IMPROVED URANIU!4RECOVERY FROM THE PROCESS STREAMS IN AN ELECTROPLATING FACILITY by

Cylindrical uraniu~ slugs are used as irradiation targets in the production reactors at the Savannah River Plant. These slugs are first chemically etched, nickel plated, encased in aluminum, inspected, and individually pressure tested. An improved process was”developed to recover the uranium from the acidic etching streams by controlling pH and the P(lqto U ratio so that the precipitation of the uranium as hydrogen uranyl phosphate was maximized. Bench scale tests demonstrated that the recovery of uranium could be increased to greater than 99.9% (vs. the current level of about 95% recovery). The recommended changes involved the addition of process effluent “hold” tanks. The addition of the various process streams to the neutralization/precipitate on tank could therefore be controlled to maintain a consistent ratio of uranyl nitrite and phosphoric acid. Also, it was determined that a strong caustic solution (resulting from the dissolution of rejected aluminum slugs) could be utilized to neutralize the nitric and phosphoric acid solutions. The buffering action of the al”uminum in the “caustic recovery solution” would reduce the sensitivity of the hydrogen uranyl phosphate precipitation to the phosphate ion concentration. INTRODUCTION The fuel and target elements used in the SRP production reactors at the Savannah River Plant (SRP) are prepared in the fuel fabrication facilities (300-M Area). Depleted uranium is used for the target elements, and after irradiation, the product plutonium is separated in the Separations facilities (ZOO-F Area). The target elements consist of hollow cylinders, approximately 8-1/4 inches long and 3 inches in diameter, with about l/2-inch-thick walls. The uranium cores are received from NLO, Inc., Fernald, Ohio, and processed through an automated plating line. The cores are first etched in a strong (6 N) nitric acid solution to remove the oxidized surface. The cores are then anodically etched in a strong phosphoric acid {17 N)/hydrochloric acid (0.5 N) solution to prepare the surface for the subsequent nickel plating operations. The phosphoric/hydrochloric acid solution is removed with a nitric acid rinse prior to the nickel plating. The nickel plated cores are encapsulated in aluminum, inspected, and pressure tested to ensure the integrity of the aluminum cladding before emplacement in the reactor. u ● * The information contained in this article was developed during the course of work under Contract No. DE-AC09-76SROOO01 with the U.S. Department of Energy. The plating line solution process is a batch-type operation, with the spent solutions being transferred to a solution recovery operation. Each of the acidic solutions is dumped when its dissolved uranium concentration becomes too high for effective use. For example, the initial nitric acid pre-etch tank is dumped when the uranyl nitrate reaches a concentration of approximately 300 g/L. The spent acidic solutions are combined and neutralized with sodium hydroxide. The resulting precipitate is filtered, with the filter cake going to low-level waste burial, and the filtrate being released to a surface impoundment. This system normally recovers about 95% of the uranium in the spent solutions. In 1983, a program was initiated by the Savannah River Plant to try to improve the percent recovery ”ofthe uranium. Use of slightly enriched uranium feed material (-1% U235) for the target elements (vs. depleted uranium, --0.20%U235) was being considered. However, since the slightly enriched material is more radioactive than the depleted material, and since the geometry of the enriched cores had more surface area, the amount of radioactivity released to the settling basin would have exceeded the Du PontSRP operating guidelines. The value of the recovered enriched uranium was also significant, at approximately 190/kg. This work defined the precipitation chemistry pertinent to the process conditions, and developed process modifications which, if implemented, were predicted to improve the uranium recovery to >99.9%. EXPERIMENTALMETHODS Samples of actual process solutions were used for all experiments. The acidic solutions were neutralized to the desired pH and mixed until a uniform dispersion was obtained. A sample of the slurry was vacuum filtered through No. 42 \fhitmanfilter paper. The filtrates were analyzed by a direct current argon plasma spectrometer. RESULTS AND DISCUSSION The experimental results indicated that the maximum precipitation of uranium occurred at a pH of 6.0 tl.O (Figure 1). An extensive x-ray spectrophotometric investigation of the precipitate showed that hydrogen autenite [1 (hydrogen uranyl phosphate H (lJ02) (pOq)208H O) was the primary precipitate] in that pH range. HU02POk04H28, NaU62POq and ~U02)3(POq)204H20 were also faund. The chemistry of the precipitation process 1s quite complex, with more than 30 chemical equations contributing to the overall reaction. Two of the primary reactions [2,3] are the PH sensitive precipitation of HU02p04, (U02)(N03)2 + H3POq ~ HU02POq+ + 2HN03 (1) and a completing reaction in which excess phosphate redissolves the HU02P04 precipitate: HU02POq + HPOq=~U02(HP0q)2= (2) -0