Experimental and theoretical studies have shown the potential of structural material aerosols to react with fission products, affecting their speciation and deposition, and thus the source term associated with a severe reactor accident. Since a PWR contains up to 2 tons of Silver-Indium-Cadmium (SIC) alloy (a neutron absorber in control rods), it is very important to understand and model the behaviour of this material (particularly silver, which is likely to react with iodine, a major contributor of the source term).
For several years, experiments have been performed in different countries to study the degradation of SIC control rods under severe accident conditions, and the release of gaseous species and aerosols during and after this degradation. Data from these dedicated experiments and from general thermodynamic databases have been used to develop models for control rod degradation and SIC release.
Comparisons between models and experimental data have shown some discrepancies. Uncertainties arise particularly concerning the interactions amongst the relocation of molten materials (disposition of the absorber material in-core) and the chemical and thermal conditions of the absorber and surrounding atmosphere.
Cooperative efforts are being made to understand the phenomena better by comparing the experimental results and models used in different computer codes, and hence to progress towards better models of the relevant processes. The present study aims to be a significant step along this way.
(1) : EDF
(2) : GRS
(3) : IRSN
(4) : PSI