Preparation of monodisperse cerium oxide particle suspensions from a tetravalent precursor.

Cerium oxide particles are a unique material that enables studying the intersection of metal oxides, f-elements, and nanomaterials. Distinct from diverse applications in catalysis, energy, and medicine, cerium possesses additional influence as a non-radioactive actinide surrogate. Herein, we present a synthesis for sub-micron cerium particles using hexamethylenetetramine and ammonium hydroxide as precipitating agents with a CeIV precursor. The combinatorial homogeneous precipitation approach yields monodisperse and moderately-stable CeO2 particle suspensions in ethanol, as determined by powder X-ray diffraction, scanning electron microscopy, dynamic light scattering, and zeta potential measurements. Various additives may be used to moderate and manipulate the surface charge of the particles. Proof-of-concept electrophoretic deposition of the particles produces a uniform layer of CeO2 on graphite. The synthesis and suspension properties are developed as a methodology towards future controlled actinide hydrolysis and film deposition.

Harvesting 88Zr from heavy-ion beam irradiated tungsten at the National Superconducting Cyclotron Laboratory.

Tungsten is a commonly used material at many heavy-ion beam facilities, and it often becomes activated due to interactions with a beam. Many of the activation products are useful in basic and applied sciences if they can be recovered efficiently. In order to develop the radiochemistry for harvesting group (IV) elements from irradiated tungsten, a heavy-ion beam containing 88Zr was embedded into a stack of tungsten foils at the National Superconducting Cyclotron Laboratory and a separation methodology was devised to recover the 88Zr. The foils were dissolved in 30% hydrogen peroxide, and the 88Zr was chemically purified from the tungsten matrix and from other co-implanted radionuclides (such as 85Sr and 88Y) using strong cation-exchange (AG MP-50) chromatographic resin in sulfuric acid media. The procedure provided 88Zr in approximately 60 mL 0.5 M sulfuric acid with no detectable radio-impurities. The overall recovery yield for 88Zr was (92.3 ± 1.2)%. This proof-of-concept experiment has facilitated the development of methodologies to harvest from tungsten and tungsten-alloy parts that are regularly irradiated at heavy-ion beam facilities.

3D printed field-deployable microfluidic systems for the separation and assay of Pu in nuclear forensics.

A compact field-deployable microfluidic system has been developed to improve timelines for the rapid analysis of debris in post-detonation nuclear forensics. We used a high-resolution 3D printer to miniaturize typical laboratory-based procedures into a fieldable platform. Microfluidic half-modules were produced for the purification of Pu from excess U, along with a portable alpha chamber for the following isotopic analysis of the Pu stream. A porous PTFE membrane is soaked with a hydrophobic tributyl phosphate (TBP) solution and is placed between two half-modules; separation is performed as a liquid-liquid extraction in an extraction channel across this membrane, where the forward and back-extractions occur within one complete module. Following separation, a 100 µL sampling of the Pu-bearing stream is injected into a small-footprint 3D printed alpha chamber for isotopic assay via alpha spectrometry as part of an online process. In this first demonstration of microfluidic separation coupled with online alpha spectrometry, high extraction yields have been obtained for Pu (98.9 ± 4.0)% and U (97.5 ± 2.5)%. The process uses less than 800 µL of solution with separation chemistry complete within 45 minutes and subsequent alpha spectrometry initiating 25 minutes after separation.

Solid-phase isotope harvesting of 88Zr from a radioactive ion beam facility.

During routine operation of the Facility for Rare Isotope Beams (FRIB), radionuclides will accumulate in both the aqueous beam dump and along the beamline in the process of beam purification. These byproduct radionuclides, many of which are far from stability, can be collected and purified for use in other scientific applications in a process called isotope harvesting. In this work, the viability of 88Zr harvesting from solid components was investigated at the National Superconducting Cyclotron Laboratory. A secondary 88Zr beam was stopped in a series of collectors comprised of Al, Cu, W, and Au foils. This work details irradiation of the collector foils and the subsequent radiochemical processing to isolate the deposited 88Zr (and its daughter 88Y) from them. Total average recovery from the Al, Cu, and Au collector foils was (91.3 ± 8.9) % for 88Zr and (95.0 ± 5.8) % for 88Y, respectively, which is over three times higher recovery than in a previous aqueous-phase harvesting experiment. The utility of solid-phase isotope harvesting to access elements such as Zr that readily hydrolyze in near-neutral pH aqueous conditions has been demonstrated for application to harvesting from solid components at FRIB.

Production, Collection, and Purification of 47Ca for the Generation of 47Sc through Isotope Harvesting at the National Superconducting Cyclotron Laboratory.

An experiment was performed at the National Superconducting Cyclotron Laboratory using a 140 MeV/nucleon 48Ca beam and a flowing-water target to produce 47Ca for the first time with this production route. A production rate of 0.020 ± 0.004 47Ca nuclei per incoming beam particle was measured. An isotope harvesting system attached to the target was used to collect radioactive cationic products, including 47Ca, from the water on a cation-exchange resin. The 47Ca collected was purified using three separation methods optimized for this work: (1) DGA extraction chromatography resin with HNO3 and HCl, (2) AG MP-50 cation-exchange resin with an increasing concentration gradient of HCl, and (3) AG MP-50 cation-exchange resin with a methanolic HCl gradient. These methods resulted in ≥99 ± 2% separation yield of 47Ca with 100% radionuclidic purity within the limits of detection for HPGe measurements. Inductively coupled plasma-optical emission spectrometry (ICP-OES) was used to identify low levels of stable ions in the water of the isotope harvesting system during the irradiation and in the final purified solution of 47Ca. For the first time, this experiment demonstrated the feasibility of the production, collection, and purification of 47Ca through isotope harvesting for the generation of 47Sc for nuclear medicine applications.

Harvesting 48V at the National Superconducting Cyclotron Laboratory.

As part of an effort to develop aqueous isotope harvesting techniques at radioactive beam facilities, 48V and a cocktail of primary- and secondary-beam ions created by the fragmentation reaction of a 160 MeV/nucleon 58Ni beam were stopped in an aqueous target cell. After collection, 48V was separated from the mixture of beam ions using cation-exchange chromatography. The extraction efficiency from the aqueous solution was (47.0 ± 2.5)%, and the isolated 48V had a radiochemical purity of 95.8%. This proof-of-concept work shows that aqueous isotope harvesting could provide significant quantities of rare isotopes which are currently unavailable at conventional facilities.

The surprisingly large neutron capture cross-section of 88Zr.

The probability that a nucleus will absorb a neutron-the neutron capture cross-section-is important to many areas of nuclear science, including stellar nucleosynthesis, reactor performance, nuclear medicine and defence applications. Although neutron capture cross-sections have been measured for most stable nuclei, fewer results exist for radioactive isotopes, and statistical-model predictions typically have large uncertainties1. There are almost no nuclear data for neutron-induced reactions of the radioactive nucleus 88Zr, despite its importance as a diagnostic for nuclear security. Here, by exposing 88Zr to the intense neutron flux of a nuclear reactor, we determine that 88Zr has a thermal neutron capture cross-section of 861,000 ± 69,000 barns (1σ uncertainty), which is five orders of magnitude larger than the theoretically predicted value of 10 barns2. This is the second-largest thermal neutron capture cross-section ever measured and no other cross-section of comparable size has been discovered in the past 70 years. The only other nuclei known to have values greater than 105 barns3-6 are 135Xe (2.6 × 106 barns), a fission product that was first discovered as a poison in early reactors7,8, and 157Gd (2.5 × 105 barns), which is used as a detector material9,10, a burnable reactor poison11 and a potential medical neutron capture therapy agent12. In the case of 88Zr neutron capture, both the target and the product (89Zr) nuclei are radioactive and emit intense γ-rays upon decay, allowing sensitive detection of miniscule quantities of these radionuclides. This result suggests that as additional measurements with radioactive isotopes become feasible with the operation of new nuclear-science facilities, further surprises may be uncovered, with far-reaching implications for our understanding of neutron capture reactions.

Organic layer formation and sorption of U(vi) on acetamide diethylphosphonate-functionalized mesoporous silica.

Acetamide diethylphosphonate (AcPhos)-functionalized silica has been shown to have a high affinity for U(vi) in pH 2-3 nitric acid. Previous work with AcPhos-functionalized silica has focused on actinide and lanthanide extraction under various conditions, but has shown poor reproducibility in the functionalization process. For this work, four AcPhos-functionalized SBA-15 materials were synthesized and evaluated based on their U(vi) sorption capacity and their stability in nitric acid. Materials synthesized using pyridine as a basic catalyst were shown to form a greater fraction of polymeric structures at the silica surface, which correlated with higher structural integrity upon contact with acidic solutions. Single-pulse 31P and 1H NMR spectra of these materials show evidence of phosphonic acid groups, as well as hydrogen-bonding interactions either between ligands or with the silica surface. Additionally, these materials were found to have significantly higher U(vi) sorption capacities and Keq values than the materials synthesized without pyridine, most likely due to the ion-exchange properties of the phosphonic acid groups. The 31P-31P DQ-DRENAR NMR technique was used to compare the average strength of dipolar coupling interactions between phosphorus atoms for the four materials. Because the strength of dipolar coupling interactions depends on the number and proximity of neighboring spins, this technique provides information about the average density of ligands on the surface. The conventional functionalization procedure yielded materials with the lowest average surface ligand density, while those using extended reaction times and the pyridine base catalyst yielded materials with higher surface ligand densities.

Probing the interaction of U(vi) with phosphonate-functionalized mesoporous silica using solid-state NMR spectroscopy.

The fundamental interaction of U(vi) with diethylphosphatoethyl triethoxysilane functionalized SBA-15 mesoporous silica is studied by macroscopic batch experiments and solid-state NMR spectroscopy. DPTS-functionalized silica has been shown to extract U(vi) from nitric acid solutions at or above pH 3. Extraction is dependent on pH and ionic strength. Single-pulse (31)P NMR on U(vi) contacted samples revealed that U(vi) only interacts with a fraction of the ligands present on the surface. At pH 4 the U(vi) extraction capacity of the material is limited to 27-37% of the theoretical capacity, based on ligand loading. We combined single pulse (31)P NMR on U(vi)-contacted samples with batch studies to measure a ligand-to-metal ratio of approximately 2 : 1 at pH 3 and 4. Batch studies and cross-polarization NMR measurements reveal that U(vi) binds to deprotonated phosphonate and/or silanol sites. We use (31)P-(31)P DQ-DRENAR NMR studies to compare the average dipolar coupling between phosphorus spins for both U(vi)-complexed and non-complexed ligand environments. These measurements reveal that U(vi) extraction is not limited by inadequate surface distribution of ligands, but rather by low stability of the surface phosphonate complex.

Development and Testing of Diglycolamide Functionalized Mesoporous Silica for Sorption of Trivalent Actinides and Lanthanides.

Sequestration of trivalent actinides and lanthanides present in used nuclear fuel and legacy wastes is necessary for appropriate long-term stewardship of these metals, particularly to prevent their release into the environment. Organically modified mesoporous silica is an efficient material for recovery and potential subsequent separation of actinides and lanthanides because of its high surface area, tunable ligand selection, and chemically robust substrate. We have synthesized the first novel hybrid material composed of SBA-15 type mesoporous silica functionalized with diglycolamide ligands (DGA-SBA). Because of the high surface area substrate, the DGA-SBA was found to have the highest Eu capacity reported so far in the literature of all DGA solid-phase extractants. The sorption behavior of europium and americium on DGA-SBA in nitric and hydrochloric acid media was tested in batch contact experiments. DGA-SBA was found to have high sorption of Am and Eu in pH 1, 1 M, and 3 M nitric and hydrochloric acid concentrations, which makes it promising for sequestration of these metals from used nuclear fuel or legacy waste. The kinetics of Eu sorption were found to be two times slower than that for Am in 1 M HNO3. Additionally, the short-term susceptibility of DGA-SBA to degradation in the presence of acid was probed using (29)Si and (13)C solid-state NMR spectroscopy. The material was found to be relatively stable under these conditions, with the ligand remaining intact after 24 h of contact with 1 M HNO3, an important consideration in use of the DGA-SBA as an extractant from acidic media.