Investigation on retained and released $^{14}$C and effects of gamma-radiation for spent Ion-exchange resins arising from boiling water reactors

Rizzato, Corrado; Thomauske, Bruno (Thesis advisor); Scherer, Ulrich W. (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022

Abstract

A common waste management strategy for spent ion-exchange resins (SIERs), arising from light water reactors (LWR), is still under discussion. Due to the characteristics of this organic waste and to the possible $^{14}C$-releases from unconditioned SIERs, a direct of disposal of SIERs would possibly not meet the acceptance criteria of the Konrad repository in Germany. Several conditioning processes have been adopted in the past, aiming mainly to a volume reduction and to the production of a more stable waste matrix. No particular focus on the organic $^{14}C$ fraction has been considered yet due to limited experimental data. Aim of this work is to investigate the amount and speciation of $^{14}C$ on some SIERs arising from boiling water reactors (BWR), its release under conditions relevant to interim storage or final disposal and to provide an insight on some radiation-induced degradation mechanisms. The first part of this thesis is focussed on the characterization of Spent Ion-Exchange Resins (SIERs) in mixed (cation and anion) powder form from condensate clean-up systems (CCU) of two German BWRs (BWR-A & BWR-B). Experimental results indicate the effects of a drying conditioning process at moderate temperatures (around 100-110 °C) that affected the anion-exchange fraction of SIERs by e.g. reducing the anion-exchange capacity while leaving the cation-exchange fraction nearly intact. The chemical speciation of $^{14}C$ retained in SIERs was investigated. Organic 14C is present in small percentages compared to the total $^{14}C$, with about 0.4% for CCU-A and 0.5-1.1% for CCU-B SIERs, which is slightly lower than other recent results reported in literature. A direct comparison of absolute results [Bq/g] with literature data is questionable, as the adopted reference mass of SIERs (dry/wet), the ionic form, the presence of metal (oxides) and the drying procedures for the samples could be different. Total $^{14}C$ specific activities ranging between 0.14-6.2 MBq/kgdw (dw=dry weight) were generally measured, compared to the 0.6-5.3 MBq/kgdw of this study. Investigations on the chemical form of possible $^{14}C$-containing species retained in SIERs by means of High Performance Liquid Chromatography (HPLC) highlighted the presence of formic acid in small amounts, 2.0-8.0 mmol/kgdw, in all samples. These results are compatible with other investigations for the same type of SIERs used in the CCU-system of a BWR, where the activity of the formic acid has been found to be the dominant fraction of the activity in the released organic species. $^{14}C$ releases in an open and closed system in inorganic form showed the trend of SIERs to reach an equilibrium with the surrounding environment, driven by a concentration gradient. No organic $^{14}C$ was detected in the gas phase. Storage conditions, possibly together with previous drying processes, resulted to affect the releases. In particular, SIERs stored in a semi-opened system revealed lower releases compared to SIERs dried and stored in tight drums. A saturation-like behaviour of the releases from SIERs under interim storage in a closed system has been observed, confirming that the system SIERs/water/atmosphere tends to reach an equilibrium in relatively short terms (few days). Investigations on the effects of the storage atmosphere confirmed the evolution of the system independently of the presence of air or inert gases. However, in absence of water and in vacuum no releases were observed, since no ion-exchange in these conditions can take place. Higher storage temperatures (50°C) resulted in enhanced $^{14}C$ releases compared to the case of storage at room temperature, characterised by faster kinetics and possibly by a slow thermal degradation of the anion-exchange fraction. The contact of SIERs with an alkaline solution (0.1 M NaOH) revealed a rapid evolution of gases containing $^{14}CO_{2}$. The contact with alkaline solutions with higher molarity showed to compensate this effect, resulting in no generation of gases. Considering the experimental conditions and taking into account a cementation process of SIERs, the number of anions present in a cement mixture would be significantly higher than the ones employed here. Additionally, the presence of $Ca(OH)_{2}$ would fix $CO_{2}$ in form of $CaCO_{3}$.The second part of this thesis focussed on model materials (i.e. ion-exchange resins - IERs - and simulated SIERs) that were exposed to gamma radiation to investigate the radiation-induced effects on the speciation of the released compounds, together with the analysis of the solid phase evolution. During interim storage and final disposal, gamma-emitters retained in SIERs could lead to structural degradation and release of $^{14}C$-containing species. Studies on the storage behaviour of SIERs are however strictly connected to the history of the waste: reactor type, reactor operation, operational time, conditioning processes, interim storage conditions, etc. and difficult to compare. For these reasons, a dedicated gamma-irradiation campaign of IERs in controlled conditions was performed, to provide a better understanding of the fundamental degradation and release processes taking place. Two approaches were adopted for the gamma-irradiation experiments: fresh IERs for equipment testing together with a comparison with literature results and simulated SIERs to reproduce the waste form after discharge from the reactor, in a more controlled way. Some degradation mechanims like de-amination and demethylation of anion-exchange resins under gamma radiation have been observed as also reported in literature. Formic acid was detected in small amounts, increasing with the dose. Experimental results for cation-exchange confirmed the higher radiation resistance of these materials, compared to the anion-exchange resins, where a more complex degradation mechanism affecting the backbone may take place. Experiments on simulated SIERs highlighted the enhanced production of $H_{2}$ and ethane in the gas phase, together with the presence of more complex molecules as propane and acetone. Similarly, formic acid in higher amounts was observed in solution and its origin is thought to be linked to the presence of carbonate ions attached to the anion-exchange resins. When this hypothesis should be verified, this could lead to the partial radiation-induced conversion of inorganic $^{14}C$ to the organic form. Additional molecules as acetic acid were detected, but its formation mechanisms should still be clarified. The radiation-induced degradation of the functional groups through de-amination and de-methylation has been confirmed, with marginal additional impact on the functional groups but marked effects on the resins´structure due to the presence of water and $NaHCO_{3}$ in comparison to the fresh unloaded IERs. In the third part of this thesis, a theoretical approach by means of Molecular Dynamics (MD) and Metadynamics simulations on a simplified molecule, Benzyltrimethylammonium (BTMA), aimed to reproduce some degradation mechanisms of IERs, reported in literature and observed within this study. In particular, measurements of the residual exchange capacity of (anion) SIERs and on the model materials exposed to gamma radiation highlighted the presence of two degradation mechanisms, which could be either thermal- and/or radiation-induced. Simulation results revealed a marked predominance of the de-amination mechanism compared to the de-methylation one, with a reduction of the activation energy in presence of an additional electron. In detail, de-amination is characterised by an activation energy of about 0.53-0.56 eV, reduced to a close-to-zero value in presence of an additional electron. De-methylation resulted into an energy barrier of 1.5 eV, reduced to about 0.6 eV in presence of an additional electron. These results seem to be qualitatively confirmed by the experimental results found here and reported by literature. However the present study considered a very simplified model, while the actual mechanisms taking place are likely to be more complex.

Institutions

• Division of Mineral Resources and Raw Materials Engineering [510000]
• Chair of Repository Safety [512410]