[en] It is a major advantage to those that must make decisions about the implementation of an emergency plan following an accident at a nuclear power plant if the likely release of activity is predicted before it occurs. To this end, a software module, which provides a rapid estimate of the source term to the environment, has been developed by NNC Ltd. Termed the RODOS STM, the software has been developed such that it can be used with the RODOS system. Operating outside of RODOS, the software remains a useful source term prediction tool. The software employs Bayesian analysis techniques, and the results of level 1 and level 2 probabilistic safety analysis, to calculate the probability of the possible releases of activity into the environment and the potential magnitude of those releases, given a set of observations about the status of the NPP. The software has been developed using the Sizewell 'B' PWR design. (author)