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Content archived on 2024-04-15

STUDY OF LEACHING MECHANISMS OF IONS INCORPORATED IN CEMENT OR POLYMER

Objective

THE PRESENT EXPERIMENTAL STUDY AIMS AT FURTHER ELUCIDATION OF SOME IMPORTANT ASPECTS RELATED TO THE LEACHING KINETICS OF CS AND SR IONS BY WATER FROM SOLID MATRICES SIMULATING CEMENTED LOW-TO-MEDIUM LEVEL RADIOACTIVE WASTES, AS WELL AS STUDY OF SIMPLER MODEL SYSTEMS, SUCH AS A SOLUBLE OR INSOLUBLE SALT EMBEDDED IN A WELL DEFINED POLYMER MATRIX (E.G. CELLULOSE ACETATE OR AN EPOXY RESIN). THE DEVELOPMENT OF REALISTIC THEORETICAL MODELS OF THE KINETICS OF ION RELEASE IS ALSO ENVISAGED.

THIS WORK IS EXPECTED TO PROVIDE FUNDAMENTAL INFORMATION NECESSARY FOR THE SOUND EVALUATION OF THE LONG-TERM ELUTION KINETICS OF IMPORTANT IONS FROM REAL WASTE PRODUCTS.
The leaching kinetics of inactive caesium from cylindrical cement specimens containing caesium sulphate have been studied at different temperatures and thicknesses. In all cases the t{0.5} plots were reasonably linear, at least initially, in accordance with Fick's law, and the diffusion coefficients could be estimated.

The leaching kinetics of inactive caesium from caesium sulphate containing cement samples (3,4 % by weight), of caesium-137 from samples containing caesium-137, or of small quantities of caesium nitrate as a carrier and of sodium nitrate (8 % by weight) were studied. Distribution profiles were obtained during the course of the leaching process which confirmed the findings of previous studies.
Leaching tests of cement specimens containing traces of strontium-90 and sodium nitrate (8 % by weight) were also carried out. A strong supression of the leaching rate caused by exposure to atmospheric carbon dioxide (during preparation, curing and leaching of the samples) was observed. The presence of strontium nitrate (0,15 % by weight) was found to result in higher leaching rates.
Leaching experiments on sodium chloride and strontium sulphate loaded epoxy resins were also carried out.

The leaching kinetics of inactive caesium from cylindrical cement specimens containing caesium sulphate was studied at different temperatures and thicknesses. In all cases the square root of time plots were reaonably linear at least initially in accordance with Fick's Law and the diffusion coefficients were estimated. The sorption of caesium ion from aqueous solution by cement was studied by eqilibrating cement granules with aqueous caesium sulphate.

Leaching od specimens containing strontium-90 and sodium nitrate was performed under exposure to atmospheric carbon dioxide.

Low temperature differential calorimetry measurements of hydrated cement were undertaken to obtain information about the melting behaviour and hence the state of water within the cement.

Cellulose films containing calcium sulphate or strontium sulphate were leach tested in frequently renewed water at 25 C. The elution curves follow a square root of time law in conformity with the Higuchi equation.

Ellution tests of sodium chloride or strontium sulphate embedded in epoxy resin indicated that strontium sulphate elution was generally similar to cellulose.
Theoretical work involved the formulation of new models capable of describing soluble salt elution.
1. CEMENT
1.1 LEACHING KINETICS OF CS IN RELATION TO THE DISTRIBUTION OF THE ION IN THE SPECIMEN AT VARIOUS STAGES OF THE LEACHING PROCESS.
1.2 EXAMINATION OF THE EFFECT OF TEMPERATURE ON LEACHING RATE BY MEANS OF PARALLEL EXPERIMENTS AT DIFFERENT TEMPERATURES.
1.3 EFFECT OF CHANGING THE THICKNESS OF THE SAMPLE ON THE ELUTIONN CURVE.
1.4 EXAMINATION OF THE POSSIBILITY OF SETTING UP A THEORETICAL MODEL FOR THE REPRESENTATION OF THE ELUTION KINETIC CURVE OF CS.
1.5 EFFECT OF ATMOSPHERIC CO2 ON THE ELUTION OF SR, IN RELATION TO THE SR CONTENT OF THE SAMPLE.
1.6 CHARACTERIZATION OF THE STATE OF WATER AND EMBEDDED IONS IN THE CEMENT MATRIX (POROSIMETRY, DIFFERENTIAL SCANNING CALORIMETRY-DSC, REVERSIBLE SORPTION BEHAVIOUR).
1.7 DEVELOPMENT OF A MORE ADVANCED MODEL FOR THE ELUTION PROCESS.

2. POLYMER
2.1 FURTHER EXPERIMENTAL INVESTIGATION OF THE CELLULOSE ACERATE-SALT SYSTEM (EFFECT OF SALT SOLUBILITY, SALT CONTENT, SALT PARTICLE SIZE OR POLYMER WATER CONTENT).
2.2 DEVELOPMENT OF A MORE SOPHISTICATED THEORETICAL MODEL TO DESCRIBE THE ELUTIO KINETICS OF SOLUBLE SALTS IN PARTICULAR.
2.3 CHARACTERIZATION OF SORPTION AND DIFFUSION PROPERTIES OF CELLULOSE ACETATE MATRICES (WHERE POSSIBLE).
2.4 FURTHER INVESTIGATION OF THE SALT SOLUBILITY EFFECT IN THE CASE OF ELUTION OF SIMPLE SALTS FROM EPOXY RESIN MATRICES.
2.5 SIMULATION OF REAL POLYMER-SALT SYSTEMS BY THE THEORETICAL ELUTION MODEL PREVIOUSLY DEVELOPED.

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Greek Atomic Energy Commission
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Address
Attikis
15310 Athens
Greece

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