Purification of Alkaline Waste Solutions from Np and Pu
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Coprecipitation experiments for neptunium 37 2 Np traced with 23'"Npand 23'Pu traced with 238pu. Unless otherwise noted, all reagents were from commercial sources and used as received without additional purification. Methods to prepare reagents not available from commercial sources have been described in previous related studies [1]. Similarly, experimental materials and methods to prepare TRU isotope solutions used in this study also were described earlier [3]. The 23'Np tracer was added to the lxi 0 M 23 Np experimental solution to provide an 3 initial [I-activity of 2xl0 counts/(min-mL). This concentration is sufficient to obtain reliable data above background counting rates even if the decontamination factor (DF) exceeds 10'. 23 The initial Pu(IV) or Pu(VI) concentration was 1.38x107 M (including 4x10-1 M 1Pu) in all experiments. This concentration is lower than the solubility of Pu(IV) hydrous oxide over a wide range of NaOH concentrations [5,6]. The coprecipitation precursor reagents and precipitation products used in the complete study are listed in Table I. The formulas of metal hydroxides given in this table and subsequently in this section are only symbolic and may not describe the phases actually present in the precipitates. These precipitation reagents were selected for their high efficiency for TRU removal as described in previous studies [1,3]. The carrier precipitation precursors were dissolved in water at appropriate concentrations prior to addition to alkaline solutions. 1299 Mat. Res. Soc. Symp. Proc. Vol. 556 ©1999 Materials Research Society
Table I. TRU Removal Reagents Tested for Dilute Alkaline Solution Precursor Carrier formation conditions Carrier Cr(CH.COO) 3 Thermal hydrolysis Cr(OH) xH2O Co(NH,),CI. Thermal hydrolysis Co(OH). xHO Na.[Fe(CN).NO].2HO Thermal hydrolysis Fe(OH)3 -xH2 O KFeO4 Reduction to Fe(III), hydrolysis Fe(OH)3 xH2 O KMnO 4 Reduction to Mn(IV), hydrolysis MnO,'xHO KMnO4 Reduction to Mn(II), hydrolysis Mn(OH) -xH20 The TRU removal test procedure by the MAR used in the present tests was the same as described in previous reports [1,3]. Experimental solutions were held in polyethylene tests tubes. Calculated amounts of H 20 were added to each test tube. This was followed by 0.1 mL of Pu(IV), Pu(VI), or Np(V) stock solutions. The solution was stirred and a calculated
amount of NaOH or NH 4OH solution added to achieve the desired concentration. In each case the amount of NaOH added included the quantity required to form the metal hydroxide. Precursor solutions and other reagents, if necessary, were added last. After the precursor addition, the test tubes were kept thermostatted for 2 to 3 hours. The carrier formation conditions were those determined to be optimum in previous work [1,3]. After coagulation, the product precipitate was separated by five minutes of centrifugation and an aliquot of supemate solution taken for analysis. This centrifugation, sampling, and analysis was repeated twice to assure completeness of separation. The 3 activities of 239Np solutions and (x ac
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