Radiolytic Degradation of BTBP type Molecules for Treatment of Used Nuclear Fuel by Solvent Extraction

Doctoral thesis, 2011

Nuclear fuel that has been discharged from nuclear reactors is highly radiotoxic and must be kept isolated from the biosphere for very long time periods. This requires an advanced final repository. One way to decrease the long-term radiotoxicity and thermal load of the used fuel is Partitioning and Transmutation (P&T). In a transmutation process, a long-lived nuclide can be transformed into a stable nuclide, or a nuclide with a shorter half-life. Most transmutation techniques require an efficient separation (partitioning) of the elements to be transmuted. One technique suitable for separating different elements in the used nuclear fuel is solvent extraction. Because of the many highly radioactive elements in the used fuel, the solvents used in separation processes will be exposed to considerable doses of radioactivity. Solvents containing extractants belonging to the BTBP (6,6’-bis(5,6-dialkyl-[1,2,4-]triazin-3-yl)-2,2’-bipyridine) family were irradiated. Different types of radiation and different doses and dose rates were used. The effect of irradiation was studied with respect to altered metal extraction, change in BTBP concentration and formation of degradation products. In general, the BTBP concentration, the metal extraction, and the separation of different metals decreased with an increase in the absorbed dose. Degradation pathways were identified for a number of degradation products. Similar products were found regardless of whether the degradation was caused by ageing, radiolysis or hydrolysis. Products resulting from the degradation of C5-BTBP seem to extract metals. Degradation of CyMe4-BTBP, resulted in degradation products systematically different from those of C5-BTBP.