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MEDIUM-ACTIVE WASTE FORM CHARACTERIZATION: THE PERFORMANCE OF CEMENT-BASED SYSTEMS

Objective

CEMENT IS USED AS A MATRIX MATERIAL FOR THE IMMOBILIZATION OF LOW AND MEDIUM-ACTIVE RADIOACTIVE WASTES AND AS A CONSTRUCTIONAL MATERIAL. ITS PROPERTIES ARE MARKEDLY AFFECTED BY TIME AND TEMPERATURE, BY REACTION WITH ADDED MATERIAL, INCLUDING BOTH WASTES AND POZZOLANIC ADDITIVES (SLAG, FLY ASH, ETC.), BY THE CUMULATIVE IMPACT OF RADIATION, AND BY ITS INTERACTION WITH THE NEAR-FIELD ENVIRONMENT.

THE PRESENT PROJECT HAS AS ITS GOAL THE QUANTIFICATION OF RELEVANT FACTORS, WITH THE AIM OF DEVELOPING A PREDICTIVE CAPABILITY. THE PROGRAMME IS COOPERATIVE, WITH THE RADIATION STUDIES BEING CONDUCTED AT HARWELL LABORATORIES.
The properties of cement systems which contribute to their immobilisation potential for radwastes are characterised. In the short term, both physical and chemical properties of the matrix contribute to the immobilisation potential, but in the longer term, chemical effects dominate.

A rigorous framework based on accessible physicochemical variables, has been constructed. The model is capable of describing the future performance of cements when leached at 20 C by relatively pure water. It embraces mainly 6 chemical components sodium oxide, calcium oxide, magnesium oxide, silicon dioxide and water together with limited data on the effect of sulphate ions.

As a consequence of these studies, a firm scientific basis has been laid for modelling the behaviour of long duration cemented systems.

The properties of cement systems which contribute to their immobilisation potential for radioactive wastes are characterised. In the short term, both physical and chemical properties of the matrix contribute to the immobilisation potential, but in the longer term, chemical effects dominate.

Before the interactions of cement with wastes can be fully assessed and data extrapolated it is necessary to be able to describe quantitatively the behaviour of cements themselves. A rigorous framework based on accessible physicochemical variables has been constructed. The model, as presently developed, is capable of describing the future performance of cements when leached at approximately 20 C by relatively pure water. It covers primarily 6 chemical components, sodium oxide, potassium oxide, calcium oxide, magnesium oxide, silicon dioxide and water, together with limited data on the effect of sulphate. The interaction of cements with inactive waste stream constituents, principally sulphate and nitrate, is described. The interaction between steel and cement is also reexamined.

As a consequence of these studies, a firm scientific basis has been laid for modelling the behaviour of cemented systems over long periods, which extend beyond those for which test data can be obtained.
1. BASIC PROPERTY MEASUREMENTS: CHARACTERIZATION OF THE SOLID AND AQUEOUS PHASES OF PORTLAND CEMENT SYSTEM BY DEVELOPING A SOLUBILITY MODEL.
2. MODIFIED CEMENT SYSTEMS: DETERMINATION AND CHARACTERIZATION OF THE PRODUCTS OF HYDRATION OF BLENDED CEMENTS, ESPECIALLY THOSE CONTAINING GLASSY SLAGS, AND THE IMPACT OF THESE ADDITIVES ON SOLUBILITY MODELS.
3.1. CEMENT - WASTE STREAM INTERACTIONS: DETERMINATION AND CHARACTERIZATION OF CEMENT PROPERTIES IN THE PRESENCE OF WASTES, ESPECIALLY REFERENCE WASTE FORMS, CONTAINING SULPHATE, NITRATE AND BORATE.
3.2. INTERACTION BETWEEN CEMENT AND CONSTRUCTIONAL MATERIALS: THE REACTION BETWEEN CEMENT AND STEEL WILL BE STUDIED, MAINLY BY CRITICAL ANALYSIS OF THE LITERATURE.
4.1. RADIATION STABILITY AND RADIATION EFFECTS: THE IMPACT OF RADIATION EFFECTS AT REALISTIC DOSE RATES WILL BE DETERMINED THROUGH MEASUREMENT OF SUCH FACTORS AS PH, EH, SORPTIVE CAPACITY AND PHASE STABILITY.
4.2. THERMAL EFFECTS: THE EFFECT OF MODERATELY ELEVATED TEMPERATURES ON THE HYDRATE PHASES UP TO 70 CELSIUS DEGREES, WILL BE ASSESSED.
4.3. PRESSURE EFFECTS: A SCOPING STUDY WILL BE MADE OF THE EFFECT OF PRESSURE ON CEMENT SYSTEMS AT PRESSURES UP TO THE EQUIVALENT OF 500 M DEPTH.
5. SELECTED ASPECTS OF CEMENT-NEARFIELD INTERACTIONS: THE ROLE OF CORROSIVE GROUND WATERS AND OF BRINES WILL BE ASSESSED.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

University of Aberdeen
Address
Regent Walk
AB9 1FX Aberdeen
United Kingdom