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MIGRATION AND RETENTION OF RADIONUCLIDES IN THE NEAR FIELD.

Objective

PART I:
OUR PARTICULAR OBJECTIVES ARE:
I) TO CHARACTERISE AND INVESTIGATE THE MECHANISMS BY WHICH COLLOIDS MAY FORM IN CEMENT LEACHATES, GIVING ATTENTION TO THE COMPOSITION, AGE AND STRUCTURE OF THE CEMENT.
II) TO DETERMINE THE PROPERTIES OF COLLOIDS WHICH MAY OCCUR IN REPOSITORY ENVIRONMENTS AND ESTABLISH THE EFFECTS OF THE NEAR-FIELD AQUEOUS CHEMISTRY ON THEIR BEHAVIOUR.
III) TO ASSESS THE RADIONUCLIDE INCORPORATION OF SUCH COLLOIDS.
IV) TO INVESTIGATE THE POSSIBLE GENERATION OF COLLOIDS BY THE INTERACTION OF CEMENT LEACHATE WITH NEAR-FIELD MATERIALS.

PART II:
THE PRIMARY AIM IS TO DEVELOP RESEARCH MODELS OF VARIOUS PHYSICAL AND CHEMICAL PROCESSES WITHIN THE NEAR-FIELD OF A REPOSITORY. THESE RESEARCH MODELS USE EXPERIMENTAL DATA TO STUDY AREAS OF REPOSITORY BEHAVIOUR IN DETAIL TO EXAMINE ASSUMPTIONS AND APPROXIMATIONS IMPLICIT IN ASSESSMENT MODELS. THE ASSESSMENT MODELS ARE NECESSARILY RELATIVELY SIMPLE.
THE NEAR-FIELD HAS A COMPLEX CHEMISTRY WHICH DETERMINES THE SOLUBILITY AND SORPTION OF RADIONUCLIDES. THIS CHEMICAL ENVIRONMENT CHANGES IN BOTH SPACE AND TIME THROUGHOUT THE NEAR-FIELD REGION DUE TO THE INGRESS OF GROUNDWATER AND THE DEGRADATION OF THE ENGINEERED BARRIERS. THE COUPLING BETWEEN THE CHEMISTRY AND TRANSPORT PROCESSES IS VERY IMPORTANT IN DETERMINING THE EVOLUTION OF THE NEAR-FIELD. THIS PROGRAMME IS LARGELY CONCERNED WITH MODELLING COUPLED TRANSPORT AND CHEMICAL PROCESSES WITHIN A CEMENT ENVIRONMENT.

PART III:
THE SORPTION OF RADIONUCLIDES ONTO SOLID PHASES IN A RADIOACTIVE WASTE REPOSITORY CAN BE A VERY IMPORTANT MECHANISM FOR THE REDUCTION OF RADIONUCLIDE SOURCE TERMS. ONE OF THE ADVANTAGES OF USING CEMENTITIOUS MATERIALS AS COMPONENTS OF THE NEAR-FIELD, IS THAT THEY PROVIDE A LARGE SURFACE AREA ON WHICH SORPTION MAY OCCUR. THE OBJECTIVE OF THIS RESEARCH PROGRAM IS TO UNDERSTAND THE PROCESSES AND REACTIONS WHICH MAY TAKE PLACE WHEN ACTINIDES ARE SORBED ONTO A CEMENT SURFACE.
A number of synthetic compounds have been prepared which represent those which are considered to be present in a mature BFS/OPC cement. Some of these have been prepared with a lanthanum dopant to investigate the possible inclusion of sorbed species in the structure. The compounds have been investigated by the best available tools to determine the chemical state of the sorbed species and the possible incorporation of the sorbed ions into the evolving structure of the cement. The sorption coefficients of these compounds for americium and plutonium have also been determined. The structural studies were impeded by the instability of the compounds, and the surface analytical tools lacked the sensitivity to detect the sorbed elements. The extended X-ray absorption fine structure (EXAFS) technique shows considerable promise. Studies of the sorption of actinides from solution onto these compounds have shown that the sorption coefficients are no less than those determined for fresh cements.

The CHEQMATE program was developed to study the chemistry and related ionic transport processes occurring in the near field of a radioactive waste repository. The numerical method used in the program was improved and models of sorption processes were included in the chemical part of the code. A model of the degradation of cement in a repository has been developed using CHEQMATE.

The thermodynamic description of clay has been improved and tested, in a validation exercise for CHEQMATE, using information provided by the British Geological Survey.
A model of diffusion and sorption experiments through cement has also been developed. This model provides a framework for modelling transport of sorbing species through cement.

The potential role of colloids in the disposal and storage of low level and intermediate level radioactive waste immobilised in cement has been investigated.
It has been established that the generation of colloids in cement leachates can arise from a process of nucleation and growth leading to the formation of an amorphous phase which is predominantly a calcium silicate hydrate.
It has also been shown that the interaction of cement leachates with near field materials (backfill, geology, metals) can result in considerable surface changes, arising from colloid retention and possible chemical reaction, to produce highly porous deposits. Such aspects may be significant in influencing radionuclide retention.
Finally, preliminary migration experiments show that the transport of colloids through porous granite (containing wide pores) is a complicated process resulting in retardation possibly due to colloid rock interactions.

This paper reports on the progress achieved in developing models of various physical and chemical processes within the near field of a low or intermediate level waste repository. The computer code CHEQMATE has been developed further to study coupled chemistry and ionic transport processes. The numerical method used in the program has been improved recently and models of sorption have been included in the chemical part of the code.

A model of the degradation of cement in a repository has been developed using CHEQMATE. In a previous study, the scale of perturbation to the groundwater chemistry surrounding a repository due to the ingress of high pH from the backfill was investigated. The thermodynamic description of clay has been improved and used in a validation exercise for CHEQMATE from information provided by the British Geological Survey. A model of diffusion/sorption experiments through cement has also been developed. This model is intended to provide a framework for modelling transport of sorbing species through cement.

A comprehensive research investigation has been undertaken to improve understanding of the potential role of colloids in the context of disposal and storage of low level and intermediate level waste immobilized in cement. Topics which have been investigated include:
the study of the formation and characteristics of colloids in cement leachates;
the effects of the near field aqueous chemistry on the characteristics of colloids in repository environments;
colloid sorption behaviour;
interactions of nearfield materials with leachates;
characteristics of nearfield materials in EC repository simulation tests;
colloid migration behaviour.

These experimental investigations provide data and a basis for the development of transport models and leaching mechanisms.
PART I: COLLOIDS RELATED TO LOW AND INTERMEDIATE LEVEL WASTE
1. STUDIES OF COLLOIDS IN CEMENT LEACHATES.
2. CHARACTERISTICS OF COLLOIDS IN REPOSITORY ENVIRONMENTS.
3. CHARACTERIZATION OF NEAR-FIELD MATERIALS IN EC REPOSITORY SIMULATION TESTS.
4. CEMENT/STEEL INTERACTIONS IN THE NEAR-FIELD.
5. COLLOID SORPTION BEHAVIOUR.

PART II: NEAR-FIELD MODELLING IN CEMENT ENVIRONMENTS
1. IMPROVEMENT OF THE NUMERICAL SOLVING METHOD USED IN THE COMPUTER CODE CHEQMATE/1/. THIS CODE COUPLES CHEMICAL EQUILIBRIA (VIA. THE PHREEQE CODE/2/) WITH IONIC MIGRATION.
2. APPLICATION OF CHEQMATE TO COUPLED CHEMISTRY AND TRANSPORT PROBLEMS RELEVANT TO THE NEAR-FIELD OF A REPOSITORY, FOR EXAMPLE THE SPACE AND TIME DEGRADATION OF THE CEMENTITIOUS BACKFILL.
3. PARTICIPATION IN THE CODE COMPARISON EXERCISE CHEMVAL.

PART III: SORPTION MECHANISMS IN THE NEAR-FIELD
1. TO ACQUIRE SORPTION DATA FOR ACTINIDES ON TO SOME SINGLE MINERAL PHASES WHICH ARE PRESENT IN MATURE CEMENT.
2. TO STUDY THE CRYSTALLOGRAPHY OF CALCIUM SILICATE HYDRATE AND ITS CAPACITY TO INCORPORATE ACTINIDE IONS WITHIN THE LATTICE.
3. TO STUDY THE SURFACES ON WHICH SORPTION HAS OCCURED IN ORDER TO OBTAIN INFORMATION ON THE CHEMICAL STATE OF THE SORBED ACTINIDE.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

United Kingdom Atomic Energy Authority
Address
353,Harwell
OX11 0RA Didcot - Oxfordshire
United Kingdom

Participants (2)

DEPARTMENT OF THE ENVIRONMENT (DOE)
United Kingdom
NIREX LTD
United Kingdom