The simulation system named SPECTRA for a severe accident in sodium-cooled fast reactors has been developed. The SPECTRA computes in- and ex-vessel phenomena and evaluates various scenarios during the severe accident. This paper provides a newly developed computational models for the ex-vessel phenomena including gas and aerosol transport, sodium-concrete interaction, and sodium fire as a part of the SPECTRA. The base module computing thermal hydraulics behavior by a lumped mass model was verified through the analysis of a 2-cells ventilation problem. The computational result of the SPECTRA agreed with the theoretical solutions both in the case with and without temperature change. The sodium-concrete interaction model was verified through code to code comparison. The computational result showed that ablation of a concrete surface started after surface temperature reached to a certain value. The computed ablation depth almost completely agreed with the result by the CONTAIN-LMR code. The ex-vessel module was applied to the computation assuming sodium leak from a reactor vessel and a primary cooling loop. This computation demonstrated increase of temperature and pressure due to sodium-concrete interaction and sodium fire.