The Effect of Gamma Irradiation on the Ion Exchange Properties of Caesium-Selective Ammonium Phosphomolybdate-Polyacrylonitrile (AMP-PAN) Composites under Spent Fuel Recycling Conditions

Abstract

The caesium radioisotopes 134Cs, 135Cs, and 137Cs are highly problematic medium-lived species produced during nuclear fission, due to their high radioactivity and environmental mobility. While many ion exchange materials can readily isolate Cs+ ions from neutral or basic aqueous solutions, only ammonium phosphomolybdate (AMP) functions effectively in acidic conditions, removing caesium even down to trace levels. Composites of AMP in a porous polymeric support such as polyacrylonitrile (PAN) can be used to selectively remove Cs+ ions from acidic aqueous decontamination liquors as well as other liquid wastes, and are promising for the isolation of Cs+ isotopes in spent fuel reprocessing. While both AMP and PAN have demonstrable acid stability, and PAN has known resistance to gamma radiation, AMP-PAN composites have received only a limited analysis of their physiochemical and ion exchange performance following irradiation. In this publication, we explore the effect of high levels of gamma irradiation on the ion exchange properties of AMP and AMP-PAN as a Cs+-selective adsorbent under spent fuel dissolver liquor concentrations and acidity. We demonstrate no significant reduction in performance with respect to uptake kinetics or capacity upon irradiation, abiding by the same absorption mechanism observed in the established literature.

The Effect of Gamma Irradiation on the Physiochemical Properties of Caesium-Selective Ammonium Phosphomolybdate–Polyacrylonitrile (AMP–PAN) Composites

“…γ irradiation is known to induce yellowing in PAN [24]. Pawde et al suggested that this effect could be exploited as an internal dosimeter, resulting from the formation of conjugated C=N bonds and cyclisation reactions within the polymer structure, [38] though in the case of AMP-PAN, the vibrancy of AMP's hue overpowers any colour change that could be observed in the PAN matrix, though for colourless or white composites, this factor could likely be exploited in applications such as those proposed here and previously by ourselves [21,22,30]. No change is observed to the morphology or XRD powder pattern of the AMP crystallites (Figures A2 and A3 respectively) upon irradiation [28,29].…”

“…A 70% (by weight) composite of AMP contained within a porous support substrate of PAN (henceforth referred to as AMP-PAN) was prepared as described in our previous publication [30], using a well-established method developed by Park et al [35], with modifications where noted [36]. An amount of 200 mL of DMSO was heated to 50 • C in a water bath with overhead stirring at 250 rpm, and mixed with 0.8 g of Tween 80.…”

“…AMP is a dense, fine powder, often implemented as a porous composite in a polyacrylonitrile matrix (PAN) to facilitate column processing [18,19,23]. While both AMP and PAN are acid-and radiation-resistant, irradiation of both components has been characterised to a greater or lesser extent [24][25][26][27], and a significant volume of literature has studied the ion exchange performance of these compounds following irradiation [18,[27][28][29][30]. The effect of radiation on their physiochemical properties is less well understood [15,23,27]; this is of great importance when considering the practical application of such materials in the nuclear industry.…”

“…One of the primary challenges in spent nuclear fuel recycling is the radiolytic degradation of organic extractants caused by the decay of highly radioactive fission products and minor actinides present in spent fuel, which causes numerous operational difficulties and generates large volumes of highly radioactive liquid waste [21,22]. We previously proposed the use of sequential, selective chromatographic separations for the isolation of heat-generating radionuclides prior to the solvent extraction of fissile isotopes in spent fuel recycling to prevent this occurrence, which would reduce the prevalence of these issues and thus increase the safety and economy of the process while allowing for a reduction in the volume of high level waste (HLW) produced [22,23,30], in a process we have dubbed ART: alternative reprocessing technologies. The first stage of investigation into this concept has been previously reported, and is illustrated in Figure 2.…”

“…Useful radionuclides, such as Cs-137 for medical applications, could potentially be recovered this way [34], or disposed directly via encapsulation or vitrification. In a recent publication [30], we studied the effect of 100 kGy of rapid (20 kGy/hr) gamma irradiation on the Cs + ion exchange performance of AMP and AMP-PAN composites under simulated spent nuclear fuel recycling conditions (with respect to Cs + concentration and acidity, based on the concept of Eccles et al) [21], observing no reduction in capacity, kinetics, or change in uptake mechanism after irradiation. Irradiation was accompanied by a clear change in the colour of both AMP and AMP-PAN from yellow to green following irradiation, as previously reported by Sebesta and Narasimharao [15,23,29] but reversible upon immersion in nitric acid [29].…”