Objective DIFFUSION IS LIKELY TO BE THE PRINCIPLE MECHANISM OF THE TRANSPORT OF RADIONUCLIDES IN CLAY. IN ORDER TO SELECT CLAY FORMATIONS SUITABLE FOR THE DISPOSAL OF RADIOACTIVE WASTE, PRIOR TO SITE SPECIFIC INVESTIGATIONS, IT IS NECESSARY TO UNDERSTAND THE FACTORS UPON WHICH THE DIFFUSION OF RADIONUCLIDES IN CLAY MAY DEPEND. SEVERAL FACTORS MAY INFLUENCE THE MOBILITY OF IONS IN CLAY SYSTEMS. WE HAVE CHOSEN TO CONCENTRATE ON THE FOLLOWING: CLAY MINERALOGY; IONIC STRENGTH OF THE ELECTROLYTE SOLUTION; THE NATURE OF THE EXCHANGEABLE CATIONS ON THE CLAY SURFACE; AND THE PRESENCE OF ORGANIC LIGANDS BOTH IN SOLUTION AND ABSORBED ON OR COMPLEXED WITH THE CLAYS. AS THE CLAY SYSTEM IS MODIFIED BOTH THE DEGREE OF SORPTION OF AN ELEMENT, AND ITS MOBILITY IN BOTH THE SOLUTION AND THE ABSORBED PHASES WILL VARY. IN ORDER TO ELUCIDATE THE MECHANISM OF DIFFUSION IT IS NECESSARY NOT ONLY TO MEASURE THE DIFFUSION COEFFICIENT OF THE CHOSEN ION, BUT ALSO TO ASSESS THE CONTRIBUTION OF EACH SPECIES PRESENT, ABSORBED OR IN SOLUTION, ITS OXIDATION STATE, DEGREE OF COMPLEXATION, HYDROLISATION ETC. THE ELEMENT CHOSEN FOR THIS STUDY IS NEPTUNIUM, IN THE FORM NP02+, BECAUSE OF THE CONSIDERABLE HAZARD POSED BY THIS ELEMENT, AND THE STABILITY OF THE +5 OXIDATION STATE IN AQUAEOUS SOLUTION. THE EFFECT OF SIMPLE ORGANIC LIGANDS SUCH AS EDTA, CITRATE AND SACCHARIC ACID AND OF HUMIC ACIDS WILL BE STUDIED. Diffusion is likely to be the principle mechanism for the transport of radionuclides in clay. Several factors may influence the mobility of ions in clay systems.The following have been chosen for this study:clay mineralogy;ionic strength of the electrolyte solution;the nature of the exchangeable cations on the clay surface;the presence of organic ligands both in solution and adsorbed on, or complexed with, the clays.As the clay system is modified both the degree of sorption of an element and its mobility, in both the solution and the adsorbed phases, will vary. In order to elucidate the mechanism of diffusion it is necessary not only to measure the diffusion coefficient of the chosen ion, but also to assess the contribution of each species present, adsorbed or in solution, its oxidation state, degree of complexation, hydrolysation, etc. The element chosen for this study is neptunium, in the form of neptunium dioxide ion, because of the considerable hazard posed by this element, and the stability of the +5 oxidation state in aqueous solution. The effect of simple organic ligands, such as ethylenediaminetetraacetic acid (EDTA), citrate and saccharic acid, and of humic acids are studied.1.1. PREPARATION OF CA AND NA FORMS OF THE THREE CHOSEN CLAYS; ST AUSTELL KAOLINITE (TWO LAYERS, NONSWELLING), FITHIAN ILLITE (THREE-LAYER, NON-SWELLING) AND WYOMING MONTMORILLONITE (THREE-LAYER, SWELLING). 1.2. MEASUREMENT OF D-001 SPACING USING X-RAY DIFFRACTION. 1.3. DIFFUSION COEFFICIENT OF A NON-SORBED ION, 36CL-, WHICH DOES NOT FORM ORGANIC COMPLEXES AND THE ACTINIDE, 237NP(V), THROUGH COLUMNS OF THE DIFFERENT CLAYS HAS BEEN MEASURED. PORE WATER COMPOSITION COVERED A RANGE OF IONIC STRENGHTS. 2.1. PREPARATION OF CLAY ORGANIC COMPLEXES WITH SIMPLE LIGANDS SUCH AS EDTA, CITRATE AND SACCHARIC ACID, AND WITH HUMIC ACIDS. 2.2. MEASUREMENT OF D-001 SPACING USING X-RAY DIFFRACTION. 2.3. MEASUREMENT OF THE RATE AT WHICH THE ABOVE RADIONUCLIDES DIFFUSE IN THE COMPLEXED CLAYS. 2.4. MEASUREMENT OF THE SELF-DIFFUSION RATES OF THE SIMPLE ORGANIC LIGANDS IN THE CLAYS. 3.1. PREPARATION AND CHARACTERISATION OF ACTINIDE-ORGANIC COMPLEXES STARTING WITH THE EDTA, THEN CITRIC AND SACCHARIC ACIDS, AND FINALLY HUMIC ACID. Fields of science engineering and technologyother engineering and technologiesnuclear engineeringnuclear waste managementnatural scienceschemical scienceselectrochemistryelectrolysisnatural sciencesearth and related environmental sciencesgeologymineralogynatural scienceschemical sciencesnuclear chemistryradiation chemistry Programme(s) FP1-RADWASTOM 3C - Research and development programme (Euratom) on the management and storage of radioactive waste, 1985-1989 Topic(s) Data not available Call for proposal Data not available Funding Scheme CSC - Cost-sharing contracts Coordinator IMPERIAL COLLEGE OF SCIENCE, TECHNOLOGY AND MEDICINE EU contribution No data Address South Kensington Campus LONDON United Kingdom See on map Total cost No data
