The main concern of the production of energy with nuclear reactors is the radioactive waste management. The high radioactivity from the waste originates from only a few chemical elements, namely plutonium and the minor actinides: americium, neptunium, iodine and technetium. A way to reduce the radioactivity, is to use partitioning and transmutation processes by which the plutonium and minor actinides are first separated from the waste and then put back in the reactor and fissioned (transmutation), hereby losing most of its long-term radiotoxicity. One way to convert these elements into nuclear fuel for transmutation is via internal gelation processes. Internal gelation is a ‘wet’ route for fuel production that can produce microspheres of fuel by the conversion of aqueous solutions. This is different to the classical fuel production which uses powder-based fabrication of pellets which create highly radiotoxic dust during the process. The formation of radiotoxic dust is avoided by using internal gelation processes. The formation of the microspheres is initiated by an increase in pH caused by the decomposition of hexamethylenetetramine/urea into ammonia. To investigate this mechanism of hydrolysis by thermal decomposition of urea, a complexation study and hydrolysis experiments were performed on different mixtures of uranium-lanthanides solutions. The experiments were followed up with pH, UV-VIS and ICP-MS measurements. The experiments were performed with mixtures of uranyl nitrate, neodymium(III) nitrate, cerium(III) nitrate and ammonium cerium(IV) nitrate. These lanthanide metals have similar chemical properties as actinides and serve therefore as surrogates that are safer to handle. For the same reason depleted uranium was used instead of natural uranium. Neodymium and cerium acted as surrogates for americium and plutonium, respectively. Results obtained in this study will serve as a basis for future work on actual actinide-containing compounds. For the complexation study, only a difference in the UV-VIS spectrum was detected for the uranyl nitrate solution when HMTA was added. A shift in the absorbance spectrum was observed as well as an increase in absorbance. The spectrum of the solution with HMTA is similar to the spectrum of (UO2)3(OH)5+. In the hydrolysis experiments with uranyl nitrate, an increase in absorbance and a peak shift from 414 nm to 421 nm occurred after the addition of urea. The absorption spectrum before the addition of urea is similar to that of UO22+. The absorption spectrum after the addition of urea is similar to that of (UO2)2(OH)22+. This implies that the addition of those amounts of urea lead to a change in the uranium(VI) species from UO22+ to (UO2)2(OH)22+. The spectra of the uranyl solutions of the complexation study and the uranyl nitrate solution of the hydrolysis experiments were different. This could be explained because during the complexation study, an acid deficient uranyl solution was used. But the spectrum after the addition of urea looked similar to the spectra that was seen with the ADUN solution. Both of these spectra were similar to the spectra of (UO2)2(OH)22+. The hydrolysis experiments indicated that uranyl nitrate precipitated at a pH between 4.1 and 5.5. For neodymium(III) nitrate a pH range between 5 and 7 was found which was similar to that of cerium(III) nitrate which was between 5.5 and 6.3. For the two component systems where uranium was mixed with one of the lanthanides, it was observed that the metals precipitated almost separately with a little bit of overlap. For the three component system U-Nd-Ce(III), it was found that uranium precipitated first and afterwards neodymium and VI cerium(III) precipitated simultaneously. This was due to the similar pH ranges of neodymium and cerium(III). For the three component system U-Nd-Ce(IV), it was found that the metals precipitated separately. Cerium(IV) was found to precipitate first, afterwards uranium and neodymium as last. It was also observed that doubling the urea concentration reduced the time to precipitate by half. Actinides require to be handled inside a glove box because of their radiotoxic nature. UV-VIS offers an easy and relatively cheap method for qualitative and quantitative analysis. When working in a highly contaminated glovebox, it is advisable to keep as many parts possible of a complex equipment outside of it. This limits the cost for future waste of a contaminated device and facilitate greatly its maintenance. With a UV-VIS device it is possible to keep most of the equipment outside of the glovebox by diverting the light beam using optical fibers. After the comparison between measurements with and without the external setup, it was concluded that the external setup had little influence on the measurements. This means the external setup can be used for qualitative and quantitative analysis.
|State||Published - 17 Sep 2019|