We develop and experiment with new techniques to reconstruct the source of an accidental release of radionuclides at continental scale. The prior information on the source to be accounted for in the inversion is identified. Using methods based on information theory, purposeful cost functions consistent with this prior are constructed. They generalize classical variational least-square assimilation techniques. The adjoint of the numerical dispersion model POLAIR3D is used in the minimization of these cost functions. The method is tested on a set of twin experiments involving the main European civil nuclear sites as candidate sources. Several experiments are performed. They demonstrate how sensitive the retrievals are to the prior, the meteorological conditions, the source profile shape and the errors. A reconstruction is then attempted on the Algeciras incident, which took place in southern Spain in May 1998. A truly successful three-dimensional inversion is out of reach with the available observations. However, a temporal reconstruction of the event succeeds, and this exemplifies the newly designed cost functions.