On the trail of radionuclides using a NASA probe

Research in the Mont Terri Rock Laboratory on safe disposal of radioactive waste. Nagra has been carrying out diffusion experiments for 20 years in order to better understand the migration of radionuclides in the Opalinus Clay. A new aspect of the measurements is the use of an X-ray fluorescence probe based on the design of a probe developed for NASA’s Mars subsurface exploration program.

“This type of measurement probe has been developed for investigations on Mars”, explains Veerle Cloet, chemist and project manager of the long-term diffusion experiment led by Nagra (National Cooperative for the Disposal of Radioactive Waste). The probe is being used in the Mont Terri Rock Laboratory, which is operated by swisstopo. The experiment is unusual in many respects. Besides the high-tech probe, transparent carbon fibres developed specially for Nagra are also being used to stabilize the borehole so that it will remain stable for at least 10 years. The X-ray transparency of the carbon fibres means that they do not hinder the measurement of the diffusing ions in the Opalinus Clay.

In the diffusion experiment, Nagra is reviewing existing understanding on the migration of radionuclides in the Opalinus Clay. According to the multiple barrier concept, following corrosion of the disposal container (after at least 10,000 years) the radionuclides will reach the tunnel backfill (bentonite). Radionuclides that are not retained by the bentonite will migrate into the Opalinus Clay. The geological barrier provided by the clay is the most important safety barrier. Its hydraulic conductivity is around 10-13 m/s, which means that there is no flowing water in the formation. The movement of radionuclides is by so-called diffusion processes. “Because of diffusion, the radionuclides move very slowly in the Opalinus Clay”, says Cloet. While positively charged ions (cations) are able to “dock” on the negatively charged surfaces of the clay minerals in the rock, negatively charged ions (anions) are repelled by the clay mineral surfaces. This repulsion means that anions move more quickly through the rock than cations. “This experiment allows us to determine the diffusion rate of the anions”, adds Cloet.

The setup of the diffusion experiment consists of four boreholes: one central borehole (diameter 600 mm) into which a sodium iodide solution is injected and three monitoring boreholes (diameter 250 mm) in which measurements are carried out. All extend to a depth of between 8 and 10 metres into the underground of the rock laboratory. One monitoring borehole is at a distance of 0.825 m from the central borehole and the others are at a distance of 1.2 m. The experiment measures how long it takes for the iodine anions to reach the monitoring boreholes. Measurements with the X-ray fluorescence probe are carried out every four to six months to check whether iodide can already be detected in the boreholes. The evolution of the iodide concentration in the monitoring boreholes will be measured over a period of 10 years.

Stable iodine is used for the experiment rather than radioactive iodine. The two isotopes have the same chemical properties and the same transport behaviour. Radioactive iodine (I-129) is produced in nuclear power plants during nuclear fission and is contained in the radioactive waste. Besides radioactive selenium, chlorine and carbon, iodine is very important for the long-term safety of a deep geological repository because of its long half-life.

“Model calculations show that the anions will start to arrive in the first monitoring borehole in 2.7 years. Iodide takes this long to cover a distance of 0.825 m”, explains Cloet. The iodide will be detectable in the boreholes at a distance of 1.2 m after 5 years. The diffusion rate decreases continuously, i.e. the anions become slower with time. The average transport time for iodide through 45 m of Opalinus Clay is around 200 000 years.

According to Swiss nuclear energy legislation, the producers of radioactive waste are responsible for its safe management and disposal. In 1972, the nuclear power plant operators and the Federal Government set up the National Cooperative for the Disposal of Radioactive Waste (Nagra) to perform this task. Nagra, with headquarters in Wettingen (AG), is the national technical competence centre in the field of deep geological disposal of radioactive waste.

Out of a strong sense of responsibility for the long-term protection of man and the environment, 110 employees are involved daily in performing this important work. The high level of competence is secured by targeted research programmes in two Swiss underground rock laboratories and intensive international collaboration.