Mo-99 2014 TOPICAL MEETING ON MOLYBDENUM-99 TECHNOLOGICAL DEVELOPMENT June 24-27, 2014 Hamilton Crowne Plaza Washington D. C. Conversion of Uranyl Sulfate Solution to Uranyl Nitrate Solution for Processing in UREX Megan E. Bennett, Delbert L. Bowers, Candido Pereira, George F. Vandegrift Chemical Sciences and Engineering Division Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439 ABSTRACT In 2016, the Canadian research reactor at Chalk River will no long irradiate targets for 99Mo production, causing a worldwide shortage of 99mTc, unless reliable, alternative production methods are developed. SHINE Medical Technologies is developing a production method using a deuterium-tritium (DT) neutron generator and a non-critical aqueous solution of low enriched uranium (LEU) in the form of uranyl sulfate solution. In this process, the recycling of uranium solution is necessary for the next irradiation. It has been proposed that the UREX purification process be used to recycle the uranyl sulfate solution; however, UREX requires a uranyl nitrate solution. Uranyl sulfate solutions do not perform well in UREX, because sulfate forms a strong complex with the uranyl ion and uranyl cannot be preferentially extracted; therefore, the uranyl sulfate solution must be converted to nitrate. A precipitation procedure for the conversion of uranyl sulfate solution to uranyl nitrate solution is discussed. 1. Introduction The availability of metastable technetium-99 (99mTc) is critical to the medical community; it is used in more than 80% of nuclear medicine diagnostic tests, or approximately 70,000 medical imaging procedures throughout the world daily [1]. In 2009, It was reported that Canada’s Chalk River nuclear reactor and the Netherlands’ High Flux reactor produce approximately 85% of Europe’s and North America’s 99mTc supply [2]. It was recently announced that the Chalk River reactor will no longer irradiate targets for 99Mo production after 2016, creating a worldwide shortage of 99mTc unless reliable, alternative production methods are developed. 99mTc cannot be stockpiled because its parent isotope is molybdenum-99 (99Mo), which has a 66- hour half-life [3]. One possible production method is based on accelerator-driven fissioning of 235U in a non-critical (subcritical) solution. Currently, most of the 99Mo is produced in a nuclear reactor by the neutron-induced fission of high-enriched uranium (HEU; ≥20% 235U). The 99Mo is isolated from the uranium and other fission products, purified, and loaded onto a generator. The 99Mo then decays to 99mTc, which has a 6-hour half-life [4]. Production of 99Mo with HEU is a nuclear proliferation concern, because HEU can be weaponized. It is far more desirable to produce 99Mo from either non-uranium-based sources, such as Mo irradiation, or using depleted uranium (DU < 0.7% 235U), natural uranium (NU ≈ 0.7% 235U), or low enriched uranium (LEU ≤ 20% 235U) [4]. For systems designed to operate with HEU, the major downside of 99Mo production using LEU is that ~5 times more uranium is required to produce an equal amount of Mo as with HEU. Production using non-HEU methods, whether with a reactor or an accelerator, is greatly preferred because nuclear proliferation concerns are greatly reduced [5]. SHINE Medical Technologies is developing a production method that uses a deuterium-tritium (DT) neutron generator and a noncritical aqueous solution of LEU uranyl sulfate. After 5–7 days, the irradiated solution is given an 8–10 hour cooling period and then run through a Mo-recovery column. The effluent is then recycled for the next irradiation. Periodically after Mo recovery, the target solution will be processed to purify the uranium of unwanted fission and activation products. During irradiation, a deuterium (2H) beam is accelerated toward a chamber containing tritium (3H) gas, generating high-energy neutrons when the 2H and 3H interact. These neutrons then impinge on the LEU solution, inducing fission of the 235U, and thus producing 99Mo. The vessel containing the LEU solution is subcritical by geometry to obviate concerns of a criticality accident (uncontrolled nuclear reaction) [5]. Two different LEU salts have been considered for use in SHINE, uranyl nitrate and uranyl sulfate. Currently, uranyl sulfate is preferred, because pH control is required when irradiating nitrate solutions due to generation of nitrate radiolysis products [6, 7]. Because sulfate stable to radiolysis, the use of uranyl-sulfate solution significantly reduces unwanted pH; however, uranyl-sulfate solutions do present a technical challenge to the periodic cleanup of the irradiated target solution from fission and activation products. Several methods for the purification of uranyl sulfate from fission and activation products were evaluated. They included: (1) Direct solvent extraction of uranyl sulfate; (2) Precipitation of the uranyl ion as uranyl peroxide; (3) Anion exchange of uranyl sulfate complexes; (4) Conversion of uranyl sulfate to nitrate followed by the UREX (URanium EXtraction) process. The first option evaluated for the purification of uranium from fission and activation products was direct solvent extraction of uranyl sulfate. This is based on synergetic extraction of the uranyl disulfate anion using trioctyl ammonium sulfate (TOA) and trioctyl phosphate (TOPO), in a 2:1 ratio. A diluent of either an alkane or tetrachloroethylene is used. Extraction of this complex is easily achieved, but the stripping of uranium from the solvent becomes the technical challenge. In order for this system to be viable for the purification of uranium from fission and activation products, a complexing stripping agent is required. This complexant must have three important properties: (1) Forms strong complexes with uranyl ion; (2) Will not displace sulfate in the organic-phase trioctyl ammonium sulfate salt; (3) Can be destroyed in the aqueous uranyl-sulfate product solution without leaving residue in the solution. A literature survey and preliminary experimental work did not find a stripping agent that met the three criteria, and, therefore, direct extraction of uranyl sulfate was deemed not a viable system for the purification of uranium from fission and activation products. The second option evaluated for the purification of uranium from fission and activation products was precipitation of the uranyl ion as uranyl peroxide. The 2+ 2- -36 solubility product (Ksp = [UO2 ] x [O2 ]) for uranyl peroxide is 1.3x10 .[8] 2+ However, there are many factors that influence the concentrations of free UO2 ion 2- and free O2 ions, and therefore the completeness of the precipitation. For the uranyl 1+ ion, there are pH-dependent hydrolysis reactions that form the species UO2OH , 1- 2- UO2(OH)2, and UO2(OH)3 and sulfate complexes UO2SO4, UO2(SO4)2 , and 4- UO2(SO4)3 . The formation of these species decreases the amount of free uranyl ion and thus the precipitation of uranyl peroxide. In addition to the formation of these complexes, the free peroxide-ion concentration is also effected by the pH, due to - formation of the weak acids H2O2 and HO2 . The relative concentrations of uranyl- sulfate complexes are also related to pH through the equilibria of sulfate ion with - HSO4 and H2SO4. Further complications to the precipitation are that peroxide ion 2- 4- can also form complexes with uranyl ion UO2(O2)2 and UO2(O2)3 . Furthermore, low pH increases the uranyl-ion solubility by keeping the concentration of peroxide ion very low. As the pH is increased, the free uranyl ion concentration drops, due to an increase in the importance of hydrolysis reactions, complexation with sulfate, and eventually complexation with peroxide ion. Despite all of these potential complications, cursory experiments were performed to assess the viability of this system for the purification of uranium from fission and activation products. Based upon these it experiments it was concluded that this system is not viable, as it results in a colloidal suspension of uranyl peroxide that was nearly impossible to centrifuge or filter. The third option evaluated for the purification of uranium from fission and activation products was anion exchange of uranyl sulfate complexes. Due to the high concentration of uranyl sulfate (~130-150g/L), and the low capacity of anion exchange resins, compared to those of solvent extraction, this process is not viable[9]. The fourth option evaluated for the purification of uranium from fission and activation products was the conversion of uranyl sulfate to uranyl nitrate followed by the UREX process. The first step in this process is to convert uranyl sulfate to uranyl nitrate. In order to do this, soluble alkaline earth nitrate salt(s) are introduced into the uranyl sulfate solution. The introduction of these salts causes the precipitation of alkaline-earth sulfate salts. If enough alkaline-earth cations are introduced into the sulfate media, all of the sulfate anion will be precipitated out (as alkaline-earth sulfate salt) thus leaving behind uranyl ion in a nitrate media. Initial experimental studies suggested that this conversion process could be performed easily, readily, and reliably. Because of this, this system was selected for further development. Once the uranyl sulfate solution is converted to uranyl nitrate the UREX process can be performed. The UREX process is well-understood , so it was selected to purify uranium of fission products [10]. The UREX process selectively extracts uranium (with partial extraction of technetium and iodine)