Release Behaviour of Sr-90 From Hydraulically Retrieved ILW Sludge
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Release Behaviour of Sr-90 From Hydraulically Retrieved ILW Sludge J J Hastings Nexia Solutions Ltd, Sellafield, Seascale, Cumbria, CA20 1PG, UK. ABSTRACT Globally, the nuclear industry has a large number of legacy wastes that are stored in ponds, silos and tanks that are nearing the end of their design lifetime and hence said wastes need processing. In the UK there are significant quantities of radioactive sludge that have arisen from the corrosion of early Magnox fuel cans which have been stored underwater. As part of the present aggressive clean-up programme these materials will be retrieved, separated, processed and immobilised as dry waste forms for long-term storage. It is envisaged that hydraulic retrieval will be used for these ILW sludges resulting in some activity being released from the sludge phase to the process liquors challenging downstream ion exchange effluent treatment plants. In order to understand this challenge, experiments have been conducted on sludge in ILW storage ponds and during sludge transfer operations to study the activity released from said sludges. In particular the solubility, adsorption behaviour of Sr-90 is discussed and how this and other aspects of the sludge chemistry impact upon the ion exchange effluent treatment process. The novel methodologies employed to obtain this data is also discussed. INTRODUCTION There are a variety of legacy sludge/slurry wastes that currently reside in ponds, silos and tanks on nuclear sites across the world. At Sellafield they largely arise from Britain’s early Magnox and AGR nuclear generation programmes. Fuel from the reactors was shipped in waterfilled transport flasks and stored in ponds at Sellafield. Some of the fuel remained in the ponds for many years and the Magnox cans corroded to form significant quantities of sludge now classified as Intermediate Level Waste. These pond facilities are now nearing the end of their service life and thus there is requirement to retrieve the wastes and decommission them. Figure 1 illustrates the stages of fuel can corrosion and Figure 2 shows a typical pond facility. Corrosion of the Magnox cans produces a sludge that is predominately Magnesium hydroxide or Brucite Mg(OH)2 but also contains other magnesium compounds, such as Artenites (Mg2CO3(OH)2.3H2O) [1]. The sludge is formed as the oxidation layer on the Magnox metal surface spalls off into the surrounding liquor. Where sludge has been left undisturbed for prolonged periods of time there is a build up of activity within the sludge pore liquor. When the sludge is disturbed this pore liquor and its associated activity is released into the surrounding liquor and there are significant uncertainties in quantifying the rate and amounts of activity that will be released into the process liquor from the sludges as they are disturbed during retrieval operations. It is important that this activity release behaviour is understood so that the challenge to downstream effluent treatment plants can be estimated and the appropriate activity abatement technology can be
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