Natural Circulation Test Campaign
M. Polidori et al.
Natural Circulation Test Campaign
on HERO-2 Bayonet Tubes Test Section
M. POLIDORI, P. MELONI, C. LOMBARDO, G. BANDINI
ENEA FSN-SICNUC-SIN
Bologna, Italy
Email:
M.E. RICOTTI
POLIMI, Research Department of Energy
Milano, Italy
A. ACHILLI, O. DE PACE, D. BALESTRI, G. CATTADORI
SIET
Piacenza, Italy
Abstract
Innovative nuclear systems, such as the SMR and Gen-IV reactors, require relatively new approaches to accomplish the function of heat removal to achieve the goal of safety and economics, both in operative and accidental conditions. The heat exchanger compactness and operability in natural circulation are of great importance for such systems. ENEA in collaboration with POLIMI and SIET is leading some studies on innovative heat exchangers within the framework of a National Research Program funded by the Italian Minister of Economic Development. The SIET laboratories in Piacenza, leader in testing and development of innovative components and systems, hosted an experimental campaign to characterize a DHR system working in natural circulation with HERO-2 test section; a bayonet tubes heat exchanger composed by two parallel tubes. The natural circulation tests conducted, even in presence of non-condensable gas, have allowed the creation of a valuable database for the characterization of the heat exchange capability in passive accidental conditions, useful for the qualification of computer codes supporting the design and safety analysis of innovative reactors. The paper presents the test campaign carried-out on HERO-2.
1.INTRODUCTION
In the frame of a National Research Program funded by the Italian Minister of Economic Development, the National Agency ENEA, in collaboration with SIET laboratories and the Polytechnic University of Milan, has carried out an experimental campaign to characterize bayonet tubes for heat exchanger applications [1] at PWR SMR conditions. Such tubes have been tested, after adaptation, in the IETI facility previously used to test and characterize steam generators with helical tubes by POLIMI [2].
The test section HERO-2 consists oftwo parallel bayonet tubes, eachcomposed of an inner pipe which conveys the incoming liquid and an outer tube electrically heated. The generation of steam occurs in the interspace between the two tubes. In view of future test campaigns, the design pressure of the component is 180 bars, but the current facility is able to operate at a pressure of 70 bars and a flow rate up to 0.1 kg/s per tube. The plant is able to feed the test section with subcooled or saturated water.
In the first experimental campaign, the component HERO-2 has been tested in open circuit in order to characterize the heat exchange (with a single active tube) and to detect and quantify thermalhydraulic instabilities of the tubes under specific operating conditions (both tubes). ENEA and POLIMI conducted both the pre-test [3] and post-test analysis[4] with RELAP5 system code. The model of HERO-2 driven by boundary conditions gave quite good results even in reproducing certain instability conditions [5].
The objective of the present test campaign is the characterization of the behaviour of HERO-2 in natural circulation conditions typical of a DHR (Decay Heat Removal) system for PWR SMR. To accomplish the new task, the loop has been closed around the HERO-2 realizing the heat sink through a tube submerged in a small pool, 11 m on top of the test section. In this configuration, several steady-states have been recorded at different filling ratio and test section power: 19 single tube tests, 21 double-tube tests and 8 tests with controlled injection of non-condensable gas.
The present paper introduce the main achievements of the test campaign together with some preliminary considerations on the general behaviour of the facility.
2.FACILITY DESCRIPTION
The instrumented test section HERO-2 (Heavy liquid mEtal pRessurized water cOoled tube #2) has been designed and supplied by ENEA in Brasimone, taking advantage of the present development and testing of this solution for heavy liquid metal GEN-IV applications [6].
The component is connected to the IETI facility as shown in the sketch of Fig. 1 in which are reported also some details of a bayonet tube. HERO-2consists of a couple of parallel bayonet tubes with an overall length of about 7.3 mand external tube diameter of 1”, while the overall height of the facility is about 18 m.Each bayonet tube is made up of three concentric tubes, a slave pipe which conveys the incoming water, an inner tube to create a sealed gap filled by air in order to reduce the thermal flux to the downward water flow, and an outer tube electrically heated. The steam generation occurs in the annular space between the inner and outer tubes. At the inlet of each tube (top part), a structure is placed to housing the interchangeable orifices for water flow stabilization.
FIG. 1 – Sketch of the HERO-2 facility for natural circulation studies.
The heating is realized by a total of 210 electric resistors that surround the two outer pipes for their entire length. Each electrical heater supply 240 W at 100 V, meaning that each tube is powered with up to 25.2 kW. The maximum temperature to safely operate the heaters is 350°C. In first approximation, the power generation could be considered linear even though this type ofheatersleads toa certain discontinuityin the stream ofpower supplied, due to possible edge effect in each heater and the spaces left to give room to five pressure nozzles in each tube. The tubes, as well as the piping and the pool are thermally insulated with rock wool.
A hot leg 3/4” pipe of about 16 m connects the HERO 2 Test Section with a pre-existing condenser submerged in a small pool, whichworks at ambient pressure (1 x 0.4 m and height 0.6 m). The condenser tube is a near-horizontal pipe (inclined of 3°) of 2” diameter in AISI316 stainless steel. The cold leg piping of 3/4” of about 18 m closes the loop.
Since the facility works in natural circulation, no mechanical components are present. The parameters that completely define every steady-state are: the electrical power supplied through the bayonet heaters and the Filling Ratio (FR), the latter is calculated as, where is the maximum water mass at cold conditions (~19.5 kg) in the loop and is the water mass subtracted before the tests to reach the desired FR.
The scheme of Fig. 1 shows also the system to maintain the water pool level during the operations, where the refilling water mass flowrate is measured with a Coriolis flowmeter, and the system to reach the required FR, where the vapor extracted from the loop is condensed and weighed.
Although the facility is well instrumented in terms of temperature (T), pressure (P) and differential pressure (DP), there is no direct measurement of mass flowrates in the loop, which can be derived from the pressure drop through the orificesonce the flow factor Kv is known. The characterization made with the experimental data of the previous test campaign gives an average value of the Kv=0.122. The mass flowrate is:
where / is the relative density with water at about 20 °C.
Another derived quantity that can be evaluated is the vapor quality at the exit of the test section:
where is the electrical power supplied, total mass flowrate, outlet liquid enthalpy, outlet vapour enthalpy, inlet enthalpy, with ‘in’ and ‘out’ referring to the inlet and outlet measurements of HERO-2 test section.
The net power can be evaluated through the difference between the electrical power suppliedand the estimated overall heat losses , in turn obtained considering the difference between the vapor enthalpy that leave the pool and the enthalpy of the replenishing water to maintain the level in the pool
.
3.HERO-2 TEST CAMPAIGN
The test campaign consists of series of steady-state at different power and FR as follows:
—21 double-tube tests (DTs) at FR 0.69, 0.64, 0.5, 0.43, 0.32 and power ranging from 5.0 to 50.0 kW;
—19 single-tube tests (STs) at FR 0.72, 0.65, 0.56, 0.45, 0.35 and power ranging from 5.5 to 22.5 kW;
—8 double tube tests with mass of non-condensable (N2) of 4 and 7 g, FR 0.50, power from 11 to 50 kW.
The main results obtained in the test campaign in the double and single tube tests are shown in the following Fig. 2 and Fig. 3where the vapor chamber pressures and mass flowrates versus the net power and FR arerespectively reported forevery tested steady-state.
As expected, the facility tends to pressurize increasing the power at constant FR, and the increase in pressure becomes more evident increasing the FR due to lower amount of compressible volume.
Also the mass flowrate increases when the FR increases, except for the higher FR where the trend stops.At constant FR, the flowrate is characterized by a local minimum value when the powerincreases, resultingfrom the balance oftwo conflicting phenomena. The first is the increase of the vapor quality with the power that, reducing the hot leg average density, increases the pressure losses. The second is the pressurization, which determines an increase in the hot leg density with a consequent reduction of the pressure losses.
The ST tests arecharacterized by lower powerthan the DT ones, but a relative greater hydraulic resistance due to the exclusion of a heated bayonet tube,therefore, higher saturation pressures are recorded in STs for the same total power. Moreover, while the mass flowrate is almost halved respect the DT tests, the system behavior becomes more variable.
The Fig. 4 shows the behavior of the estimated vapor quality at the exit of the test section and the evaluation of the global heat transfer coefficient of the condenser in the double tube tests. The global heat transfer coefficient is defined throughwhere power exchanged by the condenser, exchanging surface area and logarithmic mean temperature difference.
Increasing the FR, the slope of the exit vapour quality function of power decreases due to the higher pressurization levels, as well as the global exchange coefficient decreases due to the liquid fraction increase in the condenser.
FIG. 2 – Pressure vs net power and FR in single and double tube tests.
FIG. 3 – Total mass flowrate vs net power and FR in single and double tube tests.
FIG. 4 – Quality and global heat transfer coefficient in double tube tests.
Ata given FR, the power removal from the system can be characterized by two parameters, i.e. the logarithmic mean temperature difference on the condenser and the global heat exchangecoefficient.The first tends to increase with the loop pressure that sets the saturation temperature, the second may increase or decrease depending on how the two-phase thermal coefficients vary with flowrate and temperature that mainly depends on the flooding level of the condenser tube. In the latter case, the higher the saturation temperature, the higher the amount of power exchanged and in turn the vapor condensed.
Summarizing, for lower FR with higher compressible volume available, the increase in qualityprevailover the pressurization up to a given power in which the circuit tends to pressurize more than the quality increases. The increase of quality means higher pressure drops, explaining the lower average flowrates, lower condenser flooding levels, higher global heat transfer coefficients and lower logarithmic mean temperature difference.
For higher FR with lower compressible volume available, the increase in pressure results in higher flowrates. The lower quality causes greater condenser flooding with reduction of the global heat exchange coefficient. Therefore, the logarithmic mean temperature differencethrough the pressurization drives the system behavior.
The main results of the test conducted with injection of nitrogen are shown in Fig. 4. Making the comparison against the curve FR 0.5 in Fig. 2, the higher pressurization at increased nitrogen concentration can be appreciated, meaning a deterioration of the heat removal, while the flowrate tends to increase as effect of the pressurization and the lower vapor quality that reduce the pressure losses.
FIG. 5 –Pressure and total mass flowrate vs net power and N2 mass injected (FR=0.5).
4.CONCLUSIONS
In the frame of the National Research Program funded by the Italian Minister of Economic Development, the collaboration among ENEA, POLIMI and SIET is carrying out some studies on innovative heat exchangers for SMR applications. The HERO-2 test section was previously tested to characterize the bayonet tube heat exchange capability and flow instability conditions. The presentexperimental campaign aimto characterize the operability of the bayonet tubes as DHR system working in natural circulation for PWR LWR conditions, thus creating a consistent databasefor the characterization of the heat exchange capability in passive accidental conditions, useful for the qualification of computer codes supporting the design and safety analysis of such innovative reactors.
The main achievements obtained from the series of steady-states conducted with single active tube and double-tube, with and without injection of calibrated quantities of non-condensable gas,have been briefly presented, confirming the validity of the database produced. The test campaign will be used for the assessment of RELAP5 system code.
ACKNOWLEDGEMENTS
The activity has been funded by Italian Minister of Economic Development within a National Research Program.
References
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[2]D. Colorado, D. Papini, J.A. Hernández, L. Santini and M.E. Ricotti, “Development and experimental validation of a computational model for a helically coiled steam generator”, International Journal of Thermal Sciences, 50 (4), 569 (2011).
[3]M. Polidori et al., “Design and execution of the test campaignon the bayonet tube HERO-2 component”, proceeding of ICAPP16, San Francisco, USA, 2016.
[4]M. Polidori et al, “Post-test analysis for the characterizationof the bayonet tubes HERO-2 component”, proceeding submitted to NURETH-17, Xi’an, Shaanxi, China, Sept 2017.
[5]M. Polidori et al, “Validazione dei modelli RELAP5 dello scambiatore di calore HERO-2”, Technical report ENEA ADPFISS–LP1–068, 2016.
[6]D. Rozzia, et al., "Activities in Support to the Assessment of Steam Generator Bayonet Tubes, for GEN-IV Applications," ENEA Technical Report, ENEA NNFISS - LP3 - 054, 2012.
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