Abstract
Deep borehole disposal (DBD) is being increasingly seen as a viable and potentially superior alternative to comparatively shallow mined repository concepts for disposal of some high-level radioactive wastes. We report here details of proof-of-concept investigations into the use of cementitious grouts as sealing/support matrices for use in low temperature DBD scenarios. Using the cementitious grout to fill annular space within the disposal zone will not only support waste packages during placement, but will also provide a low permeability layer around them which will ultimately enhance the safety case for DBD. Grouts based on Class G oil well cement are being developed. The use of retarders to delay the accelerated onset of thickening and setting (caused by the high temperature and pressure in the borehole) is being investigated experimentally. Sodium gluconate and a polycarboxylate additive each provide sufficient retardation over the range 90–140 °C in order to be considered for this application. Phosphonate and sulphonate additives provide desirable retardation at 90 °C. The additives did not affect grout composition at 14 days curing and the phases formed are durable at elevated temperature and pressure.
Highlights
Advantages associated with safety, cost, and ease of implementation, and the ability to drill deeper larger diameter holes (Juhlin and Sandstedt, 1989; Beswick, 2008; Beswick and Forrest, 1982; Exxon Neftegas; Sakhalin, 2013), means that the use of deep boreholes to dispose of high level radioactive wastes (HLW, including spent nuclear fuel (SF)) is being increasingly seen as a viable alternative to emplacement in geologically shallow, mined repositories (Chapman and Gibb, 2003; Beswick et al, 2014)
The products assessed that were marketed as retarders were sodium gluconate and Sika Retarder, whereas those marketed as superplasticisers were Viscocrete 3110 and CD-33L
The times at which changes in consistency occurred were recorded; t1 was the time at which minimum consistency occurred and t2 was the time for consistency to reach 70 Bearden units (Bc), the limit of pumpability (LoP) accepted in well cementing applications (Nelson and Guillot, 2006)
Summary
Advantages associated with safety, cost, and ease of implementation, and the ability to drill deeper larger diameter holes (Juhlin and Sandstedt, 1989; Beswick, 2008; Beswick and Forrest, 1982; Exxon Neftegas; Sakhalin-, 2013), means that the use of deep boreholes to dispose of high level radioactive wastes (HLW, including spent nuclear fuel (SF)) is being increasingly seen as a viable alternative to emplacement in geologically shallow, mined repositories (Chapman and Gibb, 2003; Beswick et al, 2014). The development of an alternative more advantageous concept for the disposal of HLW is of particular interest to those involved in the nuclear fuel cycle. Packages of radioactive waste are emplaced into the bottom 1e2 km of the borehole (the disposal zone) within which they are sealed using materials known as sealing and support matrices (SSMs). These SSMs fill the annular space between the waste packages and the casing, and between the casing and the borehole wall. The borehole itself is permanently sealed above the disposal zone to the surface
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