The important isotope 239Pu was discovered in 1941 as the decay product of 239Np produced with neutrons from a cyclotron. The importance of plutonium comes from its fission properties and the capability of being produced in large quantities. In 1941, Segré, Kennedy, Wahl, and Seaborg bombarded a 1.2 kg sample of uranyl nitrate with 16 MeV neutrons for two days. The uranyl species was extracted into hydrocarbon solvent and the 239Np product was separated into an aqueous phase using an oxidation-reduction and precipitation process with La and Ce fluoride carriers. Measurement of the radioactive decay demonstrated that they had produced 0.5 μg of 239Pu. On March 28, 1941, they used that 0.5 μg sample to demonstrate that 239Pu undergoes slow neutron-induced fission with a fission cross-section for 239Pu that was approximately 50% greater than for 235U, agreeing remarkably well with more accurate values determined later. This observation that 239Pu was fissionable with slow neutrons provided the information that formed the basis for the U.S. wartime Plutonium Project of the Manhattan Engineer District (MED) centered at the Metallurgical (“Met”) Laboratory of the University of Chicago. Most of these early studies were carried out under a self-imposed cover of secrecy due to the potential military applications of plutonium and were not published until after World War II.
Only tracer quantities of plutonium existed at the beginning of the Manhattan Project, therefore the initial chemistry challenges were to: develop a large-scale production method for plutonium; develop a method for its chemical separation and purification; scale up the separations from micro- to kilograms. Fermi solved the first problem by demonstrating that uranium would undergo a nuclear chain reaction on December 2, 1942: the neutrons produced in the reaction create plutonium. The solution to the second and third problems required determining the chemical properties of plutonium so that a large-scale separations plant could be designed to separate the enormous quantity of fission products and uranium. Berkeley professor Glenn Seaborg led a large group of chemists and engineers to solve this problem.
The key to plutonium separation was the oxidation-reduction cycle, in which plutonium is “carried” in its lower oxidation state(s) by chemical precipitates and not carried when it is present in higher oxidation states. Plutonium therefore becomes separated from the fission products, which do not exhibit these differences in carrying behavior. These carrier techniques had been developed for use with trace quantities of newly discovered atoms. It was unclear at the time if these techniques could be scaled up and actually used in a chemical separations plant. An entirely new effort in ultramicrochemistry was developed and led by Burris Cunningham to determine the chemical properties of plutonium because they only had sub-microgram quantities at the time. Hundreds of pounds of uranium were bombarded with neutrons at the Washington University cyclotron, and chemically separated down to 2.77 μg as the first weighable sample of plutonium on September 10, 1942.
The bismuth phosphate process
The Seaborg team had to find a way of separating plutonium in high yield and purity from the many tons of uranium in which plutonium was present at a maximum concentration of only 250–300 ppm. Because of these low concentrations, compounds of plutonium could not be precipitated directly, and any precipitation-separation process had to be based on coprecipitation with “carriers” for plutonium. Bismuth(III) phosphate was chosen as the carrier. In addition, the highly radioactive fission products had to be separated to less than one part in 107 of the original plutonium. This rigid requirement was necessary so that separated plutonium was safe to handle. Without separation from the fission products, the plutonium from each ton of uranium would have more than 105 Ci of energetic gamma radiation.
The key to the bismuth phosphate process is that it quantitatively carries Pu(IV) from acid solution but does not carry Pu(VI). Unfortunately, the process suffers from the batch nature of operations, the large amounts of chemicals used, and large amounts of waste. The Hanford site began construction of tank farms in the 1940s and 1950s to hold these large quantities of waste.
After the Manhattan Project: PUREX, the game changer
During the Cold War, the PUREX (Plutonium Uranium Redox EXtraction) solvent extraction process revolutionized plutonium separations. In solvent extraction, the species to be separated is transferred between two immiscible or partially-miscible phases, such as water and a nonpolar organic phase. The process works by selectively complexing the actinide species of interest, decreasing its solubility in water while simultaneously increasing its solubility in the organic phase. By far the most important and widely used neutral extractant is tributylphosphate (TBP). It complexes with the actinide elements Th, U, Np, and Pu by forming inner sphere chemical bonds to the actinide metal atom via the phosphoryl P=O bond. The important reactions for UO22+ and Pu4+ are shown below:
The reactions are equilibrium reactions, therefore the ratio of products, and thus the degree of extraction, can be increased by increasing the concentration of TBP or NO3 – in the organic and aqueous phases, respectively. These extraction equilibria are the basis of the PUREX process, used almost exclusively worldwide in all modern reprocessing of spent nuclear fuel. In the PUREX process, irradiated UO2 fuel is dissolved in HNO3, with uranium being oxidized to UO2(NO3)2 and plutonium to Pu(NO3)4. A solution of TBP in a high-boiling-point organic solvent such as n-dodecane is used to selectively extract hexavalent UO2(NO3)2 and tetra-valent Pu(NO3)4 from the other actinide and fission product nitrates in the aqueous phase. In the second extraction container, a TBP solution is contacted with a dilute HNO3 solution containing a reducing agent such as ferrous sulfamate, which reduces plutonium to Pu(III), but leaves the uranium as U(VI). Plutonium then transfers back to the aqueous phase leaving uranium in the organic phase. The uranium is stripped from the organic phase using water.
The Hanford PUREX plant was authorized in 1953, and hot operations began in January of 1956. The initial processing rate was 200 MT/U/month. PUREX capacity soared and by 1961, PUREX was processing 800 MT/U/month. Although the PUREX waste-to-product ratio was much lower than other processing plants, the need for waste disposal soared. Hanford responded with many different campaigns to build new waste tank farms to store the highly radioactive waste.
The tank waste legacy
Managing and treating the tank wastes stored in the farms of aging under-ground tanks at the Savannah River Site (SRS) and Hanford has been a grand challenge for the DOE Office of Environmental Management (EM) mission, posing a significant threat to environment, safety, and health. The tank farms at SRS and Hanford contained the majority of the Department of Energy (DOE) tank waste inventory with approximately 575 million curies of radioactive materials in 91 million gallons of sludge, liquid, and solid waste stored in 226 underground tanks. The majority of activity is stored in SRS tanks (400 million Ci), while the largest volume (53 million gallons) are stored in Hanford tanks (see figure on page opposite).
The costs for managing the tank farms are enormous with about $1 million per day for tanks at SRS and life-cycle costs in the billions of dollars. Estimates for life-cycle costs reach nearly $250 billion with completion of the cleanup of SRS and Hanford tank farms at the latest by 2062. Although EM has made significant progress in its cleanup mission, the majority of the tank wastes remain untreated. Only seven tanks have been emptied and two closed at SRS; no tanks have been closed at Hanford. Given the enormous task to retrieve, treat, and dispose of the large volumes of highly complex and highly radioactive tank wastes, opportunities exist to invest in the development of advanced technologies and scientific understanding of tank waste issues that can accelerate the cleanup mission and reduce life-cycle costs.
Savannah River Site
The Savannah River Plant was built and operated as a second production site for plutonium and other nuclear materials producing well over 100 million gallons of radioactive waste stored in underground tanks. The main process used for treating spent nuclear fuel and separating plutonium was PUREX, described on the previous page. The wastes were made alkaline for storage in carbon steel tanks, producing an insoluble sludge consisting of actinide and fission products and a supernatant liquid containing the majority of the 137Cs. To date, SRS underground tanks received about 140 million gallons of radioactive waste, which was reduced to approximately 36 million gallons by evaporation. The radioactive waste is currently stored in 49 under-ground tanks containing approximately 350 million curies of radioactive material. The SRS tanks reportedly contain about 16.9 million gallons of supernate, three million gallons of sludge, and 16.6 million gallons of salt cake. Of the underground tanks, 27 have full secondary containment in compliance with the site’s Federal Facility Agreement (FFA). The remaining 22 tanks have only one or partial second containment and, therefore, are considered non-compliant tanks.
Some of the SRS waste has been treated by incorporating the radioactive components into borosilicate glass at the Defense Waste Processing Facility (DWPF) and decontaminated supernate into a cement-based waste form referred to as saltstone. In 2008, the DOE entered into a contract with Savannah River Remediation LLC to accelerate closure of the tanks, and requires that all waste must be removed from all tanks by 2028. As of 2016, the DWPF had produced 4,000 glass canisters. Final closure and grouting of the final H-area East Hill tank is scheduled for fiscal year 2032.
The Hanford Reservation was the first industrial-scale plutonium production site in the world including multiple reactors and reprocessing facilities. Plutonium and spent fuel were processed in five reprocessing plants, creating large volumes of liquid and solid radioactive wastes. Past waste disposal management involved disposal into the environment and storage in large underground tanks. The Hanford tanks contain 53 million gallons of highly radioactive and chemical waste, only about 10% of the originally generated waste volume. The high-level waste (HLW) is stored in 177 single- and double-shell tanks containing approximately 175 million curies of radioactive constituents. Nearly 70 single-shell tanks have or are suspected to have leaked up to 1.5 million gallons of waste into the surrounding soil, while none of the 28 newer, double-shell tanks have lost their integrity.
Most of the waste removal and tank closures have yet to be performed, awaiting the operation of the large Hanford Tank Waste Treatment and Immobilization Plant (WTP). The plant will use vitrification technology, which involves blending the waste with glass-forming materials and heating it to 1,150°C. The molten glass mixture is then poured into stainless steel canisters to cool and solidify. In this glass form, the waste is stable and impervious to the environment, and its radioactivity will safely dissipate over hundreds to thousands of years. The plant is scheduled to begin operations in 2023, but has been plagued by setbacks.
The creation of atomic weapons and the buildup of the U.S. Cold War nuclear arsenal has left an environmental cleanup legacy of enormous cost and scope—the largest environmental cleanup program in the world. Through science, technology, and engineering, the U.S. has developed innovative solutions and reduced the legacy footprint by 90% to less than 300 square miles at 16 sites in 11 states—no other country has done this. Legacy cleanup is necessary to transform the U.S. nuclear weapons complex and provide stewardship of a smaller U.S. stockpile. Future challenges at Hanford and SRS will give the U.S. experience in HLW treatment, essential for managing the legacy of future wastes and spent nuclear fuel (a separate challenge). Finally, integrating worker safety and environmental protection into processes and facilities is an essential element of maintaining a modern stockpile.
- G.T. Seaborg, “The Chemical and Radioactive Properties of the Heavy Elements,” Chem. Eng. News 1945, 23, 2190–2193.
- G.T. Seaborg, “Origin of the Actinide Concept,” Chapter 118 in Handbook on the Physics and Chemistry of Rare Earths, Vol. 18—Lanthanides/Actinides: Chemistry, K.A. Gschneidner, Jr., L. Eyring, G.R. Choppin and G.H. Lander, Elsevier Science B.V., Amsterdam, The Netherlands, 1994.
- G.T. Seaborg, “Transuranium Elements. A Half Century,” Chapter 2 in “Transuranium Elements: A Half Century,” L.R. Morss and J. Fuger, eds., American Chemical Society, Washington, D.C., 1992, 10–49.
- GAO-19-223, Report to Congress, “Nuclear Waste Cleanup,” Feb 2019.
- GAO-19-460T, Report to Congress, “Environmental Liability Continues to Grow, and Significant Management Challenges Remain for Cleanup Efforts,” May 2019.
- GAO-18-241, Report to Congress, “Hanford Waste Treatment Plant,” April, 2018.