Simulation and mockup tests for developing TRR-II CNS

Physica B 311 (2002) 173–181

Simulation and mockup tests for developing TRR-II CNS

C.H. Leea,*, T. Kawaib, Y.K. Chana, W.T. Hongc, D.J. Leea, T.C. Guunga,K.C. Lana

a Institute of Nuclear Energy Research, P. O. Box 3-3, Lungtan 32500, TaiwanbResearch Reactor Institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka 590-0449, Japan

cGengood Engineering Corporation, No. 36, Lane 362, Ming-Sheng W. Road, Taipei, Taiwan

Abstract

The Taiwan Research Reactor improvement and the utilization promotion project (TRR-II) with Cold Neutron

Source (CNS) was carried out at Institute of Nuclear Energy Research. The CNS with a two-phase thermosiphon loop

consists of an annular cylindrical moderator cell, a single moderator transfer tube, and a condenser. The self-regulating

characteristics of a two-phase thermosiphon loop are investigated against variations of heat load. The experiments on

the thermal-hydraulic characteristics have been performed using a full-scale mockup loop and a Freon-11 was used as a

working fluid. Two cases were evaluated by the simulation and experiments. One case is an ORPHEE-type moderator

cell in which an inner shell is open at the bottom, the other case is one with an inner cavity with no hole at the bottom

but a vapor inlet opening at the uppermost part of the cavity. The flooding limitations, liquid level and void fraction in

the moderator cell as a function of the initial Freon-11 inventory and the heat load are also reported. r 2002 Elsevier

Science B.V. All rights reserved.

Keywords: Cold neutron source; Simulation

1. Introduction

The Taiwan Research Reactor improvementand the utilization promotion project (TRR-II)was carried out by Institute of Nuclear EnergyResearch (INER). The TRR-II reactor is a pool-type of the thermal power of 20MW with a light-water coolant and a heavy-water moderator. Oneof the TRR-II major tasks is to install a ColdNeutron Source (CNS) with a competent bright-ness of cold neutron. The main part of the TRR-IICNS facility [1] consists of a buffer tank ofhydrogen gas, a hydrogen cold loop and thehelium refrigerator.

Using MCNP code [2], the brightness of coldneutron and the nuclear heating are calculated. Anaverage thermal flux over the CNS moderator cellis about 1.45� 1014 cm�2 s�1. It is close to themaximum thermal neutron flux in D2O tank. Themaximum brightness is about 1� 1012 n cm�2 s�1 -(A�1 sr�1 at 4 (A at the entrance of the cold neutronguide. The heat load of moderator cell is estimatedto be 678W of which 578W is due to nuclearheating and 100W is due to the non-nuclearheating.

The hydrogen cold loop is composed of thecondenser, the moderator transfer tube and themoderator cell. A moderator transfer tube ofthe single tube provides the counter-current two-phase thermosiphon in which the liquid hydro-gen liquefied at the condenser streams down into

*Corresponding author. Fax: +886-3-471-1404.

E-mail address: [email protected] (C.H. Lee).

0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 1 3 4 - 6

the moderator cell and the hydrogen vaporevaporated in the moderator cell blows up to thecondenser. For extracting stable cold neutron fluxfrom the CNS, the liquid level in the moderator cellshould be kept stable without a sudden bubblingagainst the heat load disturbances. Kawai et al.[3–5] had performed a series of analytical work forKyoto University Reactor (KUR) CNS with aclosed two-phase thermosiphon. Their approach isadopted for the TRR-II CNS to analyze itsthermodynamic characteristics [6].

The ORPHEE-type CNS has an annuluscylindrical-type moderator cell. The innershell is open at the bottom. The thermo-dynamic considerations show that such a state,in which the inner shell contains only hydrogenvapor and the outer shell only liquid hydrogen,is maintained only when a liquefaction capacityof condenser is large compared to the heatload and three parameters are optimized in abalance. We proposed another type of moderatorcell, with a cylindrical cavity with no hole at thebottom, but a vapor inlet opening at its uppermostpart takes the place in the inner shell in case ofthe ORPHEE-type moderator cell [7]. In thisstructure, a self-regulating characteristic isestablished easily.

Recently, a moderator transfer tube of thesingle tube was designed in FRM-II CNS facility[8]. A single-tube will not only make the structureof the moderator transfer tube simple but alsothe fabrication easy. The TRR-II CNS designedthe single moderator transfer tube withoutthe onset of flooding in the moderator transfertube during operation. The Counter-CurrentFlow Limitations (CCFL), or the flooding isassociated with carrying away the liquid flowup due to the upward high-speed gas flow.The flooding in the moderator transfer tubewill prevent liquid hydrogen falling back tomoderator cell.

The purpose of the mockup test is to validatethe self-regulating characteristics of a two-phasethermosiphon loop against thermal disturbancesfor the CNS design. Moreover, this loop will beused to demonstrate no onset of flooding in asingle transfer tube under the normal and abnor-mal conditions.

2. Experimental apparatus and operating procedure

Fig. 1 shows a schematic diagram of the mock-up test equipment composed of an annuluscylindrical moderator cell, a transfer tube, acondenser, and a buffer tank. The transfer tubemade of SUS 304 is composed of a curved sectionand a snake-shaped section, and 45mm in outerdiameter and 1.2mm in thickness. Two types ofthe moderator cell, as shown in Fig. 2, wereevaluated through experiments and simulation.Type I is an ORPHEE-type moderator cell ofwhich an inner shell is open at the bottom, andtype II has an inner cavity with no hole at thebottom but a vapor inlet opening at its uppermostpart.

The mockup equipment is a full-scale model, buta Freon-11 (R-11) is used as a working fluid forconvenience. The Wallis’s correlation equation isthe most important factor for simulating the onsetof flooding [9]. In order to do the relativeevaluation regarding the Wallis correlation, thesame density ratio (rliquid=rvapor) and a relativeheat load shown in Table 1 are applied to the test.The hydrogen density is that for the saturatedhydrogen at the pressure of 1.2 bar, and a heatload is scaled up in the case of R-11 so thatthe mass-fluxes of the liquid and the vapor in theWallis correlation equation become equal to thehydrogen case.

In order to visualize the flow patterns in themoderator transfer tube, two view ports areinstalled at the upper stream of the moderatorcell and at the down stream of the condenser,respectively. The relationship between the liquidlevel to be stored in the moderator cell and thetotal inventory of R-11 is checked before themockup test.

Fig. 2 also shows that 12 heater rods areinstalled at the bottom of moderator cells forsimulating heat load. The condenser is a shell andtube-type heat exchanger connected to the transfertube, the cooling water system, and a buffer tank.There are 32 liquid tubes in the condenser andeach tube is 1200mm in height, 12mm in diameterand 1mm in thickness. The cooling water flows inthe shell side of the condenser at a temperature ofabout 293K.

C.H. Lee et al. / Physica B 311 (2002) 173–181174

LT

-201

Moderator Cell

EL.1 200

1

2

EL.7 000

LT

-502

LT-2

02

TE-201PT-201

3

EL.3 800

4

Buffer Tank16

CoolingInlet TE-301

2

5

LT-3016

P4

TE-502

Steel Floor

TE-503

PT-501Vent

10" Flange

1 1/2" SCH 10S

Volume ~1.3 m3

1 1/4" SCH 10S, Type I, II1 1/4" SCH 10S, Type I, II

Girth Flange

Drain Device

Heater RoomSealing Gland

Venting PipeSpool Piece Pipe

Buffer Tank

View Port

Moderator Cell

Transfer TubeS-Type Transfer Tube

Components Name

Buffer Tank Supporting Frame

Condenser Supporting Frame

SUS TP304

SUS TP304

MemoMaterials

Glass

SUS 304SUS 304SUS 304

30 kW

1"

SUS 304

SUS 304

No.

Condenser

1

4

23

7

56

Heater

Orifice

10

89

13

1112

16

1415

TE-306

LT

-501

LowerTransferTube

Upper Transfer Tube

TE-501

CoolingOutlet

Condenser

Drain

SUS 304ID

TEH-501TEH-502

Fig. 1. Schematic of mockup test facility for TRR-II CNS hydrogen loop.

C.H. Lee et al. / Physica B 311 (2002) 173–181 175

The instruments include K-type thermocouples,pressure transducers, differential pressure trans-ducers, and the orifice flow meter. The layouts ofinstruments on the loop and the moderator cell are

also shown in Figs. 1 and 2, respectively. Thermo-couple accuracy is 2.2K or 0.75% of the fullscale. Pressure and pressure difference transducersare used to measure the system pressure and

Fig. 2. Schematic of two types of moderator cell for mockup test facility.

Table 1

Comparison of operation conditions between TRR-II CNS and mockup test

TRR-II CNS (H2) Mockup test (R-11)

Operating pressure (kPa) 120 569

Operating temperature (K) 20.8 356.5

Density ratio (rf=rg) 44.4 44.4

Heat load (kW) 0.7 4.7

Mass flux (g/s) 1.6 30

Wallis flooding correlation

J *1=2g þ mJ *1=2

f ¼ C

Vapor ( j *g ) 0.164 0.164

Liquid ( j *f ) 0.0248 0.0248


the local pressure drops in the loop. The accuracyof pressure and pressure difference transducersare 0.25% and 0.75% of their full range,respectively.

The collapsed liquid level is obtained fromthe measured differential pressure divided byliquid density, and gravity in low-flow conditions.The estimation accuracy of the collapsedliquid level is 1.8% of the range. The averagevoid fraction in the outer shell of the moderatorcell is calculated from the ratio of the collapsedliquid level to the two-phase mixture level inthat part.

The operating procedures are illustrated asfollows: (1) evacuate air from the loop using avacuum pump; (2) fill R-11 into the loop up to afixed level in moderator cell; (3) input a heaterpower in the moderator cell to boil R-11 andincrease the pressure of the loop up to theoperating pressure 5.69 bar; (4) control the sec-ondary water flow rate into the condenser inorder to maintain a constant pressure of 5.69 barand a saturated temperature of 357K duringa test.

3. Results and discussions

3.1. Experimental results for type I moderator cell

Initial R-11 inventory was changed from 5.05to 15.33 kg and the heat power was increased stepby step until the onset of flooding. The onset offlooding is considered as the point at which thefalling liquid film becomes a chaotic flow and alarge amplitude wave appears at the interface. Theconstant pressure is maintained at 5.69 bar byadjusting the secondary water flow rate to keep thebalance between the applied heat power and thecooling capacity of the condenser. Fig. 3 showsthe effect of heat power on the liquid level in themoderator cell in the case of the initial inventory6.28 kg. The liquid level in the moderator cellbegins to increase suddenly at the 1.6 kW heatpower and at time elaps of 750 s as shown inFig. 3(b). Then, liquid level in the moderatortransfer tube (see Fig. 3(c)) begins to increaserapidly at a time elaps of 1200 s, and this is

considered partly because some liquid is expelledfrom the inner shell into the outer shell.

Fig. 3(a) shows that the heat power of the onsetof flooding is 4.73 kW for R-11 corresponding to710W in the case of liquid hydrogen. Figs. 3(b)and (c) show the sharp decrease of liquid level inthe moderator cell as well as the rapid increase ofliquid level in the transfer tube at the floodingpower condition. The decrease of liquid level in themoderator cell is also visualized from glass sighttubes at the moderator cell.

Fig. 4 shows the effect of heat power on thelocal void fractions in the lower, middle, andupper regions of the outer shell at the sameinventory. The average fluctuations of void frac-tions in the middle and upper regions are from30% to 35%. The average fluctuation of voidfraction in the lower region is in the range of20–25%. From Fig. 4, we could evaluate the dis-tribution of void fractions in moderator cellas being one of the important parameters fordesigning CNS.

Fig. 3. The effect of heat power on the collapsed liquid levels in

moderator cell and transfer tube for type I experiment with the

initial inventory 6.28 kg.


3.2. Experimental results for type II moderator cell

The initial R-11 inventory is varied from 4.1 to5.0 kg and the heat power is increased step by stepuntil the onset of flooding. Fig. 5 shows the effectof the heat power on the liquid level in the outershell of the moderator cell in the case of the initialinventory 4.23 kg.

A stable annular flow is visualized fromboth view ports located at the down stream ofthe condenser and the upper stream of themoderator cell in non-flooding condition. Theliquid level in the moderator cell is very stable asshown in Fig. 5(b) before the onset of floodingbecause the inner cavity has no hole at the bottom,but a vapor inlet opening only at its uppermostpart.

Fig. 5(a) shows that the flooding power of R-11is 6.16 kW corresponding to 924W in the caseof liquid hydrogen. Fig. 5(b) shows that theliquid level in the outer shell of the moderatorcell is decreased suddenly at the floodingpoint. This sharp decrease of liquid level is also

visualized from the glass sight tubes of themoderator cell. Fig. 5(c) shows that the systempressure is maintained at 5.69 bar during theexperiment.

Fig. 6 shows the comparison of the floodingpower vs. the R-11 inventory in both cases oftypes I and II. The effect of R-11 inventory onthe flooding power is insignificant for bothtypes. The average flooding power for type I is4.9 kW corresponding to 735W in the case ofliquid hydrogen, and for type II it is 6.2 kWcorresponding to 930W for liquid hydrogen.From the current experimental results, thelocation of the flooding onset is at the jointportion between the moderator cell and themoderator transfer tube. The space of the jointportion in type II moderator cell is larger than thatof type I moderator cell. It was shown that thestructure around the joint portion should bedesigned carefully for preventing the onset offlooding.

Fig. 4. The effect of heat power on the moderator cell local

void fractions for type I experiment with initial inventory

6.28 kg.Fig. 5. The effect of heat power on the collapsed liquid level in

moderator cell for type II experiment with the initial inventory

4.23 kg.


3.3. Verification of self-regulating simulation

Kawai et al. had proposed an idealizedsystem having no flow resistance in the moder-ator transfer tube for a closed two-phasethermosiphon loop as shown in Fig. 7. No flowresistance is established as there is no moderatortransfer tube. The mass balance equationfor the mass of vapor, M (g), can be written asfollows:

dM

dt¼

½Q � UAðT � TwaterÞ�hfg

; ð1Þ

where Q (J s�1) is the heat load, U (W cm�2K�1)the total heat transfer coefficient of the condenserevaluated at the bulk fluid temperature T (K), A

(cm2) the surface area, Twater (K) the temperatureof water coolant, and hfg (J g�1) is the latent heatfor R-11 evaporation.

The first term on the right-hand side of Eq. (1)represents the increment quantity due to evapo-ration per unit time and the second term representsthe condensing vapor quantity. Assuming thatthe state equation holds for an ideal gas andthe Clausius–Clapeyron equation holds forR-11 vapor, the dynamic equation for the

idealized closed two-phase thermosiphon is givenas follows:

hfgdm

dtþ Zm ¼ q; ð2Þ

where Z (Wg�1) represents the liquefaction capa-city of the condenser and m ¼ M � M0 denotes asmall deviation of the vapor quantity from thesteady state and q ¼ Q � Q0 indicates a smalldeviation of heat load from the steady state.

When the heat load q1 ¼ Q1 � Q0 is appliedstepwise at t ¼ 0; q ¼ q1; for t > 0; and q1 > Zm; thevariation of the vapor quantity is given by

m ¼q1

Zð1� e�t=tÞ; ð3Þ

where t ¼ hfg=Z (s) indicates the relaxation timewhich is related to the liquefaction capacity of thecondenser, and 1=t represents the self-regulatingpower of the idealized closed two-phase thermo-siphon. The greater the value of 1=t; the morerapidly a new steady state will be established.

Eq. (3) leads to a new steady state, m ¼ q1=Z; ifthe time elapses infinitely. The result shows thatthe variation of the vapor quantity m becomessmall with the increase of the liquefaction capacityof the condenser Z: The relaxation time t alsobecomes small with increasing liquefaction capa-city of the condenser.

Fig. 6. Comparison of the flooding power vs. mass inventory

for type I and type II experiment.

Fig. 7. Single-volume model for idealized mockup test facility.


If the heat load q ¼ Q � Q0 is applied stepwiseunder the condition qoZm; then the dynamicequation becomes as follows:

hfgdm

dt� Zm ¼ �q: ð4Þ

The variation of the vapor quantity is given bysolving Eq. (4) with t ¼ hfg=Z as

m ¼q1

Zð1� et=tÞ: ð5Þ

The stepwise increase of the heat load is applied tothe mockup test equipment for checking the self-regulating characteristics. Heat load was variedfrom 2 to 4 kW for both types of the moderatorcells. During the experiments, the operator adjuststhe liquefaction capacity of the condenser varyingsecondary cooling flow rate to maintain the

operating pressure constant. Eq. (3) was appliedfor simulating the variation of the R-11 vaporquantity under the condition q > Zm; in which theincrease of the vapor quantity is shown by theincrease of system pressure. Eq. (5) was applied tothe case under the condition qoZm in which thedecrease of R-11 vapor quantity is shown as thedecrease of system pressure.

Figs. 8 and 9 show the comparison of themeasured data regarding the system pressure,liquid level in the moderator cell, and thesimulation results by the idealized model, forwhich the stepwise heat load is applied. Theseresults show that the idealized single-volumemodel explains the behavior of the variation ofthe vapor quantity and the liquid level in themoderator cell under the stepwise increase of the

Fig. 8. Comparison of the measured data and idealized model

simulation for type I moderator cell in stepwise increase heat

power from 2 to 4 kW.

Fig. 9. Comparison of the measured data and idealized model

simulation for type II moderator cell in stepwise increase heat

power from 2 to 4 kW.


heat load, although the peak of the pressurerise appears behind the rise of the heater input.This time lag depends, of course, on the liquefac-tion capacity, the structure of the moderatortransfer tube and the quantity of the heatload. However, these results show that the mockuptest equipment, which has a large liquefactioncapacity of the condenser compared with heatload, has self-regulating characteristics in bothtypes of the moderator cells. We need only tocontrol the system pressure by the feed backcontroller for maintaining the liquid level in themoderator cell almost constant against thermaldisturbances.

4. Conclusions

The self-regulating characteristics of the full-scale mockup test equipment for developing TRR-II CNS are investigated. The moderator transfertube is designed to avoid the flooding and dry-outphenomena under the heat load estimated in thedesign of the CNS. Experimental results wereexplained by using the simulation of the idealizedsingle-volume model for the two-phase thermo-siphon loop. These results show that the mockuptest equipment, which has a large liquefactioncapacity of the condenser compared with heatload, has self-regulating characteristics in bothtypes of the moderator cells. Therefore, in thiscase, we need only to control the system pressureby the feed back controller for maintaining theliquid level in the moderator cell almost constantagainst thermal disturbances. Another importantresult is that we need to pay attention to the designof the structure at the joint portion between themoderator cell and the moderator transfer tube.

The small flow area in the joint portion could leadto the onset of flooding and dry-out phenomena inthe moderator cell.

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Simulation and mockup tests for developing TRR-II CNS

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