Controlling accidents in nuclear power plants: researchers test simulation codes for passive safety systems


Nuclear power plants have an extensive repertoire of safety systems. These safety systems ensure that the nuclear power plant will be taken to a safe condition in the event of an anticipated operational occurrence.

Here, a differentiation is made between two kinds of systems: active and passive safety systems. The main difference between the systems lies in the way they operate. Active systems require an electric power supply, e.g. for pumps and valves, to fulfil their function. Passive systems are characterised by the fact that no external energy supply is necessary for their activation or operation. The driving forces rest solely on physical regularities such as gravitation, condensation or evaporation. The safety systems in German nuclear power plants are mainly active ones. The international trend in the development of advanced reactor concepts is towards an increased use of passive systems as these are less dependent on external influences and the operation of necessary additional systems.

Experiments at the largest test facility for passive systems world-wide

At present, researchers from different German institutions are taking a closer look at the passive safety systems within the framework of a research project. An association of five project partners is working on the "EASY - Integral experimental and analytical demonstration of the ability to control design basis accidents solely with passive systems" project:

  • Gesellschaft für Anlagen- und Reaktorsicherheit (GRS)
  • AREVA GmbH
  • Technischen Hochschule Deggendorf - Fakultät für Bauingenieurwesen und Umwelttechnik
  • Technische Universität Dresden
  • Rheinisch-Westfälische Technische Hochschule (RWTH) Aachen

What is special about this project is that the researchers verify the results of their analyses by comparing them with experiments that were carried out at the INKA test facility of AREVA GmbH in Karlstein. That test facility is the largest of its kind in the world. It allows the testing of the interaction of various different passive components of AREVA's advanced boiling water reactor concept, like e.g. the emergency condenser or the passive core flooding system, on a 1:1 vertical scale under conditions that are typical of an accident. At the heart of INKA are four pressure vessels measuring up to 30 metres in height and having a volume of up to 350 cubic metres.

The simulation codes of GRS: ATHLET and COCOSYS

The aim of the project is to continue developing the demonstration tools for the computation of accidents in nuclear power plants with passive systems and validate them with the help of INKA test results. Here, the researchers also want to test two GRS-developed simulation codes: ATHLET and COCOSYS. ATHLET is a program for analysing the dynamic behaviour of the reactor and the cooling loops of an NPP during events involving leaks and transients. COCOSYS can be used to run extensive simulations of event and accident sequences in light water reactor containments. Both codes are essential elements of GRS's AC2code system and are used e.g. for examining whether certain accidents can be controlled.

Codes – like ATHLET and COCOSYS – were originally developed for reactors with mainly active safety systems. Owing to - amongst other things - the lesser driving forces, passive systems pose a challenge for calculations with simulation codes. Hence the capability of these codes to calculate the characteristics of passive systems (e.g. small differences in temperature or pressure, temperature stratifications in large water pools) has to be developed further and validated. The research project is to clarify how capable the two simulation codes are when it comes to predicting the behaviour of the passive systems in dependence of a large number of continuously changing parameters.

Germany supports the development and use of passive safety systems with its own research projects. Even though passive systems are only used to a minor extent in Germany itself, this research allows Germany to make independent assessments of the safety of nuclear power plants abroad. Furthermore, it increases the general acceptance of the simulation codes developed by GRS and thus their world-wide dissemination.

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Simulation codes of GRS