Understanding the accident at the Fukushima Daiichi Nuclear Power Plant is important for safe and timely decommissioning of the reactors. This objective, together with the development of better computer codes for analysis of severe accidents, was the aim of the benchmark study conducted under the auspices of the OECD Nuclear Energy Agency. Through the diversity of the modelling codes and approaches, and the use of parametric studies, it has been possible to identify the more likely scenarios that can fit with the limited data available from the accident. The insights gained from the project will help guide research into severe accident behaviour, improve severe accident computer codes, develop accident mitigation and response at nuclear power plants, support regulatory oversight related to severe accidents, and inform policies on the development and deployment of nuclear technology.