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On 11 March 2011, the worst nuclear accident since Chernobyl (INES Level 7) occurred at the Japanese nuclear power plant site of Fukushima Daiichi. Since that day, GRS has been dealing with the accident and its scientific analysis in many different ways.

The GRS Emergency Centre
Just a few hours after the start of the accident, the Emergency Centre of GRS began its work. Being an expert advisory organisation to the German Federal Government, GRS constantly provides a team of experts of various disciplines for the Federal Ministry for the Environment, Nature Conservation, Building and Nuclear Safety (Federal Environment Ministry – BMU) to obtain and evaluate information and make predictions about possible developments in the event of a severe nuclear accident or other such events. From midday on 11 March until the beginning of July 2011, the Emergency Centre team prepared more than 200 situation reports for the Federal Environment Ministry, describing and assessing what was going on at the Fukushima site. On behalf of the Federal Environment Ministry, these situation reports were also made available by GRS to the general public – initially on the GRS website and later on the Fukushima Information Portal website which GRS has operated until February 2015.

Technical support for “stress tests”
On 17 March 2011, the Federal Environment Ministry commissioned the Reactor Safety Commission (RSK) in agreement with the Länder to perform plant-specific safety reviews (“stress test”) for all German nuclear power plants (NPPs). The objective of this review was to examine the robustness of the design of the NPPs and the planned accident management measures against increased impacts not considered so far in the light of the events in Fukushima. In support of the RSK, the required reviews were organised under the leadership of GRS together with other technical and expert organisations (TÜV Nord, TÜV Süd, Oeko-Institut, Physikerbüro Bremen, EnergieSystemeNord, Stangenberg & Partner) and carried out based on a catalogue of requirements of the RSK. The results of these reviews were evaluated by the RSK. The final report of the RSK is available here.

Moreover, since May 2011, GRS had been supporting the Federal Environment Ministry in the preparation and drafting of the National Report (Country Specific Report) within the framework of the so-called European stress test. On 24 May 2011, the EU had decided that all EU Member States would have to subject their NPPs to such a stress test, which was organised by ENSREG (European Nuclear Safety Regulators Group). The National Reports of the participating countries were subsequently subjected to reviews by experts from other nations. The results had to be available by 31 December 2011 in the form of National Reports that were subsequently subjected to reviews by experts from other countries. Based on the results of the stress tests, the “ENSREG Action Plan” and a system with peer reviews to follow up the implementation of the results in the individual countries have been established. A comprehensive documentation of the European stress tests, including all National Reports, can be found here.

GRS information notice
On behalf of the Federal Environment Ministry, GRS prepared an information notice on the effects of the earthquakes at the Japanese NPP sites Fukushima Daiichi and Daini as well as Kashiwazaki-Kariwa (Weiterleitungsnachricht zu den Auswirkungen der Erdbeben an den japanischen Kernkraftwerksstandorten Fukushima Daiichi und Daini sowie Kashiwazaki-Kariwa – in German) in early 2012. Information notices (Weiterleitungsnachrichten) are prepared by GRS if an event occurs in a German or foreign NPP that is of safety significance and if the operating experience gained by it could be of interest for the safe operation of German NPPs. The information notice on the accident in Fukushima contains 22 recommendations for German NPPs, among other things on electric power and cooling water supply, on the seismic design and on aspects of accident management and fire protection.

The recommendations of GRS and the recommendations formulated by the RSK based on the national stress test to improve the robustness of the German NPPs form the basis for the National Action Plan of the Federal Environment Ministry. The annually updated National Action Plan, provides information on the status of implementation of the individual technical and organisational measures for each NPP.

National research projects
Since 2011, GRS has been investigating, in addition to its general topics, a wide range of issues related to the Fukushima accident in various research projects. At the national level, these projects are funded by the Federal Environment Ministry (BMU) and the Federal Ministry for Economic Affairs and Energy (BMWi).

BMU. On behalf of the Federal Environment Ministry, GRS analysed the nuclear accident in the Fukushima Daiichi NPP in detail on the basis of all information available. By means of thermal-hydraulic analyses using the GRS code system ATHLET-CD/COCOSYS, which focused on the events in Units 2 and 3, additional insights could be gained, e.g. with regard to core degradation and the conditions in the containment during the first days of the accident. In-depth studies were also carried out, among other things, on topics such as natural external hazards, electric power supply or organisational measures. Overall, it was shown that implementation of most of the lessons learned from the project have already been initiated within the framework of the above-mentioned GRS information notice. The final report on this project is available here for download (in German).

BMWi. In various research projects funded by the BMWi, GRS simulation programs are applied and further developed with regard to different aspects of such nuclear accidents. This concerns, for example, the adaptation of analysis methods and programs with regard to analyses of mechanical loads on components and materials due to multiple earthquakes, as occurred in Fukushima.
The analyses of the nuclear accident in Units 2 and 3 in Fukushima, performed within the framework of the above-mentioned BMU project, and knowledge gained from experimental research projects showed the need for further development of models in the GRS code system ATHLET-CD/COCOSYS. Part of this work has already been completed (see report on COCOSYS); further aspects are incorporated in the continuous advancement of these codes.
Another project comprises, among other things, the application and development of methods for the calculation of the radioactive inventory in the reactor cores and spent fuel pools especially of boiling water reactors. Shortly after the nuclear accident in Fukushima has happened, experts assumed that accident sequences with fuel degradation may also have occurred in the spent fuel pools of one ore more of the Fukushima units, analyses were performed with regard to such events and the general applicability to German plants.

Research within the framework of the OECD/NEA
Funded by the BMU and BMWi GRS has been participating in the project ”Benchmark Study of the Accident at the Fukushima Daiichi Nuclear Power Station” (BSAF) initiated by the Nuclear Energy Agency of the OECD (OECD/NEA). The objective of the analyses performed by the partners from meanwhile ten countries is to obtain detailed insights into the nuclear accident in Fukushima which can be used to support the Japanese organisations involved in preparing the decommissioning of the reactors. Moreover, the findings from the comparison of the analyses performed with different codes serve to improve them and to deepen the understanding of the accident phenomena occurred. The work in Phase 1 of the project, which was completed at the end of 2014, focused on the phenomena of core degradation in the reactor pressure vessels and the impacts on the containments of Units 1 to 3 during the first six days of the accident. Phase 2 of the project lasted from April 2015 until the beginning of 2018. In this phase the scope of analyses to include the behaviour of radioactive fission products released during the accident within and outside the containments was extended. In addition, the time span for analyses was extended to the first three weeks of the accident. The results of the BSAF project will be published by the OECD/NEA.

Moreover, from November 2013 until December 2016 GRS experts participated in the ”Senior Expert Group on Safety Research Opportunities Post-Fukushima” of the OECD/NEA. The task of this working group is to identify research opportunities and safety issues to be handled, based on the assessment of the events in Fukushima. In addition, proposals were made on possible analyses (e.g. sampling of molten core material) within the framework of the decommissioning of the Fukushima units to fill in gaps in the knowledge about the nuclear accidents. A report on this was published by the OECD/NEA in 2017.

Participation in IAEA report
In addition to its own research, GRS participated in the drafting of the report ”The Fukushima Daiichi Accident” of the International Atomic Energy Agency (IAEA) on behalf of the Federal Environment Ministry. Consisting of the “IAEA Director General's Report” and five technical volumes, the report, published in August 2015, provides the currently most comprehensive summary of the findings on the accident and its causes, evolution and consequences.

Series of reports for the interested public
For the interested public, GRS also published the report “Fukushima Daiichi 11. März 2011 – Unfallablauf, radiologische Folgen“ on the accident sequences and radiological consequences of the Fukushima accident. The report, which was published on the fourth anniversary of the accident in a fourth, completely revised edition (in German), provides an overview of the knowledge on the accident causes and accident sequences gained until early 2015, the information on the radiological consequences at the site and in its surroundings available by the end of 2014, as well as a presentation of the work that has been carried out at the plant site since 2011. Moreover, the current edition for the first time presents results of simulation calculations performed by GRS within the BSAF project on sequences and extent of the core damages in Units 2 and 3.

Reporting on latest developments and findings
On its website and its Twitter channel, GRS reports on latest developments at the Fukushima site and findings on the accident and its consequences. For this purpose, GRS provides its monthly overview of the radiological situation at the Fukushima Daiichi site (“Überblick zur radiologischen Situation am Standort Fukushima Daiichi” – in German) with information and web links to current measured values of dose rates and contamination.