Sustainable waste

What to do with the radioactive waste produced by nuclear power? This is a major issue, given that some of it will continue to emit radioactivity for hundreds of thousands of years. The solution? Isolate it and store it to avoid exposure. Xavier Deschanels, a researcher at the Institute for Separative Chemistry at Marcoule*, explains how these special materials are handled.

Although nuclear power accounts for just 10% of electricity production worldwide, it is by far the most important source of electricity produced in France, with 70% of the country's electricity coming from the 18 nuclear power plants spread across the country.

A production method that emits very few greenhouse gases, but leaves a cumbersome residue: radioactive waste. How can we safely manage this particular type of waste? " It all depends on the type of waste we're talking about, because there are actually different categories of radioactive waste," explains Xavier Deschanels, a researcher at the Institut de Chimie Séparative de Marcoule (ICSM). They are classified according to two parameters: the intensity of their radioactivity and the lifetime of the radionuclides, which determines how long they will remain radioactive. Whatever the type of waste, the principle is the same: isolate it as far as possible from the environment to keep exposure to radioactivity to a minimum. But isolation methods differ according to the type of waste.

Confined packages

"The vast majority of nuclear waste is short-lived. It represents over 90% of the total volume of radioactive waste in France, but only 0.03% of total radioactivity", stresses the researcher. The chosen solution for this waste is surface disposal. "It's a highly organized process, far from a simple landfill. The waste is first conditioned in the form of packages, i.e. enclosed in a steel or concrete container after having undergone various treatments such as incineration and compaction", explains the researcher.

These unique packages are then confined to the earth's surface in reinforced concrete storage structures. " These structures are then sealed with a final cover made of several layers of natural materials such as clay, to protect them from the elements and guarantee the long-term safety of the repository. They are then monitored for 300 years, after which time they will no longer contain any radioactivity," explains Xavier Deschanels.

Deep burial

300 years is a fraction of a second compared with the thousands of years during which high-level waste will remain radioactive. While its activity represents over 95% of the total activity of nuclear waste, its total volume represents only around 0.2% of the total volume of this waste. " That's the equivalent of an Olympic-sized swimming pool since the beginning of nuclear power in France," says the researcher.

And for this category of waste, considered the most hazardous, surface storage is out of the question. The solution? Deep underground burial. This waste is not buried as such, but goes through complex technological processes. "Initially, it is in the form of a highly radioactive liquid, which is evaporated to obtain what we call calcinate, a real concentrate of radioactivity which is then mixed with a glass powder to stabilize the inorganic materials".

The end result is glass packages weighing around 400 kilos each, with such a high level of radioactivity that their temperature is close to 350°C, a heat that will take around sixty years to diminish through radioactive decay. " For the time being, they are kept in permanently cooled pools, then stored in concrete shafts in the La Hague processing plant, pending the forthcoming opening of the Cigeo industrial geological disposal center at Bure, where they will be buried at a depth of 500 meters on a geologically selected site to ensure that there is no release of radioactivity to the surface," explains Xavier Deschanels.

Transmutation

What other approaches are being considered to deal with nuclear waste? " At ISCM, we're working to develop solutions such as transmutation, which involves extracting certain radionuclides known as minor actinides and then transforming them, through a series of nuclear reactions, into fission products similar to those produced by uranium fission, but with lower radiotoxicity", adds the researcher. A process that would require new nuclear facilities, which would in turn generate waste that would also have to be stored deep underground.

Another avenue being explored by the researchers is "the use of mesoporous materials which have the particular ability to encapsulate radioelements. In practice, the radioactive liquid passes through a selective membrane that retains the radionuclides, and the radioactivity then collapses the porosity of the material, trapping them. For the time being, these innovations will not replace the current principle: "Isolating and storing nuclear waste remains the universal solution", concludes Xavier Deschanels.

*ICSM (CNRS, CEA, UM, ENSCM)