Development and validation of key aerosol models for source term predictions in sodium-cooled fast reactors during beyond design basis accidents

Resumen

During Sodium-cooled Fast Reactors (SFRs) severe accidents, it is foreseen that material in the form of liquid sodium mixed with fuel and fission products would be ejected into the containment. In the presence of oxygen, combustion of sodium (Na) results in the conversion of a large fraction of the burnt Na into Na-oxide aerosols that would govern the suspended radioactivity inside the containment; this together with the potential harm associated with the chemical species resulting from the Na-oxides reaction with water vapour present in the atmosphere would be responsible to a great extent for the radiological and chemical impact of any potential source term. In this sense, the characterization and behaviour of Na-based aerosols generated during in-containment Na-fires is of fundamental importance for the assessment of the radiological consequences in SFR severe accidents. The work in this thesis presents a step forward in modelling in-containment source term during potential severe accidents in Na-cooled reactors. A phenomenological model for sodium-oxide particle generation during sodium pool-fires has been developed (PG model). The model covers sodium-vapour evaporation from a sodium pool and formation of sodium-oxide aerosols and calculates the characteristics (number and size) of the particulate source term to the containment. It consists of a suite of individual models for Na vaporization (diffusion layer approach), O2 transport by air natural circulation (3D flow pattern modelling to capture the associated turbulence foreseen right above the reaction region), Na-O2 chemical reactions (instantaneous reactions and energy of reaction) and vapour-to-particle conversion of Na-oxides (i.e., nucleation and/or condensation). A partial validation with available experimental data showed a consistent model response in terms of burning rates. As using 3D computational fluid dynamics in analysis of Beyond Design Basis Accidents at present is unsuitable (lack of validated SFRs severe accident tools) and impractical (expensive computer resources), a zero-D (lumped) approach has been developed. Subsequently the model has been adapted to be implemented in the severe-accident computer code ASTEC-Na CPA by transposing the PG formulation into a form with those specific variables included in the code. The performance of the ASTEC-Na CPA with the proposed correlations implemented has been tested against some of the more sound available data in the open literature. In conclusion, the new correlations derived from the PG model are very suitable for use in a severe-accident code in terms of the negligible additional computational burden. The new correlations, by originating from simplifications of soundly-based physical modelling, avoid the arbitrary assumption of a fixed primary-particle size in the existing modelling. Limited comparisons with experiments imply that use of the new correlations increases confidence in prediction of the pool-fire particulate source term to the containment. The work performed in this thesis is framed in the CIEMAT contribution to the JASMIN project from the 7th Framework Programme of the European Commission (contract number 295803).