Abstract: Currently when in-vessel retention (IVR) is evaluated, two-layer melt pool model is usually applied for point estimate analysis. After years of intensive study on IVR, it is found that existing code could not simulate three-layer melt pool model accurately. Thus a modular IVR analysis code using three-layer melt pool model, SPIRE, was developed and validated. The results show that the calculation results of SPIRE code have a good agreement with the data from references. Thus the SPIRE code is feasible to perform IVR analysis. Based on simulation results, the heat fluxes at the bottom of the vessel and light metallic layer notably increase in three-layer melt pool configuration compared with two-layer melt pool configuration. According to sensitive analysis, uranium oxidation rate and total mass of stainless steel have larger influence on heat flux distribution and maximum critical heat flux (CHF) ratio.