natural-convection studies for advanced candu reactor concepts

Nuclear Engineering and Design 215 (2002) 27–38

Natural-convection studies for advanced CANDU reactorconcepts

G.R. Dimmick, V. Chatoorgoon *, H.F. Khartabil, R.B. DuffeyAECL, Chalk Ri�er Laboratories, Chalk Ri�er, Ont., Canada K0J 1J0

Abstract

AECL is studying advanced reactor designs where natural convection is an important design feature in heatremoval processes. The use of a flashing-driven, natural-circulation system to remove moderator heat is beingconsidered. Experiments and code simulations have shown that a flashing-driven system is feasible at normaloperating power, but is prone to flow instabilities at low powers. Vapor flashing at superheated conditions and thepresence of nucleation sites were found to be important for stable operation over the whole power range. Adevelopment concept for CANDU® is to increase the primary coolant pressure and temperature to supercriticalconditions. With a natural-convection-driven primary flow, the large variations in fluid properties near the criticalpoint introduce the potential for flow instabilities. Analyses have shown that flow instabilities can occur under certainconditions. Experimental and analytical results on the flashing system are described. The experiments and initialanalytical results for the supercritical concept are discussed. © 2002 Elsevier Science B.V. All rights reserved.

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1. Introduction

AECL is studying advanced reactor designswhere the removal of heat by natural convectionis an important design feature. This paper de-scribes two applications that are being considered:the flashing-driven passive moderator circulationsystem (PMCS), and the natural-circulation su-percritical reactor concept. The PMCS is ad-dressed in Section 3 and the supercritical reactorconcept in Section 4. A brief description of bothsystems is given below.

1.1. PMCS

During operation of a CANDU reactor, someof the heat generated by fission is deposited in therelatively cool (�70 °C) low-pressure moderatorsurrounding the high-pressure fuel channels. Theheat contained in the heavy water flowing throughthe heat exchangers is removed by pumped servicewater (Fig. 1). However, heavy water circulationby natural convection has certain advantages, anda system that rejects heat passively using a flash-ing-driven natural-circulation loop is being devel-oped. An obvious advantage of a natural-cir-culation system is the elimination of the pumps.This will result in some cost-savings and improve-ments in reliability (Spinks, 1993). Another ad-vantage lies in the role of the moderator as an

* Corresponding author. Present address: Department ofMechanical Engineering, University of Manitoba, Canada.

E-mail address: [email protected] (V. Chatoor-goon).

0029-5493/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.

PII: S0 029 -5493 (02 )00039 -0

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G.R. Dimmick et al. / Nuclear Engineering and Design 215 (2002) 27–3828

emergency core-cooling system. Elimination ofthe need for emergency-power supplies to operatethe circulation pumps would improve the re-liability of both emergency core-cooling systems:the moderator and the emergency coolant-injec-tion system.

In the PMCS design, the moderator ismaintained close to saturation at the calandriapressure, and heat is transported from thecalandria to an elevated passive heat exchanger ina natural-circulation loop (Khartabil and Spinks,1995). Under normal operating conditions, heathas to be continuously removed from themoderator. This is done in existing plants byusing a forced-convection loop. An attractivefeature of the PMCS (see Fig. 2) is that flashingoccurs close to the calandria exit. This provides alarge driving force, because of the large densitydifference between the cold-leg subcooled liquidand the hot-leg two-phase mixture. This designfeature facilitates removal of moderator heatunder both normal and accident conditions bynatural-convection. Moreover, having themoderator temperature close to saturationprovides the option of utilizing the moderatorheat for feedwater heating, which improves plantefficiency.

The main feature of this design is the vaporgeneration in the hot leg. Simulations using theCATHENA code (Hanna, 1998) have shown thata flashing-driven natural-circulation loop can beused to remove moderator heat under normaloperating conditions without any flow instabil-ities. Under some conditions, limit-cycle oscilla-tions were observed experimentally. With a fewexceptions, these were successfully described bythe CATHENA code. The tests and analysesdescribed in this paper investigated the feasibilityof the flashing-driven natural-circulation concept.

1.2. Supercritical reactor concept

The second potential application of natural cir-culation is an advanced CANDU reactor conceptwith supercritical water as coolant (Dimmick etal., 1998). The coolant, being a high-density gas ata pressure above 22 MPa and temperature abovethe critical temperature, does not encounter two-

phase conditions. Increased coolant temperatureleads directly to increased plant thermodynamicefficiency, thereby reducing unit energy costthrough reduced specific capital cost and fuelingcost. Utilizing the large density change with tem-perature around the critical point, and the gasexpansion coefficient characteristic, supercriticalwater coolant also introduces the possibility ofusing natural-circulation to drive the primaryflow.

Studies of light-water reactors (LWRs) operat-ing with supercritical light water are currentlyunderway in Japan and Russia. These studiesshow that such reactors are feasible with currenttechnology, or modest extrapolations of currenttechnology. The Japanese concept (Oka andKoshizuka, 1993; Oka et al., 1995) utilizes apumped direct cycle and their studies addressboth thermal and fast reactors. The Russian con-cept of Silin et al. (1993) utilizes primary- andsecondary-coolant circuits, where the primary cir-cuit is driven by natural convection.

Two options are currently being studied for theadvanced supercritical CANDU reactor concept:a conventional pumped version and a natural-convection version. General design details aregiven in Table 1. The focus in this paper is on thenatural-convection option.

Fig. 1. CANDU calandria schematic.

G.R. Dimmick et al. / Nuclear Engineering and Design 215 (2002) 27–38 29

Fig. 2. Flashing-driven PMCS concept.

2. Natural circulation and parallel-channelinstability

The conservation relations for the flow of athermally expandable fluid are well known andare not repeated here. Heat addition, or pressuredrop, would cause a density difference via thermalexpansion, or phase change by boiling and/orflashing, and provide the differential head re-quired to drive the flow.

The overall loop flowrate, W, in a natural-cir-culation system at subcritical pressure and tem-perature is obtained by combining the continuity,momentum and energy equations and utilizing theBoussinesq approximation, ��=��T, for theexpansion coefficient, �. This is,:

W1�=�2A2�g��

2ZQCPK

�1/3

(1)

where A is the flow area, K the loop loss coeffi-cient, Z the effective driving head, Q the looppower, g acceleration due to gravity, �l is theliquid density and Cp is the heat capacity.

If the applied power causes boiling of the flow,and if homogeneity is assumed and the axial heatinput is uniform, the two-phase flow rate W2� canbe approximated by:

W2��A2gZ��

2Qhfg�

2K�1/3

(2)

or

W2��W1�

� CP

2�2hfg�

�1/3

(3)

where �2=�l/�l−�g and hfg is the latent heat.The term (CP/hfg�) is a dimensionless evaporationnumber.

Thus we expect to find that most loop andsystem parameters have a relatively weak (one-third power) influence on the flowrate. Near andabove the critical point at an absolute tempera-ture, T, where there is no phase-change, we willhave a thermally expandable near-perfect gaseousfluid, with ��1/T.

Thus, to first order, we have the supercriticalflowrate WSC given by:

Fig. 3, based on the properties of light water,illustrates how the CANDU design could evolvein terms of coolant temperature and enthalpy,from conventional pressures and temperatures tosupercritical pressures and temperatures. In thefirst evolutionary step labeled Mark 1, the heat istransferred from a heavy-water primary system toa light-water secondary system at 19 MPa. This isexpected to operate with conventional, or near-conventional, zirconium alloy-clad fuel. This de-sign is similar to a conventional CANDU circuitdesign, except the steam generator becomes asupercritical heat exchanger. In subsequent evolu-tionary steps, labeled Mark 2, both the primaryand secondary systems would under operate un-der supercritical conditions.


Table 1CANDU-X design values

Mark 1n nat con CANDU 6Mark 1 pumped

2280Reactor thermal power (MW) 2280 2159�900�910 668Reactor electrical power (MW)

40Efficiency (%) 35402525 10Inlet header pressure (MPa)

380Core inlet temperature (C) 350 266440430 310Core outlet temperature (C)625Inlet density (kg m−3) 781446115122 690Outlet density (kg m−3)

1768Total core flow 7700253066 5.4Average channel power (MW)4.7Average channel flow (kg s−1) 246.667.27.2 6.5Peak channel power (MW)

465Max sheath temp (C) 530 �320

WSC�W1�

� 1T�

�1/3

=W1�gas (4)

However, the thermal expansion coefficient is,in fact, non-linear with temperature near the criti-cal point, as are many other properties, and theBoussinesq approximation is no longer a goodapproximation. The virial coefficients accommo-date this deviation from perfect-gas behavior, butthe properties are extremely non-linear near thecritical point. The general flow variation withmajor loop parameters (elevations, losses etc.)follows Eq. (4), but with a non-linear expansioncoefficient.

As shown, for example, in Rohatgi and Duffey(1994)), for non-linear static instability of parallelchannels, the condition for instability is given by:��(�P)

�G�

=0 (5)

Applying the necessary constant pressure dropboundary condition for parallel channels, and dif-ferentiating the integral form of the mixture mo-mentum equation, one may solve for the criticalmass velocity, �G�, when the flow is unstable.The result is a cubic in the mass velocity, corre-sponding to the inflections in the pressure dropversus flow rate curve for the heated channels.

For adiabatic supercritical flow in parallelchannels, the Eq. (5) criterion yields, after muchalgebraic manipulations, the following mass ve-locity relationship for the stability boundary:

�G��C�

De

,

where � is the kinematic viscosity, De is theequivalent diameter, and C is the constant ofproportionality for the Reynolds number depen-dency of the friction factor. Thus, this wouldindicate that for the adiabatic case, the depen-dency of the friction losses and the viscosity varia-tion are quite important.

3. PMCS experimental set-up and result

3.1. PMCS experimental set-up

The concept feasibility tests reported inKhartabil and Spinks (1995) showed oscillationswith a varying calandria inlet temperature. Toisolate the effect of flashing, the loop wasmodified to maintain a constant inlet temperature

Table 2SPORTS flowrate predictions for simple loop (Fig. 9)

Power (MW) Outlet temperatureFlowrate (kg(°C)s−1)

5.16 3894.03955.094.54044.955.0

Inlet temperature, 350 °C; Inlet and outlet pressure, 25 MPa;flow area, 4400 mm2.


Fig. 3. CANDU evolution.

to the calandria. A schematic of the modified testloop is shown in Fig. 4. It consists of a calandria(with a mixing loop), a glass riser (8.5-m-longglass pipe with an inner diameter of 10 cm), acondenser/tank, and a steel downcomer. Follow-ing accepted practice (D’Auria et al., 1991), thetest loop was scaled by keeping the height similarto the reactor’s height and reducing the volume ofall pipe components by a factor of 60. The calan-dria volume was scaled by a factor of 600.

For the separate-effect tests, one requirementwas to maintain a constant calandria inlet temper-ature. This isolated the effects of varying inlettemperature on loop stability. The simplest way toachieve this was to keep the cold-leg at the con-denser saturation temperature (�100 °C), whichis well below the saturation temperature at thecalandria exit (�117 °C). The water volume inthe tank below the condenser helped absorb varia-tions in the condensate temperature and ensuredthat the cold-leg temperature remained constant.

The reduction of three-dimensional effects inthe calandria was achieved by adding a mixingloop. The mixing loop circulated water throughthe calandria and had a negligible effect on theoverall loop behavior. The loop measurementswere, the total calandria power, flowrate into thecalandria, average void fraction (7.90 m from thecalandria exit), and temperatures and pressures atvarious locations (see Fig. 4). Flowrate was mea-sured using a turbine flowmeter with a range of1.8–36.5 kg s−1. Average void fraction was mea-sured using a �-densitometer system. Tempera-tures were taken using Type K thermocouples,while pressures were measured using Rosemountpressure cells.

3.2. PMCS experimental results

The first test was done with the mixing loopturned off to provide a reference point for runswith a mixed calandria. The results, given in Fig.


5, show that the flow is unstable for powers lessthan 112 kW (approximately 25% simulated fullpower). These flow oscillations were accompaniedby oscillations in the outlet temperature, whichcaused the location of the flashing point tooscillate.

In subsequent tests, the mixing loop was oper-ated to better simulate a mixed calandria. Theeffect of calandria mixing can be clearly seen inFig. 6, which shows the pressure drop across thevalve in the mixing loop, and the flowrate, at aconstant power of 73 kW. When the mixing loopwas operated (positive value for �P), the flow wasstable. When the mixing loop was turned off(valve �P=0), the flow was unstable and then

Fig. 5. Results without calandria mixing.

Fig. 4. Schematic of test loop for separate-effect tests.

became stable when the mixing loop was operatedagain.

Although mixing the calandria eliminated mostof the oscillations, there were some minor oscilla-tions at power levels below 108 kW. Visual obser-vations in the top part of the riser showed thepresence of slug flow when two-phase conditionsexisted in the riser. Closer examination of thetwo-phase region showed that discontinuities inthe glass riser (at the flanges connecting the 1m-long glass segments) acted as preferential loca-tions for void generation. This was not surprising,because, the glass surface was very smooth and a

Fig. 6. Effect of calandria mixing on flow stability at 73 kW.


Fig. 7. Effect of increased nucleation sites.

10 to 90 kW. Visual observations during the testindicated single-phase flow at 10 kW followed bytwo-phase at 20 kW and higher powers. TheCATHENA predictions significantly overpre-dicted the flowrate. The discrepancy (not shownhere) were much greater than the uncertainties inthe experimental measurements.

Examination of the experimental results showedthat a relatively high superheat (�2.4 °C) wasrequired to initiate flashing, whereas the CA-THENA code predicted the onset of flashing closeto saturation conditions. These experimental re-sults are consistent with other critical two-phaseflow experiments by Reocreux (Reocreux, 1974),which were also described in Downar-Zapolski etal. (1996). Those experiments reported that flash-ing occurred at temperatures that exceeded thesaturation temperature by 2–3 °C. CATHENA,on the other hand, predicted a much smallersuperheat (a fraction of a degree). This dis-crepancy in the onset of flashing explains whyCATHENA predicted more void in the riserwhich, in turn, caused the flowrate to beoverpredicted.

The effect of delayed flashing was investigatedby imposing a higher pressure at both boundaryconditions, and fixing the calandria inlet tempera-ture at 100 °C to artificially raise the flashingpoint. A value of 111 kPa was chosen, because,the saturation temperature (102.4 °C) at thatpressure equaled the experimental calandria outlettemperature at 10 kW, where single-phase flowwas observed. The improved CATHENA predic-tions are shown in Fig. 8, where the differencebetween predicted and measured flowrate wasnow greatly reduced. While the imposed higherpressure did not result in the flashing pointmatching exactly the experimental values at allpowers, the comparisons confirm that the differ-ence between the predicted and measuredflowrates was caused primarily by incorrect pre-dictions of the superheat temperature at the onsetof flashing.

It should be noted that the CATHENA codewas used as a ‘black box’ in the simulationsreported here. CATHENA is a licensing code thatallows the user only to control the data input. Notuning of the code was done, and no enhancement

larger superheat was required to generate void atthe continuous glass surface than at the disconti-nuity between the two glass pipes. This disconti-nuity caused non-uniform void generation alongthe pipe (in the direction of decreasing pressure),and resulted in slug flow and small-scale flowoscillations at low powers.

This phenomenon was investigated experimen-tally by inserting a rough wire in the middle of thetop glass section, where the effect of preferrednucleation sites caused slug flow. Tests with thewire insert clearly showed the formation of voidon the wire surface, but not on the smooth glasssurface. This uniform void generation eliminatedslug flow and the associated flow oscillations atlow powers. The results, given in Fig. 7, show thatthe flow is stable at all powers (what appears tobe flow oscillations at powers less than 50 kW areflow transients associated with the powerincreases).

3.3. PMCS CATHENA simulations

The geometry modeled by CATHENA isshown in Fig. 4. Results from the wire-insertexperiment (with calandria mixing) were used tocompare against CATHENA’s predictions. Thecase of a gradual power change was simulated.This corresponded to the test results shown inFig. 7, where power was increased gradually from


Fig. 8. Low-power comparison with CATHENA using a con-denser pressure of 111 kPa.

percritical water. The initial and boundary condi-tions were specified, as well as the core andheat-exchanger power and the loop’s resistancecoefficients and geometric details. SPORTS iter-ates on the inlet flowrate until the specified outlet-pressure boundary condition is matched. Therebyall properties, such as density, velocity, pressure,temperature and enthalpy at every node in thecircuit is obtained.

A simple configuration was chosen with similaroverall dimensions and loss-factor distribution toa CANDU configuration, as such a system isamenable to an analytical solution. In naturalcirculation, global dimensions, mean pipe diame-ter and representative loss-factor distributions areall that are required to determine the approximateflow rate and temperature rise across the channel.Thus, the qualitative parametric trends shownhere should approximate the parametric trends ofan actual (and more complicated) CANDU loop.A lot of insight can be gleaned from analyticalsolutions of a simplified system as this.

To understand the parametric trends, manythousands of these calculations were done fordifferent conditions and loop resistance. The out-put results are plotted in Fig. 10 to show thetrends. Fig. 10 confirms the expected trends— fora given inlet temperature, the outlet temperatureincreases with increasing channel power, with de-creasing elevation difference between the core andheat exchanger, and with increasing circuit losscoefficient.

Care was taken to ensure that the solutionswere truly converged solutions and were not af-fected by numerical effects, such as diffusion,damping, etc. For these steady-state results,smaller node sizes were taken until the yieldedresults appeared independent of node size.

The effect of the large density and enthalpychanges around the critical temperature can beseen in Fig. 11. Below about 370 °C the outlettemperature/channel power surface is relativelyflat (except for the high loss coefficient combinedwith high power), whereas when the inlet temper-ature exceeds 380 °C the outlet temperature risessharply regardless of the channel power and lossfactors. If the core inlet temperature is below thecritical temperature and the core exit temperature

to the code was made to match the experimentalresults. On occasions smaller time steps were triedto confirm that the results were time step indepen-dent and the solutions obtained were convergedsolutions.

4. Supercritical water-cooled CANDU

4.1. Steady-state calculations

To investigate the feasibility of natural-convec-tion cooling for the primary circuit of a supercrit-ical water-cooled CANDU reactor, calledCANDU-X, a steady-state calculation of the sim-ple natural-circulation loop, shown in Fig. 9, wasperformed using the SPORTS code (Chatoor-goon, 1986). SPORTS includes the full physicalvariations of the thermophysical properties of su-

Fig. 9. Simple natural-convection loop.


Fig. 10. CANDU-X natural circulation, inlet temperature 380 °C.

is above the critical temperature, there would be avery large density difference between the inlet andoutlet. This large density difference would pro-mote a large natural-convection driving force,which would be compensated by the increasedoutlet pressure drop resulting from the high coreoutlet velocity.

To optimize the effects of elevation and losscoefficients within a specified maximum outlettemperature, with a high-powered channel, it isnecessary to keep the inlet temperature below thecritical temperature and the outlet temperatureabove the critical temperature. If the inlet temper-ature is allowed to rise above the critical tempera-ture, this will result in a very much higher outlettemperature. Although this would enhance thethermodynamic efficiency, it also would requirelow neutron cross-section materials capable ofoperating in the 600 °C range. This is beyond thecapability of untreated and uncoated zirconiumalloys. Even operating in the 450–500 °C range,and at 25 MPa, will require significant designchanges to the channel.

To address the thermalhydraulic issues raisedby the CANDU-X concept, experimental and an-alytical programs are underway. A large pumpedloop using supercritical CO2 is being used to studyheat transfer and pressure drop characteristics, aswell as fluid-to-fluid modeling studies. This loopwill subsequently be converted to natural circula-tion to study flow instability in single- and paral-lel-channel assemblies. Additionally, a smallnatural-convection supercritical water loop hasbeen constructed as a material test facility.

4.2. Supercritical stability analysis

The SPORTS code (Chatoorgoon, 1986) wasalso used to perform stability assessment of theCANDU-X concept. SPORTS performs a stabil-ity analysis by first computing the system’ssteady-state solution, then introducing a perturba-tion in the inlet flowrate, and a real-time simula-tion is performed by solving the non-linearconservation equations. If the initial perturbationdies out and the system returns to the original


Fig. 11. CANDU-X natural circulation—significance of T.

steady state, the system is deemed ‘stable’. If,however, the initial perturbation grows causingoscillations or divergence from the initial steadystate, the system is deemed ‘unstable’.

A simple natural-convection loop with an inletflow temperature of 350 °C, and an inlet andoutlet pressure of 25 MPa was modeled. Theinterior pipe walls were considered ‘rough’, andthe pressure loss was calculated by standard pro-cedure based on whether the flow was laminar orturbulent. Table 2 gives the steady-state flow ratesand outlet temperatures calculated by SPORTS.Fig. 12 depicts SPORTS stability simulations forpowers of 4.0, 4.5 and 5.0 MW. At 4.0 MW thesystem is stable, but at 4.5 MW the system isunstable, as diverging oscillations become evident.Fig. 13 plots the steady-state characteristic of thesystem. It is interesting to note that for powersgreater than about 4.0 MW, this natural-circula-tion system shows a decreasing inlet flow rate withincreasing power-quite unusual! Chatoorgoon(Chatoorgoon, 2000) concluded that this is a newtype of flow instability, different from the modes

that exist in two-phase flow, and developed ananalytical solution for a single-channel, natural-convection loop.

5. Summary and conclusions

A technical program is underway on applica-tions of natural-circulation concepts to reactorsystems at both low and very high (supercritical)

Fig. 12. SPORTS stability assessment of simple loop (Fig. 9)(see also Table 2).


Fig. 13. SPORTS stability assessment of simple loop.

channel inlet temperature is just below the criticalpoint. If the inlet temperature rises significantlyabove the critical point, the high outlet tempera-tures, which although precluding the use of con-ventional materials, still enables natural-cir-culation flow to be adopted as in some supercriti-cal boiler systems.

Analytical studies into the stability of supercrit-ical systems have been initiated. A new type offlow instability was found and reported byChatoorgoon (Chatoorgoon, 2000). An experi-mental program using a natural-convection super-critical water loop and a pumped supercriticalCO2 loop for thermalhydraulic and materialsstudies have also been assembled.

References

Chatoorgoon, V., 1986. SPORTS, a simple non-linear ther-malhydraulic stability code. Nucl. Eng. Des. 93, 51.

Chatoorgoon, V., Stability of supercritical fluid flows in asingle-channel natural-convection loop, Int. J. of Heatand Mass Transfer, accepted for publication May 2000.

D’Auria, F., et al., 1991. Scaling of natural-circulation inPWR systems. Nucl. Eng. Des. 132, 187–205.

Dimmick, G.R., Spinks, N.J., Duffey, R., An AdvancedCANDU Reactor with Supercritical Water Coolant:Conceptual Design Features, The 6th International Con-ference on Nuclear Engineering, San Diego, California,10–14 May 1998.

Downar-Zapolski, P., et al., 1996. The non-equilibrium re-laxation model for one-dimensional flashing flow. Int. J.Multiphase Flow 22 (3), 473–483.

Hanna, B.N., 1998. CATHENA—A thermalhydraulic codefor CANDU analysis. Nucl. Eng. Des. 180 (2), 113–131.

Khartabil, H.F., Spinks, N.J., An Experimental Study of aFlashing-driven CANDU Moderator Cooling System,16th Annual Conference, Canadian Nuclear Society,Saskatoon, Canada (1995).

Oka, Y., Koshizuka, S., 1993. Concept and design of a su-percritical-pressure, direct-cycle light water reactor. Nucl.Technol. 103, 295–302.

Oka, Y., Koshizuka, S., Jevremovic, T., Okano, Y., Kitoh,K., 1995. Direct-cycle, supercritical-pressure, light-water-cooled reactors for improving economy and plutoniumutilization. Proc. Global ’95 I, 930–937.

Reocreux, M., Contribution a l’etude des debits critiques enecoulement diphasique eau vapeur, Ph.D. thesis, Univer-site Scientifique et Medicale de Grenoble, France (1974).

Rohatgi, U.S., Duffey, R.B., Natural Circulation and Stabil-ity Limits in Advanced Plants: The Galilean Law, Newtrends in Nuclear System Thermohydraulics, Pisa, MayJune 1994.

pressures. These activities are aimed at possible orpotential increases to the thermodynamic and op-erating efficiencies of the system, simplifying thesecondary moderator heat removal, and the pri-mary-side flow.

Experiments have demonstrated that the con-cept of a flashing-driven natural-circulation loopfor passive moderator cooling is feasible. CA-THENA simulations were reasonably successfulin predicting the loop behavior and were instru-mental in guiding the experimental program.Refinements to the CATHENA model led to spe-cial-effect tests to further understand the flashingphenomenon. These special-effect tests demon-strated that two factors were important for flowstability. They were, (1) flow patterns and temper-ature distribution within the calandria; and (2)flashing superheat requirements and the presenceof nucleation sites.

When the temperature distribution within thecalandria was such that temperature oscillationsoccurred at the calandria outlet, the flow oscil-lated, because, the elevation of the onset of flash-ing changed with temperature. Mixing thecalandria water eliminated this problem. This sig-nificantly improved loop stability.

The long-term evolution of the CANDU systemmay be towards operation with supercritical-watercoolant. Preliminary calculations have shown thatfull-power operation with natural-convectioncooling is feasible with existing materials if the


Silin, V.A., Voznesensky, V.A., Afrov, A.M., 1993. The lightwater integral reactor with natural circulation of thecoolant at supercritical pressure B-500 SKDI. Nucl. Eng.Des. 144, 327–336.

Spinks, N.J., Passive Emergency Heat Rejection Conceptsfor CANDU Reactors, Proceedings of International Nu-clear Congress 93, vol. 3, Paper No. C22.1, Toronto,Canada (1993 Oct. 3–6).

natural-convection studies for advanced candu reactor concepts

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