[en] After a nuclear reactor accident, various fission products are dispersed into the atmosphere creating a radioactive plume, which can cause exposure to the public. A risk assessment can be performed more accurately if the radioisotopes content of the plume is known. Gamma spectroscopy is the obvious way to evaluate the radioisotope composition of the plume, but the presence of many isotopes, mostly emitting photons of several energies, makes the gamma spectrometry in this case a cumbersome task. The expected spectra are more complicated in the first hours following an accident, as the short live radioisotopes are also present. A computer program was developed which can predict pulse height spectra from a 2 inch*2 inch NaI(Tl) detector which monitors the plume. Because of the limited resolution of the NaI( Tl) detector it is expected that only a limited number of nuclides can be distinguished. The fission products inventory, cooling times, specific release fractions, air attenuation, detector response functions and gamma energy data of the various radioisotopes are taken into consideration to calculate and display the expected pulse height spectra. The spectra are evaluated to identify regions where isolated peaks would indicate the presence of the appropriate isotopes. An estimation is provided of the isotopes which are expected to be identified. (authors)