Abstract: Under total instantaneous blockage of single subassembly accidental condition in the fast re-actors,the sodium will be boiling soon,which results in the relocation of material and pressure,and has great effect on the development of the accident.To present an integral analysis for this accident,the mechanism of sodium boiling has to be understood firstly.The two-fluid and six-equation model is adopted,and the sub-channel approach was used to discretize the computational volume,at last the model is solved by the semi-implicit numerical method developed by D.R.Liles et al.After validated on the BE+1 experiment,the thermal-hydraulic behavior of sodium under TIB accident is interpreted in detail.