kim - cfd analysis of calandria candu-6

8/9/2019 Kim - CFD Analysis of Calandria CANDU-6

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Nuclear Engineering and Design 236 (2006) 1155–1164

Analyses on uid ow and heat transfer insideCalandria vessel of CANDU-6 using CFD

Manwoong Kim ∗ , Seon-Oh Yu, Hho-Jung KimKorea Institute of Nuclear Safety, 19 Guseong-dong, Yuseong-gu, Daejeon 305-338, Republic of Korea

Received 25 January 2005; received in revised form 24 October 2005; accepted 31 October 2005

Abstract

In a CANada Deuterium Uranium (CANDU) reactor, fuel channel integrity depends on the coolability of the moderator as an ultimate heatsink under transient conditions such as a loss of coolant accident (LOCA) with a coincidence of a loss of emergency core cooling (LOECC), aswell as a normal operating condition. This study presents the assessments of moderator thermal–hydraulic characteristics in the normal operatingcondition and one transient condition for CANDU-6 reactors, using a general purpose three-dimensional computational uid dynamics code. Thisstudy consists of two steps. First, an optimized calculation scheme is obtained by many-sided comparisons of the predicted results with the relatedexperimental data, and by evaluating the uid ow and temperature distributions. Then, in the second step, with the optimized scheme, the analysesfor real CANDU-6 of normal operating condition and transition condition have been performed. The present model has successfully predicted theexperimental results and also reasonably assessed the thermal–hydraulic characteristics of the real CANDU-6 with 380 fuel channels. Flow regimemap with major parameters representing the ow pattern inside Calandria vessel has also proposed to be used as operational and/or regulatoryguidelines.© 2005 Elsevier B.V. All rights reserved.

1. Introduction

Like a loss of coolant accident in a pressurized water-cooledreactor, loss of coolant accident (LOCA) in CANada DeuteriumUranium (CANDU) reactors can be the precursors to fuel dam-age, which can result in severe radiologicalconsequences. How-ever, a CANDU reactor has the unique safety features with theintrinsic safety related characteristics that distinguish it fromother water-cooled thermal reactors. One of the safety featuresis that the heavy water moderator is continuously cooled, pro-viding with a heat sink for decay heat produced in the fuel if there is the LOCA with the coincident failure of the emergencycoolant injection system. Under such a dual failure condition,the hot pressure tube (PT) would deform into contacting withthe Calandria tube (CT), providing with an effective heat trans-fer path from the fuel to the moderator. Following the contactbetween the hot PT and the relatively cold CT (call as PT/CTcontact), there is the spike of the heat ux in the moderator

∗ Corresponding author. Tel.: +82 42 868 0473; fax: +82 42 861 9945. E-mail address: [email protected] (M. Kim).

surrounding the CT, which could lead to sustained CT dryout,as shown in Fig. 1. The prevention of CT dryout due to PT/CTcontact dependsonavailable localmoderatorsubcooling.Highermoderatortemperature (or lower subcooling) would decrease themargin of the CTs to dryout, and it is considered that the fuelchannel integrity depends on the coolability of the moderator asan ultimate heat sink.

In this regard, to estimate the local subcooling of the moder-ator inside Calandria vessel under normal operational conditionand/or transient conditions is one of the major concerns in theCANDU safety analysis, so the Canadian Nuclear Safety Com-mission (CNSC), a regulatory body in Canada, categorized thetemperature prediction of moderator for fuel channel integrityas a general action item and recommended that a series of exper-imental works should be performed to verify the system eval-uation codes comparing with the results of three-dimensionalexperimental data. In terms of predicting the moderator temper-ature in a CANDU reactor, some studies have been performedin Canadian nuclear industry. Koroyannakis et al. (1983) exper-imentally examined the ow phenomena formed by inlet owsand internal heating of a uid in a Calandria-like cylindricalvessel of Sheridan Park Engineering Laboratory (SPEL) exper-

0029-5493/$ – see front matter © 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.nucengdes.2005.10.018


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1156 M. Kim et al. / Nuclear Engineering and Design 236 (2006) 1155–1164

Fig. 1. PT/CT contact phenomena: (a) before and (b) after

imental facility. They observed three ow patterns inside testvessel and their occurrence was dependent on the ow rate and

heat load. Quraishi (1985) simulated the uid ow and pre-dicted temperature distributions of SPEL experiments using thecomputational codes, 2DMOTH. Carlucci and Cheung (1986)investigated the two-dimensional ow of internally heated uidin a circular vessel with two inlet nozzles at the sides andoutlets at the bottom, and found that the ow pattern wasdetermined by the combination of buoyancy and inertia forces.Collins (1988, 1995) carried out the thermal hydraulic analy-ses for SPEL experiments and Wolsong units 2–4, respectively,using PHOENICS code using porous media assumption for fuelchannels.

Although some computational codes were used to predict

moderator temperature for some experimental works and realCANDU-6 nuclear power plants, they could not simulate theowbehaviors inside Calandria-likevessel three-dimensionally.Moreover, although some studies were performed with three-dimensional computational codes, the porous media approxi-mation instead of real heaters or fuel channels located in thevessel as used. However, the secondary ow can be observedamong the horizontal fuel channels, which plays an impor-tant role in uid ow and heat transfer characteristics. There-fore, it is necessary to establish the analyses model whichcan simulate the ow behaviors and predict the temperaturedistributions reasonably inside Calandria vessel using the realgeometry of fuel channels. Yu and Kim have conducted mod-erator temperature prediction under 35% RIH Break LOCAwith LOECC in a CANDU-6 reactor using the real geometry(2004).

The objectives of this study are to establish an adequate the-oretical basis for the models, and to analyze the uid ow andtemperature distributions inside Calandria vessel of CANDU-6.This study consists of two steps. First, an optimized calculationscheme is obtained by many-sided comparisons of the predictedresults with the related experimental data, and by evaluating theuid ow and temperature distributions. Then, in the secondstep, with the optimized scheme, the analyses for real CANDU-6 of normal operating condition and transition condition have

been performed.

2. Thermal–hydraulic analyses

2.1. Analyses model

Duringthe normaloperation of a CANDU-6 reactor,therearetwo major ows inside the Calandria vessel, i.e., buoyant uidow formed by internal heating and momentum uid ow bythe jet ows through inlet nozzles, respectively. The cold uidis heated up with the heat generated not only by slowing downof neutron and directed radiation heating of gamma rays in thevessel, but also by the convective heat transfer from fuel channelsurface. As a large amount of heat load to uid is continuouslytransferred, the heated uid ows upward due to the buoyancyforce. So it is noticeable that both the total heat load to uid andinlet ow rate through the nozzles are the major parameters toaffect the ow characteristics inside Calandria vessel. However,the ow characteristics formed by the combination of heat loadand inlet ow are very complicated to predict analytically in agiven operation condition. Moreover, as the uid passes among

theCalandria tubes,the pressuredrop andmany secondaryowsnear thefuel channels make theexactand detailed analysesmoredifcult. Therefore, in this study, the numerical model for ana-lyzing the thermal–hydraulic behaviors inside Calandria vesselincluding the buoyancy and momentum forces is considered.

All thehorizontal fuel channels as theheaters in theCalandriavessel are modeled in such a way to analyze as a real geome-try, since they, acting as ow resistance, play an important roleto affect the heat transfer and the ow eld. The uid ow isassumed to be incompressible and single-phase ow. The owis simulated using the standard k –ε turbulence model associatedwith logarithmic wall treatment to model turbulence genera-

tion and dissipation rate within the vessel. As to the inner wallsurface of Calandria vessel, an adiabatic boundary condition isapplied. The buoyancy effects are accounted for density changeassumed as the Boussinesq approximation. The conjugated heattransfer analysis method is used to calculate the asymmetryconvective heat transfer on the fuel channel surface so as tomatch the uid–wall interface conditions adequately. To solvethe governing equations, a general purposed three-dimensionalcomputational uid dynamics code, FLUENT6.0, is used. Com-putational domain is discretized into nite control volumes (orcells) using the SIMPLE algorithm the unstructured meshesmostly for ne calculation except the vicinity of fuel channelsand reector region modeled with the structured ne meshesfor calculating the turbulence generation and dissipation rate inaccuracy. In addition, several grid layers are applied to considerthe wall effect of the fuel channels. In the calculation, a numeri-calcalculation is conductedusing a rst order upwind scheme ingeneral and the iteration is progressed until the absolute sum of dimensionless residuals in the numerical solution of governingequations is less than 10 − 3 .

2.2. Analyses of SPEL experiments

To experimentally investigate the owphenomenaby moder-ator inlet jets and internal heating in the Calandria-like cylindri-

cal vessel as the CANDU reactor, a series of experiments were


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M. Kim et al. / Nuclear Engineering and Design 236 (2006) 1155–1164 1157

performed at SPEL in Canada ( Koroyannakis et al., 1983 ). Toverify the present analysis model, the analyses of SPEL experi-ments have been performed.

2.2.1. SPEL experiment The test vessel of SPEL is not a scaled facility of a real

CANDU Calandria vessel, but it has the features of a typicalCANDU reactor, such as a re-circulating jet induced ow, heat-ing of the water by volumetric heat generation and a matrix of horizontal tubes parallel to the cylindrical axis. The vessel con-sists of a transparent acrylic cylindrical shell with 0.737 m innerdiameter by 0.254m long, and 52 copper tubes (38 mm outerdiameter by 254mm long) are arranged on a 75 mm squarepattern and installed inside the tank. Two inlet nozzles weredesigned and installed along the horizontal centerline at eachside of the vessel with an angle of 14 ◦ from the vertical direc-tion. An outlet nozzle was also installed at the bottom of the

Fig. 2. Schematic diagrams of SPEL experiments and calculation domain. (a)

SPEL experimental facility and (b) mesh for calculation on central plane.

Fig. 3. Comparison of experimental and predicted temperatures along a verticalcentreline. (a) Along the port 4 and (b) along the ports 2 and 6.

vessel. The high amperage, low voltage alternating current waspassed via the tubes through the working uid, which acted as auid resistor to generateheat from coppertubesactedas theelec-trodes. A chemical ow visualization technique was employedto determine the predominant ow regime inside the vessel.Detailed temperature proles inside the vessel were obtainedusing an optical ber probe.

To simulate the SPEL experiments, all dimensions are set tothe experimental apparatus as shown in Fig. 2, and the numberof cells is about 53,000 with unstructured meshes in the tuberegion andstructured meshes in theouter region as an optimizednumbers in accordance with the mesh sensitivity analyses.

2.2.2. Results and discussionNumerical calculations were carried out for the Calandria-

like vessel of SPEL and present analysis model predicted theuid temperature reasonably, i.e., the maximum temperatureinside Calandria-like tank is 40.3 ◦ C, which is similar to theSPEL experimental result of 41 ◦ C. Fig. 3 shows the compar-isons of experimental and computed temperatures along a verti-cal centerline. As shown in Fig. 3(a), the temperature decreasesslightly at upper region and decreases gradually along the ver-tical centerline. In the meanwhile, the temperature predicted by2DMOTH is underestimated at the bottom region of the Calan-dria compared with that of the SPEL experiment. However, as

shown in Fig. 3(b), the temperature decreases sharply due to the


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Fig. 4. Typical ow patterns for SPEL experiments. (a) Buoyancy-dominated ow ( V in = 0.13 m/s, T in = 30◦ C and heat load= 10kW), (b) mixed type ow

(V in = 0.25 m/s, T in = 30◦ C and heat load= 10 kW) and (c) momentum-dominated ow ( V in = 0.40 m/s, T in = 30

◦ C and heat load = 10kW).

momentum of inlet ow at the regions on the ow passage of uid. However, the temperature increases slightly due to theheatgeneration, and decreases again at the bottom region due to theforcedconvection.As theabove results, it canbe considered thatthepresent model has thecapability to properly analyze theuidow subject to the combined buoyancy and momentum forcessimultaneously.

The uid ow inside the vessel is very complex due to theinteraction between the momentum force generated by the inlet jets and the buoyancy force by heat load to uid. So, to identifytheow patterns formed inside thevessel with some major input

conditions, some calculations have been performed. According

to the computational results for SPEL geometry, it is found thatthreeow patterns, e.g.,momentum-dominated ow, buoyancy-dominated ow and mixed type ow, respectively, are observedinside the Calandria vessel as shown in Fig. 4. It is also noticedthat theonset conditions of theseow patterns mainlydependonthe heat loadand inlet ow rate. In the condition of low heat loadand high ow rate, the effect of momentum force on the mainstream inside Calandria vessel is more dominant than that of buoyancy force. Although the entering uid is heated up by heatgeneration, the uid ows toward the outletdue to small amountof heat load and strong inlet jets. Two inlet ows encounter

each other at the nearly center region, and discharge through the


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Fig. 5. Effect of inlet velocity on temperatures.

outlet. In this condition, the location of high temperature of uidis the near-bottom of Calandria, and the momentum-dominatedow forms. In the condition of high heat load and low owrate, however, the effect of buoyancy force on the main streambecomes more signicant than that of momentum force. Due tothe strong heat generation, the entering cold uid is heated upenoughto ow upwardbydensity gradient,and it ows along theinner wall of vessel anddischarges through theoutlet, producingthe secondary ows in the vicinity of the Calandria wall andheaters. Thelocationof thehigh temperature of uid is theupperregionof Calandria, andthe buoyancy-dominated force is found.Inthe condition ofcertain heatloadandowrate, due tothe forcebalance of momentum andbuoyancyforces, themixed type owformsandhigh temperature of uid isplacedat thecenter region.

It is found that the inlet velocity and heat load are the majorparameters affecting on the formation of ow patterns. So, to

understand theeffect of the twomajor parameters quantitatively,some sensitivity analyses were performed. In Fig. 5, as the inletvelocity is increased, the maximum and outlet temperatures aredecreased due to the increasing effect of inlet jet momentum. Itis also noticed that the ranges of inlet velocity where the mixedtype ow is observed, broaden as the heat load is increased.When the heat load is increased, the maximum and outlet tem-peratures are increased, as shown in Fig. 6. In particular, in thecondition of high moderator ow rate (momentum-dominatedow), the difference between maximum and outlet temperatureis increased with the increased heat load.

2.3. Analyses of CANDU-6 reactor

The analyseson the thermal hydrauliccharacteristicsof mod-erator inside Calandria vessel in the normal operating condition

Fig. 6. Effect of heat load on temperatures.


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Fig. 7. Calandria vessel of CANDU-6.

of a real CANDU-6 reactor have been carried out using thepresent optimized model. In addition, transient analyses werealso performed to estimate the fuel channel integrity during theLOCA with the loss of emergency core cooling (LOECC).

2.3.1. Calandria vessel of CANDU-6 reactor A Calandria vessel of CANDU-6 has the shape of the cylin-

drical tank, 6 m long and 7.6 m diameter at its widest point. Inthe core region, there are 380 fuel channels with 0.131 m of outer diameter which accounts for about 12% of the Calandriavessel volume and acts as hydraulic resistance of the uid ow.In addition, there are a number of horizontal and vertical reac-tivity mechanisms. As shown in Fig. 7, the eight inlet nozzlespointing upward with an angle of 14 ◦ from the vertical direc-tion are installed at the middle of each sidewall of Calandriavessel symmetrically in the axial–center plane, but are asym-metrically placed axially. Two outlet ports are symmetricallylocated axially but areasymmetrically placedin theaxial–centerplane.

Fig.8(a) shows thecomputational mesheson theaxial–centerplane to analyze theuid dynamicsandheat transfer characteris-tics inside theCalandriavesselof CANDU-6. In thecore region,380 fuel channels with 0.286 m of square pitch are simulated soit is possible to observe the ow behaviors around the channels.The total cell number is about 830,000 consisting of unstruc-tured meshes in the core region and structured meshes in thevicinity of fuel channels and reector region to save the calcula-tion time. In addition, to consider the channel effects, ve gridlayers are averagely applied, as shown in Fig. 8(b).

2.3.2. Results and discussion2.3.2.1. Steady-state condition: normal operation. Under thenormal operating condition of CANDU-6, most of heat isdeposited in the moderator by direct heating of neutrons, decayheat from ssionproducts,and/or gamma rays, while theheat byconvection from thesurfaceof fuel channels accounts for a smallportion of the total heat load. Forconservative analyses, the totalheat load to the moderator is taken to be 103 MW (about 103%

Fig. 8. Computational Meshes for calculation of a CANDU-6 reactor. (a) Inside calandria vessel, (b) fuel surface.


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Fig. 9. Velocity and temperature distributions in normal operating condition. (a) Velocity magnitude and (b) temperature.

of full power) consisting of 98.7MW by volumetric direct heat-ingand4.3MW by convective heat from fuel channel surface. Inaddition, it is assumed that the convective heat was transferredto the moderator uniformly with axial direction. The total owrate through the eight inlet nozzles is taken tobe the design valueof 940 l/s with 45 ◦ C of inlet temperature. The thermal bound-ary condition of the Calandria outer wall is assumed adiabaticconservatively.

Under normal operating condition, the predicted maximum,outlet and average temperatures of the moderator are 95.10,68.99 and 75.21 ◦ C, respectively, and no boiling is predicted.Especially, the present analysis model could predict properlythe moderator outlet temperature compared with those of therecommended design value in the technical guides; CANDUreactor should be operated with about 74 ◦ C of outlet tempera-ture and should not be operated with exceeding 83 ◦ C of outlettemperature.

Fig. 9 shows the complex and asymmetric ow patterninside Calandria vessel in the condition of the normal opera-tion, induced by the combination of the buoyancy and the inletmomentum forces and by the geometric effects such as the exis-tence of fuel channels actingas theow resistance andtheasym-metric installation of outlet ports. Moreover, the ow reversesits direction near the left inlet nozzles and goes down toward the

outlets, and the temperature and velocity proles show a steep

change around the ow reversal region. Fluid heated in the coreregion ows upwardby thebuoyancy force,andchanges itsowdirectionbecause of encounteringwith the inlet cold ows at thecircumferential upper region. The hottest spot is located at theupper regionof thecore region, which slightly leans to right side.Therefore, the ow pattern in the normal operating condition of CANDU-6 is predictedas mixed type ow combined buoyancy-dominated ow and momentum-dominated ow. Although theassumption of uniform heat transfer from the fuel channels tomoderator, the predicted axial distributions of temperature isnot uniform, which is also induced by the existence of fuelchannels and is combined effect of buoyancy and momentumforces.

Fig. 10 shows the temperature and velocity distributionsalong a vertical centerline of the Calandria vessel. Moderatorwith relatively high temperature is placedupper regionof Calan-dria vessel, due to buoyancy force and its temperature decreasesgradually along the vertical line. Moreover, the two temperatureproles along the symmetric vertical centerlines are asymmet-ric each other. Moderator velocity magnitude changes sharplynear the wall of Calandria vessel, so the velocity gradient is con-centrated on there. The velocity in the core region is relativelysmall due to the hydraulic resistance, i.e., fuel channels, as wellas the force balance between buoyancy and momentum forces.

In addition, the secondary ows with small velocity among the


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Fig.10. Velocityand temperature prolesalong thevertical centerline in normaloperating condition. (a) Velocity magnitude and (b) temperature.

fuel channels are found, andplay important roles of heat transferand ow behaviors.

Three ow patterns inside Calandria vessel are also observedaccording to the variation of inlet velocity, which is similar withthose of SPEL experiments as shown in Fig. 11. However, atthe bottom region the temperature and velocity distributions areslightly lean toward theoutletsdueto theasymmetric installationof two outlets.

2.3.2.2. Transient analyses: 35% reactor inlet header (RIH)with LOECC. To perform the transient analyses, the transientcondition after the 35% reactor inlet header break LOCA withthe LOECC is considered. The initial condition for the transientanalysis is the steady-state solution of normal operation of theCANDU-6 reactor. Fig. 12 shows the typical prole of total heatload transferred to moderator in that transient condition. It alsoshows the many peaks of total heat load between about 20 and40 s due to PT/CT contact. Based on 40 s, the transient condi-tion can be divided into the blow-down phase (up to 40 s) andthe post-blow-down phase (after 40s).

The heat load generated in blow-down phase consists of twoheat sources: (i) one is the volumetric ones such as ssion prod-uct decay heat and neutronic power, which are exponentially

decreasing and (ii) the other is the convective heat transfer by

PT/CT contact in the critical pass. After reactor trip initiated,the increase of void generation due to the leakage of primarycoolant during LOCA without ECC injection induces the powerpulse caused by void reactivity. The sharp peak of heat load tothe moderator takes place at about 1 s and the total amount of theheat load to moderator amounts to about 250 MW. However, theeffect of the sudden heat load increase due to the power pulse isnot signicant because the heat sink capability of the moderatoris large enough compared to the effect of this pulse. As shown inFig. 12, while a slight increase of the maximum temperature isobserved due to sudden increment of heat load at about 1 s, theow pattern is maintained the same as that of the normal oper-ation. During the time span of 1–20s, PT/CT contact does notoccur and the temperature of moderator is decreasing continu-ously until PT/CT contact occurs because of the decreasing heatload ofvolumetricheat sources.Around20 s, thelargeamount of heat load generates from the fuel channels due to PT/CT contactand oscillates severely. The predicted maximum local temper-ature of moderator reaches to about 95.2 ◦ C, so the moderator

canbe maintained subcooledconditionwithout boiling since themaximum temperature is lower than the saturation temperature.In the post-blow-down phase (after 40 s), although the PT/CTcontact could be taken place in each of the 190 channels of thebroken loop, the total heat load is not large enough to cause themoderator to boil and the moderator temperature continuouslydecreases.

3. Proposed ow regime map

According to the previous and present analyses, three owpatterns of moderator, i.e., buoyancy-dominated ow, mixed

type ow and momentum-dominated ow are, in general,observed in the Calandria vessel. Among these ow patterns,the maximum temperature predicted in the buoyancy-dominatedow is the highest among three ow patterns. To ensure thesafe operation of the CANDU reactor, so, it is necessary to pro-pose themoderatorow regimemap inside theCalandriavessel,which may be helpful to predict the ow pattern in a given con-dition in advance. This ow regime map could be applied toprotect and mitigate the steady spot in the buoyancy-dominatedow andtheoscillation possible feedback to theneutron kineticsdue to the oscillatory circulating ow pattern by controlling themajor parameters affecting the ow pattern.

To develop the ow regime map, it is necessary to explorethe main parameters representing the ow elds reasonably.In the ow elds inside the Calandria, there are three forcesconsidered: the inertia force, viscous force and buoyancy force.Therefore, two dimensionless numbers, Re and Ar , can be con-sidered to describe the ow regime map explicitly as follows:

Re =inertia forceviscous force

=ρVD

µ

and,

Ar =buoyancy force

inertia force =

gα TD

V 2 =

gαQD

Cp

ρAV 3


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Fig. 11. Typical ow patterns for CANDU-6. (a) Buoyancy-dominated ow ( V in =4.5 m/s and heat load= 103MW), (b) mixed type ow ( V in = 5.3m/s and heat

load = 103 MW) and (c) momentum-dominated ow ( V in = 10.0 m/s and heat load= 103 MW).

where A is the inlet area, m2; C p the specic heat, J/(kg K); Dthe Calandria vessel, m; g the gravity, m/s 2; Q the heat load, W;T the temperature, K; V the inlet velocity, m/s; α the thermalexpansion coefcient, K − 1; µ the viscosity, Pa s and ρ is thedensity, kg/m 3 .

Fig. 13 is the ow regime map for SPEL experiments andalso shows the comparison of the results of SPEL experimentsand present prediction. The present model could predict theexperimental results reasonably, as mentioned before, so it is

considered that the present model is reasonable to estimate

the ow eld where buoyancy and momentum forces coexist.According to the present and experimental results, it could beconcluded that the transitions of ow patterns inside Calandriavessel can be described on the basis of a certain constant Ar number. As to the CANDU-6 reactor, the ow patters under thenormal operational condition for the CANDU-6 reactor are pre-dicted as the mixed type ow and are well agreed with thoseof the CFD simulation. As a result, the proposed ow regimemap could be applied to predict the ow patterns under the some

operational conditions.


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Fig. 12. Heat load proles and temperature variation during transient.

Fig. 13. Proposed ow regime map.

4. Conclusion

To investigate the thermal–hydraulic characteristics of mod-erator ow subject to two forces, i.e., momentum force by inlet jets and buoyancy one by heat load inside Calandria vessel of CANDU-6, the analyses model has been established throughthe comparison with the experimental data and a series of thenumerical simulation has been performed for the normal oper-ating condition and the transient condition of 35% RIH break LOCA with LOECC. The real geometry of CANDU-6 with 380fuel channels is simulated in this work to investigate the oweld near the fuel channels. The present model can be expectedto be of great help to resolve the moderator temperature predic-tion item, which is the one of the generic action items issued by

CNSC. The main conclusions are as follows:

(1) The comparison with the SPEL experimental data show thatthe present model can reasonably predict the temperature

distributions of moderator, which means that the presentmodel has the good capability to properly analyze the uidow subject to the buoyancy and momentum force simulta-neously.

(2) It is found that the major parameters affecting the ow pat-terns are the inlet ow rate and heat load to the uid. Withinlet ow rate, heat load, or both, three ow patterns can bepredicted, that is, momentum-dominated ow, mixed typeow and buoyancy-dominated ow.

(3) In the normal operation of CANDU-6, due to the combinedeffect of momentum and buoyancy, the mixed type owis predicted. Moreover, many secondary ows with smallvelocity in the vicinity of Calandria wall and fuel channelsare found and play an important role of heat transfer.

In the transient condition of 35% RIH break with LOECC,no boiling was predicted although there are a sharp peak of heatload about 1 s and the large amount of heat load due to PT/CTcontact. So, since the moderator within Calandria vessel has

enough coolability as the ultimate heat sink, the fuel channelintegrity can be maintained and assured.

With the major parameters representing reasonably the owpattern, the ow regime map has been proposed for the safeoperation of CANDU reactor and can be applied to predict theow patterns under some operational conditions.

Acknowledgement

The authors would like to acknowledge the support of theMinistry of Science and Technology (MOST) of the Republicof Korea.

References

Carlucci, L.N., Cheung, I., 1986. The effects of symmetric/asymmetric bound-ary conditions on the ow of an internally heated uid. NumericalMethods Partial Differential Equation 2, 47–61.

Collins, W.M., 1988. Simulation of the SPEL Small Scale Moderator Experi-ments using the General Purpose Fluid-ow, Heat Transfer Code PHOEN-ICS. AECL Report. TTR-213.

Collins, W.M., 1995. PHOENICS2 Model Report for Wolsong 2/3/4 Moder-ator Circulation Analysis. Wolsong NPP 2/3/4, 86-03500-AR-053.

Koroyannakis, D., Hepworth, R.D., Hendrie, G., 1983. An ExperimentalStudy of Combined Natural and Forced Convection Flow in a CylindricalTank. AECL Report. TDVI-382.

Quraishi, M.S., 1985. Experimental Verication of 2DMOTH Computer CodeTemperature Predictions. AECL Report. TDAI-353.Seon-Oh, Yu, Manwoong, Kim, 2004. Temperature Prediction under 35%

RIH Break LOCA with LOECC in CANDU-6. In: Proceedings of theNTHAS 4 Symposium.

kim - cfd analysis of calandria candu-6

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