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Title: A Review Of The Treatment Of Criticality In Post-Closure Safety Assessment Of Radioactive Waste Disposal
Author: T W Hicks
Author: Environment Agency
Document Type: Monograph
Annotation: Environment Agency Project ID:EAPRJOUT_448, Representation ID: 135, Object ID: 1787
Extensive use of cement and concrete is envisaged in the construction of geological repositories for low and intermediate-level radioactive wastes, both for structural, and encapsulation and backfilling purposes. Saturation of these materials with groundwater may occur in the postclosure period of disposal, producing a hyperalkaline pore fluid with a pH in the range lo13.5. These pore fluids have the potential to migrate from the repository according to local groundwater flow conditions and react chemically with the host rock. These chemical reactions may affect the rockas capacity to retard the migration of radionuclides released from the repository after the degradation of the waste packages. The effects of these chemical reactions on the behaviour of the repository rock as a barrier to waste migration need to be investigated for the purposes of assessing the safety of the repository design (so-called asafety assessmenta or aperformance assessmenta). The objectives of the work reported here were to: identify those processes influencing radionuclide mobility in the geosphere which could be affected by plume migration; review literature relevant to alkali-rock reaction; contact organisations carrying out relevant research and summarise their current and future activities; and make recommendations how the effects of plume migration can be incorporated into models of repository performance assessment. The following paragraphs summarise the principal results and conclusions of the project. A number of key processes affecting radionuclide migration has been identified which will be perturbed by the migration of a hyperalkaline pore fluid plume. These are: porosity and permeability properties; precipitation and co-precipitation behaviour; sorption; matrix diffusion; colloid stability and mobility; the mobilisation of organics from the host rock; and the behaviour of bentonite seals. A review of literature pertaining to alkali-rock reactions in the radioactive waste, cementjconcrete, and oil industries, and elsewhere was conducted. This confirmed the existence of much relevant data concerning: alkali-rock reaction mechanisms and mineral alteration sequences; changes in porosity and permeability; sorption of radionuclides; matrix diffusion behaviour; the thermodynamics and kinetics of likely reactions; and the mobility of organics. Countries with current and planned research activities on alkaline plume migration were identified to include: Japan; Sweden; Switzerland; and the UK. These countries have many common interests on the issue of plume migration due to similarities in planned repository design, waste characteristics and potential repository host rocks. Therefore there is considerable opportunity for shared funding of research projects. With regard to research carried out thus far, there are major uncertainties for performance assessment still remaining for: the overall spatial scale of alkaline alteration; the effects of 18 April 1997 vii Technical Report P60 aearlya Na-K dominated cement pore fluids; speciation-solubility behaviour of radionuclides at pH fronts; the mechanism of sorption behaviour in rock-water systems at high pH; matrix diffusion; colloid abundances at pH fronts; and the behaviour of bentonite at high pH. Recommendations for performance assessment are that geosphere sub-models are produced for PA with variations of the above processes/properties to scope the potential effects upon radionuclide migration with regard to a reference case which assumes little or no perturbation of the far-field due to alkaline plume migration. Better definition of the scale of alteration is dependent upon the acquisition of better analogue data for repository scenarios and the use of more realistic models for chemical reaction and transport. Key words radioactive waste; geological disposal; repository performance assessment; radionuclide migration; cement/concrete; alkali-rock reaction; radionuclide sorption behaviour; rock matrix diffusion; Colloids; organics 18 April 1997 . .. Vlll Technical Report P60 1.
Publisher: Environment Agency
Subject Keywords: Radioactivity; Geological disposal; Repository performance assessment; Radionuclide migration; Cement/concrete; Alkali-rock reaction; Radionuclide sorption behaviour; Rock matrix diffusion; Colloids; Organics
Extent: 77
Total file downloads: 41

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