Long-Term Interactions of Full-Scale Cemented Waste Simulates with Salt Brines (KIT Scientific Reports ; 7721)

Since 1967 radioactive wastes have been disposed of in the Asse II salt mine in Northern Germany. A signifi-cant part of these wastes originated from the pilot reprocessing plant WAK in Karlsruhe and consisted of cemented NaNO3 solutions bearing fission products, actinides, as well as process chemicals. With respect to the long-term behavior of these wastes, the licensing authorities requested leaching experiments with full scale samples in relevant salt solutions which were performed since 1979. The experiments aimed at demonstrating the transferability of results obtained with laboratory samples to real waste forms and at the investigation of the effects of the industrial cementation process on the properties of the waste forms. This research program lasted until 2013. The corroding salt solutions were sampled several times and after termination of the experiments, the solid materials were analyzed by various methods. The results present-ed in this report cover the evolution of the solutions and the chemical and mineralogical characterization of the solids including radionuclides and waste components, and the paragenesis of solid phases (corrosion products). The outcome is compared to the results of model calculations. For safety analysis, conclusions are drawn on radionuclide retention, evolution of the geochemical environment, evolution of the density of solutions, and effects of temperature and porosity of the cement waste simulates on cesium mobilization

Understanding the mobility of caesium, nickel and selenium released from waste disposal : chemical retention mechanisms of degraded cement

Cementitious materials are used to condition or stabilise waste and to build infrastructure in disposal sites. Moreover, they are envisaged to form part of engineered barrier systems as container, backfill or liner materials in radioactive waste disposal concepts. In the event of contact with water, contaminants dissolve and their mobility is influenced by the employed cementitious materials. Therefore, sound understanding of the interactions between contaminants and degrading cementitious materials in flowing water is essential for safety assessment. The aim of this study was to identify the processes affecting retention of caesium, nickel and selenium on Hardened Cement Paste (HCP) during its degradation, from sane to severely degraded states. The focus was put on the underlying mechanisms and possible remobilisation of previously retained contaminants due to the changing composition of the HCP. Caesium, as Cs(I), nickel, as Ni(II) and selenium, as Se(VI), were chosen because they are considered as safety relevant radionuclides for nuclear waste disposal, represent different chemical characteristics and their stable isotopes can be used in experiments. To address shortcomings of previous studies in this field a combined approach was developed. First, a previously used thin-layer flow-through reactor was adapted and improved for the needs of studying contaminant retention and release during degradation of the multiphase material HCP. Second, retention and degradation were studied in equilibrated batch systems as well. Regarding degradation of HCP the following results were obtained: (1) The thin-layer flow-through setup was established for degradation of HCP at far-from-equilibrium conditions and a number of relevant experimental data were obtained. (2) A kinetic degradation model satisfactorily reproduced experimental results on HCP degradation. For this, a set of dissolution rate constants of cement phases was optimised which can also be used for other modelling studies. (3) The same model also satisfactorily reproduces results from experiments with different aqueous CO2 concentrations and with different solution types, i.e. synthesised granitic groundwater (GG water) and deionised (DI) water. (4) When quantitatively comparing different solution types, degradation of HCP equilibrated with GG water is stronger than after equilibration with DI water, due to higher aqueous CO2 concentration. Further, the effects of carbonate buffering and carbonation on HCP at far-from-equilibrium conditions were identified and quantified. (5) Four characteristic stages of HCP degradation in flow-through conditions were classified, taking into account carbonate buffering effects. The different stages can be discerned on-line by measurement of pH, Ca and Si concentrations in outflow solutions. Regarding retention of Cs, Ni and Se(VI) in HCP conditioned systems the following results were obtained: (1) Caesium and selenate distribution coefficients were determined in equilibrated systems at different degradation states of HCP. (2) Caesium and selenate retention was quantified at flow conditions during continuous degradation of HCP in DI and GG water and likely retention mechanisms were narrowed down.(3) In the case of nickel, the solubility limiting phases formed in presence of HCP were identified at different degradation states. (4) The formation of a so far non-described nickel-silicate-hydrate was observed in the more degraded system at pH around 11.6. (5) The influence of different aqueous CO2 concentrations on Cs, Ni and Se(VI) retention was demonstrated to be minor. This study showed that the persistency of contaminant retention by adsorption in degrading cementitious systems is not only a question of distribution coefficients at different degradation states, but also a question of how fast these degradation states are reached.Los materiales cementosos son usados para acondicionar y estabilizar los residuos, así como para construir las infraestructuras de los almacenes en los cuales son depositados. Dichos materiales están concebidos para formar parte de las barreras de ingeniería para residuos radiactivos. En el caso de contacto con el agua, los contaminantes presentes en los residuos se disolverán y su movilidad se verá afectada por los materiales cementosos utilizados. Por lo tanto, un profundo conocimiento de las interacciones entre los contaminantes y los materiales cementosos degradados por el agua corriente es esencial para evaluar la seguridad de los almacenes de residuos. El objetivo del estudio fue la identificación de los procesos de retención que afectan al Cs, Ni y Se durante la degradación del cemento endurecido (CE), desde estados sanos hasta estados de degradación elevados. El foco fue puesto en los mecanismos subyacentes y en la posible removilización de los contaminantes, previamente retenidos, debido a la cambiante composición del CE. Se desarrolló un enfoque combinado: Por un lado, se adaptó un reactor de flujo de capa fina, para abordar las necesidades requeridas en el estudio de la retención y la liberación de los contaminantes durante la degradación del material de CE multifase. Por otro lado, los procesos de retención y degradación fueron también estudiados mediante el uso de sistemas de tipo ¿batch¿. Con respecto a la degradación de CE se obtuvieron los siguientes resultados: 1) Se estableció la configuración de flujo continuo en capa fina para la degradación del CE en condiciones alejadas del equilibrio. 2) Los resultados experimentales obtenidos para la degradación del CE fueron reproducidos mediante un modelo de degradación cinética. Para ello, se optimizaron un conjunto de constantes de velocidad de disolución de las fases de cemento, las cuales también pueden ser utilizadas para otros estudios de modelización. 3) El mismo modelo también reprodujo, satisfactoriamente, los resultados de experimentos realizados con diferentes concentraciones acuosas de CO2 y con dos tipos de agua, la subterránea granítica (SG) y la desionizada (DI). 4) Se demostró una mayor degradación del CE cuando está equilibrado con agua SG comparado con agua DI. Este efecto es debido a una mayor concentración acuosa de CO2 en el agua SG. Además, se identificó y cuantificó el efecto tampón del carbonato y de la carbonatación del CE en condiciones alejadas del equilibrio. 5) Se clasificaron cuatro etapas características de la degradación del CE en condiciones de flujo continuo, teniendo en cuenta los efectos del tampón carbonato. Las diferentes etapas se pueden distinguir durante el experimento mediante la medición del pH y de las concentraciones de Ca y Si en la solución a la salida del reactor. Con respecto a la retención de Cs, Ni y Se(VI) en sistemas acondicionados con CE se obtuvieron los siguientes resultados: 1) Se determinaron los coeficientes de distribución de Cs y Se(VI) en sistemas equilibrados a diferentes estados de degradación. 2) Se cuantificó la retención de Cs y Se(VI) en condiciones de flujo durante la degradación continua de CE en agua del tipo DI y SG, y los posibles mecanismos de retención se acotaron. 3) En el caso del Ni, se identificaron las diferentes fases limitantes de la solubilidad formadas en presencia del CE a diferentes estados de degradación. 4) Se observó la formación de un silicato de níquel hidratado, no descrito hasta la fecha, en el sistema más degradado con un valor de pH alrededor de 11.6. 5) Se demostró que la influencia de diferentes concentraciones acuosas de CO2 en la retención de Cs, Ni y Se(VI) es mínima. Este estudio ha demostrado que la persistencia de la retención de contaminantes, atribuida a la adsorción, no es sólo debida a los coeficientes de distribución a diferentes estados de degradación, sino que depende también de la velocidad a la cual los diferentes estados de degradación son alcanzado

Ein Beitrag zur Refraktometrie des Chlorcalciumserums der Milch

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