Slow disappearances

Trapping radioactivity until it disappears

A scenario for the long-term future of the waste

It will take many years for the radioactivity of waste destined for burial at great depths to become harmless. But will the barriers erected by humans to prevent the residual radioactivity from reaching the environment last for this long? Between the radioactive decay and the degradation of these barriers, which will win?

The focus of laboratory research into disposal, such as that carried out in Bure, France, on the boundary between the Meuse and Haute-Marne départements, is to understand the long-term impact of waste buried at great depths. At the outset, the obstacles put in place to protect the environment are the glass or ceramic package incorporating the radioactive atoms, the containers and the structure holding these packages, and finally the host rock containing this structure.

These barriers are designed to last much longer than us, but however strong they are, they will deteriorate. There will therefore be a race to see which is slowest: this deterioration or the reduction in radioactivity over time.

In the first 200 years, the activity of the fission products, which will initially be dominant, will diminish by a factor of 500. During this phase – long by human standards – when the activity is at its peak, all three barriers need to be properly sealed. Much later, when the activity has decayed more – at the end of 5000 years – the impermeability of the host rock will be relied upon to contain the last few radioactive atoms.

One scenario for vitrified highly radioactive waste stored in an underground site consisting of argillaceous barriers could be as follows: in the beginning, in the first few hundred years, the high activity packages will heat the surrounding rock until it reaches a temperature of around a hundred degrees. Barriers designed for these conditions will withstand this.

The rock will then slowly cool at the rate of the radioactive decay of the waste. The metal containers will corrode and the underground structures will settle, potentially opening up cracks in the nearby rock.

Much later on (at least ten thousand years later), the radioactive atoms still contained in the glass or the uranium oxide will start to dissolve in any water present, but by this time the radioactivity will have vastly decreased. This dissolution in groundwater will cause the remaining radioelements to disperse, leading to their slow migration into the subsoil. The groundwater will percolate through the pores and cracks in the host rock before reaching a groundwater reservoir and finally ending up at a river.

Experts estimate that after ten thousand years spent in underground rock impregnated with water, one thousandth of the total mass of the glass will have dissolved. The residue of radioactive material released will have to work its way up to the surface. This will be difficult to achieve through geological formations chosen for their impermeability to water.

This plethora of precautions might seem excessive, but very safe containment of radioactive material is necessary if nuclear energy is to be developed.