Volume 88, Issue 6 -

Yields of 7 Be, 10 Be, 22 Na, 24 Na and 28 Mg produced from nat B, nat C, nat O, 23 Na, 27 Al, nat Si, nat S, nat Cl, nat K, nat Ca, nat V, 55 Mn, nat Fe, 59 Co, nat Ni, nat Cu, 89 Y, nat Ag, nat In, 141 Pr, 159 Tb, 165 Ho and 197 Au in irradiations with bremsstrahlung having maximum end-point energies up to 1200 MeV were measured by γ-ray spectrometry ( 7 Be, 22 Na, 24 Na and 28 Mg) and accelerator mass spectrometry ( 10 Be). The yields of heavier fragments, 39 Cl, 43, 44m, 44g, 46, 47, 48 Sc, 59 Fe, and 56, 57, 58, 60 Co, were also measured by γ-ray spectrometry in order to survey the relative contribution of fragmentation to spallation and/or fission in the mass yield curves. A distinct difference of the target mass-dependence of these yields has made it possible to disentangle the contributions of photospallation and photofragmentation processes. It was found that the neutron-to-proton ratios of targets strongly affect the formation of the nuclei by fragmentation as revealed in the yields of the isotopic pairs as well as of the single isotopes. The photon results were compared with those of proton-induced reactions, and no clear effect due to the difference of the initial interactions of photons and protons with nuclei was found in the fragment yields.

The kinetics of the reduction of NpO 2 2+ to NpO 2 + in nitric acid aqueous solution by formohydroxamic acid (FHA) were studied to allow more accurate flow sheet modelling of neptunium separation from uranium and plutonium in an Advanced PUREX process. The rate of the reaction was monitored using stopped-flow spectrophotometry with near-infrared detection. The conditional reduction rate in 2 M nitric acid at 22°C could be described by the following equation: -d [NpO 2 2+ ]/dt = k [NpO 2 2+ ][FHA](M/s) with k = 1.17×10 3 M -1 s -1 at [H + ] = 2.0 M. Comparison with other data available in the literature, indicates that formohydroxamic acid is an unusually strong reducing agent for NpO 2 2+ .

The binding properties of [1 n ]metacyclophane-based cation exchangers (n = 5, 6, 7) towards trivalent lanthanides and americium were investigated by means of solvent extraction from water to chloroform and xylene. Extraction constants for four ligands were calculated for Ln(III) and Am(III). Three of the ligands lack Bu t groups in 4-position, giving them higher molecular flexibility compared with the corresponding t-butylcalixarenes. Due to this reason, changing the cavity size lead to small changes in metal extractabilities compared with series of Bu t analogue, while the metal extractabilities themselves are very high. The order of extractability, being Am 3+ > Nd 3+ > Eu 3+ > Tb 3+ > Dy 3+ > Er 3+ > Yb 3+ is not influenced by the molecular structure. In the series of t-butylcalix[n]arenes (n = 4-7) the extraction of Am(III) improves with n, the improvement is significantly higher for CHCl 3 than for xylene as diluent. The crystal structure of one of these ligands (n = 7) shows a distorted cone conformation. Dimerization in the crystal occurs due to the formation of 8 intermolecular and 4 intramolecular hydrogen bonds, two dimers form a cavity which contains 10 molecules of CDCl 3 .

The binding of uranium(VI) by Suwannee River humic and fulvic acids was studied at pH values of 4.0 and 5.0 in 0.10 M NaClO 4 using an ion-exchange technique. Few data sets currently exist for metal binding to different molecular weight fractions from the same source. The complexation of U(VI) by citric acid was also studied under the same experimental conditions in order to "calibrate" the experimental and modeling approaches. For the citric acid system, the experimental results were analyzed using Schubert´s ion-exchange method, which indicated the formation of only a 1:1 uranyl-citrate complex. Close agreement was found for the values of log β 1,1 (6.69±0.03 at I = 0.10) determined from nonlinear regression of data collected at pH values of 4.0 and 5.0. This value represents a more direct measurement of the binding constant for the 1:1 uranyl-citrate complex than do other existing literature values derived from experimental data requiring the simultaneous consideration of 1:1 and 2:2 species. Both humic and fulvic acids were demonstrated to strongly bind U(VI), with humic acid forming slightly stronger complexes and exhibiting greater pH dependence. Analyses of the data for the humic and fulvic acid systems using the Schubert´s equation previously applied to the citrate system result in an apparent nonintegral number of ligands binding the uranyl ion. Schubert´s method is only appropriate for interpreting mononuclear complexes with integral moles of binding ligands. Thus, a more elaborate binding model was required and the data were interpreted assuming either: (1) a mixture of 1:1 and 1:2 uranyl-ligand complexes or (2) a limited number of high affinity sites forming a 1:1 complex. While both of these modeling approaches are shown to provide excellent fits to the data, the second is deemed more appropriate given the large size of humic and fulvic acid molecules as well as previous results obtained with other metal cations, such as Cu(II).

The synergistic extraction of uranium(VI) from nitric acid aqueous solution with a mixture of n-octyldecylsulfoxide (ODSO) and petroleum sulfoxides (PSO) in carbon tetrachloride was studied. It has been found that the maximum synergistic extraction effect occurs where the molar ratio of PSO to ODSO is three to five. The composition of the complex of synergistic extraction is UO 2 (NO 3 ) 2 · ODSO · PSO. The formation constant of the complex K OP = 57.6±5.19. The effect of extractant concentration, nitric acid concentration, salting-out agent concentration, complex anion concentration and temperature on the extraction equilibrium of uranium(VI) was also studied.

Six organic and five inorganic ion exchangers were studied for the removal of natural radionuclides from drinking water. In the first phase, distribution coefficients (K D ) of uranium and radium were determined in three model solutions: a good quality water, an Fe-Mn-rich water and a saline water simulant. The exchangers showing highest K D ´s for U and Ra were tested in column mode experiments for the removal of U, 226 Ra, 210 Po and 210 Pb from a real ground water. Considering the total 1800 processed bed volumes, the best decontamination factors (DF) for U, 20-120, were obtained with the strong base anion exchange resin. The most efficient exchangers for the removal of 226 Ra were the weak and strong cation resins and zeolite A. With use of long regeneration intervals, conventional organic resins are thus most suitable for the removal of radionuclides. When only a few hundred bed volumes were processed, with the corresponding regeneration interval about one week, the aminophosphonate resin was most effective for the removal of both uranium and radium. 210 Po and 210 Pb in ground water were mainly bound in particles and adsorbed on the surface of the anion resins. The use of the aminophosphonate resin to remove uranium and radium would therefore require an anion resin to take up polonium and lead.

Boehmite and Al-oxyhydroxide gels sorb ReO 4 - , a non-radioactive analogue of TcO 4 - from NaNO 3 solutions. Sorption appears to be substantially electrostatic (though there appears to be a specific preference for ReO 4 - over NO 3 - ) and is most effective at pH < 8. Measured K d ´s lie between 5 and 105 ml g -1 , depending on the solid, pH, and ionic strength. ReO 4 - and TcO 4 - are both partially removed from high pH Hanford-type acid waste simulants upon neutralization and formation of Al-rich sludges. We therefore propose that sequestration of Tc by boehmite limits dissolved Tc levels in the near and sub-surface environment and for that purpose boehmite might be relied on as a backfill, or reactive barrier, to limit environmental transport of Tc.