The preparation and characterisation of highly selective adsorbents for fission product removal from acid solutions

Abstract

Nuclear fuel reprocessing of fissile materials is carried out in order to provide recycled fuel for existing and future nuclear power plants. One aim of reprocessing is to recover unused uranium (U-238 and U-235) and plutonium isotopes thereby preventing them from being wasted. This can save up to 30% of the natural uranium that is required each year for the fabrication of new nuclear fuel. A second aim is to reduce the volume of high-level radioactive waste. Along with the separation of uranium and plutonium there has been a significant interest in the extraction of short-lived fission products such as caesium and strontium, which play critical role during high-level waste handling and disposal.
The PUREX process for reprocessing of irradiated fuel has been unchallenged for more than half a century even though it has several deficiencies such as flexibility, non-specificity of Tri-Butyl Phosphate (TBP), degradation of the extractant, TBP, and diluent. This project addresses the development of an alternative separation process to either replace and/or complement the PUREX process. Our process is based on the chromatographic separation of fission products from U and Pu. This research focuses on the synthesis of highly stable and selective materials which could be used as a stationary phase in a continuous chromatographic separation for short lived fission products (Cs, Sr); a technique patented by UCLan.

The objectives of this project were to synthesis highly selective adsorbents for fission products (primarily Cs and Sr) capable of extracting these cations from acidic liquor (up to 3 M HNO3). In addition to selectivity (specificity) and acid stability, the materials under investigation would require fast cation uptake and high capacity.
The research explored three key approaches for ion sorption:
(1) Creating charge imbalance into ordered mesoporous MCM-41 structure (chapter 4),
(2) Examination of molecular sieves based on their size exclusive property (chapter 5), and,
(3) Preparation of ammonium phosphomolybdate (AMP) encapsulated polymeric composites (chapter 6).
Various physical and chemical properties of the materials were characterised by XRD, SAXS, surface area, pore volume, pore size distribution, SEM, TEM, ATR-IR, 29Si NMR and TGA techniques. The cation uptake performance of the materials were evaluated for single ion and mixed ions against various nitric acid conditions. The study was further extended to rate of uptake in the best performing AMPPAN composites and identified area of improvement.
Insertion of heteroatom e.g. boron into silicate structures, did not produce the desired effect; selectivity and capacity for the target fission products (Cs and Sr) were negligible compared with the required criteria. The incorporation of a mesoporous shell around zeolite structure was effective but the uptake of fission products from nitric acid solutions was again disappointing. The uptake of fission products from slightly acid solutions (pH value ~5) was more encouraging but not specific to any single ion (e.g. Cs or Sr) and this approach could form the basis of further studies.
The preparation of AMP composites addressed both inorganic and organic substrates; AMP alumina composites in a suitable form i.e. spheres/beads was challenging and produced materials that were unsuitable and incorporated low AMP concentrations. This produced composites with low Cs uptake. The use of an organic substrate such as polyacrylonitrile (PAN) produced a composite that had a high selectivity for Cs, near specific, from nitric acid solutions but with comparatively low capacity and rate of uptake compared to pure AMP. These properties could be improved by manipulation of the composite structure; future work in this area is recommended.