Stainless steels (SS) exhibit intergranular corrosion in hot, strong, nitric acid (HNO 3 ) solutions. The intergranular corrosion rate is infl uenced by the orientation of the microstructure, the highest rates typically being observed on plate, tube, and forging surfaces that are perpendicular to the hot-working direction. This is known as end-grain corrosion. In addition, pit-like tunnels can penetrate deeply beyond the broad front. In this study, a method is presented to deconvolute the corrosion rate obtained from the weight loss of coupons to investigate the differences in end-grain behavior for two austenitic SS, NAG (nitric acid grade) 18/10L (a Type 304L variant) and Uranus 65 (a Type 310L variant). In a corrosion simulant for dissolver liquor, the broad-front end-grain corrosion rate for the former steel was found to be ~2 mm/y, whereas for the latter it was ~1.1 mm/y. In addition, it is shown that measurement of the size and shape of drilled holes can be used to assess the effect of end grain on gross metal loss. For end-grain pits, neither of the above methods was found to be useful. Instead, sectioning or radiography is required. It was found that in the same test conditions, the maximum observed end-grain pitting rates were 7.0 mm/y for NAG 18/10L and 4.5 mm/y for Uranus 65. Uranus 65, however, showed a much smaller number of end-grain pits compared with NAG 18/10L. The cause of enhanced broad-front intergranular corrosion in the end-grain direction, as opposed to the pitting-like attack, is unknown to us.
In the UK, blended high level nuclear waste (HLW) streams from the Magnox and THORP reprocessing plants are currently vitrified using a lithium sodium borosilicate base glass frit. Laboratory and full size non-radioactive simulations (produced on the Vitrification Test Rig at Sellafield [1]) of these compositions have shown that these glasses need to be melted at circa 1050°C to obtain a reasonable viscosity for pouring. Also, at high waste loadings an alkali molybdate phase (termed “yellow phase”) can form in these glasses [e.g. 2, 3]. Vitrification flowsheets are set to avoid yellow phase formation as this phase is highly corrosive to the inconel melter in the molten state and is partially water soluble at ambient temperature and so may challenge product quality.Ca and Zn additions to the base glass frit have been found to reduce viscosity and allow melt homogeneity and pouring at lower temperatures. It was also theorised that Ca additions could increase the solubility of Mo and thus reduce the likelihood of yellow phase formation. The composition of the phase separated material in as-cast and heat treated specimens of Ca and Zn HLW glasses produced at both laboratory and full scale is examined in this work
Sellafield Ltd operates a Waste Vitrification Plant (WVP) to immobilise the arisings from the reprocessing of spent nuclear fuel. Washout of solids from the base of waste storage tanks in preparation for decommissioning is likely to produce feeds enriched in molybdenum to the WVP. Vitrification of such feeds in the borosilicate glass formulation currently used by the WVP for vitrification of reprocessing waste has been investigated to determine the maximum achievable loading of MoO3.The vitrification of molybdenum in the absence and presence of reprocessing waste was studied. A number of glasses were manufactured in the laboratory containing various waste loadings. The resultant glasses were examined both visually and under the scanning electron microscope for the presence of any phase separation. Additional aluminium was added to the glasses manufactured in the absence of reprocessing waste to improve the durability of the glass. In borosilicate glass containing 3.5 wt% Al2O3 the onset of a molybdenum phase separation was observed in glasses containing 2.6 wt% MoO3. In the presence of Magnox reprocessing waste, phase separation was observed when the product contained >3.8 wt% MoO3. Soxhlet durability testing of a selection of the glasses manufactured was carried out. The Soxhlet durability of glasses in the absence of phase separation was good.
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