There is an urgent need for nations such as the UK to deal with the legacy waste that has arisen from decades of nuclear power production, particularly the higher activity wastes (HAW). The reference strategy employed by the Nuclear Decommissioning Authority (NDA) is to achieve passive safety as soon as reasonably possible. Passive safety is mainly achieved by immobilisation through the process of vitrification (high level reprocessing wastes) or encapsulation in cement grout (intermediate level waste). In the UK (and France), spent fuel (SF) is not formally classified as waste. However, the current NDA position is that any un-reprocessed AGR SF together with new build SF will eventually go to the GDF. The UK has the world’s largest stockpile of civil separated plutonium (classed as a zero value asset). Current UK government policy involves re-use of a large quantity of this Pu as MOX fuel, leaving the question of how to dispose of spent MOX unresolved.
The preferred long-term option in most countries for dealing with this waste entails some form of geological disposal in a mined, engineered repository a few hundred metres deep - often referred to as a geological disposal facility (GDF). In the UK, the NDA plans to construct a single GDF (once a geologically and politically acceptable site has been found) and then to emplace suitably treated high level waste (HLW) and spent fuel (SF) co-located with a much greater volume of intermediate level waste (ILW). The NDA’s current estimate is that the earliest this GDF would take the first containers of HLW is not until at least 2075. An alternative approach that could lead to earlier implementation would be a positive benefit.
Deep Borehole Disposal (DBD) is just such an alternative disposal route for managing high‑activity, moderate‑volume components of radioactive waste inventories. It entails emplacement of waste packages in the lower portions of vertical holes (< 0.66 m diameter) drilled 4 to 5 km into crystalline basement rock. The combination of the large disposal depth, groundwater salinity gradient and density stratification and the low lateral hydraulic conductivity of the host rock makes DBD a much safer option compared with disposal in a geologically shallow, mined, engineered repository for a large range of high level wastes (HLWs) and spent fuel. DBD has further advantages: the cost is much lower, the environmental impact is greatly reduced and the time taken to drill, fill and seal a deep borehole could be as little as 2 years once all necessary licensing applications have been approved.
In this presentation we explain the basic concept of DBD and discuss the various adaptations for dealing with different wasteforms, including the sealing systems. We present results of numerical modelling work showing the thermal envelopes in various DBD disposal scenarios as well as a model for predicting the time taken to re-establish the groundwater salinity gradient perturbed by the construction of the borehole.
