A method has been developed that is based on simplified reliability models and that allows us to estimate the risk of nuclide release from underground nuclear waste repositories. The prototype repository is treated as a combination of geological and man-made barriers. Risk depends on time and is expressed as failure probability of the barrier system for all possible initiating events, multiplied by the inventory to be released. The results include, for each nuclide, the time and amount of the maximum probable discharge rate, which can be used in a biosphere transport model. They also illustrate the effectiveness of single barriers in the barrier system, and therefore allow a preselection among alternative barrier concepts, barrier qualities, and repository sites. The probabilistic failure models for the single barriers and the entire barrier system depend on only a few parameters; therefore, the application of the method is fast and inexpensive. It has to be stressed, however, that this simple method cannot replace more detailed and sophisticated risk studies, but allows concentrating them on preselected repository concepts. It therefore represents a useful tool in the early design and site evaluation phase for all kinds of repositories and waste types. Its usefulness has been demonstrated by performing several case studies with the computer program WRISK on some typical nuclides in high level waste, bearing in mind that for a repository concept all nuclides of possible importance should be considered.