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52 Int. J. Nuclear Energy Science and Technology, Vol. 5, No. 1, 2010

Copyright © 2010 Inderscience Enterprises Ltd.

Behaviour of a natural circulation facility with a two-phase flow and in the presence of noncondensable gases for research and development of an emergency cooling system of advanced nuclear reactors

L.A. Macedo* and B.D. Baptista Filho Nuclear Engineering Center – CEN Nuclear and Energy Research Institute, IPEN – CNEN/SP Av. Prof. Lineu Prestes, 2242 – Cidade Universitária CEP 05508–000 São Paulo – SP, Brazil E-mail: [email protected] E-mail: [email protected] *Corresponding author

Abstract: This work presents the results of tests and simulations of the behaviour of an experimental facility called Natural Circulation Loop (NCL) when different volumes of noncondensables are trapped in the top header of the heat exchanger and with the system operating in a two-phase flow. The main objective of this project is to study the behaviour of a passive emergency cooling system of advanced reactors. The results show that, for lower air mass fractions in the heat exchanger, the behaviour of the NCL is equivalent to the operation of a single-phase flow. For higher air mass fractions in the heat exchanger, the behaviour changes radically, presenting higher temperatures, flows and pressure variations. The air mass fraction and the geometry of the circuit at the inlet of the heat exchanger determine the two-phase flow transient. The NCL simulations are performed with the RELAP5/MOD3.3 code presenting good agreement with experimental data for smaller air mass fractions. For higher air mass fractions, the results are satisfactory only after the circuit reaches the saturation temperature.

Keywords: natural circulation; two-phase flow; advanced reactors; noncondensable gas.

Reference to this paper should be made as follows: Macedo, L.A. and Baptista Filho, B.D. (2010) ‘Behaviour of a natural circulation facility with a two-phase flow and in the presence of noncondensable gases for research and development of an emergency cooling system of advanced nuclear reactors’, Int. J. Nuclear Energy Science and Technology, Vol. 5, No. 1, pp.52–65.

Biographical notes: L.A. Macedo is a Researcher at the Nuclear Engineering Center of the Nuclear and Energy Research Institute (IPEN – CNEN/SP), Brazil. He graduated as a Mechanical Engineer from the Engineering School of the Mackenzie University, Brazil in 1984; he obtained his MSc from the São Paulo University, Brazil in 2001 and PhD from the São Paulo University in 2008. He has been working in the experimental area with a two-phase flow in the presence of noncondensable gases.

Behaviour of a natural circulation facility with a two-phase flow 53

B.D. Baptista Filho completed his BE in Mechanical Engineering from the Industrial Engineering College (FEI), Brazil in 1977 and both of his MSc and PhD from the São Paulo University, Brazil in 1979 and 1998 respectively. He is a Senior Researcher at the Nuclear Engineering Center of the Nuclear and Energy Research Institute (IPEN – CNEN/SP), Brazil and is a Specialist in the area of artificial neural networks, nuclear engineering, heat transfer and advanced nuclear reactors.

1 Introduction

The presence of noncondensables in steam greatly inhibits the condensation process. This inhibitive effect is a major concern in the design of much condensing equipment in the chemical process industry where condensation is employed such as the evaporation, distillation and crystallisation processes (Muñoz-Cobo et al., 1996). The difference between the condensation of a pure and saturated steam and the condensation of steam in the presence of noncondensables is the resistance to the heat transfer (Collier, 1980). In the nuclear industry, the effect of noncondensables on steam condensation is very important in accident analyses. In a nuclear containment building, air and the accidental presence of hydrogen represent the main noncondensable gases. While air exists naturally in the containment building, hydrogen can exist in the case of a Loss of Coolant Accident (LOCA) or steam line break accident. The principal sources of hydrogen are the exothermic fuel cladding chemical reaction with steam, the radiolytic decomposition of water and the corrosion of certain metallic species present in the containment (Hasanein et al., 1996).

The advanced reactors are a new concept of nuclear reactors for energy generation. They are characterised by a standardised design for each type to expedite licensing, reduce capital cost and reduce construction time. Such a simpler and more rugged design makes them easier to operate and less vulnerable to operational upsets; they allow higher availability and longer operating life (typically 60 years), reduced possibility of core melt accidents, resistance to serious damage that would allow radiological release from an aircraft impact, higher burn-up to reduce fuel use and the amount of waste, burnable absorbers (poisons) to extend fuel life and resistance to proliferation and passive security (World Nuclear Association, 2007).

The design of light water-cooled advanced reactors (3rd generation) foresees the use of Passive Emergency Heat Removal Systems (PEHRS) to cool down the core by natural convection (Macedo et al., 2004). In such kind of circuits, the noncondensable gases lodge, for gravity, in the upper part of the circuit, usually in the top header of the heat exchanger.

There is an increasing interest in the study of the effects caused by the presence of noncondensables in the behaviour of these circuits (PEHRS) operating in conditions of a two-phase flow. An experimental facility called Natural Circulation Loop (NCL) was modified to allow the study of the behaviour of such systems in transient two-phase flow conditions involving steam condensation in the presence of high concentration of noncondensables at the top header of the emergency cooler.

54 L.A. Macedo and B.D. Baptista Filho

Several simulations of NCL were made using the thermal-hydraulics code RELAP5/MOD3.3 to analyse the processes involved with planned experiments. RELAP5 is the most detailed code developed to perform accident analyses (NUREG/ CR-5535-Rev.1, 2001). The performance and the behaviour of computational code RELAP5/MOD3.3 during the simulations with a two-phase flow in the presence of noncondensables were evaluated. The theoretical results were compared with the experimental data obtained in the NCL experiments.

Many experimental and theoretical efforts have been addressed to study the effects of noncondensables on the system behaviour. However, most investigations focused on the determination of the average values of condensation heat transfer coefficients. Colburn and Hougen (1934) developed the first work in this field. They developed a stepwise iterative solution method for predicting the condensation heat transfer rate from a steam/noncondensable mixture based on the heat and mass transfer analogy. Sparrow and Lin (1964) performed an analytical model of the condensation phenomenon in the presence of a saturated steam-air mixture with constant fluid properties and was based on conservation laws. Minkowycz and Sparrow (1966) developed a work on condensation in unconfined spaces such as on flat plates or outside horizontal tubes. The influence of a noncondensable gas on the heat transfer rate in the case when the steam-air mixture was either stagnant or flowing was shown by Sparrow et al. (1967). They observed reductions in the heat transfer rates due to the presence of small mass fractions of noncondensables and these reductions were becoming more pronounced as the total pressure was reduced. An analytical model to include the effect of small amounts of a noncondensable gas on laminar filmwise condensation of a steam-gas mixture flowing turbulently in a vertical tube was developed by Wang and Tu (1988). In agreement with Sparrow’s results, they found that the reductions in heat transfer due to the noncondensable gas were more significant at low pressure and at low Reynolds numbers of the mixture. Vierow (1990) measured the local heat transfer coefficient of steam condensation in the presence of a noncondensable gas in a vertical NCL. A new correlation that includes the effects of the noncondensable gas and interfacial shear on condensation was provided by Vierow and Schrock (1991) which has been used in the alternative RELAP5/MOD3.2 model. Dehbi et al. (1991) performed several experiments for external condensation in the presence of the steam-air and steam-air-helium on a vertical wall under turbulent flow. Steam-air and steam-helium experiments in a vertical tube under forced flow conditions were performed by Ogg (1991). Peterson et al. (1993) developed a semi-empirical diffusion layer model for condensation with noncondensable gases inside vertical tubes. These analytical models are dependent on the vertical orientation. An experimental study to investigate the effects of a noncondensable gas in the steam condensing system was performed by Oh and Revankar (2005). The condensation heat transfer coefficient and heat transfer rate decrease with a noncondensable gas. The condensation heat transfer coefficient increases with the inlet steam flow rate; however, it decreases with the system pressure. The condensation heat transfer rate is enhanced by increasing the inlet steam flow rate and the pressure.

In Section 2, the methodology adopted for the accomplishment of this work is described, delineating the original experimental facility (NCL), the required modifications in NCL for the performance of experiments at two-phase flow conditions and noncondensable presence, and the required instrumentation and experimental devices. In Section 3, the experimental routines are described together with the accomplished test relations and obtained results. After this, a detailed experimental


analysis is performed, presenting the behaviour of the system in conditions of a two-phase flow and in the presence of high concentration of noncondensables. Section 4 presents the performed NCL simulations with models developed in RELAP5/MOD3.3 and further compared with experimental data. The conclusions are in Section 5.

2 Experimental facility

The flow sheet shown in Figure 1 represents the experimental facility NCL that was designed to simulate a residual heat removal system of an advanced Pressurised Water Reactor (PWR). An electrically heated heater is the system heat source which is a heat exchanger composed of two horizontal pipes with a vertical tubes bundle immersed in a water tank, fed by gravity from a large elevated reservoir. The piping of NCL is in copper which is 22 mm in external diameter and 0,6 mm in thickness. The experimental circuit was assembled with welded connections and three-fourths-inch screwed joints compatible with valves, instruments and equipment nozzles. The heater and all lines were thermally insulated by using glass-wool ducts which are 25 mm thick with corrugated aluminium foil liners. Ball valves were used to reduce flow restrictions in the major circuit lines. The secondary cooling flow is controlled by a globe valve and a calibrated flow metre. The heater is of a single-pass type with ‘U’ type immersion electrical heaters. This heater was designed for a maximum thermal power of 10 KW, but the power was limited to 4.2 KW for the two-phase flow experiments (Macedo, 2001). The heat exchanger design is based on similar concepts considered in the AP600 project (Westinghouse Electric Company, 2007). It is made of copper with two one-and-three-fourths-inch headers interconnected by 18 three-eighths-inch tubes immersed in a 0.202 m3 water reservoir. The flow rate at the NCL is measured with a magnetic flow metre, Krohne Conaut, 10 mm in diameter and calibrated to 300 l/h maximum flow range (Krohne Conaut, 2003). The tank of expansion accommodates the variations of liquid volume in NCL due to temperature variations.

Twenty-four thermocouples were installed along the circuit (OMEGA Engineering Inc., 1987; IOPE Instrumentos de Precisão, 2005), five of ‘T’ type which are 1.5 mm in diameter and 19 of ‘K’ type (12 1.5 mm in diameter and seven 0.5 mm in diameter). Two Validyne pressure sensors, model DP 15, were chosen due its fast response (Validyne Engineering Inc., 2005) and were installed in two strategic points. P1 sensor was installed at the heater outlet and is used to measure pressure fluctuations in the hot leg. P2 sensor was installed at the heat exchanger inlet and measures the pressure in the top header of the heat exchanger. The data acquisition system was developed in a PC platform, with a National Instruments data acquisition board controlled by a programme which is developed using the Laboratory Virtual Instrument Engineering Workbench (LabVIEW) programming environment (Bertolace and Baptista Filho, 2003).

2.1 Instrumentation

Thermocouples were installed in the heat exchanger of the NCL in the positions indicated in Figure 2. The thermocouples TC6, TC7, TC9, and TC12 were installed on the upper header pipe internal surface. The thermocouples TC8, TC9 and TC11


were installed in the vertical pipes of the heat exchanger. There was a vent for air release. The thermocouples TC23, TC25 and TC25 were installed in the secondary coolant side of the heat exchanger and their positions were changeable in the heat exchanger.

Figure 1 Natural Circulation Loop (NCL)

Figure 2 Localisation of the thermocouples in the heat exchanger


2.2 Experimental devices

NCL was equipped with experimental devices to make possible the reproduction of the described phenomena expected during the actuation of a PEHRS. One of these devices is the system for setting the air level at the top header. The experiments were initiated from different levels of accumulated gases in the top header of the heat exchanger to make possible a detailed study of the condensation phenomena. The level and pressure measurements allowed the flow characterisation and the phenomenological study of this kind of systems.

Figure 3 shows the system for setting the air level at the top header of the heat exchanger that is based on the ‘principle of the interconnected vessels’. The initial air level was determined by the following procedures. Figure 3A shows the open valves A, B and C in the heat exchanger. Figure 3B presents the initial liquid level in the heat exchanger that was determined by the injection of liquid in the tank of expansion. The negative liquid column at the P2 pressure sensor was levelled with a centre line in the upper header pipe in the heat exchanger via valve B. The negative liquid column level for the pressure sensor P2, for the tank of expansion and for the scale level were the same (see Figure 3C). The hose of positive liquid column for the pressure sensor (P2) should be connected to valve A. The tank of expansion would be again completed with liquid at desired level, determining a differential pressure at P2 pressure sensor (see Figure 3D).

Figure 3 System for setting the air level at the top header

3 Experimental results

Every experiment in natural circulation in the presence of a noncondensable gas (air) consisted of a power step from stationary fluid initial conditions at room temperature. Table 1 presents power level, initial air mass fraction at the upper header of the heat exchanger, secondary side coolant water flow rate and secondary side coolant inlet


temperature. This work presents, for the first 3000 sec of each test, the behaviour of the NCL for different initial air mass fractions (3% and 99%) considering the same power level (2500 W).

Table 1 Conditions of the tests

Parameter Range

Power (W) 2500

Initial air mass fraction – upper tube heat exchanger (%) 3, 99

Secondary side coolant water flow rate (kg/s) 0.03

Secondary side coolant inlet temperature (°C) 18–20

Figure 4 presents the observed outlet temperature of the heater (TC1), its inlet temperature (TC17), the mass flow and the pressure P2 for an initial air mass fraction equal to 3%. The outlet temperature of TC1, the mass flow and pressure P2 show similar behaviour during all the transients. The behaviour of NCL is equivalent to the behaviour of a PEHRS with a single-phase flow (Baptista Filho and Macedo, 2001). The temperature TC1 increases from 18°C to 53°C, up to the instant 490 sec. There is an initial mass flow in the circuit determined by the difference between the density of the fluid in the hot leg (warm fluid) and in the cold leg (cold fluid). The mass flow in the NCL reaches an initial maximum value equal to 0.02 kg/sec in the instant 497 sec. The heating of the fluid in the hot leg determines the increase of the fluid volume in the NCL. The pressure P2 increases from 3690 to 3830 Pa in the instant 595 sec. There is a decrease of temperature at TC1, also for the mass flow and pressure P2 after the entrance of cold fluid in the electric heater (behaviour of temperature TC17). After 700 sec, the temperature TC1, TC17, the mass flow and the pressure P2 restart to increase. In the instant 3000 sec, TC1, TC17, the mass flow and the pressure P2 are equal to 55°C, 28°C, 0.02 kg/sec and 3890 Pa, respectively.

Figure 4 Inlet and outlet temperatures of the heater (TC17 and TC1), mass flow and pressure P2 – 3% initial air mass fraction


Figure 5 shows the behaviour of the outlet temperature of electric heater (TC1), the inlet temperature of the electric heater (TC17), the mass flow and the pressure P2 for an initial air mass fraction equal to 99%. The behaviour among tests was quite different according to the initial air mass fractions from 3% and 99%. The delay of the flow rate rise was due the initial level of liquid contained in the hot leg. The application of the power step by the electric heater determines an increase in the temperature of the fluid in the electric heater and hot leg that causes the expansion of the fluid volume. In this interval, the free convection in the electric heater is observed through the characteristic behaviour of temperature TC1 with ascending fluid lines (hot fluid) and descending fluid lines (cold fluid). The behaviour of the temperature TC1 presents a larger increasing rate after the end of the initial period of free convection. The temperature TC17 increases from 18°C–28°C in the instant 1183 sec, indicating heat transmission by conduction at the inlet tube of the electric heater. In the electric heater, with the continuous power supply, the temperature TC1 reaches the saturation temperature (107°C), establishing a two-phase transient. It starts the generation of a larger amount of steam, determining a fast increase in the specific volume within the electric heater. The mass flow reaches a maximum value of 0.092 kg/sec in the instant 1253 sec. The pressure P2 increases linearly until reaching the value of 4176 Pa at the instant 1190 sec, and then increases to 14 971 Pa up to the instant 1239 sec. The increase of the pressure P2 is due the steam formation in the electric heater (increase of the void fraction in the hot leg of the system). The entrance of cold liquid in the electric heater determines a decrease of the flow rate to 0.008 kg/sec at the instant 1281 sec. Then the flow increases to 0.034 kg/sec up to the instant 1309 sec. After reaching a saturation temperature (107°C), the temperature TC1 presents an abrupt decrease, reaching a value equal to 55°C in the instant 1512 sec and increases again to a value equal to 58°C in the instant 1813 sec. Then, the temperature TC1 reaches 56°C at the end of the transient. After the start of the two-phase flow transient, the temperature TC17 increases abruptly (with the entrance of hot liquid in the electric heater) and reaches a value equal to 33°C in the instant 1561 sec. Then the temperature TC17 presents a slow behaviour until the end of the transient. During the two-phase flow transient, the pressure P2 decreases strongly to a value equal to 4068 Pa in the instant 1260 sec. The abrupt decrease of the pressure P2 is determined by the steam condensation in the heat exchanger. Then the pressure P2 presents a stable behaviour registering 3566 Pa up to the instant of 3000 sec. The analysis of the pressure P2 shows that a part of the mass of the noncondensable stays in the heat exchanger and a fraction of this is dragged from the heat exchanger and released to the ambient by the expansion tank. The liquid level at the expansion tank also indicates the release of the noncondensable to the ambient.

Figure 6 presents the behaviour of the outlet temperature (TC25) of the secondary side of the heat exchanger for air mass fractions equal to 3% and 99%. At the start of the transient, the temperature TC25 is equal to 20°C and 18°C for the initial air mass fractions of 3% and 99%, respectively. In the instant 525 sec, the temperature TC25 increases slowly, reaching a maximum value of 27°C in the instant 3000 sec for the initial air mass fraction of 3%. The temperature TC25 for the initial air mass fraction of 99% remains stable until 1253 sec, which increases in a step of 5°C, with the increase of the temperatures in the primary side of the heat exchanger in the instant 1239 sec (Figure 5). The temperature TC25 is 25°C at the end of the transient.


The heat transfer rate to the secondary side of the heat exchanger is smaller for higher initial air mass fractions, showing a lower performance of the heat exchanger.

Figure 5 Inlet and outlet temperatures of the heater (TC17 and TC1), mass flow and pressure P2 – 99% initial air mass fraction

Figure 6 Outlet temperature (TC25) of the secondary side coolant of the heat exchanger – 3% and 99% initial air mass fractions


The comparison of the results obtained in these two tests with different initial air mass fractions (3% and 99%) shows that the behaviour of the temperature, flow and pressure is completely different. In the test with the higher mass fraction, there is no continuity (connection) between the liquid in the hot leg and in the heat exchanger, separated by the noncondensable plug (see Figure 7). Higher initial air mass fractions determine larger values of outlet temperatures of the heat exchanger and larger mass flow (with a two-phase flow). Higher initial air mass fractions also determine higher pressures along the circuit.

Figure 7 Geometry of heat exchanger primary and initial air mass fraction

4 Numerical simulations and experimental data

The computer code for thermo-hydraulics and accidents analysis RELAP5/MOD3.3 (R5) was adopted for simulations of the experimental data. The code is based on a nonhomogeneous and nonequilibrium model for the two-phase system that is solved by a fast, partially implicit numerical scheme to permit economical calculation of system transients. NCL nodalisation is shown in Figure 8. The model is divided into the following six calculation regions:

1 electric heater

2 the hot leg

3 the heat exchanger

4 the cold leg

5 the surge line

6 the expansion tank.

Figure 9 presents the comparison among the temperatures TC1, TC17 and the mass flow obtained from the experimental data and the calculated results obtained from the R5 for the test with initial air mass fraction equal to 3%. Simulations with low initial air mass fraction in the heat exchanger show a good agreement with the experimental data. The calculated temperatures present larger values than those obtained experimentally at the instant of 3000 sec (8% for temperature TC1 and 13% for temperature TC17).


Figure 8 NCL nodalisation

Figure 9 Comparison of the model R5 and experimental data – test with initial air mass fraction = 3% – TC1, TC17 and mass flow


Figure 10 presents the comparison between the temperatures TC1, TC17 obtained from the experimental data and the calculated results from the R5 for the test with initial air mass fraction equal to 99%. Before the phase change in the transient, simulations with higher initial air mass fraction in the heat exchanger show the same behaviour but did not present a good agreement with the experimental data. The calculated temperature TC1 does not satisfactorily represents the natural convection existing in electrical heater at the start of the transient, and calculated results before the phase change are early, approximately 200 sec in relation to the experimental data. The code R5 is one-dimensional (x). Analytical models with natural circulation, with a two-phase flow and in the presence of the noncondensable should be at least two-dimensionally modelled. After the phase change, the system returns to a single-phase flow, presenting a good agreement between calculated results and experimental data.

Figure 10 Comparison of the model R5 and experimental data – test with initial air mass fraction = 99% – TC1, TC17

5 Conclusions

The behaviour of a natural circulation circuit with a two-phase flow and with noncondensable gases concentrated at the top header of heat exchanger has been experimentally studied. The temperatures, mass flow and pressures were measured for different initial air mass fractions in the heat exchanger.


The experimental tests presented showed that for lower air mass fractions in the heat exchanger, the behaviour of the NCL is similar to systems with a single-phase flow. For higher air mass fractions in the heat exchanger, the behaviour is quite different, presenting high temperatures, flow rates and pressures that follow almost a step variation.

For higher initial air mass fractions, the temperatures at the secondary side of the heat exchanger grow in a slower way, showing the reduction in the heat transfer rate to the secondary side of the heat exchanger.

There is a relation between initial air mass fraction in the heat exchanger and circuit geometry; considering the outlet of hot leg and the inlet of the upper horizontal tube in heat exchanger; this relation determines the two-phase flow transient.

Simulations with low and high initial air mass fractions in the heat exchanger were performed using the computer code for thermo-hydraulics and accidents analysis RELAP5/MOD3.3. The simulations with a low initial air mass fraction presented a good agreement between calculated and experimental data. For a higher initial air mass fraction in the heat exchanger, the calculated results did not agree with experimental data before the phase change. The code RELAP5/MOD3.3 does not represent the natural convection phenomenon satisfactorily. The calculated results are early, approximately 200 sec in relation with the experimental data because the code is one-dimensional (x) and simulations with natural circulation, with a two-phase flow and in the presence of the noncondensable gas should consider at least two-dimensional models (x and y). When the process returns to a single-phase flow, the simulation presents a good agreement between the calculated and experimental data. Although the code RELAP5/MOD3.3 is being developed for a transient simulation of nuclear plants, it also proved to be a useful tool for predicting the transient behaviour of a natural circulation facility with a two-phase flow and in the presence of a noncondensable.

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