A computer code, ANEXDI (analysis of extended disassembly), has been prepared for scoping studies of hydrodynamic interactions in typical core disruptive accidents in a fast power reactor. A two-phase compressible thermohydrodynamic model is coupled with neutron point kinetics equations and solved numerically, employing the well-known implicit multifield Eulerian technique for the hydrodynamics and an integrating factor method for the neutronics. Hydrodynamics of the ANEXDI code includes, at least parametrically, (a) interphase momentum transfer depending on the phase velocity difference, the phase acceleration difference, the radius of the dispersed phase particles, the viscosity coefficient of the continuous phase, and the drag coefficient, (b) intra-and interphase heat transfer depending on the various conductivity coefficients, and (c) local vapor generation and the concurrent pressurization. A good agreement is shown between some analytically solvable, one- and two-phase shock wave problems and the numerical solutions of the ANEXDI hydrodynamics and also between ANEXDI and VENUS calculations for a typical hypothetical core disruptive accident (HCDA) in a small 40-MW(thermal) fast reactor. Some calculations along with a simple mathematical theory are presented to emphasize the effect of certain interphase phenomena and of a modeling uncertainty of the two-phase flow hydrodynamic equations on a typical HCDA. This uncertainty does not visibly affect the shock tube simulation results due to the diffused shock wave fronts produced by the computer code, but it does affect some HCDA results quite significantly, as the reactivity calculation and hence the fission power calculation are very sensitive to the density profiles of a disassembling reactor system.