Application of the general microstructural model to erosion phenomena—mechanisms for the chemical-hydrodynamic conversion of bentonite a pumpable slurry in conjunction with retrieval

摘要

One of the requirements of a system for disposalof spent nuclear fuel is that it should be possible an feasible toretrieve the canisters even after they have been put in place andthe surrounding bentonite has absorbed water and developed aswelling pressure. This 'gripping' of the canister must be releasedbefore the canister can be retracted from the deposition hole.One way to achieve this is to convert the bentonite into a slurrybe exposing it to a flow of water containing dissolved salts suchas sodium or calcium chloride. The potential efficiency of such aprocess is remarkable in view of the well-known tardiness of thesaturation of compacted bentonite with pure water. The presentstudy of the mechanisms involved was prompted by the need tounderstand to prerequisites and limitations of such a conversion process.Thus, the literature on the molecular structure ofmontmorillonite (the major constituent of bentonite) wasreviewed as well as the literature on the microstructure ofmontmorillonite—water systems. A review was also made ofsome chemical literature which led to identification of two rate-limiting factors for montmorillonite microstructure conversion:diffusion over large distances and association—dissociation ofprimary montmorillonite particles.The knowledge compiled was then used in analyses of thekinetics involved and the following conclusions wee made: (1)Exposure of the compacted bentonite with freshwater causes it toswell and to produce free particles by exfoliation. They form gelswhich cause closure of the pores so that further uptake of waterbecome limited by diffusion. (2) Exposure of the compactedbentonite with water containing dissolved salt causes theexfoliated material in the microstructure to shrink (or at leastswell less than in the fresh water case). Thus more water canpenetrate into the pores and cause differential expansion in theaggregate residues which, in turn, leads to further widening of thepores. (3) The gel formed in the above described process may beremoved by the flow of the water thus exposing fresh bentonitesurface to continued attack.The paper is based on the general microstructural model butgoes beyond it by including also dilute systems.It is concluded that the chemical-hydrodynamical method forremoving bentonite from around a deposited canister might beshown to be robust and efficient one provided that differentialexpansion of the individual grains in the microstructure isaccomplished, that flocculated conditions can be avoided and thatthe chemically modified material can be removed by flushing