Development of methodology for evaluation of long-term safety aspects of organic cement paste components

Long-term safety aspects of superplasticizers (SP) and other cement paste components were studied in this joint Nagra - NUMO - SKB - Posiva project with aim to develop a methodology for the evaluation of the long-term safety aspects of superplasticizers (SP) and other organic components of cement pastes. The study also evaluated the effects of SPs and other cement paste components that have already been used or that are most likely to be used in the construction of the high-level nuclear waste repositories in Sweden, Switzerland, Finland and Japan. The main long-term safety issue of concern is whether the superplasticizers and/or other organic components of cement pastes might affect the transport properties of radionuclides. A full evaluation of whether the superplasticizers can be used in a high-level nuclear waste repository cannot be answered based on the studies but a classification of the superplasticizers based on their impact on sorption of radionuclides has been done. The basic methodology for testing, leaching and analyzing of leachants and solid samples of different types was developed at CRIEPI. Two different methodologies for studying the impact of SPs on the sorption of Eu on crushed rock were tested and developed by Helsinki University (HU) and Chalmers University of Technology (CTH). Methods for analyzing organics leaching from grouts were successfully tested by CRIEPI and CTH (Chalmers University of Technology). At CRIEPI the total organic content (TOC) of the leachants was analyzed by Infrared absorption spectrometry (IR) followed by Gel Permeation Chromatography (GPC) for the identification of the organic compounds. At CTH several different analytical methods were tested (e.g. IR, UV spectroscopy, NMR, MALDI-TOF), but these methods still require improvement. In addition to SPs, organics are present in several components of cement pastes, for example in cement grinding aid (CGA) and micro silica slurry. The results suggests that the main high molecular weighted organics will not elute from the cementitious material, but the low molecular weighted cement grinding aid will elute. Some indications of small amounts of raw materials or decomposed materials of SPs are seen leaching out from some samples. Based on the tests performed at CTH, the sorption of Europium (Eu) on rock samples decreased clearly in the presence of the polycarboxylate (PC) type SPs: EVO 26 and Glenium 51. The effect was clearly lower for poly naphthalene sulphonate (PNS) type SPs: Mighty 150 and Rheobuild 1000. For Mighty 150 the impact on sorption of Eu on crushed rock is fairly small, the effect was strongest at the highest pH (pH 10) tested. Consistent results for the effect of Mighty 150 and Glenium 51 were obtained at HU when comparing the effect of SP at 1g/L. Discrepancies were observed at high pH (pH 10), which may be attributed to different configurations of the experiments. Possible factors having an impact on the results are e.g. Eu colloid formation and consequently lower Rd and Ld values for CTH results. The presence of smaller granite particles in the rock material used for sorption tests, since no washing procedure was used, resulted in stronger Eu sorption and higher Rd and Ld values for HU results. Additional factors noted are e.g., filter sorption resulting in lower Eu concentrations in HU measurements and consequently higher Rd and Ld values for HU results. Sorption experiments with Nickel Ni(II), Eu(III) and Thorium Th(IV) on a cation exchange resin at Empa/PSI indicate that the admixtures (Glenium 51, Rheobuild 1000 and Sigunit) may form complexes in solution with the above elements. Nevertheless, no influence on the uptake of radionuclides by cement pastes was observed in systems where the ratio of admixtures to HCP (hardened cement paste) was selected according to the expected ratio in the cementitious near field of a repository. GPC, IR and NMR analyses of field samples collected from monitoring boreholes in grouted areas in ONKALO did not show any SPs in the sampled waters. (orig.)