For screening purpose,
 - Phast : http://www.essco.co.uk/products/phast.html
 - Shell's FRED
will be fine.

However, for detail modelling following will be best.

CFD(Computational Fluid Dynamics)
 - FLACS (for explosion) : http://gexconus.com/FLACS_CFD
 - KFX(for fire modelling) : http://www.computit.no/en/Product_+_services/KFX/