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Title: Use of Nitrogen Trifluoride To Purify Molten Salt Reactor Coolant and Heat Transfer Fluoride Salts

Abstract

Abstract: The molten salt cooled nuclear reactor is included as one of the Generation IV reactor types. One of the challenges with the implementation of this reactor is purifying and maintaining the purity of the various molten fluoride salts that will be used as coolants. The method used for Oak Ridge National Laboratory’s molten salt experimental test reactor was to treat the coolant with a mixture of H2 and HF at 600°C. In this article we evaluate thermal NF3 treatment for purifying molten fluoride salt coolant candidates based on NF3’s 1) past use to purify fluoride salts, 2) other industrial uses, 3) commercial availability, 4) operational, chemical, and health hazards, 5) environmental effects and environmental risk management methods, 6) corrosive properties, and 7) thermodynamic potential to eliminate impurities that could arise due to exposure to water and oxygen. Our evaluation indicates that nitrogen trifluoride is a viable and safer alternative to the previous method.

Authors:
ORCiD logo; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1364001
Report Number(s):
PNNL-SA-123463
Journal ID: ISSN 0888-5885; 830403000
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Industrial and Engineering Chemistry Research; Journal Volume: 56; Journal Issue: 19
Country of Publication:
United States
Language:
English
Subject:
21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS; fluoride salt purification; molten salt reactor; nitrogen trifluoride fluorination

Citation Formats

Scheele, Randall D., Casella, Andrew M., and McNamara, Bruce K.. Use of Nitrogen Trifluoride To Purify Molten Salt Reactor Coolant and Heat Transfer Fluoride Salts. United States: N. p., 2017. Web. doi:10.1021/acs.iecr.7b00374.
Scheele, Randall D., Casella, Andrew M., & McNamara, Bruce K.. Use of Nitrogen Trifluoride To Purify Molten Salt Reactor Coolant and Heat Transfer Fluoride Salts. United States. doi:10.1021/acs.iecr.7b00374.
Scheele, Randall D., Casella, Andrew M., and McNamara, Bruce K.. 2017. "Use of Nitrogen Trifluoride To Purify Molten Salt Reactor Coolant and Heat Transfer Fluoride Salts". United States. doi:10.1021/acs.iecr.7b00374.
@article{osti_1364001,
title = {Use of Nitrogen Trifluoride To Purify Molten Salt Reactor Coolant and Heat Transfer Fluoride Salts},
author = {Scheele, Randall D. and Casella, Andrew M. and McNamara, Bruce K.},
abstractNote = {Abstract: The molten salt cooled nuclear reactor is included as one of the Generation IV reactor types. One of the challenges with the implementation of this reactor is purifying and maintaining the purity of the various molten fluoride salts that will be used as coolants. The method used for Oak Ridge National Laboratory’s molten salt experimental test reactor was to treat the coolant with a mixture of H2 and HF at 600°C. In this article we evaluate thermal NF3 treatment for purifying molten fluoride salt coolant candidates based on NF3’s 1) past use to purify fluoride salts, 2) other industrial uses, 3) commercial availability, 4) operational, chemical, and health hazards, 5) environmental effects and environmental risk management methods, 6) corrosive properties, and 7) thermodynamic potential to eliminate impurities that could arise due to exposure to water and oxygen. Our evaluation indicates that nitrogen trifluoride is a viable and safer alternative to the previous method.},
doi = {10.1021/acs.iecr.7b00374},
journal = {Industrial and Engineering Chemistry Research},
number = 19,
volume = 56,
place = {United States},
year = 2017,
month = 5
}
  • This report provides an assessment of the use of nitrogen trifluoride for removing oxide and water-caused contaminants in the fluoride salts that will be used as coolants in a molten salt cooled reactor.
  • The Molten Salt Reactor Experiment (MSRE) was a prototype nuclear reactor which operated from 1965 to 1969 at Oak Ridge National Laboratory. The MSRE used liquid fluoride salts as a heat transfer fluid and solvent for fluoride based {sup 235}U and {sup 233}U fuel. Extensive research was performed in order to optimize the removal of oxide and metal impurities from the reactor's heat transfer salt, 2LiF-BeF{sub 2} (FLiBe). This was done by sparging a mixture of anhydrous hydrofluoric acid and hydrogen gas through the FLiBe at elevated temperatures. The hydrofluoric acid reacted with oxides and hydroxides, fluorinating them while simultaneouslymore » releasing water vapor. Metal impurities such as iron and chromium were reduced by hydrogen gas and filtered out of the salt. By removing these impurities, the corrosion of reactor components was minimized. The Univ. of Wisconsin - Madison is currently researching a new chemical purification process for fluoride salts that make use of a less dangerous cleaning gas, nitrogen trifluoride. Nitrogen trifluoride has been predicted as a superior fluorinating agent for fluoride salts. These purified salts will subsequently be used for static and loop corrosion tests on a variety of reactor materials to ensure materials compatibility for the new FHR designs. Demonstration of chemistry control methodologies along with potential reduction in corrosion is essential for the use of a fluoride salts in a next generator nuclear reactor system. (authors)« less
  • Control of a molten sait reactor by adjustment of the concentration of a nuclear poison that can be readily added to and removed from the circulating fuel is studied. This chemical control can be used, if necessary, in addition to the negative temperature coefficient of reactivity that plays a major role in the control of liquid fueled reactors. Gaseous boron trifluoride (enriched in B/sup 10/ if desired) may serve as a nuclear poison soluble in the fuel mixture of a molten salt reactor. The concentration of boron trifluoride in the liquid is proportional to and can be controlled by itsmore » partial pressure over a free surface in the circulating system. The solubility behavior of BF/sub 3/ as a function of pressure and temperature defines the capacity of the fuel for BF/sub 3/ and establishes the limits of possible usefulness of this compound as a removable reactor poison for reactor control. Experimental measurements of the solubility of BF/sub 3/ in a molten fluoride mixture suitable for Molten Salt Reactor Experiment (MSRE) applications are determined. The MSRE is a 10 Mw(th) circulating fuel reactor moderated with unclad graphite in contact wlth a mixture of molten fluoride salts containing uranium tetrafluoride. Values of Henry's law constants are obtained (in moles of BF/sub 3/ per liter of solution per atmosphere) of 0.265, 0.145, 0.0846, and 0.0351 at 500, 550, 600, and 700 deg C, respectively. These high solubilities indicate that BF/sub 3/ should be suitable for control purposes of a molten salt reactor even at very moderate partial pressures. At reactor temperatures, and with a BF/sub 3/ partial pressure or one atmosphere, an average atomic ratio of boron to U/sup 2356/ of 0.4 in the liquid is estimated. Simplified calculations of the distribution of boron between the fuel and the graphite moderator are made. Indications are obtained, neglecting adsorption effects, that 85% of the beron remains in the liquid even assuming complete equilibrium intrusion of the graphite pores by gaseous BF/sub 3/. (auth)« less