Partial melting and recrystallization of granite and their application to deep disposal of radioactive waste: Part 1—Rationale and partial melting

A high temperature, deep borehole disposal scheme for nuclear waste is outlined. This requires that granite host rock can be partially melted by the heat from the waste then completely recrystallized, both on the time scale of the cooling of the waste. Experimental studies of non-equilibrium partial melting of a typical S-type crustal granite were carried out under the conditions likely to arise during implementation of this scheme (P=0.15 GPa, T=700–850 °C). The melting relations were determined under H2O-saturated and H2O-undersaturated conditions. Melting begins just under 700 °C and the amount of melt increases with temperature and especially H2O content (up to saturation) such that with no added H2O only 5% of melt is produced even at 850 °C but the addition of 1% H2O will generate 40% of melt 50 °C lower at 800 °C. The partial melts formed at high degrees of melting (e.g. H2O-saturated; T>800 °C) approximate to equilibrium melting but the smaller amounts of melt produced at lower temperatures under H2O-undersaturated conditions clearly do not. On the basis of these experiments, it is concluded, from the point of view of partial melting, that an S-type granite similar to the one studied would be a suitable host rock for the proposed high temperature, very deep disposal scheme for radioactive waste.