The Application of a Coupled Chemical Transport Model in a Trial Assessment of Deep Disposal of Low and Intermediate Lev
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THE APPLICATION OF A COUPLED CHEMICAL TRANSPORT MODEL IN A TRIAL ASSESSMENT OF DEEP DISPOSAL OF LOW AND INTERMEDIATE LEVEL RADIOACTIVE WASTE STEVE K. LIEW, VEA ECONOMIDES, ADRIAN DAWES AND DAVID READ WSAtkins Engineering Sciences Limited, Epsom, Surrey, KT18 5BW
United
Kingdom,
ABSTRACT A series of trial assessments are being undertaken within the United Kingdom to rehearse procedures for post-closure radiological risk analysis of low and intermediate level radioactive waste disposal sites. These trial or "Dry Run" studies draw together workers from a variety of scientific disciplines allowing simultaneous rather than isolated development of assessment procedures. This paper illustrates the use of chemical codes and data within a probabilistic risk assessment framework through a demonstration exercise performed on a hypothetical repository located beneath Harwell, Oxfordshire. The implications for risk assessment practices reliant on the assumption of "conservative" parameters are highlighted by comparison with a more mechanistic approach which takes into account competitive effects.
BACKGROUND The United Kingdom Department of the Environment (DOE) is funding an extensive programme of research to develop a probabilistic risk assessment (pra) capability for evaluation of deep radioactive waste disposal on land [1]. Consideration of chemical processes has formed an essential part of this programme since its inception in 1982, culminating in a structured methodology derived to allow representation of chemical effects at each stage of the assessment procedure [2]. A series of trial or "Dry Run" assessments are now being performed to test the adopted methodology. The first in the series clearly demonstrated the need for a pra rather than "best estimate" or "worst case" deterministic calculations [3]. Dry Run 2 [4] built on this and
highlighted the difficulties associated with combining contributions to risk from more than one scenario or postulated sequence of events. Both "assessments" addressed a hypothetical intermediate level waste (ILW) disposal facility situated in Cretaceous clay 140m below Harwell, Oxfordshire. However, neither study considered changes to the repository environment with time due either to climatic or human-induced effects. Prior to embarking on a full time-dependent pra [5], it was felt desirable that further attention be given to chemical aspects of the programme since these have been found to exert a dominating influence on dose and risk estimates in studies to date [3, 4].
METHODOLOGY The
purpose
of the work described in this paper has been to formalise
and rehearse the transfer of chemical data between detailed Mat. Res. Soc. Symp. Proc. Vol. 176. ©1990 Materials Research Society
process
models
770
and the pra objectives:-
code
VANDAL [5].
In particular, the study had the following
to demonstrate the adequacy of current models for producing input to time-dependent pra; to compare geochemical data predicted by thermodynamic modelling with earlier estimates based solely on "e
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