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Actinide Boride Ceramics Offer Alternatives for Safer Plutonium Storage

Los Alamos Team Investigates Low-Temperature Preparation Technique

Anthony Lupinetti, left, of Actinide Processing Chemistry (NMT-2), and Kent Abney of Isotope and Nuclear Chemistry (C-INC) examine plutonium boride encased in a glass tube. The researchers are investigating ways to convert actinide metals into less-reactive, safer storage forms. Initial results indicate that actinide borides are stable in water, which should give them an advantage over plutonium metal or plutonium oxide as a storage form.

Safe storage of excess plutonium is the goal of a Los Alamos effort aimed at defining alternative actinide stabilization technologies. Under a treaty with the Russian Republic, the United States has declared approximately 38 metric tons of plutonium as excess to the nuclear weapons program; the Russians have declared a nearly equal quantity of material to be excess.

The majority of the U.S. material exists as highly purified metal, which poses a proliferation risk. It also poses a chemical risk because plutonium metal slowly reacts with water to form hydrogen if it is not properly stored. Over time this can result in overpressurization of the storage container or a buildup of flammable concentrations of hydrogen.

To overcome these and other issues, Los Alamos scientists are looking at new ways to convert actinide metals into less-reactive, safer storage forms. It has long been known that plutonium can be combined with boron, a solid semi-metal that shares some of the properties of metals as well as nonmetals, to form stable, water-insoluble plutonium borides. But these compounds could only be formed under high temperatures‹in excess of 1,500 degrees Celsius. The Los Alamos researchers have explored a low-temperature method to create stable actinide ceramics. Initial results indicate that actinide borides are readily prepared and stable in water, which should give them an advantage over plutonium metal or plutonium oxide as a storage form.

Plutonium and boron form seven different binary compounds. One of the compounds, plutonium diboride, is composed of plutonium atoms (green) sandwiched between two-dimensional networks of boron atoms (blue). This form of plutonium boride encapsulates a large amount of plutonium in a compact form (92 percent plutonium by weight). The team of Los Alamos researchers has succeeded in synthesizing this compound at low temperatures.

Actinide borides are typically composed of two- or three-dimensional arrays of linked boron clusters in which the metal atoms may be located between layers or encapsulated in the boron matrix. These networks create a class of materials with high plutonium content, high densities, and high melting points.

There are only 11 known binary plutonium, uranium, and thorium boride phases; examples include plutonium diboride, uranium tetraboride, and thorium hexaboride. These materials are known to be ceramics, but other chemical properties, such as reactivity, must still be explored.

In contrast, many transitional metal and lanthanide borides have been extensively studied. These borides have been used as nonreactive corrosion-resistant coatings. The Los Alamos researchers expect that many plutonium boride phases will also prove to be nonreactive, making them strong candidates as plutonium storage forms.

Until now, plutonium borides were made exclusively by high-temperature methods. To get the two elements to mix, plutonium metal would be combined with varying amounts of boron, heated to greater than 1,500 degrees Celsius, then ground and reheated repeatedly until a homogeneous mixture was obtained. Another method to react plutonium with boron is arc melting, but this method produces even more extreme temperatures‹above 3,000 degrees Celsius.

The structure of uranium tetraboride consists of uranium atoms (yellow) encapsulated in a three-dimensional network of boron atoms (blue). This ridged structure is one of the features that makes this compound such a stable storage form for actinide elements.

The new Los Alamos method uses a reactive process to combine actinide halides with reactive boride compounds that requires far lower temperatures-between 400 and 850 degrees Celsius-and doesn't require repeating the process over and over again.

The researchers began their experiments on a small scale by combining 250 milligrams of uranium tetrachloride with two equivalents of magnesium boride in a vacuum-sealed quartz tube and heating it to 850 degrees Celsius for one day. After the reaction, the tube is opened and the contents washed with water to remove the magnesium chloride byproduct and any unreacted uranium halide.

The result is pure uranium tetraboride, a stable, insoluble compound that is not easily converted to a pure actinide form that can be used in weapons. The uranium tetraboride was identified using powder x-ray diffraction.

Experiments were also performed to determine the effect of time, temperature, and excess magnesium boride on the reaction products. The major difference in these reactions is the crystallinity of the uranium tetraboride; longer reaction times and higher temperatures yield a more crystalline product. Higher crystallinity aids in the chemical characterization of the products by helping to identify other potential products.

This graph shows two powder x-ray diffraction patterns. The top pattern is from the washed product of the reaction of uranium tetrachloride and magnesium diboride; on the bottom is a calculated pattern of uranium tetraboride. The two sets of data match exactly, confirming that uranium tetraboride is the only product.

In addition to the studies using uranium tetrachloride, similar studies were carried out with thorium tetrachloride and plutonium trichloride, producing thorium hexaboride and plutonium di- and tetraboride, respectively. The researchers are also investigating ways to produce actinide borides at even lower temperatures by using fluxing agents like lithium chloride and potassium chloride, which when combined melt near 350 degrees Celsius.

The Los Alamos team is pursuing next-generation reactions that will allow them to further understand actinide chemistry and provide routes to other actinide ceramics including ternary materials containing plutonium, boron, and other elements like oxygen. Little is known about this class of actinide-containing three-component materials, and they are being investigated to broaden the fundamental knowledge of actinide materials science.

One avenue the team is investigating is the synthesis of molecular actinide compounds that contain novel boron-containing ligands, where the actinide is bonded to a boron cluster or clusters in a molecular fashion. The molecule is then heated to decompose the molecule and form a two- or three-dimensional array.

This structure was produced using data from a single-crystal x-ray structure experiment. Uranium is shown in yellow, oxygen in red, and boron in blue. The hydrogen atoms bound to the cluster oxygen atoms (one each) and to the water molecules (two each) have been omitted for clarity. The Los Alamos team expects this uranyl borane compound to be a useful precursor to next-generation actinide boride storage forms.

This approach allows for lower reaction temperatures and better control of the final product. As an example, the researchers recently synthesized a new uranium borane complex, which should be an excellent candidate for preparing new ternary actinide materials.

The Los Alamos team presented their findings in April at the 221st American Chemical Society national meeting in San Diego.

Contributors to this article are: Anthony Lupinetti and Eduardo Garcia (NMT-2); and Julie Fife and Kent Abney (C-INC).


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