P i Abstract of paper proposed for the American Nuclear Society 1997 Winter Meeting Albuquerque, New Mexico November 16-20, 1997 JUL 2 1 1997 OSTI Converting wmo Production from High- to Low-EnrichedUranium George F. Vandegrift, C. J. Comer, J. Sedlet, and D. G. Wygmans Chemical Technology Division RERTR Program Argonne National Laboratory Laboratory ( Argonne ) under Contract No. W-31-109ENG-38 with the U.S.Department of Energy. The US. Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and pefforrn publicly and display publicly, by or on behalf of the Government. H OF rn!$ 1-
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Converting 99Mo Production from High- to Low-Enriched Uranium George F. Vandegrift, C. J. Comer, J. Sedlet, and D. G. Wygmans Chemical Technology Division RERTR Program Argonne National Laboratory Molybdenum-99 (tl,2 = 66.02 h) decays by beta emission to 99mTc (fi,2 = 6.02 h). The latter nuclide is used in many nuclear medicine applications. Much of the world s supply of 99M0 is produced from fissioning of high-enriched uranium (HEU). To supply the United States market requires about 3000 6-day Ci of 99M~ per week. The Reduced Enrichment for Research and Test Reactors (RERTR) program has been active for 19 years in modifying reactor and fuel designs to allow the switch from HEU to low-enriched uranium (LEU) with little or no loss in flux or cycle time. Many reactors have converted to LEU, and many more are in the process. While conversions of reactor fuel have proceeded, the amount of HEU being exported from the United States for use in 99M0 production has become an ever more visible proliferation concern. To meet this concern, Argonne National Laboratory is developing alternative targets and processes for conversion to LEU. Conversion to LEU necessitates changes to the target being irradiated and to the process that recovers and urifies the 99Mo. To produce the same amount of 99M0 as an HEU target (generally, 93% 23& enrichment), an LEU target (~20% 235U enrichment) will require about five times more uranium. To minimize modification to target geometry, the chemical and/or physical form of the uranium must be changed. Changes in the form and quantity of the irradiated uranium force changes to target dissolution and to subsequent 99Mo-recovery and -purification steps. The greater quantity of 238U in the LEU target, which generates 20-50 time more 239Np/Pu during irradiation than HEU, also challenges target processing. This paper discusses our efforts towards LEU substitution in two HEU targets. One type is the Cintichem target, a closed cylinder with a thin coating of U0,electroplated on the inside wall. Loading the target with more than 30 g of UO, is technically difficult. Therefore, to successfully increase the amount of uranium per target, we are developing a target that uses LEU metal foil. Processing of both the HEU Cintichem target and the LEU metal foil target begins by dissolving the irradiated uranium in acid. Electroplating barrier-metal films onto the uranium foil in the LEU target is also being investigated. These barriers absorb fission-product energy and prevent bonding of the uranium foil to the target walls. The barrier metals being studied are Ni, Zn, and Cu. To act as fission-product barriers, the films need to be >8 pm thick: we are preparing barriers 10-15 pm thick. The uranium foil is 130 pm thick. Two primary challenges in electroplating barrier layers to uranium foil are (1) preparing the uranium surface for electroplating and (2) forming a mechanically strong bond. Uranium surface preparation is by far the greatest challenge. Abundant literature exists describing methods for preparing uranium for plating by other metals. Elaborate schemes using six different treatment solutions and 20-30 steps are common. In general, they call for (1) removing organic residues, (2) removing the oxide layer, (3) etching, and (4) activating the uranium surface, in that order. Surface preparation for a thin uranium foil requires etching the foil to meet the needs of electrodeposition without significantly reducing the thickness of the foil. The best conditions to date have made excellent plates while removing about 20% of the uranium, The other HEU target is a rolled plate containing UAl, dispersed in an aluminum-powder matrix. Processing this target type begins with dissolving the entire target in base. We are -2-
pursuing two different LEU targets for this substitution. One is a dispersion target much like the current target, but using a uranium compound with a greater density than UAl,. Initial work was with U,Si,, but we are currently studying UO,. A uranium metal foil is the second LEU substitute, with a zinc fission-recoil barrier required for base dissolution. In substituting LEU for HEU targets, the first two processing steps target dissolution and the initial molybdenum-recovery procedure) are the keys to maintaining 6 Mo yield and purity. Following these steps, processing should be identical. The dissolver solution must be changed to dissolve the different chemical form of the LEU target. However, to keep process modification to a minimum, the feed to the first processing step should be chemically identical to that from the HEU target. This requirement has been the primary chemical challenge. For the acid-side Cintichem processing, LEU metal foil substitution requires using a more concentrated acid solution and equipment modification. Tracer experiments have shown that effects of LEU substitution should be minimal. A full-scale demonstration is planned for August 1997. Dissolution on the base side requires the addition of hydrogen peroxide to the dissolver solution. Following dissolution, the peroxide must be destroyed to precipitate uranium from the fission-product solution. At this point, except for possible higher concentrations of 239Np and 239pU in solution, LEU and HEU processing should be identical. In conclusion, substituting LEU for HEU appears to be technically feasible. The RERTR program is cooperating with 99M0 producers worldwide to demonstrate LEU processing under realistic conditions and with minimal economic penalty for conversion.