Deep borehole disposal (DBD) is being increasingly seen as a viable and potentially superior alternative toudcomparatively shallow mined repository concepts for disposal of some high-level radioactive wastes. Weudreport here details of proof-of-concept investigations into the use of cementitious grouts as sealing/udsupport matrices for use in low temperature DBD scenarios. Using the cementitious grout to fill annularudspace within the disposal zone will not only support waste packages during placement, but will alsoudprovide a low permeability layer around them which will ultimately enhance the safety case for DBD.udGrouts based on Class G oil well cement are being developed. The use of retarders to delay the acceleratedudonset of thickening and setting (caused by the high temperature and pressure in the borehole) isudbeing investigated experimentally. Sodium gluconate and a polycarboxylate additive each provide suf-udficient retardation over the range 90e140 °C in order to be considered for this application. Phosphonateudand sulphonate additives provide desirable retardation at 90 °C. The additives did not affect groutudcomposition at 14 days curing and the phases formed are durable at elevated temperature and pressure.
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