(eng): The task of the decision support in the case of a radiation accident is to provide up-to-date information on the radiation situation, prognosis of its future evolution and possible consequences. The reliability of predictions can be significantly improved using data assimilation, which refers to a group of mathematical methods allowing an efficient combination of observed data with a numerical model. The dissertation concerns application of the advanced data assimilation methods in the field of radiation protection. We focus on assessment of off-site consequences in the case of a radiation accident. The main contribution of this thesis is the development of sequential data assimilation methods for the early and the late phase of a radiation accident. Data assimilation is understood here as a particular case of recursive Bayesian estimation.