A simple dynamic model of a nuclear reactor with temperature dependent reactivity and six groups of neutron emitting nuclei is developed in this paper based on first engineering principles. The proposed model contains the possible minimal number of differential equations and it is capable of describing important dynamic phenomena such as load change transients between day and night periods. Because the model is nonlinear in its parameters, an optimization-based parameter estimation procedure has been applied by using the Nelder-Mead algorithm and measured data from the Paks Nuclear Power Plant. The estimated parameter values fall into physically meaningful ranges and the response of the constructed reactor model shows a good fit to the measurement data.