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The paper deals with the evaluation of the Fukushima-Daiichi Nuclear Power Plant (NPP) accident in Units 1 to 4: an attempt is made to discuss the scenario within a technological framework, considering precursory documented regulations and predictable system performance. An outline is given at first of the NPP layout and of the sequence of major events. Then, plausible time evolutions of relevant quantities in the different Units, is inferred based on results from the application of numerical codes. Scenarios happening in the primary circuit and containment (three Units involved) are distinguished from scenarios in spent fuel pool (fourUnits involved). Radiological releases to the environment and doses are approximately estimated. The event is originated by a natural catastrophe with almost simultaneous occurrence of earthquake and tsunami. These caused heavy destruction in a region in Japan much wider than the land around the NPP which was affected by the nuclear contamination. Key outcome from the work is the demonstration of strength for nuclear technology; looking at the past, misleading Probabilistic Safety Assessment (PSA) data and inadequacy in licensing processes have been found. Looking into the future keywords are Emergency Rescue Team (ERT), Enhanced Human Performance (EHP), and Robotics in Nuclear Safety and Security (RNSS). [ABSTRACT FROM AUTHOR]

• DEC safety evaluations. • Challenging issues for DEC assessements. Safety Criteria for DEC Analyses. Experiments for Computational tools validation. After the Fukushima-Daiichi accident, the need to consider safety analyses under design extension conditions of the nuclear power installations has largely increased. The objective is to reassess and investigate the possibilities of the current nuclear installations to withstand accidental conditions that were not originally covered by the design basis accident. In Belgium, new regulation introduces the Design Extension Conditions (DEC) in the requirements for the design and operation of nuclear power plants. The purpose is to assess the safety of the current operating nuclear power installations and to identify design improvements, procedural measures and additional practicable provisions or mitigation means that could be used to prevent severe accidents. This paper provides a brief overview of the issues and challenges related to DEC safety evaluations in Belgium for the Licensee and also for the Safety Authority. [ABSTRACT FROM AUTHOR]