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A Chemical Analysis of the Thermodynamic properties of

FLiNaK

Gilliam van Oudenaren

December 2013

Abstract

A research project involving the basic properties surrounding FLiNaK, a eutectic mixture of LiF-NaF-KF. Density, Melting Point and Heat Capacity were determined to give insight into the usefulness of FLiNaK in a Molten Salt Reactor.

Keywords: FLiNaK, MSR, Density, Melting Point, Heat Capacity

1 Introduction

Fluoride salt technology has recently been getting a large amount of attention as a side effect of Molten Salt Reactors (MSRs) receiving renewed publicity. Designs such as the LFTR (Liquid Fluoride Thorium Reactor) are of increasing interest and molten salts play a key role in these concepts. In this research I have looked at one of the candidate salts for use in MSRs: FLiNaK. This is a eutectic mixture of Lithium-, Sodium-, and Potassium Fluoride, respectively 46.5-11.5-42-mol %.[1] Various research involving these salts has been done at Oak Ridge National Laboratory, Tennessee in the mid 20th century. During my analysis of FLiNaK I aimed to produce new results and to put these into perspective with the original research.

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2 Investigating FLiNaK

Image: LiF, NaF, KF used to make FLiNaK (solid mixture displayed on the right, pre-heating)

In order to investigate the salt, I first had to produce it. I acquired all the individual components: LiF, NaF, KF. These were homogenously mixed before being heated to approx. 800°K. The newly formed eutectic was then cooled to below its melting point. Small samples were taken for use in determining the different properties for my research.

2.1 Melting Point

A setup was made to determine the melting point of the salt mixture, this contained a thermocouple for measuring temperature, a digital signal receiver for displaying the value, a ceramic crucible for containing the salt, tripod with wire gauze etc. and a Bunsen burner. The crucible containing the salt was heated used the burner and the temperature was closely inspected. Once the melt set it I varied the temperature till I approached a satisfactory melting point with decent accuracy. I repeated this experiment with a 2nd sample of previously mixed salt.

2.2 Heat Capacity

In order to investigate the heat capacity I made a setup involving a sample of the salt mixture – known mass, and water. The setup was similar to the one mentioned above, but now included a beaker with 250ml of distilled water connected to a 2nd thermocouple. I heated the salt mixture to a temperature of 70 ±0.1 °C after which I added to it the beaker with water. I took note of the temperature of the water after 30 seconds. I repeated this experiment with a salt mixture of 75 ±0.1 °C.

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2.3 Density

To determine the solid, room temperature density I took a sample of the mixture and weighed it. I then submerged the sample in a non-solvent and determined the difference in volume. By dividing the mass by the volume the density is easily derived. I repeated this process with a sample of different mass.

3 Results

3.1 Melting Point

1Tmelt = 455.4 ±0.3°C 2Tmelt = 456.2 ±0.3°C ____________ avgTmelt = 455.8 ±0.3°C  

3.2 Heat Capacity

Qwater = -Qsalt

C·m·ΔT = C·m·-ΔT 1.043·ΔT = C·m·-ΔT (1.043·ΔT)/(m·-ΔT) = Csalt 1C = 1.959J·g-1 2C = 1.918J·g-1

___________________ avgC = 1.94 ±0.1J·g-1

3.3 Density

1m = 0.68 ±0.1gram, 1ΔV = 0.30 ±0.1ml, 1𝜌 = 2.27g·cm-3 2m = 0.56 ±0.1gram, 2ΔV = 0.25 ±0.1ml, 2𝜌 = 2.24g·cm-3 ___________________________________________________ avg𝜌 = 2.26 ±0.1g·cm-3

Cwater = 4.18J·kg-1 ρwater = 0.998kg·m-3 Vwater = 250ml

ΔT = Tend – Twater/begin

-ΔT = Tsalt/begin - Tend

Tbegin/water = 20.2 ±0.1°C 1msalt = 0.46 ±0.1gram, Tbegin = 75 ±0.1°C → Tend = 45.6 ±0.1°C

2msalt = 0.51 ±0.1gram, Tbegin = 70 ±0.1°C → Tend = 44.3 ±0.1°C

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4 Conclusion

I successfully derived all three values I set out to obtain. They are:

Tmelt = 728.95 ±0.3°K C = 1.94 ±0.1J·g-1 at T:[293.15;368.15]°K 𝜌 = 2.26 ±0.1g·cm-3 at T=293.15°K These results can be compared to those mentioned in the Engineering Database of Liquid Salt Thermophysical and Thermochemical Properties[2], which is a document comparing the values found in different researches and aims to provide a standardized value. It should be noted that the majority of these values were obtained at 600-800°K. These are the values stated: Tmelt = 727°K C = 1.883J·g-1 𝜌 = 2.020·cm-3 All of my values, except for perhaps density, are within a reason range of the ones citied in the Engineering Database. Regarding the density; this might have to do with the fact that the values cited were obtained in a higher temperature range. This leads me to believe that FLiNaK expands upon heating.

5 Discussion

I could have likely obtained more accurate values if I improved upon a number of factors in my research. These include: purity of chemicals, accuracy of weighing scale, improved thermocouple (alternatively a calorimeter). Despite the shortcomings of a basic lab I still obtained a fairly accurate result so I can be pleased.

6 References

[1] Lane, James A. "Chemical Aspects of Molten Fluoride Salt Reactor Fuels." Fluid Fuel Reactors. Reading, MA: Addison-Wesley Pub. (1958).

[2] Sohal, Manohar S., et al. "Engineering database of liquid salt thermophysical and thermochemical properties." Idaho National Laboratory, Idaho Falls (2010).

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7 Acknowledgements

I would like to thank a number of people. Firstly Brian Kelleher at Wisconsin-Madison who provided me with valuable information surrounding molten salts and instructed me on how to prepare it. Secondly the people at the University of Amsterdam for supplying me with chemicals to make this project possible and thirdly the Hervormd Lyceum Zuid for allowing me to undertake my experiments in our school lab.