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European Commission

Short description of the work
During this stay, the first X-ray Absorption Spectroscopy measurements on nuclear fuel samples at high temperature and under controlled atmospheres were performed. We modified our experimental set-up to accept slices of sintered nuclear fuel pellets. Initially, UO2 and (U,Pu)O2 samples were planned for this experiment but due to safety issues, no sample containing plutonium could be sent to ANKA-INE. Nevertheless, in situ XANES (X-ray Absorption Near Edge Structure) spectra were successfully collected on UO2 samples and also on U0.75Ce0.25O2 and U0.50Ce0.50O2 samples as surrogates for (U,Pu)O2 compounds. During this experiment, data at uranium L3 edge were collected for the 3 compositions from 300 K up to 2023 K for a wide range of oxygen partial pressure: from air (pO2=0.21 atm) to Ar-4% H2 (pO2=1.7E-26 atm). Based on XANES data, the evolution of uranium oxidation state, thus the O/U ratio, could be determined in situ as a function of both temperature and µO2. These data will be compared to thermodynamic modelling.

 

Short description of the work
We carried out a series of laser melting experiments at the ITU to confirm these findings on our homogeneous MOX samples fabricated in the LEFCA laboratory in CEA Cadarache (Pu/U+Pu at% = 14, 24, 35, 46, 54 and 62). To achieve this aim we used pressurized argon to fill the sample chamber during the experiments. The oxygen potential of this gas mixture does not induce a strong departure from stoichiometry of MOX during melting. Another series of experiments were performed to study how the departure from the thoroughly investigate pseuo-binary (UO2-PuO2) might affect the melting behaviour. For this purpose we have melted the samples in pressurized air, which is highly oxidizing for MOX in a certain temperature domain, which lies within the whole considered range of temperature, that the sample experiences during laser melting (300-3000K).

 

Short description of the work
Nowadays PUREX process is widely used in the spent nuclear fuel (SNF) reprocessing. In this process the key components, shuch as U and Pu, are extracted with tri-n-butyl phosphate from nitric acid solitions, wherein the main impurity is Tc. In nitric acis solutions Tc usually presents as pertechnetate- ion. For reprocessing of SNF solutions containig Tc we proposed Ferroin nitrate, complex of iron(II) with o-phenantroline, which could form poorly soluble compounds with pretechnetate ions and, consequently, could be used as a to precipitate Tc.
The experiments carried out in Chalmers University were focused on determination of the coprecipitation of technetium with other elements occuring in the SNF solutions, the irradiation stability of the Ferroin nitrate and its influence on Tc precipitation and its coprecipitation with other metals.
For these purposes precipitation of Mo with Feerroin nitrate from the solution with the concentration of nitric acid 0.5, 1, 1.5, 2, 3 M at different mole ratios Ferroin/Tc was carried out. The coprecipitation of Mo with Tc in such nitric acid solutions was studied at different mole ratios Ferroin/Tc, as well. The same experiments were carried out for Ru. Ru was also precipitated with Ferroin nitrate from nitric acid solutions in the presence of Tc and with out it. It was found that Ru couldn’t be precipitated using Ferrroin nitrate from nitric acid solution and it is not coprecipitated with Tc, as well. But in the case of Mo Ferroin nitrate could be used as a precipitant to recover it from the solutions with the concentration of nitric acid from 0,5 up to 3 M.
The precipitation of Tc from the concentrated high active raffinate (HAC- high active concentrate) of PUREX process was carried out at different Ferroin/Tc mole ratios. This raffinate contained Sr, Zr, Mo, Pd, Sn, Cs, Ba, La, Ce, Pr, Nd, Sm, U, Pu (which was replaced with Th in these experiments) and Am.
These experiments showed that other elements also coprecipitated simultaneously with Tc using Ferroin nitrate, however all Am stays in mother solution.
To study the influence of the irradiation of the precipitant on Tc precipitation from pure nitric acid solution,the solution of Ferroin nitrate were irradiated. These initial series of irradiation were 50 kG and 150 kG. The irradiated solutions of the precipitant were used to precipitate Tc in the presence of Mo from nitric acid solutions and from HAC solution. The precipitation was carried out at different Ferroin/Tc mole ratios.
These experiments showed that Ferroin nitrate after irradiation oxidizes quicker in nitric acid solutions than nonirradiated one.
Also, practically all received sediments were studied with XRD and SEM analyses. SEM analyses showed that different poorly soluble compounds with variable composition form during the precipitation.

 

Short description of the work
We have previously shown that the tetraphenylimidophosphinate ligand (TPIP) stabilises neptunium and uranium exclusively in the +VI oxidation states including complete conversion from their +V states. (1,2) Moreover, TPIP is able to form oligomeric species assembled by actinyl Lewis acid base adducts (cation-cation interactions) for uranyl (VI) ions and preliminary evidence suggests such interactions occur in mixtures of neptunyl (VI) and uranyl (VI) (i.e. a mixed metal cation-cation aggregate). The study of such cation-cation interactions are extremely important in evaluating and developing new separation processes based on simplified versions of the PUREX process.

 
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