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INTERCOMPARISON OF PREDICTIVE COMPUTER PROGRAMS FOR RADIONUCLIDE MIGRATION IN THE GEOSPHERE (CHEMVAL PROJECT).

Objective

PREDICTION OF THE TRANSPORT OF RADIONUCLIDES THROUGH THE GEOSPHERE REQUIRES DETAILED KNOWLEDGE OF AQUEOUS GEOCHEMISTRY, HOSTROCK MINERALOGY AND THE SPECIFIC MECHANISMS OF SOLID-SOLUTION INTERACTION. SUCH KNOWLEDGE OFTEN NEEDS TO BE OBTAINED USING COMPUTER MODELS AND ASSOCIATED DATABASES, WHICH MUST BE SHOWN TO PROVIDE ACCURATE RESULTS, I.E. ARE VALIDATED FOR USE. INTERNATIONAL CODE VERIFICATION AND PROVING EXERCICES HAVE BECOME ACCEPTED AS AN IMPORTANT MEANS OF ASSISTING THE VALIDATION PROCESS.
THIS CONTRACT WILL ESTABLISH AN INTERNATIONAL PROJECT CALLED CHEMVAL, AIMED AT REVIEWING CURRENT PROGRESS AND ESTABLISHING RESEARCH NEEDS IN THE AREAS OF CHEMICAL AND CHEMICAL TRANSPORT MODELLING. THE OBJECTIVES OF CHEMVAL ARE AS FOLLOWS :
I) TO PRODUCE THE BEST POSSIBLE OVERALL THERMODYNAMIC DATABASE FOR USE WITH AQUEOUS SPECIATION AND COUPLED CHEMICAL TRANSPORT CODES, CONSISTENT WITH PROJECT RESOURCES AND TIME-SCALES.
II) TO APPLY AQUEOUS SPECIATION COMPUTER MODELS TO A RANGE OF REALISTIC WASTE DISPOSAL SITUATIONS, AND HENCE TO ESTABLISH AND/OR CONFIRM AREAS OF RESEARCH REQUIREMENT.
III) TO PROVIDE VALIDATION BOTH FOR AQUEOUS SPECIATION MODELS AND COUPLED CHEMICAL TRANSPORT CODES.
CHEMVAL is an international exercise aimed at the verification and validation of predictive models describing groundwater speciation and geochemical transport.
A methodology was established for verifying disparate coupled models and predictions made were compared both with each other and against analytical solutions using a variety of numerical methods. Finally the adequacy of current coupled models for simulating chemical transport in well defined column experiments was assessed.

6 computer programs were employed, 3 directly coupled, whereby mass action terms are incorporated within transport equations and 3 iteratively coupled, relying on iteration betweendiscrete speciation solubility and transport modules. Verification results obtained for the separate aspects of equilibrium chemistry and hydrodynamic transport were investigated before proceeding with fully coupled problems.

The results for uncoupled test cases posed few problems for the modelling groups and the results obtained generally display an excellent level of agreement. Comparison with analytical solutions for decay transport calculations provide further evidence of the accuracy of the numerical methods. Agreement was also satisfactory for the majority of fully coupled cases bearing in mind the development status of some of the codes. No systematic differences were apparent between the direct as opposed to the iterative approach and discrepancies could be ascribed, in the main, to the variable time/space grids used. However, it is not possible to say that coupled chemical transport models have been fully validated. Although column test results were reproduced adequately, solutions were obtained by back fitting rather than through prediction. Further, for neither of the 2 case systems were the resultant numerical solutions unique. More work is needed, therefore, before the realism of coupled transport simulations can be properly assessed.
1. COMPARISON, SENSITIVITY AND EXTENSION OF DATABASES. THE RESULTS OF OTHER PARTS OF THE WORK WILL BE USED IN DETERMINING THE MOST APPROPRIATE COURSE OF ACTION.
2. THE APPLICATION OF AVAILABLE AQUEOUS SPECIATION PROGRAMS AND DATABASES TO A RANGE OF HYPOTHETICAL, THOUGH REALISTIC, WASTE DISPOSAL SITUATIONS.
3. THE APPLICATION OF AVAILABLE PROGRAMS COUPLING CHEMISTRY AND WASTE TRANSPORT TO A RANGE OF TEST CASES.
4. VALIDATION OF AQUEOUS SPECIATION AND COUPLED CODES BY COMPARISON, WHERE POSSIBLE, WITH A) EXPERIMENTS, B) FIELD TESTS AND C) NATURAL ANALOGUES.
THE EXACT SCOPE AND NATURE OF WORK WILL BE AS AGREED AT PLENARY MEETINGS OF CHEMVAL PARTICIPANTS, TO BE HELD THROUGHOUT THE DURATION OF THE PROJECT.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Atkins Wootton Jeffreys Consultants Ltd
Address
Woodcote Grove Ashley Road
KT18 5BW Epsom
United Kingdom