The details of robust computational and novel semiempirical methodologies that were developed and tested at the University of Missouri Research Reactor (MURR) to accurately determine the possibility of a “hot” startup and the estimated critical position (ECP) of the control rods following an unplanned shutdown are presented. The computations, based on a modified coupled MCNP5-ORIGEN2 code system and using ENDF/B-VII.0 and TENDL-2013 nuclear data sets, accurately simulate the MURR core operational histories while predicting the critical rod positions within ±0.001 Δk/k of experimental critical data. In this study, using the coupled MCNP5-ORIGEN2 computations, various core-specific parameters were methodically characterized and were adapted into a semiempirical formulation better suited for practical reactor operations. The predictive capabilities of these novel semiempirical approaches regarding core criticality and ECPs required utilizing, for the first time, the net transient response of the negative reactivity worth for necessary fission product poisons including 135mXe and several others. Calculation-to-experiment deviations in hot-startup criticals and the corresponding control rod positions are shown to be less than ±0.03% Δk/k and ±1% difference in relative rod position, respectively.