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Open Access Publications from the University of California

Chemical Isolation of Am-240 and the Adsorption of Europium and Americium Using Silica Supported CMPO-Calix[4]arenes

  • Author(s): May, Erin Marie Gantz
  • Advisor(s): Arnold, John
  • et al.

My dissertation is composed of two projects. The first area of study was the conclusion of a six-year project to determine the neutron-induced fission cross-section of Am-240. This cross-section is difficult to study due to the 50.8 hour half-life of Am-240 but is important for the evaluation of neutron signatures to establish the history of plutonium-containing materials.

Such a study is relevant for the fields of nuclear stockpile stewardship and nuclear forensics. Prior to the study outlined here, much of the ground work for this determination had already been provided, including the identification of the Pu-242(p; 3n)Am-240 reaction for the production of approximately 100 nanograms of Am-240 and the development of a preliminary separation procedure to isolate the Am-240 from the unreacted Pu-242 and the undesirable fission and decay products generated from other reactions with the target. However, many variables in the separation procedure needed further investigation. Therefore, a series of column chromatography experiments was performed to determine which of the two previously

designed procedures would provide the best purification of 240Am while minimizing the losses.

Once the general outline of the procedure was determined, a collaboration with Idaho National Laboratory (INL) made it possible to further refine and test it. Of the four steps in the originally designed procedure, the final step was among the most difficult to accomplish as it involved the separation of the light trivalent lanthanides from americium. Americium/lanthanide separations are dificult due to their identical oxidation states and very similar ionic radii of americium and the lanthanides. The original step utilized a column

composed of TEVA® extraction chromatography resin and a mobile phase composed of ammonium thiocyanate and formic acid. Due to the complexity of this procedure, several alternative procedures were tested at INL with the TRU® resin and a mixture of fission products more representative of those that would be found in the actual target. A gas pressurized extraction chromatography (GPEC) system was used to automate the separation and

achieve greater resolution between the lanthanide and americium elution peaks. Ultimately, it was demonstrated that the original TEVA® resin procedure was more reliable than the TRU® resin procedure, however the TRU® GPEC experiments may be relevant for future target material separations. The culmination of this project was the testing of the first step

of the procedure with one-tenth of the amount of Pu-242 that will be in the actual target. This experiment, in conjunction with the americium/lanthanide separation steps that were tested, prompted a re-evaluation and abbreviation of the overall separation procedure, decreasing the required number of total separation steps from four to two. Therefore, the purification of Am-240 could be accomplished much more quickly and easily than originally anticipated,

making the determination of the neutron-induced fission cross-section of Am-240 more of a reasonable possibility in the future.

Part II of my dissertation describes the investigation of the adsorption and complexation properties of silica-anchored carbamoylmethylphophine oxide (CMPO)-calix[4]arenes. In Part I, it was demonstrated that the difficulty encountered when separating trivalent lanthanides and actinides can be a severe impediment to accomplishing even fairly straightforward benchtop radiochemical procedures. Much greater motivation for developing new

technologies to achieve this are encountered when considering the mixtures of trivalent actinides and lanthanides produced when irradiated nuclear fuel is reprocessed. CMPO, a ligand that binds indiscriminately with both the trivalent actinides and lanthanides, was attached to a calix[4]arene scaffold, which was then attached via two different methods to a silica surface. In previous studies, it has been shown that the CMPO-calix[4]arene has a greater affinity for americium than europium. The experiments in this work aimed to establish

whether this relationship held for CMPO-calix[4]arenes anchored in two new ways to the silica surface. It was found that, for a very particular set of conditions - high salt concentration and an aqueous solution pH of 3 - that a CMPO-calix[4]arene very rigidly anchored to the surface of the silica displayed extremely high uptake for Eu(III)when sites outnumbered the number of Eu(III) atoms by at least 10 to 1. This was not found when the same material and

conditions were tested with Am(III) nor when the more flexibly anchored CMPO-calix[4]arene material and conditions were tested, leading to the conclusion that both the aqueous solution conditions and the rigidity of the grafted site affect the affinity of the CMPO-calix[4]arene toward cations of interest. The optimization of the CMPO-calix[4]arene system could have

been potentially useful for Part I of this dissertation for separating americium and the lanthanides, had it existed at the time, and could eventually be applied to the separation of trivalent actinides and lanthanides at the conclusion of nuclear fuel reprocessing.

Part I and Part II contribute to two different areas of actinide/lanthanide separation research, with Part I focused on optimizing existing resin systems to achieve separation and Part II focused on testing new materials specifically intended for separating trivalent actinides and lanthanides. Part I shows that existing resin systems can be used to rapidly purify a very small amount of americium from much greater amounts of plutonium, the lanthanides, and other elements. Part II demonstrates that the affinity of a grafted CMPO-calix[4]arene site for either trivalent actinides or lanthanides can be tailored based on the rigidity of the grafting to the solid and the aqueous phase conditions. Separately, it is hoped that these studies can be applied to work focused on detailed actinide target material purification and new innovations in actinide/lanthanide separations, respectively. However, it is also hoped that, cumulatively, these studies contribute to a broader understanding of actinide and lanthanide interactions with ligands designed to effect their separation, both for benchtop laboratory separations and nuclear fuel reprocessing.

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