Microanalysis of B, C and FE containing melts for nuclear reactor severe accident studies
Titre du congrès :EMAS 2007, 10th European Workshop on Modern Developments and Applications in Microbeam Analysis
Ville du congrès :Antwerp
Date du congrès :06/05/2007
Boron carbide control rods are used in Efench Pressurised Water nuclear Reactors (PWR). During a core meltdown accident, the boron carbide pellets react with the steel cladding, forming a mix that melts from 1200°C onwards and flows towards the bottom of the core where they may be submitted/fb oxidation by the steal atmosphère. Out of pile small scale tests hâve been carried ouf to study the oxidation by steam of B4C-steel liquid mixtures
prepared by melting powder mixes. On Une kinetic oxidation data were obtained by monitoring the gas releases with a mass spectrometer. In support to the oxidation test, detailed EPMA investigations were performed on the solidified mixes before and after the tests. Microstructures
as well as composition were investigated with a CAMECA SX 100.
The quantitative analysis of the light B and C éléments in complex B4C-stainless steel mixtures was approached gradually starting with the simplified Fe-B System. Interférences between the BKa line and the high order Fe Unes were eliminated by using the differential mode acquisition.
A parasitic contribution to the BKa peak, induced by the fluorescence of the synthetic multilayer (M0/B4C) crystal used, was observed. This contribution was estimated by quantitative analysis on a pure Fe standard and further taken into account to correct dosages.
In addition, instead of pure boron, a house-made standard of Fe2B was prepared to improve the quality of boron dosages. An absolute précision of 0.2% wt was achieved on the dosage of boron on unknown borated phases.
Phase area ratio détermination by BSE images analysis was coupled to quantitative analysis data to get the global composition of the samples. For non oxidized samples, a good corrélation was obtained between this microanalysis estimation and the actual composition of samples.
The évolution of this global composition during the oxidation process was studied by performing successive oxidation tests with increasing steam exposure duration. It was revealed a preferential oxidation of boron and chromium.