and RPV water level. The tests were performed at constant
power, balanced by a corresponding condenser heat removal capacity. The
specified RPV power was selected to match the desired steady state pressure in
the RPV. The basic test matrix is shown in Table1. For a given RPV pressure
level, the maximum energy is removed if the condenser is fully submerged in the
pool. The corresponding power for the four specified pressure levels is given
in Table 1 (column “high”). For running tests at lower power, the condenser
heat removal capacity is adjusted by lowering the water level in the pool,
thereby reducing the active condenser area. Three tests with different power
(“low,” “middle,” and “high”) were performed for each of the four different
pressure levels.Table 1
Basic test matrix (variation of pressure and power).
RPV pressure [bar]RPV power [kW]LowMiddleHigh2.01352002453.02503804805.04306608858.05949171306The core-inlet hydraulic resistance coefficientk has been varied as
follows: for the basic tests a BWR typical value was used (kbasic=30). Selected tests were performed with a
low inlet resistance (klow=7) and with
high inlet resistance (khigh=500).The RPV water level has
also been varied: most of the tests were performed with the collapsed water
level at 12.8 m above bottom of RPV. This level represents about the nominal value
for the ESBWR. For few tests the water level was reduced close
to top of riser which is at 11.0 m above RPV bottom. The selected values were
11.1 m and 11.4 m.The following three series of tests with totally 25
experiments have been performed:(i)
B-series tests (BWR-typical core-inlet flow resistancek=30):
(a)
12
tests with nominal RPV water level (12.8 m),(b)
2 tests with low RPV water
level (11.1 m and 11.4 m);(ii)
L-series tests (Low core-inlet flow resistancek=7):
(a)
3 tests with nominal RPV water level (12.8 m),(b)
1 test with low RPV
water level (11.1 m);(iii)
H-series tests (High core-inlet flow resistancek=500):
(a)
6 tests with nominal RPV water level (12.8 m),(b)
1 test with low RPV
water level (11.1 m).
An overview of all PANDA tests is included in Table 2.Table 2
Overview of the
PANDA tests and analysis results.
Test namekPPowerLevel RPVLevel poolTsubzboil.b*zflash*V1V2V3V123Var1231123fTApDRAnalysis of RPVClass(bar)(kW)(m)(m)(K)(–)(–)(m/ s)(m/ s)(m/s)(m/s)(m2/s2)(%)(Hz)(sec.)(deg)(–)pressure signalLOWk SERIES:L8.377.96131312.814.215.82.70.750.540.510.520.520.0135220.01606300.641 major peak,f=0.01455 (T=65s);
DR=0.632L5.375.0490912.824.207.73.20.790.320.310.320.320.0030170.008412000.681 major peak,f=0.0084 (T=1 20 s);
DR=0.682LL5.375.0291111.114.205.32.60.430.350.430.370.380.0037160.01119000.741 major peak,f=0.011 (T=90s); DR=0.742L2.372.0224712.834.2013.812.51.020.160.200.210.190.003029????no major peak1BASICk SERIES:B8.1308.0459412.751.735.83.00.820.290.23/0.260.002720????1 major peak,f=0.0077 (T=130s); DR=0.5?1B8.2308.0691712.812.175.82.70.770.360.310.410.360.0052200.01158700.61 major peak,f=0.0115 (T=87s); DR=0.4?2B8.3307.98130612.804.205.82.70.750.520.470.570.520.0085180.01586300.611 major peak,f=0.0158 (T=63s); DR=0.4?2B5.1305.1539112.821.737.54.00.860.190.180.180.180.001521????no major peak1B5.2305.0463712.842.177.63.10.770.220.220.230.220.001718????1 major peak,f=0.0065 (T=154s);
DR=0.85165.3304.8589612.774.207.82.80.720.300.270.260.280.0037220.008012500.661 major peak,f=0.08 (T=125s); DR=0.652BL-5.3305.0289611.114.207.03.30.700.370.330.390.360.005520????1 major peak,f=0.01225 (T=82s); DR=0.41BLL5.3304.9589711.394.207.23.20.710.350.320.360.340.0085270.01069500.851 major peak,f=0.01055 (T=95s);
DR=0.812B3.1303.0223112.821.7310.48.00.960.150.160.140.150.002634????no major peak1B3.2303.0236112.802.1710.55.80.910.180.170.160.170.001825????1 peak,f=0.0055 (T=182s);163.3302.9750212.784.2110.74.70.870.200.180.190.190.001621????1 peak,f=0.055 (T=200s);1B2.1302.0211112.811.7314.017.91.050.100.130.120.120.001735????no major peak1B2.2302.0218312.802.1713.812.51.010.130.150.130.140.002435????no major peak1B2.3302.0024112.804.2013.910.20.990.140.160.160.150.002935????no major peak1HIGHk SERIES:H8.15007.959512.811.735.21.40.25/0.130.160.140.002334????no major peak3H8.35007.83130912.784.214.61.20.03/0.260.310.29??????no major peak3H5.15005.0641912.831.737.11.80.410.080.080.100.090.001444????no major peak3H5.35004.9790212.814.206.61.30.120.150.120.170.150.003440????no major peak3HL5.35005.0090711.104.216.01.50.230.220.160.230.200.017064????no major peak3H2.15002.0313112.851.7313.67.40.950.080.050.050.060.000641????no major peak1H2.35002.0024612.834.2013.64.60.840.080.060.070.070.0080126????no major peak1


### 2.3. Experimental Results and Analysis

Natural Circulation Modes
The
coolant temperature increases in the heated section (core), but it may not
reach saturation conditions under the given conditions. A rough estimation of
the position of the boiling boundary in the core by means of a heat balance was
performed. The temperature measured at the inlet of the core was used as the
reference temperature, and the coarsest assumptions were made (no heat losses,
uniform radial and vertical distribution of power in the core, no subcooled
boiling). The nondimensional valuezboil.b* reported in Table 2
corresponds to the distance (measured from the inlet of the core, and divided
by the length of the core) at which this boiling boundary can be expected (zboil.b*≤1:
boiling in the core; zboil.b*>1: no
boiling in the core). The result is that boiling in the core did not occur in
any test except may be in tests with high core-inlet resistance and relatively
high power for which zboil.b*≅1 is found.In
the same way, a coarse estimation of the height in the riser at which flashing
may be expected to occur has been made. Using the same inlet temperature,
calculating the temperature at the outlet of the core, and assuming it is
conserved all along the riser (no heat losses), a distancezflash* (measured from inlet of riser, and
divided by length of riser) is estimated and reported in Table 2. For most of
the tests, it is expected that flashing should only occur in the upper region
or above the riser. Of course, in tests at high core-inlet resistance and
relatively high power, and also in tests with low RPV water level (and hence
lower pressure head), this basic calculation predicts flashing in lower
sections of the riser.These estimations do not take into account the 3D and
local effects that might affect the physical phenomena and the flow regime
actually occurring in the RPV. Some interesting information can be retrieved by
plotting some of the temperature profiles measured in the central axis of the
RPV. Two examples are given in Figure3, for which, a curve corresponding to
the saturation temperature is also plotted on each graph. The saturation
temperature has been estimated by using the time-averaged RPV pressure and
calculating the pressure head at each height, without considering the possible
presence of void.Measured temperature profiles in the vertical axis of the RPV.
(a)
Test H2.1(b)
Test H8.1From Figure3, it is clear that the
validity of the comparison with the saturation temperature may be limited due
to the error in temperature measurements. Due to the data reduction procedure,
this error slightly increases with temperature (Figure 3). The saturation
temperature decreases along the vertical axis as the system operates at low
pressure. Flashing should occur when the coolant reaches saturation conditions
in the riser. Following this approach, in test H2.1 (shown in Figure 3(a)), the
average height of the flashing boundary can be roughly estimated to be at about
the level corresponding to the top of the riser. In this case, flashing should
be responsible for the shape of the profile above this height, where measured
temperatures seem to lie in the vicinity of the saturation curve. This
basically confirms the previous coarse prediction (Table 2, zflash*=0.95). In test H8.1, a different situation is shown
(Figure 3(b)): the fluid temperature clearly following the saturation curve
indicates that two-phase flow should have occurred along the complete riser
length. To summarize, these different plots help to qualitatively distinguish
different cases and define three classes:Class 1.
flashing above the riser, or perhaps no flashing at all (Figure3(a));Class 2.
flashing at a lower elevation in the riser;Class 3.
two-phase flow all along the riser (Figure3(b)).
Rough estimations of time-averaged
void fraction values processed from differential pressure measurements also
indicate that the highest void fraction was assessed at the top of the riser,
and especially for cases with high core-inlet resistance/high power (Class 3).
These cases showed the largest difference between the swell level (measured by
the TDR probe) and the collapsed water level retrieved from differential
pressure measurements. For example, in tests HL5.3 and H8.3, a void fraction of
the order of 10% was estimated in the upper region of the riser. In the lower
sections, values lower than a few percent were found.Analysis of Flow Velocity Measurements in RPV Downcomer
The natural circulation flow rate is
of main interest. Hence, this short analysis concerns the velocity measurements
from 3 ultrasonic flow meters in the RPV DC. The sensors are located at
elevation 5.00 m above RPV bottom with an angle of 120° azimuthally between
each other.
The velocity signals were sampled at
a frequency of 0.5 Hz for the whole duration of the test (5 hours). It should
be noted that the time constant of the sensors was set to 6 seconds in order to
avoid aliasing problems. The autopower spectral density (APSD) of the signals
was calculated to identify resonance frequencies of the system. The autocorrelation
function (ACF) was used to calculate the decay ratio (DR) of the system. The DR
is a widely used parameter to quantify the stability of the system: ifDR<1,
the system is linearly stable; if DR>1, it is unstable. In practice, the DR
values presented in Table 2 were estimated by simply calculating the ratio
between the second and the first maxima of the ACF (Figure 4(c)). Cross-power spectral
densities, coherence, and cross-correlation functions were also calculated to
examine coherence, possible phase shifts, and so forth between signals from
different flow meters.
An overview of the results of the analysis
performed can be found in Table2. The basic parameters of each test are given
in the first columns of this table. Mean values of the velocities calculated
over the test period (V1, V2, and V3) are also
reported. V123 is the average of the three mean velocities. The
quantity I123 should provide an estimate of the intensity of the
velocity fluctuations, and was calculated as follows: (1)I123(%)=100.(Var123)1/2V123,where Var123 represents
the average of the 3 variances of the signals. It should be realized that this
relation could result in very high values for the tests with very low natural circulation
velocities, for which the signal-to-noise ratio is expected to be higher (e.g.,
test H2.3).
Concerning the spectral analysis, the frequency and the corresponding
period of the major peak are reported when possible. The value “0” in
the column “Δφ” of Table 2 means that no
significant phase shift between the signals was observed, that is, the three
velocity signals were oscillating in phase at one given frequency. When
possible, a DR was estimated from the ACF and is also reported in Table 2. One
column of the table refers to the analysis of signals from the sensor measuring
the total pressure in the RPV. These additional observations may help to
characterize some of the tests. Looking at the spectra and at the results of
this analysis based on the ACF and the estimation of DR values, the PANDA tests
can be categorized in two different groups as follows.Analysis of flow velocity signals in
the downcomer (Test L8.3).
(a)
Measured flow velocities in the downcomer(b)
Normalized power spectra of velocity signal(c)
ACF processed from one velocity signal1st Group
The rows in Table2 corresponding to these tests are
greyed. An illustration of this case is made using test L8.3 results. An
excerpt of the raw velocity signals recorded during this test is shown in
Figure 4(a). A major peak can be observed on the processed spectra of the 3
sensors reported in Figure 4(b). The time traces of the velocity signals show
that the three sensors oscillate “together,” that is, with no significant phase
shift between each other. This was confirmed by looking at the phase of the
complex cross-power spectral densities between the different signals. Just
simply looking at a number of raw signals recorded over the test period, it can
also be seen that the other measured variables (temperatures, pressure in the RPV,
condenser feed, and drain flows, etc.) show fluctuations with the same
frequency. The ACF obtained from velocity signal 1 is given in Figure 4(c).
Clearly, a DR value indicating stable behavior (DR=0.64) can be estimated from
this graph.2nd Group
An
illustration of this group is given using test B3.3 results. Observations made
on the raw signals (Figure5(a)) in this case show rather random behavior.
In some other tests belonging to the same group, it is sometimes possible to
observe significant phase shifts between the sensors. Figure 5(b) shows that
there is no longer a major peak present in the power spectrum. Using a
logarithmic scale, the low-frequency part of the spectrum appears to be flatter.
Moreover, the ACF does not really allow a DR value to be derived (Figure 5(c)),
this DR appearing to be much smaller (actually, very close to 0) compared to the
tests of the first group.Analysis of flow velocity signals in the
downcomer (Test B3.3).
(a)
Measured flow velocities in the downcomer(b)
Normalized power spectra of velocity signal(c)
ACF processed from one velocity signalIt was also observed that these two characteristic modes of behavior,
indicated by the ACF plots, did not vary as a function of time during the
course of the tests. For tests L8.3 and B3.3, this was done by calculating ACFs
over 5 successive (1-hour long) time periods and it clearly showed that, in
both cases, all the ACFs for one test at different times were very similar.Some data from other sensors
(condenser feed and drain flow rates, RPV pressure, etc.) have also been analyzed
using the same approach as for the velocity data. For the tests from the 1st group,
a resonance peak can be observed at the same oscillation frequency. No peak can
be found for the tests belonging to the 2nd group. However, in some of these
tests (e.g., test B3.3), the condenser feed flow and RPV pressure signals may
exhibit a peak in their spectrum whereas no such peak was observed for the
velocity signals (see also the second last column of Table2, concerning the
analysis of RPV pressure signals). This indicates that flashing possibly takes
place above the top of the riser, not influencing the main circulation flow.
These cases fall consistently into Class 1 (see Figure 3).From the expected mechanism of the
flow oscillations, the fluid travelling time in the heated section, and the
transit time of the enthalpy perturbations in the adiabatic section, should
play a role in determining the period of the oscillations. From Figure11, it
is clear that the oscillation period decreases with increasing average flow
rate. The total transit time can be evaluated as the sum of the transit time in
the riser section plus half of the transit time in the heated section [3]. In
PANDA, no velocity measurement is available in the riser. An estimation of the
riser velocity was made by simply using the time-averaged value of the velocity
in the DC reported in Table 2, and just considering the flow area ratio between
the riser and the DC.Following this simplified approach, the oscillation period was found to
be in the order of 1.6 times the transit time of the fluid (Figure6). The
simple way in which the transit time was estimated did not consider the
occurrence of two-phase flow in the riser. Hence, it is likely that our
assessment overestimated this time. Thus, these observations seem to be quite
consistent with those presented for the CIRCUS facility [1], for which the
period of oscillation was found to be twice the transit time.Figure 6
Period of oscillation versus fluid transit time in core and riser.Discussion
The differentiation between the two
groups of tests is based on DR values derived from the velocity signals. However,
the validity and the accuracy of this method can be questioned. Many other
methods have been used to extract stability parameters and can be compared to
the ACF-based method. For this reason, some of our experimental velocity time
traces were processed using a more sophisticated method, namely, the
autoregressive method. This method properly applied to our signals yielded
about the same DR values as those presented in Table2. This confirms the main
result from this analysis that all tests are stable.However, by having a closer look to the tests, the different flow
regimes that can be assumed, and the three classes defined, can be related to
the previous findings regarding the stability of the system. Information
reported in Table2 was used for this purpose. The plots in Figure 3 partly
illustrate the following classification.Class 4 (Figure3(a)).
High inlet subcooling and relatively low (or even very
low) flow rates characterize this class. It is likely that flashing occurred
only above the riser, the natural circulation flow still being in a
single-phase regime. This would explain why, in some of these tests, contrary
to the velocity signals in the DC, some RPV pressure signals show a peak in
their spectrum and allow the derivation of DR values (see, e.g., test B5.2). It
is also possible that in some of these tests (notably those performed at the
lowest powers), no flashing occurred at all in the whole system.Class 5.
These tests, performed at relatively high
power and low inlet resistance, have been characterized by a well-identified
oscillation period and DR values could be derived from the velocity signals.
The highest natural circulation flow rates were measured in these tests, which
indicate the combined effect of the flow enhancement by steam production and of
a low inlet resistance. According to our estimations and analyses, no boiling
occurred in the core but flashing did in the upper half of the riser.Class 6 (Figure3(b)).
Very stable behavior was found
for these tests performed with an increased inlet resistance. Under these
conditions, the flashing front fluctuates close to the bottom of the riser,
which stabilizes very much the two-phase natural circulation. The measured flow
rates are among the lowest of the test series, as a result of the high flow
resistance. Looking at the parameters (notably the expected position of the
boiling boundary) reported in Table2 and at the characteristic temperature
profiles (Figure 3(b)), it seems that stable two-phase natural circulation flow
can be assumed in these cases.The presented classification gives a
rather coherent picture of the PANDA test series. It was possible to link the
experimental observations with the expected phenomenology. No unstable tests
were recorded. However, the characterization of the tests (from single-phase to
two-phase flow conditions) is satisfying and quite consistent with what is
reported in the literature. Test BLL5.3, which shows a DR of about 0.85, should
be very close to unstable conditions.The test matrix allowed the variation
of the RPV water level. However, the actual influence of this parameter on the
flow behavior cannot be clearly assessed. Comparing tests B5.3 and BL5.3, it
seems that a lower level would have a stabilizing effect.


## 2.1. PANDA Facility

The multipurpose thermal-hydraulic test facility PANDA is located at Paul Scherrer Institute (PSI), Villigen, Switzerland.
The facility is designed and used for investigating at large-scale system
behavior and phenomena for different light water reactor (LWR) designs. PANDA
has a modular structure, which is based on six cylindrical pressure vessels
with a total volume of 460 m3 and four open pools with a total
capacity of 60 m3 (Figure 1). Four of these pressure vessels are
arranged in two vertical columns. The two lower vessels simulating the wetwell
or suppression chamber are interconnected with two large pipes. The wetwell
vessels support the two vessels, simulating the drywell. They are interconnected
at about midplane elevation with one large pipe. The fifth vessel simulates the
core flooding (GDCS) pool. The sixth vessel is configured to simulate the reactor
pressure vessel (RPV). Its dimension allows for performing natural circulation tests
at roughly 1 : 1 scale in height. The vessel has an inner diameter of 1.23 m and is
19.2 m in height. The RPV includes a heated section, a riser, and a downcomer
(DC). The heated section has a height of 1.3 m and does not reproduce the
geometry of a real core. The power is generated by 115 electrical heater
elements divided into six individually controlled groups. This allows for
conducting tests with any specified power up to 1.5 MW. The heaters are
distributed over the cross-section to obtain a uniform radial distribution over
the whole power range. The riser has a height of 9.5 m (starting at the top of
the heated section). Four rectangular open pools are placed on top of the drywell
vessels and are equipped with heat exchangers/condensers. The facility is well
insulated in order to minimize heat losses: an insulation layer of 300 mm is
applied on the RPV and 100 mm on system lines and pools. The overall height of
the facility is about 25 m. The maximum operating conditions are 10 bar and
200°C.Figure 1
PANDA facility.The basic
instrumentation of the
facility includes about 1000 sensors to measure temperatures, pressures, pressure
differences, water levels, flow rates, gas concentrations, fluid phases, and
electrical heater power. The data acquisition system maximum scanning rate is 5 Hz. The standard operating scanning rate is 0.5 Hz. The facility is remotely
controlled from a graphical-display man-machine interface with process
visualization.


### 2.1.1. PANDA Facility Configuration for Natural Circulation Tests

For the NACUSP project only
parts of the PANDA facility were used. Figure2 shows the test
configuration consisting of the RPV natural circulation loop and the
condensation/cooling loop. The steam produced in the RPV flows to the condenser
and condenses due to the temperature difference with the secondary side (water
pool at saturation temperature). The pool is open to the atmosphere and water
evaporates from the boiling pool. The condensate flows through the drain line
back to the RPV downcomer. A major
modification to the RPV was related to the core inlet region. A movable ring
was attached to
the lower edge of the shroud (separating the core from the downcomer) to allow for varying the core-inlet
hydraulic resistance. The gap height between the lower edge of the ring and the
bottom of the RPV can be varied in order to obtain the required core-inlet flow
resistance. The PANDA auxiliary systems are used for preconditioning the
facility and for adding water to keep the pool at the required level along the test duration.Figure 2
PANDA configuration for natural circulation
tests.The instrumentation in the RPV
has been significantly improved. In total about 110 sensors are used for the natural
circulation tests. Additional K-type thermocouples inside riser and downcomer have been
installed to measure local fluid and wall temperatures. In the range of 100°C–150°C a
measurement error less than 0.5°C can be expected. For higher temperatures the
error is still less than 1°C. Three ultrasonic flow meters have been added to
measure local fluid flow velocities in the downcomer. The measurement error
(for pipe flow) is typically less than 1% of the reading. Absolute pressure and
heater power are measured as well. Differential pressure sensors in the riser
and downcomer allow for the assessment of void fraction in different vertical sections
of the RPV. A time domain reflectometry (TDR) probe is used to measure the
level swelling due to the void production in the RPV.


## 2.1.1. PANDA Facility Configuration for Natural Circulation Tests

For the NACUSP project only
parts of the PANDA facility were used. Figure2 shows the test
configuration consisting of the RPV natural circulation loop and the
condensation/cooling loop. The steam produced in the RPV flows to the condenser
and condenses due to the temperature difference with the secondary side (water
pool at saturation temperature). The pool is open to the atmosphere and water
evaporates from the boiling pool. The condensate flows through the drain line
back to the RPV downcomer. A major
modification to the RPV was related to the core inlet region. A movable ring
was attached to
the lower edge of the shroud (separating the core from the downcomer) to allow for varying the core-inlet
hydraulic resistance. The gap height between the lower edge of the ring and the
bottom of the RPV can be varied in order to obtain the required core-inlet flow
resistance. The PANDA auxiliary systems are used for preconditioning the
facility and for adding water to keep the pool at the required level along the test duration.Figure 2
PANDA configuration for natural circulation
tests.The instrumentation in the RPV
has been significantly improved. In total about 110 sensors are used for the natural
circulation tests. Additional K-type thermocouples inside riser and downcomer have been
installed to measure local fluid and wall temperatures. In the range of 100°C–150°C a
measurement error less than 0.5°C can be expected. For higher temperatures the
error is still less than 1°C. Three ultrasonic flow meters have been added to
measure local fluid flow velocities in the downcomer. The measurement error
(for pipe flow) is typically less than 1% of the reading. Absolute pressure and
heater power are measured as well. Differential pressure sensors in the riser
and downcomer allow for the assessment of void fraction in different vertical sections
of the RPV. A time domain reflectometry (TDR) probe is used to measure the
level swelling due to the void production in the RPV.


## 2.2. PANDA Test Matrix

The test matrix allowed the RPV power and pressure to be varied, as well
as other parameters influencing the natural circulation behavior such as core-inlet
hydraulic resistance and RPV water level. The tests were performed at constant
power, balanced by a corresponding condenser heat removal capacity. The
specified RPV power was selected to match the desired steady state pressure in
the RPV. The basic test matrix is shown in Table1. For a given RPV pressure
level, the maximum energy is removed if the condenser is fully submerged in the
pool. The corresponding power for the four specified pressure levels is given
in Table 1 (column “high”). For running tests at lower power, the condenser
heat removal capacity is adjusted by lowering the water level in the pool,
thereby reducing the active condenser area. Three tests with different power
(“low,” “middle,” and “high”) were performed for each of the four different
pressure levels.Table 1
Basic test matrix (variation of pressure and power).
RPV pressure [bar]RPV power [kW]LowMiddleHigh2.01352002453.02503804805.04306608858.05949171306The core-inlet hydraulic resistance coefficientk has been varied as
follows: for the basic tests a BWR typical value was used (kbasic=30). Selected tests were performed with a
low inlet resistance (klow=7) and with
high inlet resistance (khigh=500).The RPV water level has
also been varied: most of the tests were performed with the collapsed water
level at 12.8 m above bottom of RPV. This level represents about the nominal value
for the ESBWR. For few tests the water level was reduced close
to top of riser which is at 11.0 m above RPV bottom. The selected values were
11.1 m and 11.4 m.The following three series of tests with totally 25
experiments have been performed:(i)
B-series tests (BWR-typical core-inlet flow resistancek=30):
(a)
12
tests with nominal RPV water level (12.8 m),(b)
2 tests with low RPV water
level (11.1 m and 11.4 m);(ii)
L-series tests (Low core-inlet flow resistancek=7):
(a)
3 tests with nominal RPV water level (12.8 m),(b)
1 test with low RPV
water level (11.1 m);(iii)
H-series tests (High core-inlet flow resistancek=500):
(a)
6 tests with nominal RPV water level (12.8 m),(b)
1 test with low RPV
water level (11.1 m).
An overview of all PANDA tests is included in Table 2.Table 2
Overview of the
PANDA tests and analysis results.
Test namekPPowerLevel RPVLevel poolTsubzboil.b*zflash*V1V2V3V123Var1231123fTApDRAnalysis of RPVClass(bar)(kW)(m)(m)(K)(–)(–)(m/ s)(m/ s)(m/s)(m/s)(m2/s2)(%)(Hz)(sec.)(deg)(–)pressure signalLOWk SERIES:L8.377.96131312.814.215.82.70.750.540.510.520.520.0135220.01606300.641 major peak,f=0.01455 (T=65s);
DR=0.632L5.375.0490912.824.207.73.20.790.320.310.320.320.0030170.008412000.681 major peak,f=0.0084 (T=1 20 s);
DR=0.682LL5.375.0291111.114.205.32.60.430.350.430.370.380.0037160.01119000.741 major peak,f=0.011 (T=90s); DR=0.742L2.372.0224712.834.2013.812.51.020.160.200.210.190.003029????no major peak1BASICk SERIES:B8.1308.0459412.751.735.83.00.820.290.23/0.260.002720????1 major peak,f=0.0077 (T=130s); DR=0.5?1B8.2308.0691712.812.175.82.70.770.360.310.410.360.0052200.01158700.61 major peak,f=0.0115 (T=87s); DR=0.4?2B8.3307.98130612.804.205.82.70.750.520.470.570.520.0085180.01586300.611 major peak,f=0.0158 (T=63s); DR=0.4?2B5.1305.1539112.821.737.54.00.860.190.180.180.180.001521????no major peak1B5.2305.0463712.842.177.63.10.770.220.220.230.220.001718????1 major peak,f=0.0065 (T=154s);
DR=0.85165.3304.8589612.774.207.82.80.720.300.270.260.280.0037220.008012500.661 major peak,f=0.08 (T=125s); DR=0.652BL-5.3305.0289611.114.207.03.30.700.370.330.390.360.005520????1 major peak,f=0.01225 (T=82s); DR=0.41BLL5.3304.9589711.394.207.23.20.710.350.320.360.340.0085270.01069500.851 major peak,f=0.01055 (T=95s);
DR=0.812B3.1303.0223112.821.7310.48.00.960.150.160.140.150.002634????no major peak1B3.2303.0236112.802.1710.55.80.910.180.170.160.170.001825????1 peak,f=0.0055 (T=182s);163.3302.9750212.784.2110.74.70.870.200.180.190.190.001621????1 peak,f=0.055 (T=200s);1B2.1302.0211112.811.7314.017.91.050.100.130.120.120.001735????no major peak1B2.2302.0218312.802.1713.812.51.010.130.150.130.140.002435????no major peak1B2.3302.0024112.804.2013.910.20.990.140.160.160.150.002935????no major peak1HIGHk SERIES:H8.15007.959512.811.735.21.40.25/0.130.160.140.002334????no major peak3H8.35007.83130912.784.214.61.20.03/0.260.310.29??????no major peak3H5.15005.0641912.831.737.11.80.410.080.080.100.090.001444????no major peak3H5.35004.9790212.814.206.61.30.120.150.120.170.150.003440????no major peak3HL5.35005.0090711.104.216.01.50.230.220.160.230.200.017064????no major peak3H2.15002.0313112.851.7313.67.40.950.080.050.050.060.000641????no major peak1H2.35002.0024612.834.2013.64.60.840.080.060.070.070.0080126????no major peak1


## 2.3. Experimental Results and Analysis

Natural Circulation Modes
The
coolant temperature increases in the heated section (core), but it may not
reach saturation conditions under the given conditions. A rough estimation of
the position of the boiling boundary in the core by means of a heat balance was
performed. The temperature measured at the inlet of the core was used as the
reference temperature, and the coarsest assumptions were made (no heat losses,
uniform radial and vertical distribution of power in the core, no subcooled
boiling). The nondimensional valuezboil.b* reported in Table 2
corresponds to the distance (measured from the inlet of the core, and divided
by the length of the core) at which this boiling boundary can be expected (zboil.b*≤1:
boiling in the core; zboil.b*>1: no
boiling in the core). The result is that boiling in the core did not occur in
any test except may be in tests with high core-inlet resistance and relatively
high power for which zboil.b*≅1 is found.In
the same way, a coarse estimation of the height in the riser at which flashing
may be expected to occur has been made. Using the same inlet temperature,
calculating the temperature at the outlet of the core, and assuming it is
conserved all along the riser (no heat losses), a distancezflash* (measured from inlet of riser, and
divided by length of riser) is estimated and reported in Table 2. For most of
the tests, it is expected that flashing should only occur in the upper region
or above the riser. Of course, in tests at high core-inlet resistance and
relatively high power, and also in tests with low RPV water level (and hence
lower pressure head), this basic calculation predicts flashing in lower
sections of the riser.These estimations do not take into account the 3D and
local effects that might affect the physical phenomena and the flow regime
actually occurring in the RPV. Some interesting information can be retrieved by
plotting some of the temperature profiles measured in the central axis of the
RPV. Two examples are given in Figure3, for which, a curve corresponding to
the saturation temperature is also plotted on each graph. The saturation
temperature has been estimated by using the time-averaged RPV pressure and
calculating the pressure head at each height, without considering the possible
presence of void.Measured temperature profiles in the vertical axis of the RPV.
(a)
Test H2.1(b)
Test H8.1From Figure3, it is clear that the
validity of the comparison with the saturation temperature may be limited due
to the error in temperature measurements. Due to the data reduction procedure,
this error slightly increases with temperature (Figure 3). The saturation
temperature decreases along the vertical axis as the system operates at low
pressure. Flashing should occur when the coolant reaches saturation conditions
in the riser. Following this approach, in test H2.1 (shown in Figure 3(a)), the
average height of the flashing boundary can be roughly estimated to be at about
the level corresponding to the top of the riser. In this case, flashing should
be responsible for the shape of the profile above this height, where measured
temperatures seem to lie in the vicinity of the saturation curve. This
basically confirms the previous coarse prediction (Table 2, zflash*=0.95). In test H8.1, a different situation is shown
(Figure 3(b)): the fluid temperature clearly following the saturation curve
indicates that two-phase flow should have occurred along the complete riser
length. To summarize, these different plots help to qualitatively distinguish
different cases and define three classes:Class 1.
flashing above the riser, or perhaps no flashing at all (Figure3(a));Class 2.
flashing at a lower elevation in the riser;Class 3.
two-phase flow all along the riser (Figure3(b)).
Rough estimations of time-averaged
void fraction values processed from differential pressure measurements also
indicate that the highest void fraction was assessed at the top of the riser,
and especially for cases with high core-inlet resistance/high power (Class 3).
These cases showed the largest difference between the swell level (measured by
the TDR probe) and the collapsed water level retrieved from differential
pressure measurements. For example, in tests HL5.3 and H8.3, a void fraction of
the order of 10% was estimated in the upper region of the riser. In the lower
sections, values lower than a few percent were found.Analysis of Flow Velocity Measurements in RPV Downcomer
The natural circulation flow rate is
of main interest. Hence, this short analysis concerns the velocity measurements
from 3 ultrasonic flow meters in the RPV DC. The sensors are located at
elevation 5.00 m above RPV bottom with an angle of 120° azimuthally between
each other.
The velocity signals were sampled at
a frequency of 0.5 Hz for the whole duration of the test (5 hours). It should
be noted that the time constant of the sensors was set to 6 seconds in order to
avoid aliasing problems. The autopower spectral density (APSD) of the signals
was calculated to identify resonance frequencies of the system. The autocorrelation
function (ACF) was used to calculate the decay ratio (DR) of the system. The DR
is a widely used parameter to quantify the stability of the system: ifDR<1,
the system is linearly stable; if DR>1, it is unstable. In practice, the DR
values presented in Table 2 were estimated by simply calculating the ratio
between the second and the first maxima of the ACF (Figure 4(c)). Cross-power spectral
densities, coherence, and cross-correlation functions were also calculated to
examine coherence, possible phase shifts, and so forth between signals from
different flow meters.
An overview of the results of the analysis
performed can be found in Table2. The basic parameters of each test are given
in the first columns of this table. Mean values of the velocities calculated
over the test period (V1, V2, and V3) are also
reported. V123 is the average of the three mean velocities. The
quantity I123 should provide an estimate of the intensity of the
velocity fluctuations, and was calculated as follows: (1)I123(%)=100.(Var123)1/2V123,where Var123 represents
the average of the 3 variances of the signals. It should be realized that this
relation could result in very high values for the tests with very low natural circulation
velocities, for which the signal-to-noise ratio is expected to be higher (e.g.,
test H2.3).
Concerning the spectral analysis, the frequency and the corresponding
period of the major peak are reported when possible. The value “0” in
the column “Δφ” of Table 2 means that no
significant phase shift between the signals was observed, that is, the three
velocity signals were oscillating in phase at one given frequency. When
possible, a DR was estimated from the ACF and is also reported in Table 2. One
column of the table refers to the analysis of signals from the sensor measuring
the total pressure in the RPV. These additional observations may help to
characterize some of the tests. Looking at the spectra and at the results of
this analysis based on the ACF and the estimation of DR values, the PANDA tests
can be categorized in two different groups as follows.Analysis of flow velocity signals in
the downcomer (Test L8.3).
(a)
Measured flow velocities in the downcomer(b)
Normalized power spectra of velocity signal(c)
ACF processed from one velocity signal1st Group
The rows in Table2 corresponding to these tests are
greyed. An illustration of this case is made using test L8.3 results. An
excerpt of the raw velocity signals recorded during this test is shown in
Figure 4(a). A major peak can be observed on the processed spectra of the 3
sensors reported in Figure 4(b). The time traces of the velocity signals show
that the three sensors oscillate “together,” that is, with no significant phase
shift between each other. This was confirmed by looking at the phase of the
complex cross-power spectral densities between the different signals. Just
simply looking at a number of raw signals recorded over the test period, it can
also be seen that the other measured variables (temperatures, pressure in the RPV,
condenser feed, and drain flows, etc.) show fluctuations with the same
frequency. The ACF obtained from velocity signal 1 is given in Figure 4(c).
Clearly, a DR value indicating stable behavior (DR=0.64) can be estimated from
this graph.2nd Group
An
illustration of this group is given using test B3.3 results. Observations made
on the raw signals (Figure5(a)) in this case show rather random behavior.
In some other tests belonging to the same group, it is sometimes possible to
observe significant phase shifts between the sensors. Figure 5(b) shows that
there is no longer a major peak present in the power spectrum. Using a
logarithmic scale, the low-frequency part of the spectrum appears to be flatter.
Moreover, the ACF does not really allow a DR value to be derived (Figure 5(c)),
this DR appearing to be much smaller (actually, very close to 0) compared to the
tests of the first group.Analysis of flow velocity signals in the
downcomer (Test B3.3).
(a)
Measured flow velocities in the downcomer(b)
Normalized power spectra of velocity signal(c)
ACF processed from one velocity signalIt was also observed that these two characteristic modes of behavior,
indicated by the ACF plots, did not vary as a function of time during the
course of the tests. For tests L8.3 and B3.3, this was done by calculating ACFs
over 5 successive (1-hour long) time periods and it clearly showed that, in
both cases, all the ACFs for one test at different times were very similar.Some data from other sensors
(condenser feed and drain flow rates, RPV pressure, etc.) have also been analyzed
using the same approach as for the velocity data. For the tests from the 1st group,
a resonance peak can be observed at the same oscillation frequency. No peak can
be found for the tests belonging to the 2nd group. However, in some of these
tests (e.g., test B3.3), the condenser feed flow and RPV pressure signals may
exhibit a peak in their spectrum whereas no such peak was observed for the
velocity signals (see also the second last column of Table2, concerning the
analysis of RPV pressure signals). This indicates that flashing possibly takes
place above the top of the riser, not influencing the main circulation flow.
These cases fall consistently into Class 1 (see Figure 3).From the expected mechanism of the
flow oscillations, the fluid travelling time in the heated section, and the
transit time of the enthalpy perturbations in the adiabatic section, should
play a role in determining the period of the oscillations. From Figure11, it
is clear that the oscillation period decreases with increasing average flow
rate. The total transit time can be evaluated as the sum of the transit time in
the riser section plus half of the transit time in the heated section [3]. In
PANDA, no velocity measurement is available in the riser. An estimation of the
riser velocity was made by simply using the time-averaged value of the velocity
in the DC reported in Table 2, and just considering the flow area ratio between
the riser and the DC.Following this simplified approach, the oscillation period was found to
be in the order of 1.6 times the transit time of the fluid (Figure6). The
simple way in which the transit time was estimated did not consider the
occurrence of two-phase flow in the riser. Hence, it is likely that our
assessment overestimated this time. Thus, these observations seem to be quite
consistent with those presented for the CIRCUS facility [1], for which the
period of oscillation was found to be twice the transit time.Figure 6
Period of oscillation versus fluid transit time in core and riser.Discussion
The differentiation between the two
groups of tests is based on DR values derived from the velocity signals. However,
the validity and the accuracy of this method can be questioned. Many other
methods have been used to extract stability parameters and can be compared to
the ACF-based method. For this reason, some of our experimental velocity time
traces were processed using a more sophisticated method, namely, the
autoregressive method. This method properly applied to our signals yielded
about the same DR values as those presented in Table2. This confirms the main
result from this analysis that all tests are stable.However, by having a closer look to the tests, the different flow
regimes that can be assumed, and the three classes defined, can be related to
the previous findings regarding the stability of the system. Information
reported in Table2 was used for this purpose. The plots in Figure 3 partly
illustrate the following classification.Class 4 (Figure3(a)).
High inlet subcooling and relatively low (or even very
low) flow rates characterize this class. It is likely that flashing occurred
only above the riser, the natural circulation flow still being in a
single-phase regime. This would explain why, in some of these tests, contrary
to the velocity signals in the DC, some RPV pressure signals show a peak in
their spectrum and allow the derivation of DR values (see, e.g., test B5.2). It
is also possible that in some of these tests (notably those performed at the
lowest powers), no flashing occurred at all in the whole system.Class 5.
These tests, performed at relatively high
power and low inlet resistance, have been characterized by a well-identified
oscillation period and DR values could be derived from the velocity signals.
The highest natural circulation flow rates were measured in these tests, which
indicate the combined effect of the flow enhancement by steam production and of
a low inlet resistance. According to our estimations and analyses, no boiling
occurred in the core but flashing did in the upper half of the riser.Class 6 (Figure3(b)).
Very stable behavior was found
for these tests performed with an increased inlet resistance. Under these
conditions, the flashing front fluctuates close to the bottom of the riser,
which stabilizes very much the two-phase natural circulation. The measured flow
rates are among the lowest of the test series, as a result of the high flow
resistance. Looking at the parameters (notably the expected position of the
boiling boundary) reported in Table2 and at the characteristic temperature
profiles (Figure 3(b)), it seems that stable two-phase natural circulation flow
can be assumed in these cases.The presented classification gives a
rather coherent picture of the PANDA test series. It was possible to link the
experimental observations with the expected phenomenology. No unstable tests
were recorded. However, the characterization of the tests (from single-phase to
two-phase flow conditions) is satisfying and quite consistent with what is
reported in the literature. Test BLL5.3, which shows a DR of about 0.85, should
be very close to unstable conditions.The test matrix allowed the variation
of the RPV water level. However, the actual influence of this parameter on the
flow behavior cannot be clearly assessed. Comparing tests B5.3 and BL5.3, it
seems that a lower level would have a stabilizing effect.


## 3. ATHLET Simulations

### 3.1. Thermal-Hydraulic Model

The thermal-hydraulic code
ATHLET [4], which has been developed by GRS (Gesellschaft für
Anlagen- und Reaktorsicherheit mbH), was used for the calculation of selected experiments
from the PANDA test matrix (see Section 2). The ATHLET input dataset models
all main parts of the PANDA configuration used for the NACUSP natural circulation
experiments (Figure 7).Figure 7
Schematic representation (a) and ATHLET model (b) of PANDA test facility.The ATHLET model consists of the lower plenum (P1-LP-1), the core
section (P1-CORE) with 115 electrical heater elements, the riser (P1-RIS1),
upper plenum (P1-UP-1), downcomer (P1-DC), and the upper part of the RPV
(P1-RIS2). The isolation condenser (IC) is not modeled. Therefore, a bypass
(CIRC-ENT, CIRC-EIN) is connected to the upper part of the RPV and to the downcomer,
modeling the IC-feed and drain lines by fills with constant mass flows and
given enthalpy (only drain line). In all control volumes, the 5-equation model
(separate conservation equations for liquid and vapour mass and energy, mixture
momentum equation) and the full-range drift-flux model of ATHLET are used. A
valve at the lower end of the downcomer (VLV, see Figure7) is used to model
different k-factors of the core inlet. The cross-section of this valve can be
changed to adjust the core inlet flow resistance.


### 3.2. Steady State Calculation

At first, a steady state calculation with constant boundary conditions
was performed for each PANDA test. With help of these calculations, the
pressure losses, heat losses, the RPV water level, and the temperature
distribution were adjusted. The steady state calculation starts with zero
power. After a few seconds, the core power is switched on and the power is increased
with time. For the calculations, it was not possible to specify a certain
k-factor for the core inlet, because the form-loss coefficients are changed within the
algorithm of the ATHLET code during the steady state calculation. Therefore, a
valve at the lower end of the downcomer with changing cross-section was used to
adjust the core-inlet flow resistance. The cross-section of this valve was
reduced to a value, which leads to DC velocities corresponding to the measured
data. The steady state calculation is stopped if stable conditions are reached. As an example, the
results of the steady state calculation for Test B 8.3 and the corresponding experimental
data are shown in Figure8.Core power (MW.RP.7), system pressure (MP.RP.1), RPV level (ML.RP.1), and downcomer velocity (MVE.DC.1) for test B 8.3: experiment and ATHLET steady state calculation.
(a)(b)(c)(d)


### 3.3. Transient Calculations

From the PANDA test matrix, 5 experiments were selected for the ATHLET
simulations. Table3 shows the initial conditions calculated with ATHLET in
comparison to the measured data. For the experiments the data were recorded
over a period of 5 hours. The experimental data in Table 3 represent average
values. The initial conditions for the ATHLET simulations correspond to the end
of the steady state calculations.Table 3
Initial conditions for the selected PANDA tests (Exp. = Experiment, Cal.
= ATHLET Calculation, Add. Calc. = Additional
ATHLET calculation).
TestPower   MW.RP.7RPV pressureRPV levelDC velocityDC temperature(kW)MP.RP.1 (bar)ML.RP.1 (m)MVE.DC.1 (m/s)MTL.RP.1 (°C)Exp.Cal.Exp.Cal.Exp.Cal.Exp.Cal.Exp.Cal.B3.35025022.972.9812.7812.780.190.19133.7133.4B5.26376375.045.0612.8412.850.220.22152.1152.3L5.39099095.045.0512.8212.830.320.32152.0152.3LL5.39119115.025.0211.1111.230.380.38151.5152.1B8.3130613067.987.9912.8012.860.520.52169.7170.5Add. Calc.—1308—4.9—11.43—0.60—151.2Each transient calculation starts from a steady state calculation as
described in Section3.2. To simulate the system behavior with respect to natural
circulation stability a short disturbance of the drain line mass flow was used
in order to stimulate oscillations in the loop (see Figure 9). The response of
the system can be seen in the calculated riser mass flow.Mass flow in IC-feed and drain lines (CIRC-EIN, CIRC-ENT) and riser mass flow (P1-RIS1).
(a)(b)As in the experiments, the DC velocities were used to calculate the spectrum
and the decay ratio. Figure10 shows the downcomer velocity, the spectrum, and
the autocorrelation function for two ATHLET calculations (Test B 8.3 and Test B
3.3). Both calculations predict stable behavior and no limit-cycle oscillations
occur. The disturbance of the drain line mass flow leads to flow oscillations
with decreasing amplitude. In both cases, the oscillations were damped due to
the higher core-inlet loss coefficient (k=30) used in the B-series experiments.
Although the oscillations were damped in the ATHLET simulations, an oscillation
period and also a decay ratio can be calculated from the DC mass flow. Table 4
gives a comparison between measured and calculated period and decay ratio for
the selected PANDA tests.Table 4
Oscillation period and decay ratio calculated from the DC mass flow (Exp. = experiment, Cal. = ATHLET calculation).
TestPeriod  (s)Decay ratio  (–)Exp.Cal.Exp.Cal.B3.3200.0204.8—0.66B5.2154.0170.7—0.61L5.3120.0113.80.680.62LL5.390.0102.40.740.55B8.363.068.30.610.60DC velocity, spectrum, and autocorrelation function for
the ATHLET calculations of tests B 8.3 and
B 3.3.
(a)(b)(c)Figure 11
Oscillation
period as a function of the DC velocity for the PANDA tests and the ATHLET
simulations.The experiments and also the ATHLET calculations show that the
oscillation period decreases with increasing DC velocity. This behavior is
illustrated in Figure11. The results of the ATHLET calculations show a good
agreement with the experimental data.To demonstrate the ability of ATHLET to calculate an unstable behavior
with the PANDA model, an additional calculation with a core-inlet loss
coefficient ofk=7, a higher core power, and a lower RPV level was performed.
The initial conditions for this calculation are given in Table 3. The results
of the simulation show an unstable behavior with strong oscillations caused by
flashing in the riser section. Figure 12 shows the calculated DC mass flow, the
spectrum, and the autocorrelation function. The oscillations have a period of
62 seconds with a decay ratio > 0.9.DC velocity, spectrum, and autocorrelation function for the additional ATHLET
calculation with higher core power and lower RPV level.
(a)(b)(c)In Figure13, the calculated temperature distribution over the height of
the PANDA model is illustrated for t=5000 seconds. At this time, the dynamic
behavior becomes unstable. The subcooling in the core is high and no void
production takes place. Only above the middle of the riser section, the
temperature reaches saturation conditions and flashing-induced oscillations
occur.Figure 13
Temperature distribution over the height of the PANDA model (additional ATHLET calculation).


## 3.1. Thermal-Hydraulic Model

The thermal-hydraulic code
ATHLET [4], which has been developed by GRS (Gesellschaft für
Anlagen- und Reaktorsicherheit mbH), was used for the calculation of selected experiments
from the PANDA test matrix (see Section 2). The ATHLET input dataset models
all main parts of the PANDA configuration used for the NACUSP natural circulation
experiments (Figure 7).Figure 7
Schematic representation (a) and ATHLET model (b) of PANDA test facility.The ATHLET model consists of the lower plenum (P1-LP-1), the core
section (P1-CORE) with 115 electrical heater elements, the riser (P1-RIS1),
upper plenum (P1-UP-1), downcomer (P1-DC), and the upper part of the RPV
(P1-RIS2). The isolation condenser (IC) is not modeled. Therefore, a bypass
(CIRC-ENT, CIRC-EIN) is connected to the upper part of the RPV and to the downcomer,
modeling the IC-feed and drain lines by fills with constant mass flows and
given enthalpy (only drain line). In all control volumes, the 5-equation model
(separate conservation equations for liquid and vapour mass and energy, mixture
momentum equation) and the full-range drift-flux model of ATHLET are used. A
valve at the lower end of the downcomer (VLV, see Figure7) is used to model
different k-factors of the core inlet. The cross-section of this valve can be
changed to adjust the core inlet flow resistance.


## 3.2. Steady State Calculation

At first, a steady state calculation with constant boundary conditions
was performed for each PANDA test. With help of these calculations, the
pressure losses, heat losses, the RPV water level, and the temperature
distribution were adjusted. The steady state calculation starts with zero
power. After a few seconds, the core power is switched on and the power is increased
with time. For the calculations, it was not possible to specify a certain
k-factor for the core inlet, because the form-loss coefficients are changed within the
algorithm of the ATHLET code during the steady state calculation. Therefore, a
valve at the lower end of the downcomer with changing cross-section was used to
adjust the core-inlet flow resistance. The cross-section of this valve was
reduced to a value, which leads to DC velocities corresponding to the measured
data. The steady state calculation is stopped if stable conditions are reached. As an example, the
results of the steady state calculation for Test B 8.3 and the corresponding experimental
data are shown in Figure8.Core power (MW.RP.7), system pressure (MP.RP.1), RPV level (ML.RP.1), and downcomer velocity (MVE.DC.1) for test B 8.3: experiment and ATHLET steady state calculation.
(a)(b)(c)(d)


## 3.3. Transient Calculations

From the PANDA test matrix, 5 experiments were selected for the ATHLET
simulations. Table3 shows the initial conditions calculated with ATHLET in
comparison to the measured data. For the experiments the data were recorded
over a period of 5 hours. The experimental data in Table 3 represent average
values. The initial conditions for the ATHLET simulations correspond to the end
of the steady state calculations.Table 3
Initial conditions for the selected PANDA tests (Exp. = Experiment, Cal.
= ATHLET Calculation, Add. Calc. = Additional
ATHLET calculation).
TestPower   MW.RP.7RPV pressureRPV levelDC velocityDC temperature(kW)MP.RP.1 (bar)ML.RP.1 (m)MVE.DC.1 (m/s)MTL.RP.1 (°C)Exp.Cal.Exp.Cal.Exp.Cal.Exp.Cal.Exp.Cal.B3.35025022.972.9812.7812.780.190.19133.7133.4B5.26376375.045.0612.8412.850.220.22152.1152.3L5.39099095.045.0512.8212.830.320.32152.0152.3LL5.39119115.025.0211.1111.230.380.38151.5152.1B8.3130613067.987.9912.8012.860.520.52169.7170.5Add. Calc.—1308—4.9—11.43—0.60—151.2Each transient calculation starts from a steady state calculation as
described in Section3.2. To simulate the system behavior with respect to natural
circulation stability a short disturbance of the drain line mass flow was used
in order to stimulate oscillations in the loop (see Figure 9). The response of
the system can be seen in the calculated riser mass flow.Mass flow in IC-feed and drain lines (CIRC-EIN, CIRC-ENT) and riser mass flow (P1-RIS1).
(a)(b)As in the experiments, the DC velocities were used to calculate the spectrum
and the decay ratio. Figure10 shows the downcomer velocity, the spectrum, and
the autocorrelation function for two ATHLET calculations (Test B 8.3 and Test B
3.3). Both calculations predict stable behavior and no limit-cycle oscillations
occur. The disturbance of the drain line mass flow leads to flow oscillations
with decreasing amplitude. In both cases, the oscillations were damped due to
the higher core-inlet loss coefficient (k=30) used in the B-series experiments.
Although the oscillations were damped in the ATHLET simulations, an oscillation
period and also a decay ratio can be calculated from the DC mass flow. Table 4
gives a comparison between measured and calculated period and decay ratio for
the selected PANDA tests.Table 4
Oscillation period and decay ratio calculated from the DC mass flow (Exp. = experiment, Cal. = ATHLET calculation).
TestPeriod  (s)Decay ratio  (–)Exp.Cal.Exp.Cal.B3.3200.0204.8—0.66B5.2154.0170.7—0.61L5.3120.0113.80.680.62LL5.390.0102.40.740.55B8.363.068.30.610.60DC velocity, spectrum, and autocorrelation function for
the ATHLET calculations of tests B 8.3 and
B 3.3.
(a)(b)(c)Figure 11
Oscillation
period as a function of the DC velocity for the PANDA tests and the ATHLET
simulations.The experiments and also the ATHLET calculations show that the
oscillation period decreases with increasing DC velocity. This behavior is
illustrated in Figure11. The results of the ATHLET calculations show a good
agreement with the experimental data.To demonstrate the ability of ATHLET to calculate an unstable behavior
with the PANDA model, an additional calculation with a core-inlet loss
coefficient ofk=7, a higher core power, and a lower RPV level was performed.
The initial conditions for this calculation are given in Table 3. The results
of the simulation show an unstable behavior with strong oscillations caused by
flashing in the riser section. Figure 12 shows the calculated DC mass flow, the
spectrum, and the autocorrelation function. The oscillations have a period of
62 seconds with a decay ratio > 0.9.DC velocity, spectrum, and autocorrelation function for the additional ATHLET
calculation with higher core power and lower RPV level.
(a)(b)(c)In Figure13, the calculated temperature distribution over the height of
the PANDA model is illustrated for t=5000 seconds. At this time, the dynamic
behavior becomes unstable. The subcooling in the core is high and no void
production takes place. Only above the middle of the riser section, the
temperature reaches saturation conditions and flashing-induced oscillations
occur.Figure 13
Temperature distribution over the height of the PANDA model (additional ATHLET calculation).


## 4. Conclusions

In the PANDA facility, 25 tests have
been carried out in order to investigate natural circulation flow behavior and
stability, under low-pressure/low-power conditions. Some parameters influencing
the natural circulation flow were varied, such as the RPV power and pressure,
the core-inlet hydraulic resistance, and also the water level in the RPV.An analysis of the data from three ultrasonic sensors installed at three
circumferential locations in the downcomer was presented. The power spectra
show, in a few tests, a major and unique resonance peak. The period of
oscillation of the flow rate seems to be of the order of twice the transit time
of the fluid in the riser. Analyses of the time series show that these cases
are stable. The tests for which more flat spectra were obtained show a behavior
that is even more stable.A phenomenological classification has been applied according to three
circulation modes that could be assumed: single-phase circulation (with
possibly flashing above the riser), two-phase circulation with flashing in the
upper half of the riser, or two-phase flow along the complete riser length. The new experimental data about the behavior of the natural circulation flow
under low-pressure conditions represent a
valuable extension of the available database, notably because the height of the
riser in PANDA is approximately preserved with respect to a real reactor. The
data have been used for assessing the capabilities and limitations of
thermal-hydraulics codes in predicting natural circulation characteristics at
low pressure/low power.The ATHLET simulations show stable behavior for the
selected PANDA tests. In all simulations, the oscillations were damped and no
limit-cycle oscillations occur. The calculated periods and decay ratios show a
good agreement with the experimental results. With help of the additional
calculation, it could be demonstrated that ATHLET can also simulate an unstable
behavior. In this case, strong oscillations occur, caused by flashing in the
riser section.



---

*Source: 874969-2008-04-30.xml*