Télécharger la vidéo (258.52 MB - mp4) (258.52 MB)Peak activity on the MARS beamline
[SOLEIL]
Bruno Sitaud, Manager of the MARS beamline at the SOLEIL synchrotron : We are about to enter the restricted area, so we will all wear an operational dosimeter....
Bruno Sitaud is coming to make sure everything is ready for the arrival of radioactive specimens. The ASN (French Nuclear Safety Authority) has just granted him special approval for this type of study.
Bruno Sitaud : This is a very recent development; it's only since last week that we have been authorised to analyse samples of higher level of radioactivity than we've been studying up to now. For three years, this beamline has been accommodating users with very slightly radioactive samples, who are mainly looking at environmental issues. But now, with our new approvals, we can turn towards nuclear fuel, such as the fuel used in nuclear power plants.
[CEA]
The long-awaited specimens are currently at CEA. They are thin , irradiated steel discs that have remained in the core of nuclear research reactors for several years. These steels, which could be used in the construction of future power plants, will be examined by SOLEIL synchrotron scientists and CEA scientists.
Jean-Luc Béchade, head of the Microstructural Materials Analysis Laboratory (LA2M) at CEA : The samples are made of steel reinforced with nanometric particles rich in oxygen, yttrium, and titanium, which give the materials excellent temperature properties, particularly under irradiation.
What we want to find out, particularly after irradiation at high doses, is: do these nanometric particles remain nanometric, does their composition change, or do they dissolve in the material, in the matrix.
The aim is therefore to study how these nanometric particles change under irradiation and to check in particular that they remain stable over time. Because if they were to disappear, the steel would lose its mechanical properties and could no longer withstand the high temperatures, around 700°C, that will be found in the nuclear power plants of the future.
Jean-Luc Béchade : These tiny particles, just a few nanometres across, can be seen with this transmission electron microscope. We can measure their size and density and even find out their chemical composition. What we cannot do, on the other hand, is find out about their crystallographic structure. To do that, we need to use synchrotron radiation on the MARS beamline.
The small steel samples are placed in shielded steel and lead shells that block radiation, and then sent safely to the SOLEIL synchrotron.
[SOLEIL]
After the radiological tests required for admission to the site, the specimens are sent to the beamline according to a very precise protocol.
Bruno Sitaud : The MARS beamline has been dedicated to the study of radioactive materials since its design. There are fewer than ten beamlines like this one all over the world.
We have completely airtight hutches in which negative pressure can be obtained, providing a confinement inside the beamline. We also have as a new feature reinforced biological shielding. This shielding is obtained from several cm of lead recently installed behind this wall.
One of the thin steel discs is installed on the beamline. Intense x-rays will be able to pass through it after its removal from its protective shell. One of the special features of the MARS beamline is that it can perform two types of measurement: diffraction measurements and x-ray absorption measurements.
In this case, the synchrotron radiation will concentrate on the nanometric particles dispersed in the steel, and more specifically on the organisation of the oxygen, yttrium, and titanium atoms in these small precipitates.
Jean-Luc Béchade : This results from the diffraction of our sample.
Our precipitates have a signature.
Each of these circles corresponds to the signature of these crystallographic structures.
And what we’ll look for is this: after irradiation, do we still see them as well as before irradiation, and don’t we have some rings that will move. If they move, it means that the structure has been altered or modified by the irradiation.
Denis Menut, Engineer in the LA2M at CEA
For these measurements, we looked at yttrium. We will look only at the first and second atoms that are neighbours in the material.
According to our initial observations, the irradiation seems to have an effect on the material, without dissolving the nanometric reinforcements. It will take several months to analyse and interpret these data, but the stability of the microstructure already seems to have been achieved, which bodes well for survival in the nuclear power plants of the future.
Bruno Sitaud : We are looking towards characterising the materials of the future. But we are already working on characterising present-day materials, meaning the nuclear fuel already used in nuclear power plants.
Everything is new; everything remains to be discovered. That is also the challenge facing us in the coming years.