Measurements of Sheath Temperature Profiles in ...

Proceedings of ICONE14 International Conference on Nuclear Engineering July 17-20, Miami, Florida, USA Proceedings of ICONE 14 14th International Conference on Nuclear Engineering July 17-20, 2006, Miami, Florida, USA

ICONE14-89621

ICONE14-89621 MEASUREMENTS OF SHEATH TEMPERATURE PROFILES IN BRUCE LVRF BUNDLES UNDER POST-DRYOUT HEAT TRANSFER CONDITIONS IN FREON Y. Guo, D.E. Bullock, I.L. Pioro and J. Martin Chalk River Laboratories, Atomic Energy of Canada Limited Chalk River, Ontario, Canada K0J 1P0 [email protected], [email protected], [email protected], [email protected] temperature obtained in the test, at an overpower level of 64%, was significantly below the acceptable maximum temperature, indicating that the integrity of the Bruce LVRF will be maintained at PDO conditions. Therefore, the Bruce LVRF exhibits good PDO heat transfer performance.

ABSTRACT An experimental program has been completed to study the behaviour of sheath wall temperatures in the Bruce Power Station Low Void Reactivity Fuel (shortened hereafter to Bruce LVRF) bundles under post-dryout (PDO) heat-transfer conditions. The experiment was conducted with an electrically heated simulator of a string of nine Bruce LVRF bundles, installed in the MR3 Freon heat transfer loop at the Chalk River Laboratories (CRL), Atomic Energy of Canada Limited (AECL). The loop used Freon R-134a as a coolant to simulate typical flow conditions in CANDU® nuclear power stations. The simulator had an axially uniform heat flux profile. Two radial heat flux profiles were tested: a fresh Bruce LVRF profile and a fresh natural uranium (NU) profile. For a given set of flow conditions, the channel power was set above the critical power to achieve dryout, while heater-element wall temperatures were recorded at various overpower levels using sliding thermocouples. The maximum experimental overpower achieved was 64%. For the conditions tested, the results showed that initial dryout occurred at an inner-ring element at low flows and an outer-ring element facing internal subchannels at high flows. Dry-patches (regions of dryout) spread with increasing channel power; maximum wall temperatures were observed at the downstream end of the simulator, and immediately upstream of the mid-bundle spacer plane. In general, maximum wall temperatures were observed at the outer-ring elements facing the internal subchannels. The maximum water-equivalent

INTRODUCTION Low void reactivity fuel (LVRF) has been designed at Atomic Energy of Canada Limited (AECL) for Bruce Power Station to improve the performance of existing CANDU reactors. The benefits of implementing the Bruce LVRF bundle, compared to the current 37-element bundle, include reductions in coolant void reactivity and element power rating, and improvement in critical channel power (CCP). The Bruce LVRF bundle uses the CANFLEX® bundle design, which consists of 43 elements in four rings, with the outer and intermediate rings comprising 21 and 14 elements and the inner ring and centre rod comprising 7 and 1 elements. The three outer rings are fuelled with slightly enriched uranium, and the centre element is fuelled with a mixture of natural uranium and dysprosium (a neutron poison). Prior to implementation of the Bruce LVRF into reactor cores, various thermalhydraulic parameters are required for qualifying the Bruce LVRF design. Fuel sheath temperature under post-dryout (PDO) heat transfer conditions is one of the key parameters that need to be quantified. CANFLEX® – (CANDU Flexible) is a registered trademark of AECL and the Korea Atomic Energy Research Institute (KAERI).

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CANDU – Canada Deuterium Uranium (a registered trademark of AECL). 1

Copyright © 2006 by ASME

In support of the quantification of the Bruce LVRF sheath temperature behaviour under PDO heat transfer conditions, an experiment was completed to obtain PDO wall-temperature measurements of a horizontal, Freoncooled, electrically heated, axially-uniform bundle string simulating a Bruce LVRF fuel string in a reference (uncrept) channel. Two radial power profiles were tested: a fresh LVRF profile and a fresh natural uranium (NU) fuel profile (which is similar to the discharge LVRF profile). The experimental data were analysed, and the temperature profiles obtained with the fresh NU fuel profile were compared with those obtained with the fresh LVRF fuel profile to examine the effect of radial power profile on fuel sheath temperature behaviour. The objectives of this paper are to describe the Freon PDO tests performed and to present typical results. Figure 1 Schematic Diagram of the MR-3 Heat Transfer Loop

FREON LVRF-BUNDLE PDO TESTS Previous studies have shown that PDO heat transfer is similar between axially uniform-heated and axially nonuniform-heated bundles in water and Freon flow [1]. Therefore, an axially uniform-heated LVRF bundle simulator was used in the experiment. The simulator was installed in the horizontal test station of the MR-3 heat-transfer loop at CRL. The loop, test station, and bundle simulator are briefly described in the following sections.

Horizontal Test Station The horizontal test station was constructed with an 8-m-long carbon steel pipe of 15-cm in diameter. It housed the flow tube and bundle string. A gap was maintained between the flow tube and the steel pipe to provide a path for pressure and differential pressure impulse lines. The gap was blocked off at the downstream end of the channel to restrict the coolant flow to the inside of the flow tube where the bundle simulator was located. The coolant entered the test station near the upstream end. It circulated around the external side of the flow tube and entered the flow tube at the upstream end. This eliminated the cross flow at the entry point and reduced entrance effects. The coolant was discharged from the flow tube at the downstream end of the test station. It circulated around the external side of the flow tube and left the test station near the downstream end. A schematic diagram of the horizontal test station is shown in Figure 2.

MR-3 Heat-Transfer Loop A schematic diagram of the MR-3 loop is shown in Figure 1. Refrigerant-134a (R-134a) was employed as working fluid to simulate PDO heat transfer in water. Subcooled fluid was delivered to the test station through two recirculating pumps. A pre-heater (or cooler) located downstream of the pumps was used to control the test-station inlet temperature. The fluid was heated when it flowed over the heated bundle simulator in the horizontal test station (the vertical test station was not used in this experiment). The two-phase mixture discharged from the test station was then directed to a vapour drum, which was used to control the system pressure, where the vapour and liquid fluid were separated. While the vapour was sent to condensers for condensing, the liquid fluid was re-circulated back to the loop through pumps.

The flow tube was made of composite epoxy fibreglass, and acted as an electric insulator between the directly heated bundle simulator and carbon-steel test station. The tube inside diameter corresponded to the design value of reactor pressure tubes. Fourteen pressure taps were installed along the flow tube for local pressure measurements (see Figure 2 for details).

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Figure 2 Schematic Diagram of the Horizontal Test Station with Pressure Tap Locations Spacer pads were installed at the middle plane of each element to maintain inter-element gaps. Non-insulating pads were used in gaps between elements in the same ring, and insulating pads in gaps between elements in neighbouring rings for electrical insulation. Using insulating pads and cross-webs at the endplate between rings allowed the use of an external resistor bank (described in the next section) to adjust the current passing through each ring of elements. This provided the flexibility to change the radial power distribution of the bundle to any desired profile.

LVRF Bundle Simulators A bundle string was designed and fabricated to simulate an aligned string of nine LVRF bundles including bundle junction and appendages (i.e., spacers, bearing pads, and buttons). It consisted of 43 heater rods made from Inconel-718 tubes with two different outer diameters for the centre rod and elements in the inner ring and for elements in the intermediate and outer rings. Figure 3 shows cross-sectional dimensions of the LVRF bundle. Power to the bundle simulator was provided by means of joule heating. Nickel-plated copper-spool pieces were used to join the heater rods forming a nine-segment bundle string. The connection facilitated electric current passing through the string from one end to the other. Each segment had an overall physical length of 500 mm nominal, and a heated length of 480 mm nominal. A copper tube was brazed to each end and extended the element string beyond the length of the test station for power connection (see Figure 2). The downstream three segments of the string (identified as Bundles “A”, “B” and “C” in Figure 2) were equipped with sliding thermocouple assemblies. A connecting rod linked all assemblies in each element together and extended outside the bundle string for the experimenter to control the movement of thermocouples.

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Element looking upstream

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Figure 3 Cross-Sectional View of LVRF Bundle Simulator (Looking Upstream)

All element strings were held together by endplate assemblies. Cross-webs between rings in the endplate were made of fibreglass to electrically isolate each ring. 3


CHF BUTTONS

inside the heater elements that allowed axial and circumferential positioning. The loop pressure, temperature and flow were monitored and controlled using the PARAGON TNT software through MTL high-speed analog and digital scan modules.

BEARING PADS

All elements in the last three bundles, “A”, “B” and “C” were instrumented with sliding thermocouples for an indication of wall temperatures and dryout. The thermocouple carriers were rotated over the complete circumference (i.e., 360º) of the elements and traversed along the bundle to detect dryout at various planes of the bundle string. These thermocouple carriers were connected to an automated drive mechanism, which controlled their movements. During each scan of the element internal-wall temperature, the thermocouples traversed axially along bundles at a rate of approximately 1 mm per second until the downstream end of the bundle-heated length was reached. The thermocouples were then rotated 10 degrees and axial travel was activated to return the thermocouples to the downstream position, then again rotated 10 degrees. This zigzag travel pattern was continued until the full circumference of the element (i.e., 360°) had been covered by the two thermocouples mounted in each of the elements. The angular position of the thermocouple was measured clockwise, as viewed looking upstream, with 0 degrees referenced at the bottom (Figure 3).

Figure 4 LVRF Bundle Simulator Bearing pads were spot-welded to elements in the outer ring at locations corresponding to the LVRF bundle design, as illustrated in Figure 3 and Figure 4. A stainless-steel spring was attached to each upstream bearing pad on the five top elements to maintain the bundle in eccentric top-to-bottom position inside the flow tube. Examination of the bundle simulator geometry showed that the external dimensions of the bundle segment and appendages were consistent with the LVRF bundle design. External Resistor Bank A variable-resistor bank was used to adjust the radial power profile in the bundle simulator. The resistor bank consisted of four adjustable resistors for the three rings and centre element in the simulator. Each resistor was connected in series to a current shunt and to the appropriate element ring. Moveable power clamps were connected to the test section and adjusted to provide the desired resistance to each of the element rings. Two settings of these clamps were used to simulate the radial power profiles of the fresh NU fuel and LVRF. The voltage drop and current through each ring were measured, providing an accurate measurement of power generated in each element ring.

Test Conditions The test covered flow conditions of interest to CANDU. Table 1 lists the tested flow conditions in Freon. The water-equivalent values, converted with fluid-to-fluid modelling parameters [2], are provided in brackets. The maximum bundle string overpower was 64%, with respect to bundle string dryout power (i.e., the power at which initial dryout occurred).

Instrumentation and Data Acquisition The MR-3 loop, test station, and bundle simulator were instrumented for temperature, flow rate, pressure, differential pressure and power measurements. Calibrated resistance temperature devices (RTDs) were installed at both the inlet and outlet ends of the test section. Calibrated pressure transmitters were used to measure the inlet and outlet pressures. Fourteen differential pressure transducers were used to measure pressure drops at various sections along the channel. Figure 2 illustrates the arrangement of pressure taps and differential-pressure cells. Test-section flow rates were measured with two in-series turbine flow meters. Individual ring powers were determined using the current shunts and a custom-designed electronic instrument. The test-section power was calculated using the voltage drop and the current through all rings. PDO wall temperatures were obtained with movable thermocouples (i.e., sliding thermocouples) installed

Table 1Freon Post-Dryout Test Conditions* Radial Power Profile

Fresh NU Fuel

Fresh LVRF

Outlet Pressure (MPa)

Overpower

12.20 (17.00)

Inlet Temp. (°C) 43.6 (274.0)

1.475 (9.00)

16%, 32%, 48%, 64%

3561Dxxx

12.20 (17.00)

45.0 (269.0)

1.832 (11.00)

6%, 12%, 18%, 24%

3E71Dxxx

18.69 (26.00)

49.4 (281.0)

1.832 (11.00)

8%, 16%, 24%, 32%

2552Dxxx

12.20 (17.00)

43.6 (274.0)

1.475 (9.00)

16%, 32%, 48%, 64%

3562Dxxx

12.20 (17.00)

45.0 (269.0)

1.832 (11.00)

5%, 10%, 15%

3E72Dxxx

18.69 (26.00)

49.4 (281.0)

1.832 (11.00)

8%, 15%, 22%, 29%

Run ID

Nominal Flow (kg/s)

2551Dxxx

*

Numbers in brackets present the water-equivalent values. “xxx” in the Run ID represents overpower in percentage.

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Each series of PDO runs was preceded by an initial dryout run to obtain the initial reference dryout data. In general, four overpower runs of approximately equal overpower increments were conducted for each PDO run series. Repeat PDO runs were performed for selected test conditions to verify the repeatability of the tests. A total of six initial dryout runs and fifty overpower runs were completed.

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BP & SP

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BT 120

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BT

BP

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Wall Temperature (°C)

FLOW

BP 60

BT 120

BP & SP 180

240

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360

BP 420

480

Wall-temperature values of elements with no or faulty thermocouples were supplemented with those of another element at the symmetrical position with reference to the vertical centre axis. The vertical symmetry of data was checked through comparisons of available symmetrical element data. The temperature scans showed that dryout initiated at the downstream end of the bundle string spread upstream with increasing overpower. At high overpowers (greater than 30%), four wall-temperature peaks, with two at each half bundle, were observed along the bundle (Figure 6 and Figure 7). The first peak was located at the bundle downstream end where the highest enthalpy (or quality) was encountered. The second peak was observed at locations immediately upstream of the button plane in the downstream-half bundle (refer to Figure 4). The reduction in wall temperature at locations downstream of the button plane was due to the heat transfer enhancement of the buttons. Strong flow turbulence induced by the middle-plane spacers led to the second reduction in wall temperature, forming the third peak at locations upstream of the spacers. The bundle upstream button plane generated the third reduction in wall temperature, leaving the fourth peak at locations upstream of the button plane. This peak did not appear at low overpowers (less than 30% overpower, as shown in Figure 5 and Figure 8. In addition to the above four peaks, wall temperature reductions were observed downstream of the bearing-pad planes. The reductions were the result of bearing-pad enhancement on heat transfer. The above temperature variation is a typical temperature distribution; it demonstrates the consistency of the measured temperatures with the bundle configuration.

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BP 420

140

Figure 6 Illustration of Scanned Wall Temperature Over Element 01 at 48% Overpower

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Axial Location (mm)


FLOW

100

BP 0

150 Element 01 (Run 2551D016 Scan 2433)

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In the Freon PDO tests, the wall temperature scan was taken using a “zigzag” pattern, as explained in the previous section. The wall-temperature data was meshed into fine grids of 2 mm in the axial direction and 10° in the radial direction. Examinations of the wall temperature scans were performed on an element-by-element basis to identify data from faulty thermocouples and data showing peculiar trends. Figure 5 and Figure 6 illustrate typical wall temperature distributions at every 20° around the circumference of Element 01 at 16% and 48% overpowers, respectively.

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Overall Examination

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Element 01 (Run 2551D048 Scan 2387)

The measured PDO heat transfer data were reduced and processed to provide relevant parameters for the analysis of PDO heat transfer. The data reduction and processing include examination of parametric trends, meshing of wall-temperature scans, generation of complete wall-temperature maps, extraction of maximum wall-temperature data, conversion of internal-wall temperatures to external-wall temperatures, calculation of local parameters at locations of interest and conversion of Freon data to water-equivalent values.

20

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150

FREON PDO HEAT TRANSFER DATA

0

0

480

Axial Location (mm)

Figure 5 Illustration of Scanned Wall Temperature Over Element 01 at 16% Overpower 5


Bundle A, Element 01 (Run 2551D048, Scan 2387, 48% overpower)

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Axial distance from end of heated length (mm)

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380

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Flow Direction

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Wall temperature (C)

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Angle (deg.)

Figure 7 Illustrated Contour Map of Wall Temperature for Element 01 at 48% Overpower


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Flow Direction

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Figure 8 Illustrated Contour Map of Wall Temperature for Element 01 at 16% Overpower 6


Wall temperatures varied with circumferential location as well. Figure 9 illustrates a circumferential walltemperature variation at the 270-mm plane (from the downstream end of the bundle, i.e., 30 mm upstream of the middle plane) of Element 01 at various overpowers. Maximum wall temperatures were circumferentially located in the sections facing internal subchannels (i.e., around 180° for Element 01, refer to Figure 3) for low overpowers (less than or equal to 16%). Within the tested conditions, no wrap-around dry-patch was observed at any single element for overpowers below 16%. At high overpowers, wall temperatures at various circumferential locations became similar. The same trend was observed in all test runs.

140 Run 3561D012, Scan 2370


100

80

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Element 10, Circumferential 290° Scan 2370: P = 1.831 MPa, Flow = 12.22 kg/s, Tin = 45.0°C, OP = 12% Scan 2612: P = 1.831 MPa, Flow = 12.22 kg/s, Tin = 45.0°C, OP = 12%

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Axial Location (mm)

Two repeat runs were performed to verify the repeatability of the PDO heat transfer data. Figure 10 compares the axial temperature distributions between Scan 2370 and its repeat Scan 2612 for Element 10 at circumferential angle of 290° where the maximum temperatures were observed. The two temperature distributions overlapped each other, indicating good test repeatability.

Figure 10 Comparison of Wall Temperature for Scan 2370 and its Repeat Scan 2612 Maximum PDO Wall Temperature Bundle maximum wall temperatures corresponding to the internal-wall values and their corresponding locations were extracted from the instrumented elements for analysis of the minimum PDO heat transfer. The extracted temperatures were each confirmed by means of examining the temperature distribution in the bundle. This confirmation process assured that no outliers were picked up for the maximum wall temperatures. The confirmed temperatures were subsequently converted to external wall temperatures using a one-dimensional heat conduction equation.

The measured wall temperature profiles, together with the corresponding flow conditions, were converted to water-equivalent values using fluid-to-fluid-modelling technique. Pressures, heat fluxes and qualities were converted using the modelling parameters described in [2]. Mass fluxes and heat transfer coefficients were converted based on Reynolds number and Nusselt number, respectively. Wall temperatures were calculated from heat flux and heat transfer coefficient. Figure 11 illustrates a contour map of element wall temperature in water-equivalent values.

The following observations were obtained from the maximum wall-temperature data. Initial dryout occurred at the downstream end of the bundle string. For the bundle string with a fresh NU fuel profile, initial dryout occurred at the inner-ring element (low flows) and the outer-ring element facing internal subchannels (high flows). For the bundle string with a fresh LVRF profile, initial dryout occurred at the outer-ring element facing internal subchannels.

150


Flow

Repeat run, Scan 2612 120

100

In general, the bundle maximum wall temperatures were observed at upstream locations of the downstream end and the spacer plane. 50

Run 2551, Element 01

The maximum wall temperatures were primarily observed at outer-ring elements in the bundle. The maximum wall temperatures were circumferentially located in the sections facing internal subchannels for low overpowers (less than or equal to 16%). At high overpowers for a few cases, the maximum wall temperature was observed at the surface facing the pressure tube after dry-patch wrapped around the element. The differences in temperature at various circumferential locations were only a few degrees.

16% overpower 32% overpower 48% overpower 64% overpower 0 0

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350

Circumferential Location (°)

Figure 9 Circumferential-Temperature Variations at Axial Location of 270 mm of Element 01 for Various Overpowers 7


480


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Flow Direction

Angle (deg.)

Figure 11 Illustrated Contour Map of Element Wall Temperature in Water-Equivalent Values very small. The bundle maximum wall temperatures for the upstream-half bundle were higher than those for the downstream-half bundle. This trend is due mainly to the higher thermodynamic quality in the downstreamhalf bundle.

150

Maximum Wall Temperature (°C)

Pout = 1.475 MPa, Flow = 12.2 kg/s, Tin = 43.6°C 140 130 120

CONCLUSIONS

110 NU fuel RFD, Downstream-Half Bundle

100

•

An experiment was completed to provide Freon PDO heat-transfer data for LVRF bundle strings of two radial power profiles corresponding to fresh NU fuel and fresh LVRF in the horizontal uncrept channel. The tested flow conditions in waterequivalent values ranged from 9 to 11 MPa for outlet pressure, from 17 to 26 kg/s for mass flowrate, and from 260 to 280 °C for inlet temperature. The maximum overpower achieved was 64%.

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The sheath temperature data exhibited consistent parametric trends with outlet pressure, mass flowrate, inlet-fluid temperature, and overpower.

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The dryout spread across the bundle followed a consistent trend. For the bundle string with a fresh NU fuel profile, initial dryout occurred at the inner-ring element (low flows) and the outer-ring element facing internal subchannels (high flows). For the bundle string with a fresh LVRF RFD,

LVRF RFD, Downstream-Half Bundle 90

NU fuel RFD, Upstream-Half Bundle LVRF RFD, Upstream-Half Bundle

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Overpower (%)

Figure 12 Variations of Maximum Wall Temperatures with Overpower The maximum water-equivalent temperature obtained in the test, at an overpower level of 64% in Freon (which was equivalent to 64% overpower in water), was significantly below the acceptable maximum temperature. Under the same channel inlet conditions (i.e., pressure, flow, and inlet temperature) and overpower ratio, maximum wall temperatures were slightly higher for the radial power profile corresponding to the fresh LVRF than the NU fuel (Figure 12), but the difference was 8


initial dryout occurred at the outer-ring element facing internal subchannels.

•

•

temperature, indicating that the integrity of the Bruce LVRF will be maintained at PDO conditions. Therefore, the Bruce LVRF exhibits good PDO heat transfer performance.

Dry-patches spread with increasing channel power. In general, the bundle maximum wall temperatures were observed at locations immediately upstream of the downstream end for the downstream-half bundle and at locations immediately upstream of the spacer plane for the upstream-half bundle. Radial locations of the bundle maximum wall temperatures were identified at outer-ring elements. The maximum wall temperatures were circumferentially located at the surface facing internal subchannels at low overpowers (less than or equal to 16%). At high overpowers for a few cases, the maximum wall temperature was observed at the surface facing the pressure tube, but the temperature difference was small around the circumference of the element where the dry-patch has wrapped around.

REFERENCES [1] Leung, L.K.H., Pioro, I.L. and Bullock, D.E., “Post-Dryout Surface-Temperature Distributions in a Vertical Freon-Cooled 37-Rod Bundle”, Proceedings of the 10th International Topical Meeting on Nuclear Reactor Thermalhydraulics (NURETH-10), Seoul, Korea, October 5-9, 2003. [2] Leung, L.K.H. and Groeneveld, D.C., “Fluid-to-Fluid Modelling of Critical Heat Flux in 37-Element Bundles”, Proceedings of the 21st Nuclear Simulation Symposium, Ottawa, Ontario, Sept. 24-26, 2000.

The maximum water-equivalent temperature obtained in the test, at an overpower level of 64%, was significantly below the acceptable maximum

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