Radioactive waste streams that include metallic uranium are incompatible with conventional ordinary Portland cement (OPC)-based encapsulation matrices. These encapsulation systems are essentially composite materials that incorporate high replacement levels of pulverised fly ash (PFA) or blast furnace slag (BFS). A potential alternative encapsulant for the treatment of problematic waste streams is magnesium phosphate cement. This paper discusses the fundamental characterisation results obtained from two magnesium phosphate cement formulations being developed in the UK for the encapsulation of metallic intermediate level waste (ILW). When compared to conventional OPC based systems, the two magnesium phosphate cement formulations investigated have lower pH, are able to chemically combine more mix water into the system and provide sufficient workability at water/solid ratios close to the theoretical confines needed for paste saturation. The results presented have confirmed compliance of this material against NDA RWMD guidelines for strength and expansion. The X-ray diffraction (XRD) and differential scanning calorimetry (DSC) results obtained for both formulations up to 360 days cure time have indicated that the cement system shows evidence of chemical stability.
Encapsulation in cement is the favoured method in the UK for disposal of intermediate and low level radioactive wastes. It is usual to use composite cement systems incorporating blast furnace slag (BFS) or pulverised fuel ash (PFA) as these offer several advantages over Portland cement, notably a lower heat of hydration. The use of these mineral additions utilises a waste product which would itself need a disposal route and, because of the decreased amount of Portland cement used, provides a reduction in cost and energy consumption. Cementitious systems have many attributes which make them suitable for encapsulation and immobilisation, including: • Inexpensive and readily available; • Assist immobilisation of radionuclides by: a) acting as a diffusion barrier, b) providing sorption and reaction sites, c) maintaining a high pH which in turn decreases radionuclide solubility; • Provide radiation shielding which is not degraded by the radiation; • Controllable permeation and diffusion characteristics over a wide range via selection of constituents and components. Where physical adsorption is a significant factor for immobilisation, the calcium silicate hydrate gel (C-S-H) formed on hydration of a Portland cement is advantageous as it has a high surface area and large micropore volume. Composite cements based on blast furnace slag will produce a higher proportion of C-S-H than ordinary Portland cement increasing the sorption capacity, and reducing the capillary porosity so that the diffusion resistance is increased. Intermediate level waste covers a wide range of materials, for example, metals and ion exchangers, each with differing chemical properties. It is, therefore, necessary to access the ability of the cementitious system to immobilise different wastes and to characterise the products formed. It is also necessary that alternative encapsulant materials be considered for immobilising wastes not suited to the composite cements already being used. The techniques employed to do this include x-ray diffraction (XRD), to identify standard and non-standard hydration products, isothermal conduction calorimetry (ICC) and scanning electron microscopy (SEM).
Legacy radioactive wastes arising from reprocessing of nuclear fuels in the UK are classified as intermediate level waste (ILW), which contain things such as aluminium and magnesium. Blast furnace slag (BFS) composite cements are used to encapsulate ILW. These cements have a high pH which is advantageous to limit the mobility of some of the radioactive species but can cause corrosion of metals. The present paper describes some fundamental aspects of corrosion of aluminium and magnesium in BFS composite cements.The corrosion of aluminium produced an interface between aluminium and cement which was porous with a series of zones containing bayerite (Al(OH) 3 ) and strä tlingite (2CaO.Al 2 O 3 .SiO 2 .8H 2 O). With magnesium, the main corrosion product was found to be brucite (Mg(OH) 2 ) and the porous zone was less pronounced. The hydration of the bulk cement did not appear to be affected by the corrosion of these metals.
6Release of gases from intermediate level radioactive wastes encapsulated in cement-based wasteform grouts is a 7 major concern during the interim storage, transportation and operational stage of a phased geological repository 8 system. The ability of the wasteform grout to retain radioactive gases depends on its mass transport properties, 9 which evolve with age, water saturation degree and any degradation. This paper reports the gas diffusion, gas 10 permeation and water absorption (sorptivity) coefficients of two mature wasteform grouts containing 75% pul-11 verised fuel ash or 75% blast furnace slag. Some of the slag grouts were found to be affected by microcracking, 12 and these were subsequently examined using backscattered electron microscopy and quantified using a specifi-13 cally developed image analysis procedure. Prior to transport testing, samples were conditioned in a realistic 14 manner to a range of water saturation levels (10-100%). The mass transport properties were found to be highly 15 dependent on the degree of water saturation, in particular for samples conditioned at relative humidity greater 16 than 55%. The microcracks have a significant effect on permeability, but less influence on diffusion and water 17 absorption. Implications of the results on the effectiveness of these grouts for radioactive waste containment are 18 presented.
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