Fabrication Process and Product Quality Improvements in Advanced Gas Reactor UCO Kernels

Abstract

A major element of the Next Generation Nuclear Plant (NGNP)/Advanced Gas Reactor (AGR) Fuel Development and Qualification Program is developing fuel fabrication processes to produce high quality uranium-containing fuel kernels, TRISO-coated particles and fuel compacts needed for planned irradiation tests. The goals of the program also include developing the fabrication technology to mass produce this fuel at low cost. Kernels for the first AGR test, AGR-1, consisted of uranium oxycarbide (UCO) microspheres that were produced by an internal gelation process followed by high temperature steps to convert the UO 3 + C "green" microspheres to UO 2 + UC x . The high temperature steps also densified the kernels.Babcock and Wilcox (B&W) fabricated UCO kernels in their Lynchburg facility for the AGR-1 irradiation experiment, which went into the Advanced Test Reactor (ATR) at Idaho National Laboratory in December 2006. An evaluation of the kernel process prior and after these kernels were produced led to several recommendations to improve the fabrication process. These recommendations included testing alternative methods of dispersing carbon during broth preparation, evaluating the method of broth mixing, optimizing the broth chemistry, optimizing sintering conditions, and demonstrating fabrication of larger diameter UCO kernels needed for the second AGR irradiation test, AGR-2.Based on these recommendations and requirements, a test program was defined and performed. Certain portions of the test program were performed by Oak Ridge National Laboratory (ORNL), while tests at larger scale were performed by B&W. The tests at B&W have demonstrated improvements in both kernel properties and process operation. Changes in the form of carbon black used and the method of mixing the carbon prior to forming kernels led to improvements in the phase distribution in the sintered kernels, greater consistency in kernel properties, a reduction in forming run time, and simplifications to the forming process. Process parameter variation tests in both forming and sintering steps led to an increased understanding of the acceptable ranges for process parameters and additional reduction in required operating times. Another result of this test program was to double the kernel production rate. Following the development tests, approximately 40 kg of natural uranium UCO kernels have been produced for use in coater scale up tests, and approximately 10 kg of low enriched uranium UCO kernels for use in the AGR-2 experiment.

Selection and properties of alternative forming fluids for TRISO fuel kernel production

“…The size of the needle orifice, broth flowrate, and vibration rate control the droplet size [12]. The HMTA serves as an ammonia donor for gelation, which increases the pH of the precursor solution, decomplexing the ureauranium mixture and forming a uranium oxyhydroxide gel [4,7] (Step 2, Fig. 4).…”

“…4) starts with a solution of uranyl nitrate, urea, hexamethylenetetramine (HMTA), carbon black, and Tamol™ at 0°C [4]. The urea prevents premature gelation at lower temperatures, accelerates HMTA decomposition at elevated temperatures, and complexes with the uranium in solution [7].…”

“…Aging in warm ammonia hydroxide (Step 3, Fig. 4) then allows for complete gelation of the hardened spheres [4] and leaches nitrate salts from the gel.…”

“…The Tamol™ acts as a dispersant for the carbon black, which reacts with UO 2 to form uranium carbide during a subsequent carbothermic reduction (Step 5, Fig. 4) [4]. A vibrating needle delivers the chilled broth into a mixture of TCE and Span™ 80 (0.5 g/l) at 60°C (Step 1, Fig.…”

“…The current kernel production process in the US uses trichloroethylene (TCE) as the forming fluid in a sol-gel production column [4]. Over time, gelation byproducts in the forming fluid inhibit complete gelation, and the forming fluid is periodically replaced [5].…”