Th-ING: A Sustainable Energy Source
Thorium is now green
Imagine an element that when used in a nuclear reactor is so safe that it may never lead to the possibility of the type of catastrophic meltdown that threatened the reactors in Japan. Picture one ton of such an element producing as much energy as 200 tons of uranium or 3,500,000 tons of coal. Imagine an element that right now is trapped in 3,200 metric tons of nuclear waste waiting for final disposition at the Nevada National Security Site.
The element is thorium, a silvery-white metal that is slightly radioactive. It was named after the Norse god Thor by Jöns Jakob Berzelius, who discovered the element in 1828. During the last decade, thorium has been labeled the "green nuke" because, unlike other actinides such as uranium and plutonium, it cannot be easily used in nuclear weapons and, if used in nuclear reactors, is so safe that it would never be the cause of a nuclear meltdown.
Anhydrous halide complexes are key starting materials for synthesizing transition metal, lanthanide, and actinide compounds. However, preparing thorium halides—the key to unlocking thorium's potential—has proved expensive and has been further complicated by environmentally harmful processes that involve tricky reactions that require harsh, unsafe reaction conditions. For example, one process costs as much as $5,000 per kilogram to yield thorium compounds and materials, whereas another process requires high temperatures and hazardous chemicals for its production yields.
To resolve these complicated matters, Jaqueline L. Kiplinger and her team at Los Alamos National Laboratory have developed a novel method known as Th-ING (Thorium Is Now Green), which circumvents the hazards and cost issues of conventional methods to produce a new thorium chloride reagent, ThCl4(DME)2. This cost-effective, safe, "green," and scalable method will revolutionize the use of thorium in nonaqueous thorium chemistry and materials science. This method also stands to play a crucial role in creating one of the world's future sustainable energy sources.
In nature, thorium (such as the sample shown here) is found as thorium-232. Countries such as Russia, India, and China have plans to use thorium for their nuclear reactors, partly because of its safety benefits.
Producing a New Thorium Chloride Reagent
The principal building block of this new method is thorium nitrate, Th(NO3)4(H2O)5, which serves as the starting material. Los Alamos scientists react thorium nitrate with aqueous hydrochloric acid under mild conditions. The reaction produces ThCl4(H2O)4 in quantitative yield. Scientists then use a novel combination of anhydrous hydrochloric acid and trimethylsilyl chloride (Me3SiCl) to remove the coordinated water molecules and replace them with dimethoxyethane (DME) to make the thorium chloride reagent, ThCl4(DME)2.
ThCl4(DME)2 is an excellent synthetic precursor to a wide range of thorium(IV) compounds containing Th-O, Th-N, Th-C and Th-X (X = F, Cl, Br, I) bonds. Overall, the reaction chemistry with ThCl4(DME)2 can be performed at multigram scales and is high yielding (>88%).
Mild and Safe Manufacturing Conditions
Conventional methods used to manufacture ThCl4 require the use of hazardous gases, such as sulfur chloride, chlorine, phosgene, and carbon tetrachloride. Such processes also require expensive custom equipment, such as tube furnaces, and operational temperatures greater than 450°C.
Manufacturing ThBr4(THF)4 requires the use of bromine, which is volatile, corrosive, and toxic. Thorium metal is pyrophoric (spontaneously ignites) and must be cleaned with nitric acid before use. Moreover, the reaction temperature must be maintained at 0°C or polymerization of the tetrahydrofuran solvent will take place.
Th-ING uses hydrochloric acid to convert thorium nitrate, Th(NO3)4(H2O)5, to ThCl4(H2O)4, which is converted to the new thorium chloride reagent. The reaction conditions are comparatively mild (temperature of 100°C) and can be performed using conventional glassware in a traditional laboratory. The reaction does produce some NOx, but a ventilation hood keeps the process safe. Subsequent drying with trimethylsilyl chloride is also done using the mild temperature of 50°C. Moreover, thorium nitrate is not pyrophoric.
This graphic shows the molecular structure of ThCl4(DME)2 with thermal ellipsoids projected at the 50% probability level. Hydrogen atoms have been omitted for clarity. The thorium(IV) metal center features a distorted dodecahedron geometry. The average Th–Cl bond distance of 2.690 Å compares well to those presented by other reported thorium(IV) tetrachloride complexes [e.g., ThCl4(O=PPh3)3, Th–Cl(ave) = 2.736 Å; ThCl4(TMEDA)2, Th–Cl(ave) = 2.689 Å], and the average Th–O bond length of 2.596 Å is consistent with those measured in ThBr4(DME)2 (Th–O(ave) = 2.588 Å).
It's an Environmental Th-ING
Synthesizing ThBr4(THF)4 also requires thorium metal, an expensive material available only at a small number of institutions. Thorium metal is not readily produced. For example, it is possible to obtain the metal by reducing thorium oxide with calcium metal or by reducing thorium tetrachloride with calcium or magnesium metal at high temperatures (~1,000°C) under an atmosphere of argon. Each of these processes requires subsequent separations and produces mixed (radioactive/hazardous) waste. The actual ThBr4(THF)4 synthesis also tends to be wasteful (maximum 60% production yield) and is usually low yielding because of the formation of unwanted byproducts caused by the ring-opening/polymerization of the solvent THF. Additionally, the ThBr4(THF)4 complex is thermally sensitive and decomposes at room temperature.
Synthesizing ThCl4 requires dangerous and environmentally harmful protocols that involve reacting thorium dioxide with hazardous sulfur chloride, chlorine, phosgene, or carbon tetrachloride vapors at elevated temperatures (450°C–1,000°C) for several days. Although these reactions produce highly pure ThCl4, they tend to give poor yields (maximum ~80% production yield) because the product ThCl4 must be sublimed from unreacted ThO2. The hazardous gases are used in excess and released into a hood and ultimately the atmosphere.
In contrast to the synthesis of ThCl4 and ThBr4(THF)4 , which are incomplete reactions resulting in thorium waste, Th-ING is superior in that the synthesis is quantitative (95% production yield). The consistent high yields provided by the Los Alamos process translate into less mixed (radioactive/hazardous) waste compared with the syntheses of ThCl4 and ThBr4(THF)4. Unlike the synthesis of ThCl4, Th-ING does not release hazardous gases into the atmosphere. Finally, Th-ING avoids the wasteful solvent ring-opening/polymerization that frequently occurs when preparing ThBr4(THF)4.
ThCl4(DME)2 is an excellent synthetic precursor to a wide range of thorium(IV) complexes containing Th-O, Th-N, Th-C and Th-X (X = F, Cl, Br, I) bonds: (i) 3 equivalent Ph3P=O, THF, 100% yield; (ii) excess TMEDA, THF, 100% yield; (iii) excess Me3SiBr, toluene, 24 h, 100% yield; (iv) 4 equivalent KOAr(Ar = 2,6-tBu2C6H3), THF, 99% yield; (v) 4 equivalent Na[N(SiMe3)2], toluene, reflux, 12 h, 93% yield; (vi) 2 equivalent (C5Me5)MgCl(THF), toluene, reflux, 24 h, 88% yield.
Taking Advantage of 3,200 Metric Tons of Waste
From 1957 to 1964, the Department of Energy's predecessor agency, the Atomic Energy Commission, acquired more than seven million pounds of thorium nitrate in more than 21,000 drums. In 2004, some 20,000 drums (3,200 metric tons) of thorium nitrate from the Defense Logistics Agency/Defense National Stockpile Center (DLA/DNSC) Depots in Maryland and Indiana were transferred to the Nevada National Security Site for disposal. The price tag for such disposal was estimated at more than $60 million, putting the effort on hold and leaving the waste in storage since that time.
Los Alamos' Th-ING process could use this waste as the starting material for producing the thorium chloride reagent. Not only would Th-ING provide a nondestructive path forward for this "waste," it would resolve a $60 million issue that has to this day been difficult to overcome.
Nuclear power plants produce electricity by using nuclear fission to heat water and produce steam. Current power plants use uranium and plutonium. However, thorium is a much safer alternative, as the metal can avoid nuclear meltdowns. Moreover, thorium cannot be used in nuclear weapons, making it an ideal alternative when it comes to addressing issues related to the proliferation of materials used in nuclear weapons.
The Thorium Fuel Cycle and Other Uses
Using the ThCl4(DME)2 reagent, it will be possible to develop a safe and secure thorium fuel cycle, which in turn could lead to a sustainable energy future. In theory, thorium is a superior nuclear fuel, one that has several important advantages over uranium for the following reasons:
- Thorium-powered nuclear reactors are more efficient and produce less than one percent of the waste of today's uranium nuclear reactors.
- Thorium reactors are safer (never the cause of a meltdown), less expensive, and smaller; they can be configured to eliminate the possibility of meltdown or other types of accidents.
- Fission of thorium does not produce much plutonium, and thus its use could effectively eliminate further weapons production in volatile regions without sacrificing energy production. The use of thorium could also help reduce the proliferation of nuclear weapons at a global scale.
According to the World Nuclear Association, as of January 2011, there were approximately 440 nuclear power plants in operation globally, with 60 more under construction. There are also 320 new nuclear plants either planned or proposed around the world, with approximately one-fifth of those plants commissioned for construction within the next seven years.
Thorium and thorium compounds have numerous applications, including aircraft engines and spacecraft, to heat-resistant ceramics, high-quality lenses for cameras and scientific instruments, and mantles for natural gas lamps, oil lamps, and camping lights. Th-ING will enable, for the first time, easy and safe access to thorium chemistry and materials research. Of particular importance is the fact that the Los Alamos Th-ING process can be performed on a large scale, which is necessary for industrial production. Possible new applications of thorium(IV) include (1) developing routes to thorium-nitride/carbide/oxide/fluoride fuels and (2) enabling sol-gel science for nuclear materials storage, processing, and fuel. Los Alamos' homogeneous thorium complex will also be invaluable for grafting thorium onto solid supports for industrial or large-scale applications.
Cost-effective, safer, and environmentally friendly, Th-ING stands to revolutionize thorium chemistry and materials science, address the elimination of waste that has been in storage since 1957, and play a crucial role in creating one of the world's future sustainable energy sources. With Th-ING, the future for thorium is now as bright as the metal's own luster.
–Jaqueline L. Kiplinger and Octavio Ramos Jr.
Approximately 20,000 drums (3,200 metric tons) of thorium nitrate await disposal. However, such waste is ideal as a starting material for producing the ThCl4(DME)2 reagent that in turn would lead to the use of such stockpiled nuclear waste as fuel for thorium-based nuclear reactors.