The effects of radiolysis and hydrolysis on the stability of extraction systems for minor actinides

Graduation date: 2011Industrial reprocessing of irradiated nuclear fuel (INF) is one of the most complex procedures performed on a large scale; the process is intricate due to the mix of radionuclides present in INF. As a global trend for nuclear power and reprocessing continues, research is geared toward optimizing the extraction of targeted radionuclides from the assortment of byproducts with the aim to decrease the radioactivity of the stored waste and recycle the targeted radionuclides in mixed oxide fuel. Currently, simultaneous separation of radionuclides in one extraction cycle is the leading approach to processing spent nuclear fuel. The process implements a universal extraction mixture for one-step extraction of all targeted radionuclides, followed with selective stripping of individual metals with aqueous solutions. \ud "Group Extraction of Actinides and Fission Products", one of the top approaches in this effort, is based on a modification of the Universal Extraction (UNEX) solvent. The process is currently performed using an extraction mixture composed of the organic complexant octyl(phenyl)-N,N-diisobutylcarbamoyl methylphosphine oxide (CMPO), the cation exchanger, chlorinated cobalt dicarbollide (CCD), and polyethyleneglycol (PEG) in the diluent phenyltrifluoromethyl sulfonate (FS-13). The solution extracts both fission products and actinides. However, this composition was initially developed for low level waste, and it is of limited use when it comes to processing solutions containing large amounts of actinides and lanthanides, such as in INF. \ud The current process is restricted by the limited solubility of CMPO and its complexes with metals. In order to more effectively process acidic aqueous solutions containing large amounts of actinides and lanthanides, modifications must be made to the current composition of the mixed solvent. Previously, it has been shown that diamides of dipicolinic acid have increased capacity to extract actinide and lanthanide metals, when compared to CMPO. Furthermore, these diamides exhibit synergistic behavior with CCD to extract cesium, strontium, and trivalent metals. \ud This study investigated the possibility of replacing CMPO with diamides of 2,6-pyridinedicarboxylic acid (dipicolinic acid) with N,N' diethyl N, N' ditolyl dipicolinamide (EtTDPA). Stability of selected diamides was tested in a simulation of the harsh environment of dissolved nuclear fuel in order to determine their viability for use in reprocessing. Acidic hydrolysis and radiolysis conditions are always present in such systems. EtTDPA, in solution with CCD and FS-13, were exposed to nitric acid and irradiation by gamma photons (Co-60). The stability of EtTDPA was determined through analysis of distribution ratios for Am-241 and Eu-252 and Eu-254 extracted from acidic aqueous solutions. Mass spectrometry was also employed to determine if any structural changes occurred in the chemicals as a result of hydrolysis or radiolysis. Results showed that Et(o)TDPA was the most stable isomer across radiolysis, and also withstood hydrolysis