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.......................... MEASUREMENT OF FBACTOR POWER . . . . . . . . . . . . . . . . . . Normal Heat Balance Calculation . . . . . . . . . . . . . . Coolant S a l t Flow Measurement by Decay of Circulating Activation Products. . . . . . . . . . . Coolant System Pressure Drop . . . . . . . . . . . . . A i r Heat Balance Calculation. . . . . . . . . . . . . . . . pEFU?ORMANCE OF THE MAIN HEAT EXCHANC%R AND RADIATOR. . . . . . . Design Review . . . . . . . . . . . . . . . . . . . . . . . Primary Heat Exchanger . . . . . . . . . . . . . . . . Coolant Radiator . . . . . . . . . . . . . . . . . . . Analysis of Performance . . . . . . . . . . . . . . . . . . Primary Beat Exchanger . . . . . . . . . . . . . . . . Coolant Radiator . . . . . . . . . . . . . . . . . . . CONCILTSIONS AND FECOMMENDATIONS . . . . . . . . . . . . . . . . . REZZRENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . INTRODUCTION

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REACTOR POWER MGASUREMENT AND HEAT TRANSFER PERFORMANCE IN THE MOLmN SALT REACTOR EXPERIMENT

C. H. Gabbard

ABSTRACT

The operating power of the MSRE was routinely determined by a heat balance on t h e f u e l and coolant s a l t systems performed by t h e on-line computer. This gave a calculated full-power l e v e l of 8.0 MW. However, changes i n the isotopic composition of uranium and plutonium i n t h e f u e l s a l t indicated a power lower than t h e 8 MW by about 7 10%. Attempts t o resolve t h i s discrepancy included a measurement of the coolant s a l t flow rate by t h e radioactive decay of a c t i v a t i o n products i n the coolant salt, a recalc u l a t i o n of the coolant system pressure drop, and a heat balance on t h e air s i d e of t h e coolant r a d i a t o r . These e f f o r t s t o date have been inconclusive, b u t t h e coolant s a l t flow rate w a s found t o be the only p o t e n t i a l source of s i g n i f i c a n t e r r o r i n the heat balance. A c a l i b r a t i o n check of the d i f f e r e n t i a l pressure c e l l s reading t h e coolant flow v e n t u r i w i l l be made during the scheduled post-operation examinations.

The heat-removal c a p a b i l i t i e s of t h e f u e l s a l t t o coolant s a l t h e a t exchanger and coolant s a l t t o a i r r a d i a t o r were below t h e predictions of t h e o r i g i n a l design calculations and l i m i t e d t h e full-power output of t h e MSRF:. I n the case of t h e primary heat exchanger, the overestimate was due t o t h e use of erroneous, estimated physical property data, t h e thermal conductivities i n particular, f o r t h e f u e l and coolant salts. When accurate, measured values of physical properties were used with the heat t r a n s f e r r e l a t i o n s h i p s f o r conventional f l u i d s , t h e calculated performance w i t h the observed value. I n the case of t h e primary heat exc t h e design w a s only p a r t i a l l y exof t h e radiator, the ov an a i r "film" temperature. plained .by t h e improper There w a s no decre exchangers over more than Keywords : MSRE, he used salts, performanc

r a n s f e r c a p a b i l i t y of t h e two h e a t rs of operation. lance, heat t r a n s f e r , heat exchanger,

ti) IXTROIXJCTION

Operation of t h e Molten S a l t Reactor Experiment (MSRE) from 1966 through 1969 provided a unique opportunity t o measure heat t r a n s f e r i n equipnent using molten f l u o r i d e salts i n the temperature range of 1000 1200째F over an extended period of time.

Analysis of these data has l e d

t o conclusions regarding t h e adequacy of conventional design procedures and t h e change (or l a c k thereof) i n h e a t t r a n s f e r r e s i s t a n c e s i n t h i s molten-salt system over more than 3 years of operation. This r e p o r t describes the problems associated w i t h measuring t h e power of t h e MSRE,

then deals with predicted and observed h e a t t r a n s f e r c o e f f i c i e n t s . The MSRE design and operation are described i n d e t a i l i n References 1 and 2.

Figure 1 shows t h e layout of t h e important components.

Fuel s a l t

was c i r c u l a t e d a t about 1200 gpn through t h e core, where it w a s heated by t h e f i s s i o n chain reaction, then through a 279-ft2,

cross-baffled s h e l l -

and-tube h e a t exchanger where it t r a n s f e r r e d h e a t t o a second s a l t flowing through 1/2-inch tubes.

Heat was removed from t h e coolant s a l t and dissi-

pated t o t h e atmosphere i n an air-cooled h e a t exchanger ("coolant radiator")

with 3/4-inch tubes. MEASURE3ENT OF REACTOR POWER The o f f i c i a l operating power of the MSJE w a s determined by making a

heat balance around t h e f u e l and coolant systems.

This heat balance,

which was r o u t i n e l y calculated by the on-line computer, is more f u l l y described i n References 3 and 4. The various nuclear power instrument a t i o n systems ( l i n e a r chambers, f i s s i o n chambers, and s a f e t y chambers)

were c a l i b r a t e d t o agree w i t h t h e nuclear power as indicated by t h e h e a t

balance. The h e a t balances calculated through March 1968 indicated a nominal full-power l e v e l of about 7.2 MW.

There was some reason t o suspect t h i s

value, however, because data from r e a c t o r operation a t d i f f e r e n t power levels strongly suggested that the coolant s a l t s p e c i f i c heat was a cons t a n t r a t h e r than t h e temperature-dependent r e l a t i o n t h a t was being used

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i n t h e heat balance.'

Two l a b o r a t o r i e s (ORNL and t h e National Bureau of

Standards) independently measured the coolant s a l t s p e c i f i c heat, a r r i v i n g a t values i n good agreement w i t h each other b u t s u b s t a n t i a l l y higher ( i n the E R E temperature range) t h a n t h e previously used value.6

The new

measurements a l s o showed v i r t u a l l y no v a r i a t i o n with temperature.

The

new, constant value of t h e s p e c i f i c h e a t was incorporated i n t o t h e computer p r i o r t o t h e beginning of operation on U-233 f u e l i n January 1969. The c a l c u l a t e d full-power l e v e l was changed from 7.2 t o 8.0 MW as a r e s u l t of the specific heat revision.

A s r e s u l t s of precision i s o t o p i c analyses of t h e heavy elements i n t h e f u e l salt became available, independent determinations of r e a c t o r power were obtained from t h e changes i n uranium and plutonium i s o t o p i c compositions.

The most recent evaluations of thesd i s o t o p i c data yielded

a full-power l e v e l of 7.34 k 0.09 MW (References 7 and 8).

The heat

balance c a l c u l a t i o n has been reviewed as described below i n attempts t o resolve t h e discrepancy between the measured h e a t balance power and t h e

power indicated by t h e changes i n i s o t o p i c composition of t h e f u e l s a l t .

Normal Heat Balance Calculation A t s i g n i f i c a n t power l e v e l s , t h e dominant term i n t h e h e a t balance was the heat removed from t h e coolant s a l t a t t h e r a d i a t o r .

From Table I,

which shows t h e r e l a t i v e importance of t h e various terms i n t h e heat balance f o r operation a t f u l l power, it is c l e a r t h a t there was l i t t l e opportunity f o r s i g n i f i c a n t overall e r r o r due t o t h e o t h e r terms. The h e a t removed a t t h e r a d i a t o r was c a l c u l a t e d from t h e mass flow

rate, t h e s p e c i f i c heat of the coolant salt, and t h e temperature drop across t h e r a d i a t o r .

Possible sources of e r r o r i n each of these were

examined. The s a l t temperature drop across t h e r a d i a t o r was measured a t thermocouple wells a t t h e i n l e t and o u t l e t .

Three c a l i b r a t e d thermocouples

were i n s t a l l e d i n each w e l l w i t h two from each w e l l b e i n g used i n t h e h e a t balance.

Although the laboratory c a l i b r a t i o n s of t h e thermocouples indl-

cated a AT e r r o r of about -O.3'F,

t h e r e was concern t h a t l a r g e r systematic

Table I E

Typical Values in MSREHeat Balance at Full Power Mw Heat removed from coolant salt at radiator Heat removed by cooling water Heat removed by component cooling air Heat removed by fuel pumpoil Unaccountable heat losses Power input Power input Power input Power input

to to to to

electric heaters fuel pump space coolers coolant pump impeller . Nuclear power generated

7.853 0.341 0.016 0.003 0.011 -0.175 -0.035 -0.007 -0.036 7.971

To test this possibility, errors might exist in the installed condition. in November, 1969 the reactor system was operated isothermally at 1210"F, 1070"F, and 1010째F to determine the error that would actually occur in operation over the temperature range of the coolant system. The indicated LSCerror for the coolant system at full power was +O,23"F which would cause a 0.4% overestimate in the calculated power. A-second test to determine the influence of the radiator air flow on the thermocouple readings showed no detectable effect. The coolant salt density-is believed to contribute a +- 1%uncertainty to the heat removal term, and the revised specific heat measurementhad a stated uncertainty of k 1.44. The only remaining potential source of a significant errorais in the measurement of the coolant-salt volumetric flow rate. The volume flow rate of the coolant salt wasmeasured at the radiator inlet by a venturi

flow meter with two channels of readout.

Each readout channel c o n s i s t s

of a differential pressure c e l l and t h e associated e l e c t r o n i c s t o supply

a l i n e a r flow s i g n a l t o t h e computer. The d i f f e r e n t i a l pressure c e l l s a r e connected t o t h e v e n t u r i pressure t a p s and are i s o l a t e d from t h e hightemperature s a l t by m e t a l diaphragm seals and NaK-filled l i n e s . A review of t h e v e n t u r i manufacturer's c a l i b r a t i o n data disclosed a n e r r o r i n conv e r t i n g t h e differential head from water t o mercury which had caused a 2 . s reduction i n t h e measured flow rate' and i n t h e r a d i a t o r heat-removal term.

Another p o s s i b i l i t y f o r error, which s t i l l e x i s t s , is i n t h e

b r a t i o n of t h e d i f f e r e n t i a l pressure cells.

Cali-

The flowmeter readings were

checked f o r evidence of trapped gas o r compressibility i n t h e NaK-filled l i n e s by observing t h e flow readings as t h e coolant system overpressure w a s increased from 5 t o 65 p s i during a coolant system pressure test. There w e r e essentially no i n d i c a t e d f l o w changes on either channel during t h i s test.

The a c t u a l range c a l i b r a t i o n of t h e d i f f e r e n t i a l pressure

c e l l s cannot be checked u n t i l later t h i s year when t h e coolant piping w i l l be c u t so t h a t known pressure s i g n a l s can be applied t o t h e c e l l s .

Two independent attempts t o determine t h e coolant s a l t flow rate are described below. However, t h e r e s u l t s of these two e f f o r t s were inconc l u s i v e and t h e f i n a l assessment of t h e flow rate w i l l be made from t h e c a l i b r a t i o n check of t h e d i f f e r e n t i a l pressure c e l l s . Taking t h e nominal f u l l power of 8.0 MW and applying t h e 2.8 flow e r r o r and t h e 0.4% AT e r r o r , t h e h e a t balance would i n d i c a t e a power l e v e l

of 8.2 5 0.16 MW as compared t o 7.34 +- .Og MW indicated by t h e i s o t o p i c a n a l y s i s of t h e f u e l .

If a l l of t h i s discrepancy were assigned t o t h e

coolant s a l t flow r a t e i n t h e heat balance, t h e flow r a t e would have t o be lowered from t h e nominal value of

850 gpn t o 770 gpn.

Coolant S a l t Flow Measurement by Decay of C i r c u l a t i n g Activation Products Because t h e accuracy of t h e d i f f e r e n t i a l pressure c e l l s reading t h e coolant s a l t flow venturi could not be checked u n t i l some months after t h e end of r e a c t o r operation, an attempt was made during t h e last power =in

December 1969 t o measure t h e coolant salt flow rate by t h e decay of

a c t i v a t i o n products i n t h e salt.

Nitrogen-16 and fluorine-20 were produced

in the coolant salt by neutron reactions with fluorine in the heat exchanger. These activities then decayed with half-lives of 7.4 set and 11.2 set respectively as the salt was pumped around the coolant loop. We were also hopeful of finding long-lived activities from impurities in the salt that would have been useful in making a geometry calibration of the equipment. A high resolution gamma spectrometer with a &OS-channel analyzer was available for detecting and counting the various energy peaks that might be present. Two holes were drilled in the high-bay floor to the coolant cell so that the coolant salt piping could be scanned at two locations while the reactor was operating at full power. These two locations were separated by a total circulating salt volume of 20.9 ft3 which included the radiator, the coolant pimp, and the 205 line. This volume would give decay times of 1 and 1.5 half-lives respectively for the 2oF and 16N at the design Correction factors were estimated to account for the effects flow rate. 5

of mixing by the side stream through the relatively stagnant coolant pump tank and for the effects of the line-205 flow that bypassed the radiator through different sections volume. The effects of possible flow variations of the radiator tube bundle were found to be negligible. Preliminary data showed that the background count was obscuring the Lead bricks were stacked around the decount from the coolant piping. tector to reduce the background count and the diameter of the collimator used to aim the detector was ~increased from l/a-in. to l/2-in. to increase the count rate from the coolant line. The background remained high compared to the count rate from the coolant cell, but the ability to resolve, the count rate into discrete energy peaks and the limited time available for the experiment led us to begin the actual data collection. Although the background count was higher than desired for the experiment, the radiation was below 2 mR/hr and was not a biplogical hazard:

5 c w

Data were collected on magnetic tape for a total active counting time of about 11 hours at each of the locations on the coolant piping. Two sets of data were taken at the second location with about six inches . of lead shielding between the detector and the hole to the coolant cell.

u The first count measured the background count i n t h e high bay, and t h e

second count was taken w i t h a small ' % o source t o c a l i b r a t e t h e gamma energy t o t h e channel number of the analyzer. The data were analyzed by a computer program which gave t h e i n t e g r a l count rate f o r each energy peak i n the spectrum.

The background count

appropriate f o r each m r t i c u l a r peak was automatically subtracted by t h e computer program.

A comparison of t h e two gamma s p e c t r a with and without

t h e lead s h i e l d i n g between t h e d e t e c t o r and coolant piping indicated t h a t only the 1.63-Mev energy peak from ?F coolant flow rate.

would be u s e f u l i n c a l c u l a t i n g t h e

None of t h e o t h e r peaks, including two that corre-

sponded t o t h e 6.13 and 7.12-Mev gammas from l%,

six inches of lead shielding.

were a t t e n u a t e d by the

These energy peaks that were not attenu-

a t e d were believed t o be capture gammas from t h e r e a c t o r c e l l shielding. The d e t e c t o r was located near t h e ends of the t o p s h i e l d blocks a t t h e

southwest corner of t h e high bay and t h e r e was a f i e l d of gamma photons P

and fast neutrons from beneath the t o p shield blocks.

The coolant s a l t flow rate as c a l c u l a t e d from t h e decay of the c

1.63-Mev photons from =F w a s 610 gpn as compared t o 850 gpn indicated by the flow v e n t u r i .

Although the 610 gpn is probably 20 t o 30% below

t h e a c t u a l flow and the measurement was not u s e f u l f o r i t s intended purpose of resolving t h e power discrepancy, t h i s method of measuring t h e flow rate appears t o be feasible i f t h e proper precautions are taken i n s e t t i n g up t h e experiment.

The l a r g e s t source of e r r o r i n t h e experiment

was probably i n t h e geometry d i f f e r e n c e s between t h e two scanning points.

This type e r r o r could possibly be eliminated by a more c a r e f u l design of

t h e counting s t a t i o n s t o ensure low background and similar counting geometries, o r by spiking t h e coolant s a l t with a longer-lived a c t i v i t y t h a t would be e s s e n t i a l l y uniform throughout t h e coolant loop.

This

a c t i v i t y could then be used t o provide a geometry c a l i b r a t i o n f a c t o r between t h e two counting s t a t i o n s .

Other important sources of e r r o r could

be i n t h e e f f e c t i v e s a l t volume between t h e two s t a t i o n s o r i n t h e e f f e c t s of t h e high background.

The r e a c t o r was s h u t down a s h o r t time a f t e r t h e

data were taken and t h e r e was no opportunity t o r e f i n e t h e experiment.

. Lid

Coolant

System pressure Drop

The coolant salt flow rate of 850 gpn measured by the flow venturi was .within the range originally predicted from the calculated coolant system pressure drop and the performance characteristics of the coolant predicted by allowing pump. A range of 850 to 940 gpn had been originally a + lO$'variation on the calculated pressure drop and a f 5%flow variation on the coolant pump. A flow rate 10s below the design range would require a large error in the head loss calculation or an unreasonably poor . pump efficiency. There is no convenient way to check the performance curve of the installed coolant'pump, but the system head loss was recently recalculated by the writer at the design flow rate of 850 gpm. Table II gives the revised head losses of the various components of the coolant system. Allowing for a i 15%uncertainty in salt viscosity, the calculated coolant system head loss ranged from 94 to 99 feet of salt as compared to the original design value of 78 ft. Actually, a somewhat greater uncertainty band is probably required to account for the selection of friction factors and coefficients for entry and exit losses. The head loss of 99 ft would give a predicted minimum flow of aTout 800 gpm based on the coolant pump water test data and allowing for a 5%lower flow in the MSl?E than in the water test pump. However, this cannot be taken as a precise flow calculation because there are a large number of assumptions in the pressure drop calculation and because the actual coolant pump characteristic curve The minimum predicted flow rate of might not be within the 546margin. 800 gpn is still above the -'j"i+Ogpn required to resolve the discrepancy in reactor

i i 6d

power.

W Table I1

Calculated Head Loss of MSRF: Coolant Systems Components A t 850

Head Loss ( f t of salt)

Item

Line 202

13.8 13.8 11.8

Heat Exchanger

28 .o

Radiator

29 .Q

Line 200 Line 201

Total

96 -4 ?

A i r Heat Balance Calculation The r a d i a t o r a i r system provided an opportunity t o make an inde-

pendent measurement of t h e operating power of t h e r e a c t o r .

Heat balances

on t h e air system were completed i n May 1966 and t h e s e were i n general agreement with t h e s a l t h e a t balances using the revised value of t h e coolant s a l t s p e c i f i c heat.

The s t a c k a i r o u t l e t temperature f o r t h e s e

h e a t balances was measured a t a s i n g l e point near t h e s t a c k w a l l and t h e

a i r flow measurement was based on t h e reading of a p i t o t - v e n t u r i flowmeter

a t t h e c e n t e r of t h e s t a c k .

The r e l a t i o n between t h e flowmeter reading

and t h e t o t a l s t a c k flow had been previously determined a t s e v e r a l flow

rates from v e l o c i t y p r o f i l e s taken w i t h a h o t wire anemometer. v e l o c i t y p r o f i l e s were taken a t ambient a i r temperature.

These

This measure-

ment r e l a t e d t h e t o t a l s t a c k air flow d i r e c t l y t o t h e computer readout of the flowmeter output and d i d not involve the.manufacturer's output vs

v e l o c i t y c a l i b r a t i o n data.

j u * f)

The only point at which air velocities could be measured was at a location about 50 feet up the 75-f-thigh, lo-ft-diameter air stack. This location would give upstream and downstream L/D ratios of 5 and 2.5 respectively. Both upstream and downstream distances are insufficient to ensure a normal flow distribution, and flow disturbances could be introduced by either the sharp 90" corner at the bottom of the stack or by wind effects at the top of the stack. Since the air stack could also have a temperature distribution as well as a velocity distribution and since the velocity distribution might change under actual operating conditions, two air heat balances were completed in the fall of 1969. -For these heat balances, the mounting of thempitot-venturi was modified and a thermocouple was added so that velocity and temperature traverses could be taken on two prpendicular diameters across the stack while the reactor was operating at power. A new velocity calibration was also completed on the pitot-venturi prior to using it in running the traverses. The new pitot-venturi calibration gave air flow rates below those obtained previously. Figure 2 shows a comparison of the new calibration and flow equation with the previously used stack flow relationship and with the manufacturer's -calibration data. The new calibration, which was in general agreement with the manufacturer's data, was adopted. -Air heat balance data were taken at two operating conditions of the reactor, one at the nominal full-power condition and the other at highest The results of the two air heat balances power attainable with one blower. Previous heat balances had-given higher results -are shown in Table III. The main difference in more in agreement with the salt heat balances. the air heat balances was in the lower air flow rates indicated by the Figure 3 shows the velocity and new~calibration of the pitot-venturi. .t temperature distributions across the radiator stack for the full-power condition. The average velocity for the two traverses shown was 3270 fpn as compared to 3475 -fpn which would have been obtained with the previously is shown as a temperaused flow measurement, The temperature distribution ture rise above ambient air temperature because the ambient temperature Similar distributions were changed during the time data were being taken.

Fig. 2.

Calibration of Pitot-Venturi A i r Flow Meter I

Table III

Results

of MSRE Air Heat Balances I

7.335 0.343 0.0166 -0.679*

4.93 0.295 0.017 -0.421*

7.01

4.82

Computer Heat Balance Power (MW)

7.96

6.31

Ratio of Air Heat Balance Power/Salt Heat Balance Power

0.881

0.764

Heat Removed by Radiator

Air

Flow (MW)

Heat Removed by Cooling' Water (MW) Heat Removed at Component Cooling Pump (MW) Electric Power Input Nuclear

Power by Air

Heat Balance (MW)

c ._ *

These values include the power input to the radiator to the main and annulus blowers in addition to the electric applicable to a salt -heat'balance. .,

heaters and power input

430 L .

L W

120

I00

3800

3600 c

E Q Z 3400

> k

3 W

’3200

cre

3000

2800 0

N AND€

Fig. 3.

STACK POSITION (ft)

10 S AND W

A i r Velocity and Temperature Distribution i n MSHE Radiator Stack a t Nominal fill Power

iL, i

obtained velocity

for the partial power operation. distributions are an indication

The large-variations in these of the difficulty in obtaining

an accurate flow measurement. At the present time, the accuracy of given precedence over the air heat balance The two air heat balances taken 1. inconsistent with each other as

the salt heat balance must be for the following reasons. at different power levels were indicated by the ratios of air

heat balance power to salt heat balance power shown in Table III. The salt heat balance power at various power levels was in agreement with the neutron flux power indication from the compensated ion-chamber and was therefore proportional to the

actual power. The difficulty in obtaining the true air flow and temperature 2. rise with the large variations as shown in Figure 3. Unaccounted heat losses and air leakages from the radiator 3. enclosure. The construction and instrumentation of the radiator air system were not intended for precision measurements as required for a.heat balance and therefore the difficulties encountered were not surprising. EERF'ORMA& OF TRE MAIN HEAT EXCRANGERAND RADIATOR

; P .u

The initial escalation of the MSRE Dower level in April and May of 1966 showed that the heat transferycapability was below the design prediction for:both the primary heat exchanger and the radiator. 'With the reactor system operating within its design temperature range, the maximum power level of the reactor as calculated by the computer heat balance was limited by these components to about 7;2 MW as compared to-the nominal full-power rating of 10 MW. Slightly higher power could have been achieved by raising the fuel temperature, but the large temperature increase required to obtain only a small~power increase made this impractical. The original designs of both the heat exchanger and the radiator were reviewed to determine the cause of the lower than expected performance. The actual operating performance was also carefully monitored

W t o determine if the reduced performance was caused by some f a c t o r associa t e d w i t h the operation.

A more complete discussion of t h e i n i t i a l evalu-

a t i o n of t h i s problem i s given i n Reference 10.

Design Review Primary Heat Exchanger

The primary h e a t exchanger is-a conventional cross-baffled, U-Tube exchanger as shown i n Fig. 4. i

Fuel s a l t c i r c u l a t e s on the s h e l l side a t

1200 gpn and coolant s a l t c i r c u l a t e s a t about 850 gpn through t h e tubes.

The exchanger now contains 159 half-inch tubes on a t r i a n g u l a r p i t c h . For a more d e t a i l e d descripbion, see Reference 1.

The methods used i n t h e design of t h e MSRE h e a t exchanger a r e those commonly followed i n designing heat exchangers of t h i s type.

The tube-

s i d e c o e f f i c i e n t was computed from t h e Sieder-Tate equation, and t h e shelI-side c o e f f i c i e n t w a s computed from a c o r r e l a t i o n by Kern.â&#x20AC;?

Im-

p l i c i t i n t h e use of these procedures i s the assumption t h a t t h e fused

s a l t s behave as normal f l u i d s . Previous heat t r a n s f e r work on fused salts had shown t h i s t o be a v a l i d assumption f o r both flow i n s i d e tubes and on t h e outside of tube bundles. 12, l3 ,

The design c a l c u l a t i o n s t e n d t o give a conservatively low prediction of the h e a t - t r a n s f e r c a p a b i l i t y ( e f f e c t i v e UA) f o r f o u r reasons. F i r s t , the c o r r e l a t i o n f o r s h e l l - s i d e c o e f f i c i e n t by Kern i s conservative, i.e., h i s design curve f a l l s below the d a t a points rather than through the mean.

T h i s would t e n d t o make t h e predicted shell-side c o e f f i c i e n t low by 0 20$.

Since t h e s h e l l - s i d e r e s i s t a n c e i s about a t h i r d of the t o t a l , the

e f f e c t of t h i s conservatism on the predicted o v e r a l l c o e f f i c i e n t , U, i s about 0

6%. Second, t h e predicted c o e f f i c i e n t would a l s o be low because

an a d d i t i o n a l r e s i s t a n c e of about 11% was added a r b i t r a r i l y t o allow f o r scale.

T h i s was done even though it has been shown, both i n and o u t of

p i l e , t h a t the salts do not corrode o r deposit s c a l e on Hastelloy-N under

MSRE operating conditions.

The t h i r d conservative approximation was i n

t h e d e f i n i t i o n of t h e e f f e c t i v e h e a t - t r a n s f e r surface area. c r e d i t was taken f o r t h e b e n t part of t h e tubes, i.e.,

Here no

the a c t i v e length

* j

, ORNL-LR-DWG SZOS6R2

FUEL INLET

cj of t h e tube was taken t o be t h e s t r a i g h t portion between t h e thermal barrier near the tube s h e e t and t h e l a s t b a f f l e .

T h i s approximation was

made i n recognition that t h e thermal e f f i c i e n c y of the r e t u r n bends might be less than t h a t of t h e s t r a i g h t b r t i o n s .

Nevertheless, t h i s region

contains 7 t o 8 percent of t h e t o t a l tube area and w i l l transfer a s i g n i f i c a n t amount of heat.

F i n a l l y an a d d i t i o n a l 8$ of a c t i v e h e a t - t r a n s f e r

area was added t o t h e computed requirement as a contingency f a c t o r . The n e t r e s u l t is that a d e l i b e r a t e margin f o r e r r o r of over 20$ was included i n t h e h e a t exchanger design. Between t h e design and t h e operation of t h e heat exchanger, some modifications were made. When t h e heat exchanger was hydraulically t e s t e d w i t h water before being i n s t a l l e d i n t h e reactor, t h e s h e l l - s i d e pressure To reduce t h e high pressure

excessive and t h e tubes vibrated.

outermost row of f o u r tubes was removed and t h e corresponding holes i n t h e baffle p l a t e s were plugged.

To a l l e v i a t e t h e tube v i b r a t i o n

problem, an impingement b a f f l e was placed a t t h e f u e l s a l t i n l e t . a d d i t i o n t h e tubes were "laced" with rods next t o r e s t r a i n t h e lateral movement of t h e tubes.

h baffle p l a t e t o

A laced s t r u c t u r e was a l s o

b u i l t up i n t h e r e t u r n bend t o m a k e t h e s e tube projections behave as a

No attempt was made t o measure t h e o v e r a l l h e a t - t r a n s f e r c o e f f i c i e n t , b u t it does not appear

u n i t , and t h e tubes e s s e n t i a l l y support each other.

t h a t t h e s e changes were enough t o a f f e c t t h e conservatism i n t h e o r i g i n a l

design. gible.

The e f f e c t of t h e rods and impingement b a f f l e was probably negli-

The loss i n h e a t t r a n s f e r by t h e removal of t h e f o u r tubes was a l s o

r e l a t i v e l y small.

The h e a t - t r a n s f e r area of t h e removed tubes was only

about 2.5$ of t h e t o t a l ; t h e e f f e c t on capacity was probably less because these B r t i c u l a r tubes, by v i r t u e of t h e i r proximity t o t h e s h e l l , would be expected t o have h e a t - t r a n s f e r c o e f f i c i e n t s below t h e average.

A t t h e time t h e design review was completed i n t h e summer of 1966, we concluded t h a t t h e design methods were appropriate, t h e assumptions conservative, and t h a t subsequent modifications should not have used up t h e margin of s a f e t y believed t o be provided i n t h e design.

i 19 i

iId

The three remaining

possible

causes of the low heat transfer

were:

1. _ That a buildup of scale was occurring on the tubes even though r this was believed impossible. 2: That the tube surfaces were being blanketed with a gas film. The physical properties of the fuel and coolant salts used in 3. the design were not the correct values. Subsequent operation of the reactor, as discussed later in the report; showed that the heat transferwas constant with time, indicating no buildup of scale and that there was no evidence of gas filming. However, a reevaluation of the physical properties showed that the thermal conductivity of both the fuel and coolant salt was sufficiently below the value used in the design to account for the overestimate of the overall coefficient. .Table IV on page 23 of this report shows a comparison of the original . physical property data to the latest values and.shows the effect on the -calculated heat transfer coefficients.

Coolant Radiator ._

The heat-transfer surfaces of the radiator consist of 120 unfinned 3/b-inch tubes, each about 30 ft long. The S-shaped tube bundle, consisting of 10 staggered banks ,of 12 tubes each, is located in a horizontal Doors can air duct so that air blows across the tubes at right angles. be lowered just upstream and downstream of the tubes to vary the air flow over them. A bypass duct with a controlled damper and the option of using either one or two blowers provide other means of varying the air pressure A detailed description of the radiator and its drop across the radiator.

i i w

enclosure is given% Reference 1; * As in the design of the primary heat exchanger, the Sieder-Tate equation was used to calculate the heat-transfer coefficient on the inside The same comments as to.validity of method and accuracy of of the tubes. salt properties apply in both designs. In the radiator, however, only 2s ~of the calculated heat-transfer resistance was inside the tubes, so no conclusions with regard to accuracy of the inside 9il.m calculations can _ be drawn from the-observed

performance,

Over 95$ of the r e s i s t a n c e i s on t h e a i r side.

v, This c o e f f i c i e n t was

calculated using an equation by Colburn recommended by McAdams

.l4

This

equation i s well-proven f o r cross-flow geometries i d e n t i c a l i n a l l ess e n t i a l s t o t h e MSRE r a d i a t o r .

The d i f f i c u l t y with applying t h e equation

t o t h e MSHE design is t h e very large difference between t h e tube temperat u r e and the bulk temperature of t h e a i r .

The physical properties of the

a i r vary so much Over t h i s range t h a t r e l a t i v e l y l a r g e v a r i a t i o n s i n the I

heat-transfer c o e f f i c i e n t can be calculated depending on which temperature is selected f o r t h e evaluation of t h e physical properties.

The

de-

sfgn calculation used a i r properties a t t h e temperature of t h e outside surface of the tubes.

The procedure recommended by McAdams i s t o evaluate

t h e properties a t a " f i l m temperature" defined as the average of the sur-

f a c e and t h e bulk air temperatures.

Had t h i s been done, t h e outside f i l m

c o e f f i c i e n t (and t h e o v e r a l l coefficient) calculated f o r the MSflE r a d i a t o r would have been lowered from 60 t o 51.5 Btu/(hr-ft2-OF).

Even lower values

would have r e s u l t e d if t h e physical properties had been evaluated nearer

t h e temperature of the bulk of t h e air. The heat-transfer c o e f f i c i e n t calculated using the recommended a i r film temperature was s t i l l g r e a t e r than t h e observed value by ab

A contingency factor of t h i s magnitude would not be unreasonable when t h e large air-to-tube surface temperature

unconventional geometry of t h e tube b sidered.

i f f e r e n c e i s considered and when the l e within i t s enclosure is con-

However, the o r i g i n a l r a d i a t o r design had included only a k$

overdesign. Analysis of Performance I

primary Heat Exchanger

The heat t r a n s f e r performance of t h e main h e a t exchanger has been

monitored throughout t h e power operation of the MSHE by two methods.

The

o v e r a l l heat t r a n s f e r c o e f f i c i e n t was evaluated by a procedure described i n References 10 and 15 which was developed t o eliminate the effects of c e r t a i n types of thermocouple e r r o r s .

The o v e r a l l c o e f f i c i e n t was calcu-

l a t e d from t h e equations:

bs.'

where a and S U A

'3 w

Temperature parameters evaluated Overall heat transfer coefficient,

by on-line

Ff*F,

Heat exchanger surface area, Mass flow rates of fuel and coolant,

Cf&

Specific

heats of fuel

computer,

and

and coolant.

The value of the derivative d(a! + S)/d(a - S) was determined from the slope of the line obtained by plotting (a! + S) vs (a - S) at several different poGer levels. Thus a short period at steady-state operation at several &wer!.levels was required for each measurement at the heat trans:; ' .',,'_ :. fer coefficient. :.. I_j ..A more con;enient metho-d-of monitoring.'the heat exchanger for changes in performance waF:the. heat transfer::~inaex~-(HTI): .The.33?1 ias evaluated _~ I.3 at full power and;.was-defined-as thekrakio. of thk'heat'balance power to .+ 3-.I .L the temperature.-difference-between-the. fuel and doolant-salts entering 2 .". .~,_(_f,..- .. .. -- -_.,- ~..--I ~. ---. -., ::, the heat exchanger.I- Figure 5 shows the;.e.I and. overa,ll .'W!'.;of the heat '. ;I, exchanger- taken'?over. the life--of-the reactor':,. T&se--plots indicate that i;: : : ,i 'there has*been noâ&#x20AC;&#x2DC; deteriorationof~perf'ormanceov~r,the..lire of the re__,:_-.-t-._. .__ ..; -..;.-... .~ ~~~~~ ---.~~--..--. - actor and~~thatithere has been fouling pr scale buildup I~.~: __ _.. _ :tube_..... -:. i._ .- -.-.. .. -~ ". I---no detectable - .-: c> in a period of"about 3;1/2 years-offre&tor~operation; :-This! would imply .< that the total oFrating lifeof'the re&torb ~in~fud~ng about 26,000 hours -_-I_~_.~.... I . ..1..-. -.. It _._.._.~ . ~_. of salt circulation, h&been%thout scale buildup. : _-. .-.~--..,-. .~.~ : The circulating gas .volume-ih~the-:fuel.&lt-has+varied--from 0 to .;- : 0.6% during different'periods-~of--~eactp~-.operatio?,.~'A IteHt$as conducted I ___j ..: during the early powersoperation %o determine filming-'of the tube 1_.^ ..i .Iif &s L-';:'i.'! :,:.,<,,q~L;-,r', ~~-:!:,t, 1.;d.:Ai,lm;. !;;A., i, .y..>" surfaces could be causing the lower than predicted heattransfer. The test was conducted by rapidly venting gas overpressure from the fuel

0 0 rn

8 5 :

O U

t-l

Ir, system and observing t h e f u e l and coolant temperatures f o r changes t h a t would be i n d i c a t i v e of an expanding gas film. P

changes i n t h e temperatures.

There were no detectable

The heat t r a n s f e r index also d i d not show

any changes t h a t could be related t o the longer term changes i n t h e gas

void. When t h e MSRE was f i r s t operated a t s i g n i f i c a n t p o w e r levels, the h e a t exchanger performance was observed t o be below the design value. Reference 10 presents a complete discussion of t h i s problem and t h e possible causes.

The conclusions reached were t h a t the h e a t exchanger had

been properly designed using t h e s a l t physical property data a v a i l a b l e

a t t h a t time b u t t h a t some of t h e physical properties, t h e thermal cond u c t i v i t i e s i n p a r t i c u l a r , were s u b s t a n t i a l l y below t h e values used i n t h e design.

Table

N shows

a comparison of t h e physical property data

used i n t h e o r i g i n a l design t o t h e current data.

The heat transfer coef-

f i c i e n t s c a l c u l a t e d by the conventional design procedures using these two sets of data are a l s o shown.

Table IV

Physical Properties of Fuel and Coolant S a l t s Used i n MSFU3 Heat Exchanger Desian and Evaluation

Fuel

Coolant

Current Fuel Coolant

0.659

23.6 123.1

0.4735

0.577 1989

Overall Coefficient, Btu *

618

The measured o v e r a l l heat t r a n s f e r c o e f f i c i e n t s have ranged from 646

t o 675 w i t h an average of 656 Btu/(hr-ft2-"F)

f o r 8 measurements.

These

measurements were made on t h e basis of nominal f u l l power a t 8.0 MW and a coolant s a l t f l o w of 850 gpn.

If t h e a c t u a l flow rate were

770 gpn,

which would be the flow consistent w i t h a power l e v e l of 7.34 MW, t h e measured overall c o e f f i c i e n t would be 594 as compared t o a calculated

value of 599 Btu/(hr-ft2-'F).

These measured c o e f f i c i e n t s were based on

a t o t a l e f f e c t i v e surface area of 279 ft".

This area includes the U-bends

b u t excludes t h e area between t h e tubesheet and i t s thermal b a r r i e r .

The

o r i g i n a l design c a l c u l a t i o n s excluded t h e area i n the bends t o provide an a d d i t i o n a l margin of s a f e t y .

The above data show that t h e conventional

heat t r a n s f e r c o r r e l a t i o n s are applicable t o molten-salt h e a t exchangers and t h a t t h e erroneous physical property d a t a used i n the o r i g i n a l design was the only cause f o r t h e overestimate of the heat t r a n s f e r capability.

I n a l l other respects, t h e heat exchanger performance has been f a u l t -

less.

There has been no indication of leakage either t o t h e outside o r

between t h e f u e l and coolant salts, no indication of tube v i b r a t i o n s after t h e modifications mentioned earlier, and no indications of flow r e s t r i c t i o n s ,

e- '

Coolant R a d i a t o r Although the r a d i a t o r was monitored continuously during power operat i o n of t h e r e a c t o r f o r changes i n performance, the only measurements of t h e heat t r a n s f e r c o e f f i c i e n t s were made i n 1966. The design and i n s t r u mentation of t h e radiator and i t s enclosure were not intended t o provide data f o r an accurate determination of t h e radiator h e a t t r a n s f e r coefficient.

However, these c o e f f i c i e n t s could be estimated from t h e a v a i l a b l e

data a t any time t h e r a d i a t o r doors were f u l l y open.

The o v e r a l l coef-

f i c i e n t s were calculated using the standard heat t r a n s f e r equation. I

Q = UAaTm where

Q =

t r a n s f e r r e d h e a t from coolant s a l t heat balance,

u =

overall heat transfer coefficient,

heat t r a n s f e r areas 706 ft2, and

mean temperature difference.

The d i f f i c u l t y i n applying t h i s equation w a s i n t h e measurement of t h e o u t l e t a i r temperature.

A d i r e c t measurement a t t h e t o p of t h e a i r stack

could be made only when the bypass damper was completely closed and then

a correction was required t o account f o r t h e a i r flow from the annulus blowers. For other conditions, t h e o u t l e t a i r temperature was calculated from a s a l t and a i r heat balance across t h e radiakor. The a i r flow and temperature measurements were made with the o r i g i n a l instrumentation and s t a c k flow c a l i b r a t i o n discussed i n t h e A i r Heat Balance s e c t i o n of t h i s report.

No attempt has been made t o reevaluate t h e r a d i a t o r heat trans-

f e r based on t h e recent flow o r temperature t r a v e r s e s because of t h e d i s -

crepancies t h a t s t i l l e x i s t i n these measurements. The r a d i a t o r o v e r a l l heat t r a n s f e r c o e f f i c i e n t s vs a i r pressure drop

assuming a nominal f u l l power of 8.0 MW are shown i n Fig. 6.

The d i s -

continuity when t h e second blower was energized w a s o r i g i n a l l y believed t o be t h e r e s u l t of d i r e c t a i r impingement from t h e second blower.

How-

ever, t h i s could be a r e s u l t of flow or temperature e r r o r s and t h e calcuThe observed o v e r a l l c o e f f i c i e n t evaluated a t f u l l

l a t i o n a l procedures.

power was 42.7 Btu/(hr-ft2-OF)

51.5 Btu/( hr-f t2-OF)

as compared t o the corrected design value

I n a l l other respects, t h e performance of the r a d i a t o r w a s completely satisfactory.

The heat t r a n s f e

remained constant through t h e l i f e of t h e

r e a c t o r and there were no s a l t leaks or other d i f f i c u l t i e s with t h e radiator itself.

There were

a r l y d i f f i c u l t i e s with the r a d i a t o r en-

closure which are discuss

another report. l6

eliminated by modif i c a t i o

t h e enclosure and doors.

These d i f f i c u l t i e s were

0-0

q R )

$ s

51)

D N -

0 0

-L

OVERALL t EAT TRANSFER COEFFICIENT (Btu/hr i t 2 O F )

*LJ

.CONCLUSIONS~AND REK!OMMENDATIONS .

The M&E heat exchanger and radiator /

-factorily In regard posed by goals of The

performed completely

satis-

except that the heat removal capability was less than intended. to the overall operation of the MSRE, the power limitation imthe heat removal system was not a serious problem. All of the the MSRRwere successfully achieved at,the attainable power level. analysis of the primary heat exchanger performance.showed that

the conventional heat transfer correlations are applicable to molten salts: the initial overestimate of the heat exchanger performance was completely resolved by the revised physical property data for the fuel and coolant salts. Operation for more than 3 years showed no loss in heat transfer

/ /

/ i

! ! j I 8 ! ! ! /

I ! /

capacity with time as a result of corrosion, scale, flow bypassing, or flow restrictions. In the case of the radiator, a discrepancy still exists between the calculated and observed performance. The cause for this discrepancy has not been definitely established. It would appear that: (1) the air-side heat transfer correlation was not completely suitable for the large. surface-to-air temperature difference that existed in the radiator, or (2) there were air flow leakages, bypassing, or air flow variations in this particular installation that caused the low heat transfer. Regardless of the reason for the overestimation of the radiator performance, it is clear that the 4% contingency factor in the original'design was insufficient. The unusually~low contingency factor for heat transfer equipment occurred because the basic radiator design was completed early in the MSRE design when the nominal design power was still 5 MW. The radiator and main heat exchanger were designed for 10 MW to ensure adequate performance. When the nominal power rating of the MSRE was later raised to 10 MW,.the radiator had only a 4% overdesign. An obvious, but often ignored design principle, would be to evaluate the performance over the maximum possible range of the physical property data and operational variables and then use the resulting performance range as &art of the basis in selecting a contingency factor. A larger degree of overdesign would have been indicated for the radiator if this procedure had been followed.

w. Thus far we have been unahle t o explain t h e discrepancy between t h e power indicated by t h e salt heat-balance and t h a t indicated by long-term

changes i n the isotopic composition of the f u e l salt.

. :

The air heat

balances indicated a lower power than the s a l t heat balance b u t the accuracy of these measurements is not s u f f i c i e n t t o override the s a l t heat balance. been

The coolant s a l t flowmeter i s t h e only element t h a t has not y e t

checked as thoroughly as possible.

A lower coolant salt flow rate

was indicated by the decay of c i r c u l a t i n g a c t i v a t i o n products.and a l s o by t h e recalculation of the coolant system head loss.

these methods is very accurate.

However, n e i t h e r of

The most reliable value of t h e flow rate

should be determined from t h e planned c a l i b r a t i o n of the flow element

differential-pressure cells. I

REFERENCES 1.

R. C . Robertson, MSRE Design and Operations Report, Part I, De-

s c r i p t i o n of Reactor Design, ORNL-?M-728, (January 1965). 2.

P. N. Haubenreich and J. R . Engel, "Experience w i t h the Molten-Salt Reactor Experiment, '' Nucl. Appl. and Tech., 8, 118 (1970).

R. H. Guymon, MSRE Design and Operations Report, Part VIII, Operating Vol. I1 (January 1966). Procedures, ORNL-TM-~O~,

G. H. Burger, J. R. Engel, and C. D. Martin, Computer Manual f o r MSRE Operators, i n t e r n a l memorandum MSR-67-19 (March 1967).

C. H. Gabbard, S p e c i f i c Heats of MSRE Fuel and Coolant Salts, i n t e r n a l memorandum, MSR-67-19 (March 1967)

6. MSR Program Semiann. Progr. Rept., August 31, 1968, ORNL-4344, P. 103. 7. MSR Program Semiann. Progr. Rept., Feb. 28, 1970, ORNL-4548, Sect. 6.2.1. 8. MSR Program Semiann. Progr. Rept., Feb. 28, 1970, ORNL-4548, Sects. 10.3 and 10.4. 9. 10.

MSR Program Semiann. Progr. Rept.,

Aug. 31, 1969, ORNL-4449, p. 12.

C . H. Gabbard, R. J. K e a , and H. B. Piper, Heat Transfer Performance of the MSRE Main Heat Exchanger and Radiator, i n t e r n a l memorandum

ORNL-(3-67-3-38 (March 1967). 11. D. W. Kern, Process Heat Transfer, McGraw-Hill Co., 1st Ed., P. 136, (1950).

Inc., New York,

ohen, Fused S a l t Heat Transfer Part 111, a n s f e r i n C i r c u l a r Tubes Containing the S a l t L-2433, (March 1960).

12.

H, W. Hoff'man and S. Forced Convection H e Mixture NaN02-NaN0

13.

rosh, Development Testing and Performance R. E . MacPherson and M. Evaluation of Liqu d Molten S a l t Heat Exchangers, i n t e r n a l -164 (March 1960). memorandum ORNL-CF

14. W. E. McAdams, Heat Transmission, M c G r a w - H i l l Co., Inc., New York, 3rd Ed., P. 272, (1954).

15.

H. B. Piper, "Heat Transfer i n t h e MSRE Heat Exchanger," Trans. Am. Nucl. SOC., Vol. 10 (supplement covering Conf. on Reactor Operating Experience), p. 29, July, 1967.

16. R. H, Guymon, MSRE Systems and Components Performance, ORNL-TM, t o be published.

OWL-TM- 3002

I”AL 1. J. L. Anderson 2. C. F. Baes 3. H. F. Bauman 4. S. E. Beall 5. M. Bender 6. C . E. B e t t i s 7. E. S. B e t t i s 8. D. S. Billington 9. F. F. Blankenship 10. E. G. Bohlmann 11. C. J. Borkowski 12. G. E . Boyd 13. R. B. Briggs 14. W. L. Carter 15. C. J. Claffey 16. C. W. Collins 17. J. W. Cooke 18. W. B. C o t t r e l l 19-20. D. F. cope, AEC-ORO 21. B. Cox 22. J. L. Crowley 23. F. L. C u l l e r 24. S. J. Ditto 25. W. P. Eatherly 26. D. E l i a s , AEC-Washington 27. J. R . Engel 28. D. E . Ferguson 29. L. M. F e r r i s 30. A. P. Fraas 31 J. K. Franzreb 32. J. H. Frye 33. W. K. Furlong 34-38. C. H. Gabbard 39. R. B. Gallaher 40. W. R. Grimes 41. A, G. Grindell 42. R. H. Guymon 43. P. H. Harley 44. P, N. Haubenreich 45. H. W. Hoffman 46. A. Houtzeel

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T. L. Hudson P. R. Kasten R. J. Ked1 50 M. T. Kelley 51 A. I. Krakoviak 52 T. S. Kress 53 J. A. Lane 54 K e r m i t Laughon, AEC-OSR 55 M. I. Lundin 56 R. N. won 57 R. E. MacPherson 58 H. E. McCoy . McCurdy 59 €IC. 60. C. K. McGlothlan 61-62. T. W. McIntosh, AEC-Washington 63 L. E . McNeese 64. J. R. McWherter 65 A. J. Miller 66. R. L. Moore 67 M. L. Myers 68 H. H. Nichol 69 E . L. Nicholson 70 A. M. Perry 71 B. E. Prince 72 G. L. Ragan 73 M. Richardson 74 D. R. Riley, AEC-Washington 75. R. C. Robertson 76-78. M. W. Rosenthal 79 H. M. Roth, AEC-OR0 80. J. P. Sanders 81. A. W. Savolainen 82. T. G. Schleiter, AEC-Washington 83 J. J. Schreiber, AEC-Washington 84. Dunlap S c o t t 85 R. M. Scroggin AEC-Washington 86. M. Shaw, AEC-Washington 87 M. J. Skinner 88. A. N. Smith 89 I. Spiewak 90. D, A. Sundberg

47 48. 49

-32 ORNL-W 3002 I"AL

DISTRIBUTION

(continued)

91 92. 93 94 95

96.

97-

98. 99 100-101. 102-103. 104-106. 107. 108.

'cssf I

R. E . Thoma D. B. Trauger A. M. Weinberg J. R . Weir M. E. Whatley J. C. White G. D. Whitman L. V. Wilson Gale Young Central Research Library (CRL) X-12 Document Reference Section (DRS) Laboratory Records Department (ED) Laboratory Records Department Record Copy (LIRD-RC) ORNL Patent Off ice

EX"AL

DBTRIBUTION

109-123. Division of Technical Information Extension (DTIE) 124. Laboratory and University Division, OR0