Testing of a near-field biogeochemical model against data from a large-scale gas generation experiment
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Testing of a near-field biogeochemical model against data from a large-scale gas generation experiment J.S. Small1, M. Nykyri2, N. Paaso3, U. Hovi3, M. Itävaara4, T. Sarlin4 1 Nexia Solutions Ltd, Warrington, UK 2 Safram Oy, Espoo, Finland 3 Teollisuuden Voima Oy, Eurajoki, Finland 4 VTT Biotechnology, Espoo, Finland ABSTRACT A biogeochemical model that represents processes of metal corrosion, microbial degradation of cellulosic waste and mass transfer within a heterogeneous system has been used to represent processes of gas generation in a large-scale (20m3) experiment that has studied degradation of typical nuclear reactor operating waste. The experiment has been in operation for a period of about eight years and has established a pattern of methanogenic gas generation. A “blind testing” approach has been used to develop the model of the experiment using independently derived kinetic data for corrosion and microbial processes. The model correctly represents the anaerobic conditions leading to methane generation during the course of the experiment. The overall rate of gas generation of the experiment is well represented, as is the composition of evolved gases and geochemistry of sampled liquids. The experiment and the model together build confidence in the ability to simulate processes of gas generation and variation in chemical conditions in heterogeneous repository environments. INTRODUCTION Understanding and quantifying processes of gas generation resulting from metal corrosion and degradation of organic wastes are important to many safety cases for low- and intermediatelevel radioactive waste (LLW, ILW) repositories. A repository system involves a large number of interacting processes which makes realistic predictive modelling a challenging task. Microbial activity is commonly invoked as being important to gas generation from ILW and LLW. Microbes may be directly involved in generation of gases such as methane, however they may also consume hydrogen produced through corrosion. Chemical conditions within the near field may also be strongly affected by microbial activity which is relevant not only to the living conditions of microbes but also to the release and transport of radionuclides in groundwater. Heterogeneous chemical conditions may develop as a consequence of the types of waste and packaging materials that are used. Through a combination of representative experimental simulation and detailed biogeochemical reaction-transport modelling these processes can be better understood and quantified in order that long-term behaviour can be simulated. The Finnish nuclear power generating company Teollisuuden Voima Oy (TVO) operates a large scale Gas Generation Experiment (GGE) in its VLJ Repository at the Olkiluoto site. The primary objective of the GGE is to quantify rates of gas generation from actual LLW that is disposed of in the VLJ Repository. The GGE has produced a unique set of gas generation rate and gas and water composition data since operation started in 1997. Previously the GGE formed part of the EC PR
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