Mechanisms of Dissolution of Radioactive Waste Storage Glasses and Caesium Migration from a Granite Repository
- PDF / 494,211 Bytes
- 8 Pages / 420.48 x 639 pts Page_size
- 59 Downloads / 219 Views
Inc.
347
MECHANISMS OF DISSOLUTION OF RADIOACTIVE WASTE STORAGE GLASSES AND CAESIUM MIGRATION FROM A GRANITE REPOSITORY
NEIL A. CHAPMAN, IAN G. MCKINLEY, DAVID SAVAGE, JULIA M. WEST Institute of Geological Sciences, Environmental Protection Unit, Harwell, Oxfordshire OX11 ORA, United Kingdonm
ABSTRACT Experimental and theoretical data are used to compare the effect of three possible leach mechanisms for borosilicate glass waste buried in a granite host-rock on the release and subsequent migration of 135Cs. Protracted release episodes and variations of up to an order of magnitude in groundwater transport times and five orders in output concentrations are possible.
INTRODUCTION The essence of geological disposal of high-level radioactive wastes, in mined repositories [1) or in deep boreholes [2], is that a number of barriers should operate in concert to ensure adequate containment of the wastes. However attention has focussed on the leaching characteristics of some potential waste forms in isolation [e.g.3] suggesting that disposal methods should be designed on the intrinsic integrity of the waste form itself, rather than on the ultimate performance of the complete multi-barrier system. Favouring the latter approach, that of assessing performance of a complete disposal system, hydrothermal studies of geochemical interactions in a granite repository have been combined with mathematical modelling of groundwater movement in the rock and waste dissolution, behaviour, to produce a simple model of processes leading to release of radionuclides into the rock surrounding a waste package. This has been combined with data on the sorptive behaviour of altered fracture surfaces in granite to demonstrate the manner in which such derived release rates can influence nuclide migration. A model of the release of caesium from a typical solid waste form (borosilicate glass) and its migration through fissures in the host granite has been developed. Caesium is well-characterised in terms of its sorptive properties and environmental behaviour and is used here to demonstrate the mechanisms involved in the release and migration processes. Although the 13 5 bulk of the radiocaesium in the waste decays within 1000 years or so, CS comprises 18% of the original Cs inventory and has a half-life of 2.3 x 106 y.
NEAR-FIELD RELEASE MODEL Description of geochemical fluxes in the near-field depends upon predise definition of waste composition, container emplacement configuration, host rock hydrogeology, and the effects of temperature, pressure and radiolysis. These parameters are either difficult to evaluate or will be extremely site-specific. This analysis therefore assesses a very simple system comprising a cubic metre block of borosilicate glass waste, emplaced without canister or backfill in granite at a depth of about I km. The waste composition used is UX reference borosilicate glass, 'M22' [4], with a thermal output and nuclide inventory appropriate for approximately 120 years storage prior to disposal. The glass is assumed to be fracture
Data Loading...
