160 Special Issue | October 2014

BARC NEWSLETTERFounder’s DayTHERMAL HYDRAULIC AND MATERIAL COMPATIBILITY

STUDIES IN MOLTEN SALTS

A.K. Srivastava, A. Borgohain, S. S. Jana, V.K. Gupta and N.K. MaheshwariRED

S. RangarajanWSCD

A.G. Kumbhar and S.J. KenyRPCD

Abstract

Molten salt is increasingly getting more attention as fuel, blanket and coolant for High Temperature Reactors

(HTR) as well as a coolant and storage medium for solar power tower systems. This article deals with the activities

performed on molten salt coolant technologies for high temperature reactor and solar power tower applications.

With the objectives to study the thermal-hydraulic behavior of different molten salts and compatibility of different

structural material with molten salts, two different experimental facilities namely Molten Salt Natural Circulation

Loop (MSNCL) and Molten Salt Corrosion Test Facility (MOSCOT), have been designed, fabricated, installed and

successfully operated.

Dr. N.K. Maheshwari is the recipient of the DAE Group AchievementAward for the year 2012

Introduction

BARC is developing a High Temperature Reactor

(HTR), capable of supplying process heat at 1000oC to

facilitate hydrogen production by splitting water. BARC

is also developing a potential technology to make solar

power dispatchable, such that solar heat can be used

to generate electricity at will. In both the cases it has

been proposed to use molten salts as a coolant since it

has low melting point and high boiling point, enabling

us to operate the system at low pressure. Molten

fluoride salts and molten nitrate salts are proposed

as candidate coolants for HTR and solar power tower

systems respectively. Some of the challenges related

to the use of above salts include understanding its

thermal-hydraulic behavior, development and testing

of related high temperature instruments and study

of its compatibility with different structural materials

[1,2,3]. In this regard the heat transfer and pressure

drop characteristics of molten fluoride salt as well

as molten nitrate salt has been theoretically studied

already [4,5]. A comparative study of the steady state

and transient behavior of natural circulation loop with

various heat transfer media has also been performed

[6]. Nevertheless theoretical studies are not sufficient

enough to qualify these salts for HTR and solar power

applications. Development of suitable test facilities and

detailed experimentation in these setups will create a

complete database on the characteristics of molten

salts under various operating conditions. This will also

help in validating in-house and commercially available

codes with molten salts. In view of this, two different

experimental facilities- Molten Salt Natural Circulation

Loop (MSNCL) for thermal hydraulic studies of molten

salts and Molten Salt Corrosion Test Facility (MOSCOT)

for material compatibility study in such environment

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CONTENTS

Special Issue | October 2014 161

BARC NEWSLETTERFounder’s Dayhave been setup in BARC. Pre-test analyses of MSNCL

have also been carried out using one dimensional

in-house developed code LeBENC, acronym of Lead

Bismuth Eutectic Natural Circulation [7].

Description of the Test Facilities

Molten Salt Natural Circulation Loop (MSNCL)

Considering the importance of natural circulation in

high temperature systems and taking into account the

scarcity of experimental data, a Molten Salt Natural

Circulation Loop (MSNCL) has been setup. Fig. 1 and

Fig. 2 show the isometric view and photograph of

the MSNCL respectively. Initially, molten nitrate salt;

i.e. a mixture of NaNO3 and KNO3 in 60:40 ratio;

has been taken for the experimental studies. MSNCL

comprises of five parts viz. heater section, cooler,

melt tank, expansion tank and main loop piping.

All components and piping of the loop are made of

Inconel 625 material. It has been designed in such a

way that the effect of different orientations of heater

and cooler on the mass flow rate can be studied.

An expansion tank has been provided at the top of

the loop to accommodate the volumetric expansion

of the salt. Instruments for measuring temperature,

pressure and level have been installed in the loop. A

central control system has been provided to control

the process parameters.

Molten Salt Corrosion Test Facility (MOSCOT)

Molten Salt Corrosion Test Facility (MOSCOT) consists

of two cylindrical vessels called melt tank and main

vessel. Figure 3 and Fig. 4 show the schematic and

photograph of the MOSCOT facility respectively. Both

the vessels have been fabricated with Inconel-625

material. The inner surface of the vessels was coated

with nickel to reduce corrosion. The test facility has

been designed for in-situ measurement of static

and dynamic corrosion in molten salt environment.

Eutectic mixture of lithium fluoride, sodium fluoride

and potassium fluoride (FLiNaK) has been used as a

working fluid for the study. To carry out in-situ corrosion

measurements, electrochemical technique has been

used for which a Potentiostat-Galvanostat (PG-Stat)

has been installed in the facility. The resulting data and

its interpretation provide estimation of corrosion rate

of the material in the given molten salt environment.

In order to maintain the chemistry of the molten salt, Fig. 1: Isometric view of MSNCL

Fig. 2: Photograph of MSNCL

162 Special Issue | October 2014

BARC NEWSLETTERFounder’s Dayhigh purity argon gas has been provided as a cover

gas in both the vessels, which has been purged to

atmosphere through a water scrubber.

validated with the experimental results of molten nitrate

salt. The results for the same are as follows:

Steady state analysis

In the steady state natural circulation experiment, the

loop was allowed to reach steady state conditions at

different powers. By observing the trend of molten

salt temperature at different location, the steady state

conditions can be judged. For steady state natural

circulation, Vijayan [9] showed that the flow in a

single phase uniform or non-uniform diameter natural

circulation loops can be expressed as,

(1)

Where, constants ‘c’ and ‘r’ depends upon the

nature of flow i.e. laminar or turbulent. Parameter NG

depends upon the geometry of the loop. The detailed

derivation of Eq. 1 can be found in Vijayan [9]. The

same correlation is compared with experimental data

in Fig 5. It is found that the experimental results

uniformly lag with correlation values due to the heat

losses in the loop.

Fig. 3: Schematic of MOSCOT facility

Results

Thermal hydraulic studies in MSNCL

Various steady state and transient experiments such as

loss of heat sink transient, step power increase have been

performed in the loop for Vertical Heater and Horizontal

Cooler (VHHC) orientation. An in-house developed one

dimensional code LeBENC was already validated with

water and lead-bismuth eutectic [6,8]. Further it is

Fig. 4: Photograph of MOSCOT facility

Fig. 5: Comparison of experimental Data with steady state correlation given by Vijayan et. al.

Transient Studies

Various transients’ viz. loss of heat sink, heater trip,

startup of natural circulation and step change in power

Special Issue | October 2014 163

BARC NEWSLETTERFounder’s Dayhas been performed in MSNCL. A typical temperature

profile of cooler inlet and cooler outlet in loss of heat

sink transient is shown in Fig. 6. Results obtained from

LeBENC are compared with experimental data and

found in good agreement.

Corrosion studies in MOSCOT facility

In situ corrosion tests were performed on four different

Ni-alloys viz. Inconel 625, Inconel 617, Inconel 600 and

Incoloy 800 at five different temperatures 550 oC, 600 oC, 650 oC, 700 oC and 750oC using electrochemical

polarization (Taffel plot) technique. Corrosion current

(ICorr) obtained from the intersection of cathodic

and anodic Taffel lines was used for the estimation of

corrosion rate. A typical impedance spectra and Taffel

curve of Inconel 600 are shown in Fig. 7 and Fig. 8

Fig. 6: Cooler Inlet and outlet temperature variation in LOHS transient and its comparison with LBENC

Fig. 7: Impedence spectra of Inconel 600 at different temperature

Fig. 8: Taffel curve of Inconel 600

respectively. Corrosion rates (mills per year) of all the

selected materials at different temperatures obtained

from the experiments have been listed in table.1. The

results show that the corrosion rate is temperature

dependent and at highest experimental temperature

Inconel 600 has lowest corrosion rate.

Concluding Remarks

The present Molten salt test facilities have facilitated

thermal hydraulic and material related studies at high

temperatures and development/testing of instruments

like level sensor, control valves etc. Steady state and

transient behavior of molten nitrate salt in natural

circulation flow condition have been studied in

MSNCL. On the other hand, material compatibility

studies using electrochemical technique under fluoride

salt environment have been performed in MOSCOT

facility. Both the facilities are being used to generate

Material

Temp(°C)

Inconel 600

Inconel 617

Inconel 625

Incoloy800

550 6.2 10.0 - 17.4 600 12.2 18.2 6.1 30.1 650 25.9 22.7 5.0 31.2 700 25.4 33.9 71.3 45.4 750 15.8 97.9 127.6 33.2

Table 1: Corrosion rate (mpy) of different materials in molten fluoride salt environment at different temperatures

164 Special Issue | October 2014

BARC NEWSLETTERFounder’s Dayextensive experimental data which are relevant to

high temperature coolant system design and solar

application. The experimental data obtained from

MSNCL is being used for the validation of in-house

developed computer codes and CFD codes essential

for the design of high temperature reactor systems.

References

1. Cooke J.W., Development of the Variable Gap

Technique for Measuring the Thermal Conductivity

of Fluoride Salt Mixture, ORNL-4831, Oak Ridge

National Laboratory.

2. Grele M.D., Gedoen L., Forced convection Heat

transfer characteristics of Molten FLiNaK flowing in

an Inconel X system, NACA RM E53L18, National

Advisory Committee for Aeronautics (1954).

3. Ambrosek J., Anderson M., Sridharan K., Allen

T., Current Status of Knowledge of Fluoride Salt

(FLiNaK) Heat Transfer, Nuclear Technology, 165

(2009) 166.

4. Srivastava A. K., Vaidya A. M., Maheshwari N.

K., Vijayan P. K., Heat Transfer and Pressure Drop

Characteristics of Molten Fluoride Salt in Circular

Pipe, Journal of Applied Thermal Engineering,

Volume 61, Issue 2, 3 November 2013, Page 198-

205, (2013).

5. Srivastava A.K., Vaidya A.M., Maheshwari V,

Vijayan P.K., Heat Transfer and Pressure Drop

Characteristics of Molten Nitrate Salt In Circular

Pipe, Thirty Ninth National Conference on Fluid

Mechanics and Fluid Power December 13-15

(2012) SVNIT Surat, Gujarat, India.

6. Jaiswal B.K., Srivastava A.K., Borgohain A,

Maheshwari N.K., Vijayan P.K., A Comparative

Study of the Steady State and Transient Behavior

of Natural Circulation Loop with Various Heat

Transfer Media, 38th National Conference on

Fluid Mechanics and Fluid Power December 15-17

(2011) MANIT, Bhopal.

7. Srivastava A. K., Borgohain A., Maheshwari N.

K., Vijayan P. K., Pre-Test Analysis of Molten Salt

Natural Circulation Loop Using FLiNaK Salt,

International Conference on Molten Salts In

Nuclear Technology, January 9-11 (2013) BARC,

Mumbai.

8. Borgohain A., Jaiswal B.K., Maheshwari N.K.,

Vijayan P.K., Saha D., Sinha R.K., Natural circulation

studies in a lead bismuth eutectic loop, Progress in

Nuclear Energy 53 (2011) 308-319.

9. Vijayan P.K., Experimental and numerical

investigations on the nature of the unstable

oscillatory flow in a single-phase natural circulation

loop, XVII National and VI ISHMT/ASME Heat and

Mass Transfer Conference, IGCAR, Kalpakkam, Jan

5-7 HMT-2004-C100, pp 600-606.

Nomenclature

Ress Steady state Reynolds number Grm ModifiedGrashofnumber,

D Diameter,m r Densityofworkingfluid,kg / m3

b Volumetriccoefficientofexpansion,1/K

g Accelerationduetogravity,m / s2

D Tr Referencetemperaturedifference,(QH / AmCp), K

Q Totalheatinputrate,W H Loopheight,m A Flowarea, m2

Cp Specificheat,J / kg.K

m Dynamicviscosity,Pa.s