Band 95, Heft 10

Cross sections for ( n ,2 n ) and ( n , p ) reactions have been measured on tellurium isotopes at the neutron energies of 13.5−14.6 MeV using the activation technique. Data are reported for the following reactions: 120 Te( n ,2 n ) 119m Te, 120 Te( n ,2 n ) 119g Te, 122 Te( n ,2 n ) 121m Te, 122 Te( n ,2 n ) 121g Te, 124 Te( n ,2 n ) 123m Te, 128 Te( n ,2 n ) 127g Te, 130 Te( n ,2 n ) 129m Te, 120 Te( n , p ) 120m Sb, 122 Te( n , p ) 122m+g Sb, 124 Te( n , p ) 124m+g Sb, 126 Te( n , p ) 126m+g Sb and 128 Te( n , p ) 128m+g Sb. These new data for the cross sections of tellurium isotopes have smaller uncertainties than those reported in the previous literature. The first data for the 120 Te( n , p ) 120m Sb reaction cross sections are presented. In our experiment, natural high-purity tellurium powder was used as target material and the samples were wrapped in cadmium foil during the irradiation so the influence of the ( n , γ ) reactions of thermal neutrons was reduced to a low level and the interfering reactions were avoided or subtracted. The HPGe detector employed in this work has better resolution than NaI(Tl). Furthermore, while gamma-ray yields were measured and then turned into cross sections using the formula in Sect. 2.2, the most recent and accurate nuclear data available were adopted. These cumulative improvements should make our results more accurate and reliable than those reported in the previous literature. Our results are useful for verifying the accuracy of nuclear models used in the calculation of cross sections and are expected to help new evaluations of the 14 MeV neutron cross sections on isotopes of tellurium.

The oxidation of U(IV) ions in the diluted solvent phase, 30% TBP/ n -dodecane, has been investigated in the presence of plutonium ions, which can act as catalysts for U(IV) oxidation. The reaction was shown to follow the cycle below, with the first and third stages being rate determining. U 4+ + 2Pu 4+ + 2H 2 O → UO 2 2+ + 2Pu 3+ +4H + 2Pu 3+ + HNO 3 + 2H + → 2Pu 4+ + HNO 2 + H 2 O Pu 3+ + HNO 2 + H + → Pu 4+ + NO + H 2 O 2NO + HNO 3 + H 2 O ⇔ 3HNO 2 The overall reaction stoichiometry is the same as for the oxidation of U(IV) by HNO 3 in TBP: U(NO 3 ) 4 ·2TBP + HNO 3 ·TBP + H 2 O·TBP + TBP ⇔ UO 2 (NO 3 ) 2 ·2TBP + HNO 2 ·TBP + 2HNO 3 ·TBP The rate equations of both these rate limiting steps have been determined, with that for the U(IV)-Pu(IV) reaction (5) being given by the equation below, where k 1 =74.4±6 M -1.2 min -1 at 25.2 °C and the activation energy is 72±11 kJ mol -1 (in 0.5 M HNO 3 ). The rate of the second slow stage, the Pu(III)-HNO 2 reaction, is given by the equation below, where the rate constant is k 2 =627±28 M -1 min -1 at 25.2 °C and the activation energy is 87.2±1.4 kJmol -1 (in 0.5 M HNO 3 ). Mechanistically, it was shown that the U(IV)-Pu(IV) reaction may proceed via the interaction of the hydrolysed actinide ions U(OH) 2 2+ and PuOH 3+ and the Pu(III)-HNO 2 reaction was found to most probably involve oxidation of Pu(III) ions by nitrinium nitrate (NONO 3 ) ions in its rate determining step.

The sorption of tetravalent plutonium onto kaolinite, a clay mineral, has been studied as a function of pH. The sorption studies have been performed by batch experiments under aerobic and anaerobic conditions (glove box). A pH range of 0–11 has been investigated with plutonium concentrations of 3.5 × 10 -7 −6.9 × 10 -9 M and a solid phase concentration of 4 g/L. A sorption edge at about pH=1 and maximum sorption around pH=8.5 has been found under aerobic and anaerobic conditions. In the presence of CO 2 at pH > 8.5, the sorption of plutonium is decreased due to the formation of soluble carbonate complexes. This is supported by speciation calculations for Pu(IV)-hydroxo-carbonate species in aqueous solution. Depending on the pH, 1%−10% of the sorbed plutonium is desorbed from the kaolinite and released into the fresh solution. For comparison with the behavior of Pu(IV), the sorption of the redox-stable Th(IV) onto kaolinite has also been investigated. Furthermore, the sorption of humic substances (HS) onto kaolinite has been studied as a function of pH and for varying concentrations of HS as a prerequisite to understand the more complex ternary system: plutonium, humic substances, and clay. It has been found that the sorption of Aldrich humic acid onto kaolinite is generally higher than that for Gorleben fulvic acid.

The extraction of plutonium from tissue paper matrix was carried out using supercritical carbon dioxide (Sc-CO 2 ). The removal of plutonium was also studied from other matrices such as teflon, glass and stainless steel. Initial experiments were carried out under subcritical conditions. Supercritical extraction conditions were also employed in some experiments for the recovery of plutonium from tissue paper matrix. Complete extraction of Pu(III) as well as Pu(IV) in their nitrate form from tissue paper was achieved for the first time using supercritical carbon dioxide modified with n-octyl(phenyl)-N,N-diisobutyl carbamoylmethyl phosphine oxide (φCMPO) in methanol. Pu(IV) was also completely removed from other matrices such as teflon, stainless steel and glass. Studies on the extraction of Am(III) using supercritical carbon dioxide modified with CMPO in methanol resulted in its complete recovery from tissue paper.

The determination of fluoride from diverse matrices at front and back end of nuclear technology and some studies from this laboratory on optimizations of different experimental parameters differing with multiple fuels and reactor materials, have been reviewed. The most useful techniques such as fluoride ion selective electrode (F-ISE) and ion-chromatography (IC) widely adopted as routine methods for fluoride determination in nuclear industry have been discussed. The effect of various buffer strengths on the response of the fluoride ion selective electrode has been examined. The ion chromatographic studies on mobile phase concentration, medium of sample, sample injection volume etc. to get distinct fluoride peak within optimum time in presence of other anionic species in diverse concentrations have been reported. The results of various sample matrices such as UO 2 , PuO 2 , (U,Pu)O 2 , Pu-alloy, thoria, zircaloy, slag, HLLW, LLLW etc. , analyzed after matrix separation using pyrohydrolysis setup, for both solid and liquid samples and without matrix separation by masking potentially interfering ions of liquid samples, have been presented.

The radioactive 82 Br was used as a tracer to study the bromide isotopic exchange reactions. The experiment was performed to study the effect of temperature, concentration of ionic solution, and amount of ion exchange resin on kinetics of isotopic exchange reactions. It was observed that under different experimental conditions, the amount of bromide ions exchanged and the initial rate of bromide ion exchange calculated for Duolite A 101D resins were higher than that for Duolite A 102D resins. It is expected that the same technique can be employed to assess the efficiency of various ion exchangers under different operating conditions. The probable mechanism of bromide isotopic exchange reaction was also predicted by comparing the specific reaction rate of rapid and slow exchange process.

Lead is contained as trace element in uranium ores and propagates throughout the production process to intermediate products like yellow cake or uranium oxide. The lead isotopes in such material originate from two sources: natural lead and radiogenic lead. The variability of the isotopic composition of lead in ores and yellow cakes was studied and the applicability of this parameter for nuclear forensic investigations was investigated. Furthermore, the chemical impurities contained in these materials were measured in order to identify characteristic differences between materials from different mines. For the samples investigated, it could be shown, that the lead isotopic composition varies largely from mine to mine and it may be used as one of the parameters to distinguish between materials of different origins. Some of the chemical impurities show a similar pattern and support the conclusions drawn from the lead isotope data.

The paper presents a case study for the application of environmental tritium to assess the mean residence time of water in the Tafilalet aquifer. This aquifer corresponds geologically to a tectonic depression with Quaternary fill. The Tafilalet area depends mainly on agriculture and herding. Groundwater is the main source of water supply for domestic, agriculture, and herding purposes. The data on the distribution of tritium in subsurface water indicate the modern water of the Tafilalet aquifer. The tritium transport modelling indicates that Quaternary alluvial waters are likely to have short mean residence times of a few years (14 to 32 years). The study addresses several difficulties concerning the detailed quantitative interpretations of the results, which must be the focus of future work.