Experimental observation has shown that the assumption of complete fuel fragmentation in a vapor explosion by the shock adiabatic thermodynamic model results in predicting upper bounds for the shock pressure, propagation velocity, and work output. We have modified this model by considering the condition where the assumption of complete fragmentation of the fuel is relaxed. We have adopted a methodology using experimental values of the shock pressure and propagation velocity to estimate the initial mixture conditions of the experiment and the mass fraction of the materials participating in the explosion. Analysis of a steady state subcritical vapor explosion in one dimension has been carried out by applying the conservation laws of mass, momentum and energy and the appropriate equation of state for a homogeneous mixture of molten tin and water. The KROTOS-21 experiment, conducted at the Joint Research Center at Ispra, Italy, was used as our initial benchmark experiment in this analysis. A quasi-steady explosion pressure of ~3 MPa and a propagation velocity of ~200 m/sec were obtained in this experiment. Using this model, the estimated minimum mass of the fragmented fuel was found to be 0.21 kg (3.2%) of the total mass of the fuel. The predicted work output by this model corresponding to the above fragmented fuel mass was found to be 9.8 kJ. The estimated initial void fraction of the vapor was found to be 11.5%. In these analyses we have compared the various possible closure relations applied to the detonation wave theory for a vapor explosion and associated concerns of model stability in the two-phase region.