Generally, the analysis of a sodium-water reaction (SWR) is classified by two major events. One is the very initial stage of peak pressure and shock wave propagation caused by the reaction itself, and the other is the bulk motion including the mass transfer phase in the quasi-steady state of the reaction period during several second or minute orders after leak initiation. The system dynamic response during a sodium-water reaction event shows very different characteristics between the initial stage and the long-term period. In order to investigate the later phase of a SWR event, the code SELPSTA (Sodium-water reaction Event Later Phase System Transient Analyzer) has been developed and the design and construction of the test facility has been made.