Cutting edge

Can nuclear waste be disposed of safely? A group from Sheffield is looking at the alternatives.

Over the past two decades, natural disasters, such as the recent storms in France and floods in Venezuela, have heightened awareness of the consequences of climatic changes.

There is increasing evidence that these changes can be attributed mainly to burning fossil fuels, so an alternative source of energy is needed. The only realistic option is nuclear power. But this otherwise clean and plentiful energy comes at a price - the generation of radioactive waste. It would be irresponsible to seek realisation of its potential without resolving the problem of waste. This is especially true for the various forms of high-level waste (HLW), such as spent nuclear fuel, reprocessing waste and ex-military fissile materials, for instance plutonium. Whether the generation of nuclear electricity increases, decreases or ceases tomorrow affects only the scale of the problem. There are already substantial inventories of HLW worldwide. The same applies to the continuation of spent fuel reprocessing. HLW inventories can only increase as existing stockpiles of nuclear weapons are decommissioned.

Realistically, the only option for HLW is some form of geological disposal, but the nuclear industry has no agreed solution. Most countries favour long-term storage and/or disposal in "deep" (but geologically shallow) mined and engineered repositories. This concept is not without problems - especially groundwater flow and access.

Since 1993, a small group of us in environmental and geological sciences at Sheffield University have been researching a scheme for high-temperature, very deep, geological disposal of HLW. Potentially, it is safe, economically realistic and environmentally sound. Boreholes are drilled into granite to depths greater than 4km. Special cylindrical containers filled with HLW are placed in the lower section of the hole, which is backfilled with granite and sealed. The HLW packages deliver the energy needed to produce maximum temperatures of about 900C at the container/rock interface. During this heating, which occurs over months, a zone of partial melting migrates into the host rock. As the thermal output of the HLW drops, zones of metamorphosed rock are left outside the zone of melting. The silicate melt recrystallises over a few years to seal the container and its contents into a sarcophagus of solid granite. Essential to the scheme is that melted granite can be recrystallised at a cooling rate faster than that of decaying HLW. Because natural granites take thousands of years to crystallise, this was initially in doubt. However, our experiments (supported by BNFL) have demonstrated that granite melts can be recrystallised within the time and temperature envelope provided by a container of cooling HLW.

Three main features of the scheme encourage confidence in its safety: n The sarcophagus of granite is in equilibrium with its host and effectively immune to corrosion over the time - more than 100,000 years - required to render HLW harmless

* The depth creates a geological barrier through which any escaping radionuclides would have to migrate to reach the biosphere

* Any fluids in the host rocks at such depths would be dense brines that should remain isolated from near-surface groundwater systems. Even if HLW were to be leached out it would only enter this isolated system of brine-filled fractures. Such containment could in principle survive tectonic disturbances such as earthquakes. Putting the waste back into the earth's crust is the next best thing to never having dug it up.

Details of the scheme have been published in the Journal of the Geological Society, 157.

Fergus Gibb, department of environmental and geological sciences, University of Sheffield.