barc newsletter home founder’s daybarc.gov.in/publications/nl/2014/spl2014/pdf/paper38.pdf ·...
TRANSCRIPT
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
Home
NEXTPREVIOUS ê ê
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