[en] Knowledge of the phenomena that may occur during severe accidents in a nuclear power plant is an important prerequisite for being able to predict the plant behavior, in order to formulate procedures and instructions for incident handling, for contingency planning, and to get good quality at the accident analysis and risk studies. Since the early 80's nuclear power companies and authorities in Sweden has collaborated in research on severe reactor accidents. Cooperation in the beginning was mostly linked to strengthening the protection against environmental impacts after a severe reactor accident, in particular to develop systems for filtered depressurization of the reactor containment. Since the early 90's the cooperation has partially changed and shifted to the phenomenological questions of risk dominance. During the years 2009-2011, cooperation continued in the research-program APRI-7. The aim was to show whether the solutions adopted in the Swedish strategy for accident management provides reasonable protection for the environment. This was done by gaining detailed knowledge of both important phenomena in the hearth melting behavior, and the amount of radioactivity that can be discharged to the surroundings during a severe accident. To achieve this aim, the research program has included a follow-up of international research in severe accidents and evaluation of results, and continued to support research at KTH and Chalmers Univ. of severe accidents. The follow-up of international research has promoted the exchange of knowledge and experience and has provided access to a wealth of information about various phenomena relevant to the events at severe accidents. This was important to obtain a good basis for assessment of abatement measures in the Swedish nuclear reactors. Continuing support to the Royal Inst. of Technology has provided increased knowledge about the ability to cool the molten core of the reactor vessel and the processes associated with cooling the enclosure and steam explosions. A computational tools have been developed to analyze a meltdown of the reactor vessel bottom. The calculations show that a cooling flow through the actuators should enable us to delay the melting of the tank bottom, but might hamper the further development if the tank bottom melting it occurred. Experiments and analyzes show that the porosity of the gravel bed is high and geometries with an insulating layer of fine particles on top of a gravel bed is less likely. Reducing the water depth and/or increasing diameter of the melt jet, increases the risk of a non refrigeratable configuration. Completed research on steam explosions have given us more detailed knowledge, but this is still not sufficient to provide a full understanding of the process in the context of a meltdown. Based on the results of research at KTH, it is considered that it is not realistic in a completely deterministic way to determine how an accident sequence has developed. The support to Chalmers research has given greater knowledge of breakdown chemistry, mainly jod behavior in the enclosure and the scrubber after a breakdown. The understanding of the basic relationship for the distribution of methyl iodide between water and gas phase has increased. Experiments and analyzes have made clear that the redox potential in an aquatic environment have a minor impact on the ability to retain iodine as long as the pH is kept above the acid range. There is a potential to strengthen the scrubbers iodine separation capability with the addition of an organic phosphorus compound