Investigation of Thorium Tetrafluoride Purification Using the Ammonium Bifluoride method

Bernard Fitzpatrick1

K. Foster2, T.M. Besmann2, M.H.A. Piro1

1Ontario Tech University; 2University of South Carolina


Actinoid fluorides such as thorium tetrafluoride (ThF4), are quite hygroscopic and require purification of oxygen species present in the salt in very stable compounds such as ThO2 – which would otherwise increase the corrosivity of the salt in a practical reactor. As an alternative to installation of an HF line in the laboratory, which has safety implications at a University, a small (75 mL) Ni pressure vessel is fitted with crucibles for both the fuel salt and the ammonium bifluoride. Upon heating the ammonium bifluoride decomposes into ammonia and HF, which may then purify the fuel salt specimen. This approach was first described in the literature in [1-4], although difficulties were reported in [5]. Overall, it is not yet a robust or repeatable process.
Reactants were measured out using a 0.01 mg-precision mass balance within an Ar-filled inert glovebox. They were loaded into Ni crucibles and stacked (ammonium bifluoride on the bottom, ThF4 on the top) into the pressure vessel. The vessel was then closed using a torque-wrench. The vessel is wrapped in a heater tape, as well as heated on the bottom nearest the specimens using a hotplate at 250 °C. The heater tape is automatically controlled by a thermocouple tightened under one of the six chamber bolts. Temperature is checked using an IR-thermometer to ensure a chamber valve’s temperature tolerance is not exceeded.
After allowing the vessel to heat for 12 h, a valve is opened allowing water vapour and HF gas to escape through a scrubber and out of the glovebox into the laboratory exhaust system. The specimen is removed and heated in the glovebox environment to 450 °C for 6 h in on open crucible to dehydrate it.
The purity of the resulting powder can be judged qualitatively by its colour: pure ThF4 is white, with darker shades indicating the present of impurities. For more quantitative analysis, differential scanning calorimetry is employed using a NETZSCH 449 F1 simultaneous thermal analyzer. The temperature of the melting peak measured is a well-defined quantity (~1111 °C [6]) and deviations indicate impurities.

  1. E. Capelli. Thermodynamic Characterization of Salt Components for Molten Salt Reactor Fuel. Doctoral Thesis, TU Delft, Melkeweg, The Netherlands, 2016.
  2. M. Beilmann. Thermodynamic Investigation of Fluoride Salts for Nuclear Energy Production. Doctoral Thesis, Heidelberg University, Institute of Physical Chemistry, Heidelberg, Germany, 2013.
  3. B. N. Wani, S. J. Patwe, U. R. Rao, and K. S. Venkateswarlu. Fluorination of Oxides of Uranium and Thorium by Ammonium Hydrogenfluoride. Journal of Fluorine Chemistry, 44:177–185, 1989.
  4. Che Nor Aniza Che Zainul Bahri, Aznan Fazli Ismail, and Amran Ab. Majid. Synthesis of Thorium Tetrafluoride (ThF4) by Ammonium Hydrogen Difluoride (NH4HF2). Nuclear Engineering and Technology, 51(3):792 – 799, 2019.
  5. J.A. Ocadiz-Flores. Personal Communication, TU Delft, 2020.
  6. Pavel Souček, Ondřej Beneš, Benoit Claux, Elisa Capelli, Michel Ougier, Václav Tyrpekl, Jean-Francois Vigier, and Rudy J.M. Konings. Synthesis of UF4 and ThF4 by HF gas fluorination and re-determination of the UF4 melting point. Journal of Fluorine Chemistry, 200:33-40, 2017.

Event Timeslots (1)

Wednesday – 15th September 2021
Bernard Fitzpatrick