Article pubs.acs.org/IECR
Use of Nitrogen Trifluoride To Purify Molten Salt Reactor Coolant and Heat Transfer Fluoride Salts Randall D. Scheele,* Andrew M. Casella, and Bruce K. McNamara Pacific Northwest National Laboratory, PO Box 999, Battelle Blvd., Richland, Washington 99352, United States S Supporting Information *
ABSTRACT: The molten-salt-cooled nuclear reactor is one of the Generation IV reactor types. One of the challenges of implementing 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 nitrogen trifluoride’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; (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. structural-metal fluorides resulting from the HF/H2 treatment.11 Other contaminants such as graphite dust and broken fuel pebble pieces were removed by mechanical filtration. Other contaminants not removed by the H2/HF treatment include corrosion products from the materials of construction and neutron activation products. Because NF3 is an effective temperature-sensitive fluorinating and oxidizing agent for oxides, metals, and lower-oxidation-state fluorides of elements found in spent nuclear fuels,12−20 this Article considers the use of thermal NF3 to remove oxide and water contaminants from the MSR coolant fluoride salts and to convert contaminant graphite, broken fuel pebbles, and corrosion products to soluble or volatile fluorides. Although not considered in this paper, NF3 provides a method for effectively separating dissolved bred uranium from thorium using differences in their fluoride volatilities.
1. INTRODUCTION With the resurgence of nuclear power as a potentially attractive source of energy, the molten-salt-cooled reactor (MSR), such as that conceived by researchers at University of California at Berkeley,1 has been identified by the U.S. Department of Energy’s Nuclear Energy Research Advisory Committee as one of the six Generation IV reactor types1 and has received renewed interest globally.2−9 The MSR would be cooled by a primary liquid fluoride salt coolant with the heat in the primary coolant transferred to a secondary molten fluoride salt and converted to electricity by a closed-loop Brayton electricity generation cycle. The Oak Ridge National Laboratory test reactor concept Fluoride Salt-Cooled High Temperature Test System (FHR-TS)10 includes a direct reactor auxiliary cooling system that uses a liquid fluoride salt as the primary means for removing decay heat from the reactor should the primary and shutdown cooling systems fail. Maintaining the purity of the primary coolant 7Li2BeF4 (FLiBe) and the secondary coolants is critical for limiting corrosion of the containment and piping.10 Pure, high-purity fluoride salts have low corrosivities, but when contaminated by water or oxygen, their corrosivities increase significantly. Routine replacement of FLiBe to ensure coolant purity would not be feasible because of the limited availability of enriched 7 Li. The Molten Salt Reactor Experiment (MSRE) removed oxygen and water contamination products by sparging their molten salts with a mixture of H2 and HF at 600 °C to provide an acceptable oxide content while managing the salt’s free fluorine potential by passing the salt over a bare beryllium metal surface. A final H2 purge at 600 to 800 °C removed dissolved © XXXX American Chemical Society
2. CANDIDATE COOLANT AND HEAT TRANSFER SALTS FOR FHR-TS The baseline primary coolant for the FHR-TS is 7Li2F2-BeF2 or 7 Li2BeF4.10 The leading intermediate coolant candidates for the FHR-TS are KF-ZrF4 or KZrF5, KF-KBF4 or K2BF5, and LiFNaF-KF (46.5−11.5−42 mol %) or Li4NaK3.65F8.65 (FLiNaK).10 The leading candidate materials for the direct reactor auxiliary coolant system cooling-loop coolant are lowerReceived: Revised: Accepted: Published: A
January 26, 2017 March 17, 2017 April 6, 2017 April 6, 2017 DOI: 10.1021/acs.iecr.7b00374 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research melting-point fluoride salts such as KZrF5, K2BF5, and 7LiFNaF-BeF2 or 7LiNaBeF4.10 2.1. Contaminants in Coolant and Heat Transfer Salts. Mathews and Baes21 found that water reacts with fluoride ion in FLiBe to form dissolved O2− and HF or OH− and HF. With a solubility of 0.01 mol BeO/kg FLiBe, solid BeO will precipitate. BeF2 is hygroscopic,22 making control of exposure to water critical for preventing corrosive constituents from being introduced into the FLiBe. Fortunately, LiF is not hygroscopic.23 Our thermodynamic equilibrium calculations using HSC Chemistry,24 provided in Figures 1 and 2 for 1 mol of FLiBe
The impact of H2O/O2 and H2O alone on the composition of FLiNaK based on thermodynamic calculations provided in Figures 3 and 4 shows that, when 1 mol of FLiNaK is treated
Figure 3. Predicted equilibrium composition based on thermodynamic calculations for FLiNaK exposed to H2O/O2 (1 mol of H2O/1 mol of O2/2 mol of FLiNaK).
Figure 1. Predicted equilibrium composition for Li2BeF4 exposed to H2O/O2 (1 mol of H2O/1 mol of O2/1 mol of FLiBe) based on thermodynamic calculations.
Figure 4. Predicted equilibrium composition based on thermodynamic calculations for FLiNaK exposed to H2O (1 mol of H2O/2 mol of FLiNaK).
with 1 mol of H2O and/or 1 mol of O2, the conversion of the constituent fluoride salts to their oxide or hydroxide is less than 10 mol ppm. Similar calculations for exposure of other various salt constituents to water found only ZrF4 would be converted to ZrO2 in any significant quantity. For evaluation purposes, we assume that the constituent fluoride salts form their respective oxides and hydroxides.
Figure 2. Predicted equilibrium composition for Li2BeF4 exposed to H2O (1 mol of H2O/1 mol of FLiBe) based on thermodynamic calculations.
3. COOLANT AND HEAT TRANSFER SALTS PROPERTIES Several physical properties of the coolant and heat transfer salts will be important during purification processing. Table 1 provides the melting points of the candidate coolant salts10 plus that of LiF-NaF-RbF. Williams et al.25 and Williams26 provide the melting points and additional physical properties including 900 °C vapor pressure, 700 °C density, volumetric heat capacity, viscosity, and thermal conductivity of all the candidate fluoride coolants with the exception of KF-KBOF4.
exposed to 1 mol of H2O and/or 1 mol of O2 as H2O/O2 or H2O alone, show that the H2O should convert a portion of BeF2 to BeO and HF. At 800 °C, nominally, 16% of the BeF2 will be converted to BeO by a combination of H2O and O2 and 12% by H2O alone. The equilibrium amount of BeO increases with increasing temperature. These figures also illustrate the complex chemistries of F, Li, and Be in FLiBe as a function of temperature. B
DOI: 10.1021/acs.iecr.7b00374 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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at unknown levels. Technical Resources International39 expected Advanced Specialty Gases to resume production. The three grades of NF3 supplied by Air Products are Commercial (99.7% purity), VLSI (99.9% purity), and Megaclass (99.996% purity), while Central Glass Company provides 99.995% purity NF3. Air Products purifies NF3 to the high-purity levels required by the electronics industry by removing N2F2 by pyrolysis over 200 to 300 °C (473 to 573 K) metal or metal fluoride40 and water, nitrous oxide (N2O), and carbon dioxide (CO2) by adsorption on zeolite molecular sieves. The impurity levels in the three grades of NF3 provided by Air Products are provided in the Supporting Information. 4.2.2. NF3 Delivery. Air Products delivers NF3 in highpressure cylinders or bulk containers. The high-pressure cylinders are pressurized to 100 bar (1450 psig).40 NF3 is also available in large, skid-mounted containers (Y-cylinders) that contain 195 kg (430 lb) and are mounted horizontally and tapered and threaded at both ends.45 Large amounts of NF3 can be delivered in tube trailers or International Organization for Standardization modules that are most commonly four- or eight-tube bundles of 56 cm (22 in.) diameter tubes containing up to 5440 kg (12 000 lb). 4.3. NF3 Chemical and Physical Properties. NF3 is a colorless gas at room temperature that boils at −128.75 °C (144.40 K). NF3 has a pyramidal structure with C3v point group symmetry similar to ammonia (NH3). However, in contrast to ammonia, NF3 exhibits no basic properties and is not protonated even in the superacid HSO3F-SbF5-SO3.46 Highpurity NF3 has little odor, but it can have a pungent, musty odor if it is contaminated with traces of active fluorides. Anderson et al.28 provide a broad-based general reference for NF3, but it has limited distribution and availability. Table S2 provides a selection of NF3’s physicochemical properties. 4.4. NF3 Use Considerations. NF3 has characteristics that must be managed for safe use and release. Relative to HF, which is highly toxic, NF3 is only slightly toxic and is only reactive at high temperatures and when exposed to certain physical conditions. It is believed to have significant global warming potential (GWP). The hazards of NF3 and HF are provided in Table 2 using the National Fire Protection Association’s (NFPA’s) and
Table 1. Melting Points of Candidate Primary, Intermediate, and Direct Reactor Cooling System Coolant Salts melting point, °C
salt Primary Coolant LiF-BeF2 (FLiBe)
46010
Intermediate Coolant KF-ZrF4 39010 LiF-NaF-KF (FLiNaK) 45410 KF-KBF4 46010 LiF-NaF-RbF 43525,26 Direct Reactor Auxiliary Cooling System KF-KBF4 46010 KF-ZrF4 39010 7 LiNaBeF4 31510
4. NF3 PROPERTIES AND USE CONSIDERATIONS Important factors for NF3 purification use include the current NF3 industrial uses, production levels, chemical and physical properties, chemical and reactivity hazards, environmental impacts, and effluent management strategies. 4.1. NF3 Background. NF3 is an industrially important oxidizing and fluorinating agent used by the electronics industry to etch silicon and to remove residual coatings deposited in chemical vapor deposition reactors as volatile fluorides and as a fluorine source in high-power chemical lasers,27 and it was once considered as a rocket propellant.27,28 The fiber-optic industry has investigated and successfully demonstrated the use of NF3 to remove the oxide, hydroxide, and water impurities from fluoride-based glasses such as fluorozirconate glasses based on zirconium tetrafluoride (ZrF4), whose performance is significantly affected by oxide and hydroxide impurities.29−32 Complicating NF3 use is its potential environmental impact. In the mid-1990s, NF3 replaced perfluorocarbons compounds such as CF4, C2F6, and SF6 as the silicon-etching and chemical vapor deposition reactor-cleaning agents for the electronics industry to reach Kyoto Protocol goals for reducing these gases33 because it was considered an environmentally benign material. Recent evaluations33−36 have identified NF3 as a potential significant long-lived “greenhouse” gas. 4.2. Nitrogen Trifluoride Production and Delivery. The two primary large- or industrial-scale methods for producing NF3 use either direct fluorination of ammonia or electrolysis of molten ammonium acid fluoride.37−40 Central Glass Company, Ltd.’s alternative process treats ammonium aluminum fluoride with 150 °C F2.38 Air Products and Chemicals, Inc. (Air Products) implements Tompkins and Wang’s method41 to electrochemically convert molten ammonium acid fluoride (cryolite) to NF3 using HF.37,38 Air Products also uses direct fluorination of ammonia over heated ammonium acid fluoride using molecular fluorine (F2).37,38,42 4.2.1. NF3 Production and Quality. Total NF3 production levels and capacities are not freely available, but Fthenakis et al. reported that 7000 t of NF3 was produced in 2008,43 and Prather and Hsu estimate34 that the total 2008 production was 4000 ± 25% tons and that would double in 2010. Air Products, the largest producer-manufacturer of NF3, planned to produce 3200 tons by 2009.44 Katsuhara et al.38 reported that in 2005 Central Glass Company was producing 400 t yr−1 and had plans to produce 600 t yr−1 by the end of 2006 by direct fluorination of cyrolite with F2. Other producers include Kanto Denka at 1000 ton yr−1 and DuPont in China, Formosa Plastics, Mitsui Chemicals, and Anderson Development Co.34,39
Table 2. Hazard Ratings for NF348 and HF49 a NF3
a
HF
hazard
NFPA
HMIS
NFPA
HMIS
health flammability instability/physical special
1 0 0 oxidizer
1 0 3 NA
4 0 1
3 0 2 NA
Hazard Rating Scale (0−4).
Hazardous Material Identification System (HMIS) hazard ratings. Table 2 combines the NFPA hazard “instability” with HMIS hazard “physical” since both describe a material’s chemical reactivity. Both NFPA and HMIS use a rating scale of 0−4 where 0 = no hazard, 1 = slight hazard, 2 = moderate hazard, 3 = serious hazard, and 4 = severe hazard. As Table 2 shows, NF3 is a slightly toxic gas but has no fire or reactivity hazards, while HF is very toxic and corrosive.47 These characteristics require that their chemical hazards, compatibility with materials of construction, and their release be managed. For fluoride salt purification, HF use requires H2 C
DOI: 10.1021/acs.iecr.7b00374 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research with its well-established flammable and potentially explosive character. 4.4.1. NF3 and HF Health and Safety Considerations. According to BOC Gas and Air Product Material Safety Data Sheets,48,50 NF3 is a noncorrosive, nonflammable, oxidizing, chemical asphyxiant that complexes with hemoglobin to form methemoglobin, thus reducing the capacity of blood to carry oxygen, causing cyanosis. Once exposure to NF3 stops, the methemoglobin reverts back to hemoglobin.40 NF3 may also cause eye irritation. Anhydrous HF is a highly toxic (NFPA rating 4) and corrosive gas above 19.7 °C that can have significant health effects. HF is not a carcinogen but can cause severe skin burns that may not be immediately noticeable. HF can penetrate the skin and damage underlying tissue. Severe inhalation exposure to HF can cause nose and throat burns, lung inflammation, pulmonary edema and, if not promptly treated, other systemic effects such as depletion of calcium body levels. Unlike other acid burns, specialized medical care is required. The fluoride ion is extremely mobile and can penetrate quickly and deeply into the skin.51 4.4.1.1. NF3 Purification. Air Products52 directs that dry media such as activated charcoal or molecular sieves not be used to purify NF3 because of the potential of rapid exotherms from sudden exposure to large amounts of NF3. Henderson and Woytek40 report that zeolites are used by Air Products to remove water, N2O, and CO2. 4.4.1.2. NF3 and HF Toxicology. The Occupational Safety and Health Administration (OSHA) permissible exposure limits (PELs) for NF3 and HF over an 8 h work shift are 10 and 3 ppm, respectively. Although NF3’s PEL is only a factor of 3 greater than that of HF, the consequences from exposure to NF3 are less. The health effects as identified by OSHA for HF are Irritation (Eye, Nose, Throat), Marked (HE14); Acute lung damage (HE11); Acute toxicity, Ventricular fibrillation (HE4); Cumulative bone damage (HE3).53 The significant hazards from HF exposure are captured by Air Products through the warning provided in their Safetygram: “WARNING: Burns with concentrated hydrofluoric acid (HF) are usually very serious, with the potential for significant complications due to fluoride toxicity. Concentrated HF liquid or vapor may cause severe burns, metabolic imbalances, pulmonary edema, and lifethreatening cardiac arrhythmias. Even moderate exposures to concentrated HF may rapidly progress to fatality if left untreated.”54 Exposure to HF can have immediate, nonreversible health effects. The health effects from inhalation exposure to NF3 are methemoglobinemia (HE13) and cumulative liver damage (HE3). According to Air Products, the effects of the initial red blood cell changes will clear over several hours but should still be monitored for secondary effects,45 and “at the cessation of NF3 exposure, methemoglobin spontaneously reverts to hemoglobin. While methomoglobinemia clears over several hours, hemolytic anemia may take several weeks to resolve.”52 Exposure to NF3 can have immediate health effects, but depending on the severity of the exposure, these can be reversed. 4.4.1.3. NF3 Reactivity Hazards. The primary hazard from NF3 arises if sufficient energy is provided to release fluorine and produce a self-propagating reaction with materials that are not compatible with fluorine. The NF3 hazard can be safely managed through proper design of equipment and appropriate
management of factors such as temperature, pressure, adiabatic compression, and velocity in pipelines. Barbier et al.55 provide a code of practice for safe use of NF3. NF3 should be managed as a mildly toxic oxidizer with a relative oxidation potential of 1.6 where O2 has an oxidation potential of 1.0. Although NF3 is relatively inert at atmospheric pressure and ambient temperature, the autoignition temperature of some combustible materials may decrease with increasing NF3 pressure as illustrated in Table 3.55 To manage the NF3 Table 3. Ignition Temperatures of Cu, Fe, and Ni at 1 and 7 bar NF355 ignition temperature, °C NF3 pressure, bar
Cu
Fe
Ni
1 7
550 475
817 612
1187 967
reactivity hazard, precautions should be taken to prevent inadvertent heating of NF3 such as operating at an as-low-aspractical temperature, managing the NF3 pressure and velocity, using clean and compatible materials of construction, preventing mechanical shocks, preventing adiabatic compression, controlling flow friction, and preventing localized hot spots in equipment. Chen56 reports a 5.0% H2 lower flammability limit for the NF3/N2 mixtures used in the semiconductor industry. On the basis of this reported reactivity of NF3 with H2, this gas mixture should not be used to purify coolant salts. 4.4.1.4. Materials of Construction for NF3 Use. NF3 is noncorrosive to common metals below 70 °C (343 K) and can be used with steel, stainless steel, and nickel. Corrosion increases significantly if moisture or HF is present. NF3 is compatible with the fluorinated polymers such as Teflon, Kel-F, and Viton at ambient conditions.40 Air Products52 reports that static exposure to NF3 containing ≤0.1% active fluorides as HF did not corrode aluminum, stainless steel, Inconel, Monel (a nickel−copper alloy), nickel, titanium, copper, beryllium copper, aluminum bronze, or tungsten at penetration rates greater than 0.43 mils a−1 (0.011 mm a−1). These 270 day tests were performed at temperatures ranging from −78 to 71 °C (195 to 344 K) at pressures ranging from 1 × 10−8 to 1.7 × 10−7 bar (2 × 10−7 to 2.5 × 10−6 psi). Air Products reports that carbon steel, stainless steel, nickel and its alloys, and copper are suitable for use at low pressures,
