Jack L. Collins

Uranium kernel formation via internal gelation

Summary In the 1970s and 1980s, U.S. Department of Energy (DOE) conducted numerous studies on the fabrication of nuclear fuel particles using the internal gelation process. These amorphous kernels were prone to flaking or breaking when gases tried to escape from the kernels during calcination and sintering. These earlier kernels would not meet today´s proposed specifications for reactor fuel. In the interim, the internal gelation process has been used to create hydrous metal oxide microspheres for the treatment of nuclear waste. With the renewed interest in advanced nuclear fuel by the DOE, the lessons learned from the nuclear waste studies were recently applied to the fabrication of uranium kernels, which will become tri-isotropic (TRISO) fuel particles. These process improvements included equipment modifications, small changes to the feed formulations, and a new temperature profile for the calcination and sintering. The modifications to the laboratory-scale equipment and its operation as well as small changes to the feed composition increased the product yield from 60% to 80%–99%. The new kernels were substantially less glassy, and no evidence of flaking was found. Finally, key process parameters were identified, and their effects on the uranium microspheres and kernels are discussed.

Preparation of spherical, dense uranium fuel kernels with carbon

The internal gelation process and a suitable broth formulation with an uranium concentration of 1.3 M was used to produce air-dried uranium trioxide dihydrate (UO3·2H2O) and carbon microspheres with crush strengths greater than 600 g per microsphere. The addition of carbon lowered the slow-pour densities of the air-dried microspheres by a minimum of 9% if all other conditions were held constant. The crush strengths of the air-dried microspheres with and without carbon remained very good. These microspheres were not prone to leach when they were washed with ammonium hydroxide, and they did not have the tendency to crack during subsequent heat treatments. For the UO3·2H2O microspheres with and without carbon, dehydration occurred at the same rate. The dehydration was accompanied by spontaneous reduction of the urania to UO2.67. In the same temperature range, hydrogen and carbon can be used to further reduce the urania to uranium dioxide. Therefore, the loss of carbon during calcination appears to be unavoidable. The current recommendation on calcinations is to use a temperature of 600 °C or higher to minimize the loss of carbon. Dense and strong uranium fuel kernels with carbon were produced in argon at 1680 °C.

Monosodium Titanate in Hydrous Titanium Oxide Spheres for the Removal of Strontium and Key Actinides from Salt Solutions at the Savannah River Site

Abstract Fine powders of monosodium titanate effectively remove strontium and plutonium from alkaline salt supernatant. At the Savannah River Site, larger, porous particles with monosodium titanate were desired for continuous column operations. The internal gelation process was used to make hydrous titanium oxide microspheres with 32 and 50 wt% monosodium titanate. With actual supernatant, the microspheres with 50 wt% monosodium titanate produced average batch distribution coefficients of 35,000 mL/g for plutonium and 99,000 mL/g for strontium. These microspheres were tested using a simulant and a flow rate of 5.3 bed volumes per hour. The plutonium removal dropped from 99% to 94% while the strontium removal remained nearly 100%. The microspheres exhibited good flow performance and no particle degradation.

Determination of Ideal Broth Formulations Needed to Prepare Hydrous Aluminum Oxide Microspheres via the Internal Gelation Process

A simple test-tube methodology was used to determine optimum process parameters for preparing hydrous aluminum oxide microspheres by the internal gelation process. Broth formulations of aluminum, hexamethylenetetramine, and urea were found that can be used to prepare hydrous aluminum oxide gel spheres in the temperature range of 60-90 C. A few gel-forming runs were made in which microspheres were prepared with some of these formulations in order to equate the test-tube gelation times with actual gelation times. These preparations confirmed that the test-tube methodology is reliable for determining the ideal broths.

Effects of Sodium Hydroxide and a Chelating Agent on the Removal of Aluminum from Radioactive Sludge

Abstract: An essential step during the remediation of nuclear waste by the U.S. Department of Energy involves the separation of nonradioactive components such as aluminum from high-level waste sludges to minimize the ultimate volume to be stored in a nuclear waste repository. Plans for waste treatment at Hanford and the Savannah River Site include the use of 1 to 3m sodium hydroxide (NaOH) at an elevated temperature to leach the aluminum from the sludge. Triethanolamine (TEA) was added to caustic leaching solutions in an effort to improve the solubility of aluminum from authentic tank-waste sludge. High-level radioactive waste sludge with significant amounts of gibbsite and hard-to-dissolve boehmite phases was used in these tests. In concept, a chelating agent such as TEA can both improve the dissolution rate and increase the aluminum concentration in the liquid phase. However, TEA could also increase the solubility of other sludge components that are potentially problematic to downstream processing. Six tests were performed with leachate concentrations ranging from 0.1 to 3.0m NaOH, 0 to 3.0m TEA, and 0 to 2.9 m NaNO3. One test was performed using 3.0m NaOH at 80°C in order to simulate the baseline process, while the other tests were performed at 60°C. As expected, more aluminum entered the solution at 80 C than at 60°C when other test conditions were held constant. With caustic alone, equilibrium was achieved at both temperatures within 10 days. The addition of TEA significantly increased the concentration of aluminum in the leachate, and the aluminum concentration continued to increase even after 11 days of processing. The fraction of aluminum dissolved at 60°C increased from 35% using 3.0 in NaOH alone to 87% using a combination of 3.0 m NaOH and 3.0m TEA. The high-nitrate, low-hydroxide solutions did not significantly dissolve the aluminum because aluminate ion could not be produced. A small addition of TEA to these low-caustic solutions had no effect on aluminum removal. The use of TEA also increased the solubility of copper, nickel, and iron, which are only minor constituents. The TEA also had a significant effect on the solubility of the radionuclides 137Cs and 60Co. The significant presence of 137Cs in the leachates was expected with and without TEA. The high-nitrate leaches, which were the least effective of the leaching solutions, removed 69% of the 137Cs from the washed sludge, while a combination of 3.0m NaOH and 3.0m TEA removed 96%. Very little 60Co was removed from the sludge except with the use of the 3.0/h NaOH-3.0m TEA solution, which removed 53%. These results indicate that only TEA and 60Co need to be examined for potential chemical and radiological impacts, respectively, on downstream processes.