Analytical characterization of nuclear accident corium materials
Résumé
As part of the studies concerning possible serious accidents in a pressurized water reactor, corium coming from the meltdown of reactor core material (i.e. UO2–based fuel, zirconium-based fuel cladding, and elements from the steel-based structure) was studied in order to determine the phase diagrams of the materials and to predict the material’s behavior at high temperature. Corium formation can give rise to a gravity segregation of two liquid phases, molten oxides and metals. Should the reactor vessel be breached with a resulting spread of the core content over the concrete foundation basemat, these phases can also react with the elements within the concrete. Data on the local and overall chemical composition of the phases provided by post-accident analyses are very important in the determination of the physico-chemical properties of corium, as are the phase diagrams of systems representative of these complex materials. The objective of this paper is to present different analytical approaches to the analysis of corium trialed with simulants based on natural uranium, to prepare the way for an application on real corium samples.
The first part of this work describes the WDS EPMA methodology employed in order to obtain highly targeted characterizations of sample phases with chemical compositions which are either from a U-Zr-Fe-O system fabricated by arc melting, or from a U-Zr-O system prepared in a Joule furnace. There are numerous difficulties in EPMA analysis of these types of materials, due to:
- the heterogeneity of the samples bearing phases of micrometric dimensions,
- native oxidation at ambient temperature after the metallographic preparation,
- the measurement of oxygen in the presence of heavy elements, particularly uranium in this case, which requires an optimization of the analysis conditions with fine-tuning of the electron beam and of the spectral analysis of the elements.
A second section of this work describes a semi quantitative assessment of the overall chemical composition, which was carried out by a multi-analytical approach on a simulant sample. Analysis techniques used included SEM/EDS, XRF WDS, and laser ablation coupled with inductively coupled plasma mass spectrometry. The XRF WDS measurement was carried out on a few cm2 of Corium after pre-cutting to ensure a flat surface for the X beam exposure. The analytical approach was carried out by semi quantitative analysis. The ICP/MS/LA enabled the 238U / 91Zr ratio (the main components of the sample investigated) to be recorded over a length of about fifty µm. It was thus possible to compare the elemental content data found by the 3 different analysis techniques.