Località. Piacenza. Progetto ECATE. Progetto E.C.A.T.E. Efficienza e Compatibilità Ambientale delle Tecnologie Energetiche - PDF

Località Piacenza Doc. n. R CONSORZIO LEAP Laboratorio Energia Ambiente Piacenza Progetto ECATE Re. Progetto E.C.A.T.E. Efficienza e Compatibilità Ambientale delle Tecnologie Energetiche COMPORTAMENTO

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Località Piacenza Doc. n. R CONSORZIO LEAP Laboratorio Energia Ambiente Piacenza Progetto ECATE Re. Progetto E.C.A.T.E. Efficienza e Compatibilità Ambientale delle Tecnologie Energetiche COMPORTAMENTO DI UN GENERATORE DI VAPORE A TUBI ELICOIDALI IN UN SISTEMA DI SICUREZZA DI TIPO PASSIVO, IN CIRCOLAZIONE NATURALE L.Santini, M.E.Ricotti Prima emissione M.E. Ricotti REV DESCRIZIONE ELABOR VERIFICATO APPROVATO DATA LEAP / REPORT Sottoprogetto 3. Obiettio Realizzatio 1. ANALISI CFD E PROVA A GRANDE SCALA DI GENERATORE DI VAPORE INNOVATIVO R3.1/5 Comportamento di un Generatore di Vapore a tubi elicoidali in un Sistema di Sicurezza di tipo passio, in circolazione naturale (sostituisce: R3.1/5 Confronto tra dati sperimentali e simulazioni CFD per moto interno e moto esterno in tubi elicoidali) NOTA: dopo alcuni mesi di laoro, è risultato eidente che i dati sperimentali a disposizione nella letteratura aperta internazionale, per quanto riguarda scambiatori di calore a tubi elicoidali e con fluido in moto esterno in cross flow, sono praticamente inesistenti, mentre alcuni dati sperimentali per moto interno, ottenuti anche dal gruppo di ricerca, riestono in effetti un interesse assai contenuto. In tale situazione, la messa a punto di una facility sperimentale al solo scopo di ottenere dati per il confronto e la alidazione dei modelli e dei calcoli CFD, per moto esterno, è apparsa non giustificata e non compatibile con il budget disponibile per il sottoprogetto. Si è quindi deciso di tralasciare le analisi CFD, peraltro già sufficientemente trattate nel Report 3.1/3, e di affrontare un tema assai più interessante, dal punto di ista delle conoscenze scientifiche e dell impiego di Generatori di Vapore a tubi elicoidali: l inserimento di tale componente quale elemento fondamentale di un sistema di sicurezza di tipo passio, funzionante con miscela bifase in circolazione naturale e a pressione ariabile, per reattori innoatii (es. IRIS). Il report/paper qui riportato, sostituisce pertanto il documento R3.1/5 inizialmente preisto. 1 INSTABILITIES IN A TWO-PHASE FLOW NATURAL CIRCULATION SLIDING PRESSURE LOOP Lorenzo Santini a, Antonio Cammi b, Marco Ricotti c a Politecnico di Milano, Dipartimento di Energia, b Politecnico di Milano, Dipartimento di Energia, c Politecnico di Milano, Dipartimento di Energia, ABSTRACT A natural circulation sliding pressure power imposed loop that simulates the behaiour of an Emergenecy Heat Remoal System of an innoatie nuclear reactor, IRIS reactor, has been built and operated at SIET thermohydraulics labs in Piacenza, in the frame of the ECATE R&D programme managed by the LEAP lab. The loop is scaled 1:13 on power with respect to the real one and has a full eleation (~2 m). Two types of two-phase flow instabilities of the loop were detected during the operation. A first kind of instability characterized by high frequency oscillations of loop flow rate (with periods of the order of 1 seconds) were detected in all the tested runs. Fast Fourier Transform were applied in order to reduce flow rate time recordings and the most significant frequencies were collected as a function of loop operatie parameters. A strong link between loop high frequencies oscillations first armonica and operatie pressure were found and suggested an interpretation of the phenomenon in terms of compressibility of the apour phase. A simple mechanistic model for predicting high frequencies oscillations first armonica is proposed. A second kind of instability characterized by ery low frequencies oscillations (hundreds of seconds) has been detected in the highest explored filling ratio, i.e..79. This second type of instability is characterized by the presence of a ery low quality (few percents) flowing in the riser of the circuit. At low qualities a small steam generator inlet subcooling ariation brings together huge ariations in riser oid fraction that change its graitational pressure drops. The changing graitational pressure drop influence loop flowrate that impacts also on pool condenser outlet subcooling. The coupling and the time delays between flowrate, pressure drops and downcomer subcooling cause the obsered phenomenon of the low frequency oscillations. INTRODUCTION Two phase natural circulation (NC) has been widely applied in the past in seeral power conersion systems. NC found applications in the past in automotie industry for engine cooling when power densities were sufficiently low. Today NC in presence of a centrifugal field is extensiely applied in gas turbines blade cooling. Both subcritical and supercritical fossil fuelled power stations with power higher than 6 MW e hae been built and operated in the past using natural circulation for the boiler flow driing mechanism. The main adantage of using NC in boilers design is related to its simplicity, reduction in operation costs related to the absence of pumps and better flow distribution between seeral parallel tubes [1] if compared with forced circulation. Nuclear industry made and make extensie use of NC principle for core cooling. Small nuclear power reactors like Dodeaard reactor and VK-5 power stations hae been built 2 and operated from the sixties using natural circulation principle for core cooling during full power operation. All pressurized water reactors use the natural circulation principle for steam generator functioning (with the exception of Babcock and Wilcox once-through steam generators) during normal operation. Neertheless the many adantages in reality coer seeral drawback related to the complex physics inoled in NC systems that renders their modelling a challenging task. In particular the problem of thermohydraulics instabilities is one of the most crucial drawbacks of natural circulation systems. Instabilities could cause oscillations of the main loop parameters in terms of pressure, flow rate and temperatures. Seere oscillations can induce fatigue problems to loop structures. Mechanical fatigue could rise due to pressure oscillations, while thermal fatigue could be induced by premature and oscillating dryout of the heating surfaces. Today seeral project of innoatie nuclear power plants include NC principle for core cooling during shutdown and post accident scenarios. Between them IRIS project makes extensie use of natural circulation principle for safety reasons. IRIS reactor [2] is a low/medium power (335MW e ) pressurized water reactor for electricity production deeloped by an international consortium led by Westinghouse. IRIS deelopment started in late 1999 as part of the NERI program and has rapidly progressed to a nuclear reactor market entry targeted for deployment in the time frame. The plant conceptual design was completed in 21 and the preliminary design is currently underway. The pre-application licensing process with NRC started in October 22 and IRIS is one of the designs considered by US utilities as part of the ESP (Early Site Permit) process. The main safety system of IRIS reactor is the so called Emergency Heat Remoal System (EHRS), a system composed by 4 parallel natural circulation loops whose scope is eacuating primary circuit internal energy to the enironment. IRIS EHRS starts its operation during transients, accidents or wheneer the normal heat remoal paths are lost. An EHRS subsystem is a closed circuit with two parallel steam generators, an hot leg (the riser of the circuit), the heat sink composed by an heat exchanger bundle submerged in Refueling Water Storage Tank (RWST) and the cold leg that close the circuit by bringing cold condensed water to the steam generators (schematic principle in figure 1). The major functions of IRIS EHRS are the following: -Emergency Core Decay Heat Remoal -Emergency Reactor Coolant System Water Inentory Control (LOCA mitigation) -Emergency Containment Pressure Reduction During plant normal operation ales V1 and V2 are open and ale V3 is closed (figure 1), preheated water coming from the regenerator line is pushed into the steam generators where is eaporated, slightly superheated and sent to the high pressure turbine. If a reactor trip occurs the core decay heat will normally be remoed by the steam generators with feed water supplied by the start up feed water system and the steam is directed to the condenser ia the steam dump ales. In case of malfunction of the start up feed water system, the EHRS is aailable to remoe the decay heat following the closure of V1 and V2 ales and the opening of V3 ale. 3 EHRS hot leg Heat sink EHRS cold leg Reactor V1 To turbine V3 V2 From pre-heater line Steam Generator (SG) Fig. 1-Schematic of IRIS Emergency Heat Remoal System (EHRS) In this paper the results of an experimental campaign on a mockup of IRIS EHRS will be presented. Steady state results hae been already described in a preious paper [3] and will not be recalled here. The focus will be put on the thermalhydraulic oscillations obsered during loop operation and inoling pressure, flow rate and temperatures excursions of the loop. Experimental facility The facility built and operated at SIET thermohydraulics labs (Piacenza) is an extension of an electrically heated test section used for forced flow experiences on an helically coiled steam generator [4]. The experimental loop is briefly composed by a heat source, a riser, a heat sink and a downcomer. The tube of the steam generator is entirely insulated with rock wool and the small thermal dispersions were accurately characterized as a function of the temperature difference between external tube wall and ambient air. The riser is a 21.3 meters long AISI 316 stainless steel tube with an inner diameter of 2.93 mm and an outer diameter of mm. Downcomer tube is a is a 31.8 meters long AISI 316 stainless steel tube with an inner diameter of 2.93 mm and an outer diameter of mm. Pool condenser tube is 1m long with 59 and 73 mm of inner and outer diameters. The tube of the condenser is slightly inclined (3 ) to aoid water draining during condensation. Riser and downcomer diameters hae not been scaled with respect to IRIS EHRS riser and downcomer expected pressure drops. All loop pipes were accurately insulated with rock wool. Neertheless in a part of the experimental runs (nearly one half), the small thermal losses along riser and downcomer were compensated with an electrical wire coiled along the tubes whose power can be regulated during operation. 4 Extracted steam condenser Loop Filling Ratio Controlling Vale V3 Riser throttling ale Vapour to atmosphere V4 Condenser tube. Din=59 mm Dout=73mm L=1m Inclination=3 Balance Riser Din=2,93mm Dout=26,27mm Total Lenght=21,25m Loop Height =19,9m Downcomer Din=2,93mm Dout=26,27mm Total Lenght=31,75m Coiled Steam Generator Din=12,53mm Dout=17,15mm Toltal Lenght of the tube=32m Heated Length=24m Total Height=7,9m DC Electric Heating V2 V1 Test section inlet throttling ale Flowmeter Orifice Fig.2-Natural circulation loop test section The pool condenser tube is submerged into a 25 liters pool. A metallic slab is placed few centimeters under the apor escaping duct of the pool to reduce the presence of liquid droplets in the exit stream. The eaporating water in the pool is continuously replaced ia a submerged drilled tube placed in the bottom of the pool. The tube in the bottom of the pool is connected to a tank whose water leel is maintained to a constant alue thanks to a floating deice. The measured quantities in the loop (more than 2 measuring points) are flow rates, pressures (absolute and differential), temperatures and powers. The circuit flow rate has been measured by a calibrated orifice of 5 mm placed at steam generator inlet and instrumented with a differential pressure transducer calibrated at SIET labs (SIT certified) with an estimated maximum uncertainty of 2%. The circuit absolute pressure is measured at steam generator inlet ia an absolute pressure transducer calibrated at SIET labs and with a maximum uncertainty on read alue of.1%. Differential pressure transducer are placed across the throttling ales and along the downcomer with the scope of ealuating the possible presence of mixture at condenser tube outlet. 5 Fluid temperature measurement is obtained with K-class thermocouples (calibrated in SIET labs and with a maximum error at 1 C of.4 C). Fluid temperatures are measured at steam generator inlet and outlet headers, at condenser tube inlet and outlet and inside pool condenser. The electrical power is measured ia a olt-amperometric digital instrument with a relatie uncertainty guaranteed of 2.5%. The filling procedure One of the main goal of the experimental campaign was to inestigate the effect of the so called filling ratio (FR) on the behaior of the system. The filling ratio is defined as the ratio between the water inentory stored during loop operation and the maximum water mass that could be stored in cold conditions by completely filling the loop. In the real EHRS loop the desired filling ratio will be acquired by properly timing the closure of the two ales that isolate the passie system from the remaining portion of the secondary circuit; in Piacenza s test section the desired filling ratio was obtained with the following procedure starting with an empty loop (with reference to figure 2): -ales 1 e 3 open, the loop is completely filled with cold water ia an external feed pump 1. -ale 1 closed -worm up of the steam generator -extraction, condensation and weight of the extracted steam by means of the balance placed at the top of the circuit -ale 3 close when the desired filling ratio is obtained -operation of the loop at the desired power leel conditions. The compensation of the heat losses The electrical power gien to the steam generator generate heat in the tube that partially is transferred to the fluid and a small amount (nearly 1% in our typical operating conditions) goes to the enironment as thermal losses. Part of the power gien to the fluid is transferred to the pool thanks to the circulation and a small amount is dispersed in the enironment due to the dispersions of riser, downcomer and headers (nearly 1,5 times the heat losses of the steam generator). Due to the fact that this thermal losses are not negligible, if compared with IRIS EHRS ones, we tried to compensate them by coiling riser and downcomer tubes with an electrical wire whose power can be regulated. Some difficulties arises in the application of this thermal dispersions inhibitor. The main difficulty is related to the strong effect of thermal dispersions on system operating pressure. In fact the choice of the proper power needed to compensate the dispersions is a function of system operating pressure that will be reached after the compensating procedure is fully accomplished, that is an unknown ariable. This fact impose the necessity of a dynamic procedure of compensation as described in the following: -once the normal steady state (without thermal compensation) is reached, the difference between inlet thermal power (at steam generator) and outlet thermal power (at pool condenser) is computed so that an estimation of compensating power is aailable. 1 The total measured mass of cold water storable in the system was nearly 25 kg 6 -the electrical wires are turned on and a new steady state is reached with an higher system pressure and thus higher thermal losses to the enironment -a small increase in wire electrical power is needed in order to oercome the increased thermal dispersions -continuous increase of compensating power until a balance between inlet and outlet power is reached. The intrinsically iteratie nature of the procedure and the long waiting times for reaching steady state, make the procedure time consuming and complex at least at the beginning of test campaign. Luckily the first experiences with this compensating procedure allowed us to quickly estimate the needed amount of electrical power without the necessity of the long iteratie procedure. RESULTS OF THE EXPERIMENTAL CAMPAIGN Experimental loop operation showed that it was prone to two types of oscillations that will be described in the following. High frequencies oscillations Fast Fourier Transform (FFT) has been applied to time recordings of flow meter orifice in order to capture the period (first harmonic in Fourier analysis) of such oscillations. Next figure (figure 3) shows a typical plot from Fourier analysis. 1 6 analysis of orifice pressure drops 1 4 First Frequency Power Spectral Density Frequency [Hz] Fig.3-FFT applied to loop flow meter orifice time recording In the following table (table 1) the periods of the oscillations obtained with FFT analysis are collected for all the runs that hae shown only high frequency oscillations. 7 Table 1-High frequencies oscillations: runs, filling ratios and detected periods (first armonica in FFT) Run FR P[bar] Period[s] 26712_ _ _ _ _ _ _ _ _ _ _ _ Here we present a ery simplified model proposed with the scope of mechanistically interpret the high frequency oscillations experienced during loop functioning. The true origin of the oscillations must probably be searched in the density waes type instability [5] affecting loop heat source (the steam generator). Neertheless, with no pretension to following the true physical explanation, we will show that it is possible to introduce a more simple schematization capable of capturing the obsered period of the oscillations. The model assumes that the liquid part of the loop stored mass behaes as a free oscillating mass and the apour portion as a spring whose stiffness is somehow related to its compressibility. Liquid water has been assumed in saturated conditions at the working pressure (assumed eerywhere in the circuit at a constant alue). The total olume of the circuit is the sum of the olume occupied by the apour and the one occupied by the liquid, assuming liquid and apour in saturated conditions: V = V + V = M + M (1) t l l l Being i the specific olumes, functions of saturation pressure, and M i the masses. Haing preiously defined the filling ratio (FR) it is possible to write: FR M = M M + l = (2) M M Being M the maximum mass of liquid storable in the circuit in cold conditions (2 C). Combining (5) and (6) we obtain: 8 M M l V FR M t l = l FR M = l V t (3) (4) So that it is possible to express the total apour and liquid inentories inside the circuit as a function of external ariables (pressure and filling ratio). Assuming that apour behaes as a perfect gas it is possible to express its compressibility (assumed isothermal) in the following way: d dp t Rϑ 1 = 2 (5) MM p It is simple to demonstrate that with this assumptions the mass-spring system has a natural frequency expressed by the following formula: T 2π 2π = = ω Ap Rϑ M MM M l (6) Where A is a reference tube free cross sectional area, MM is water molecular mass, R is gas uniersal constant and θ is loop saturation temperature. The simple relation (6) shows the main elements on which the high-frequency oscillations depend: -Pressure: an increase in the pressure of the system has the effect of decreasing the period of the oscillations (higher frequencies). The physical meaning of this conclusion is that the increased pressure make the stiffness of the apour-spring higher, thus increasing the frequency of the oscillations. -Liquid mass: an increase in the liquid content, ceteris paribus, has the effect of increasing the period and this is due to the higher inertia of the system. The characteristic cross sectional area of the circuit (A in eq. (6)) must be carefully chosen. The circuit has three fundamentals cross sectional areas: steam generator tube area, riser tube area and condenser tube area. Depending on which area is chosen, equation (6) gies different alues for the period of the oscillations. What we found is that the first oscillation frequencies (first armonic in Fourier analysis) are well predicted by using steam generator cross sectional area in equation (6). A possible explanation of this result is th
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