Code package AC²

GRS has bundled its software for the simulation of incidents and accidents into one large code package: AC². AC² combines the calculation codes ATHLET, ATHLET-CD and COCOSYS and thus allows cross-system examination of nuclear protection goals.

Areas of application of AC²

The three simulation codes ATHLET, ATHLET-CD and COCOSYS, which were developed by GRS in co-operation with national and international partners, are the eponyms and at the same time the main components of AC². They are coupled with each other in AC² and thus allow the simulation of thermohydraulic behaviour in the cooling circuit in direct interaction with the effects in the containment during operation, during anomalies as well as during incidents and accidents in nuclear installations.

ATHLET can be used to analyse the plant’s reaction to different events (leaks, failure or malfunctions of various systems) in nuclear power plants as well as in research reactors. The ATHLET-CD extension allows the calculation of fuel destruction during an accident and the release of radioactivity within the plant. With this, for example, the formation and dislocation of melts in the core area and in underlying areas of the reactor building can be modelled and calculated. COCOSYS can be used to simulate the processes in the containment during operation and operational anomalies as well as during incidents and accidents involving the release of radioactive substances. Another component of AC² is the visualisation tool ATLAS

For which reactor types can AC² be used?

In addition to conventional light-water reactors, such as those operated in Germany, AC² can also be used to simulate light-water reactors currently under construction or development that have passive systems for residual-heat removal. Other areas of application are so-called innovative reactor concepts. These include the lead-bismuth-cooled MYRRHA reactor, which is currently being planned in Belgium, as well as small modular reactors (SMR).

What are the extensions of the latest AC² version?

For the new version of AC², improvements have been made and extensions added to the individual code components.

The new version of ATHLET allows i.a. a more accurate simulation of heat transfer in passive heat-removal systems. In modern reactors or reactor concepts, these systems are increasingly taking over the tasks of the active safety systems, which are usually operated with pumps. The main advantage is that these passive systems function solely on the basis of the laws of nature, e.g. gravitation, free convection or evaporation and condensation. They do not depend on their own energy supply (e.g. electricity), which - as happened at Fukushima Daiichi - can fail. While the European Pressurised Water Reactor (EPR) under construction in France, the UK and Finland has both active and passive systems, the AP1000 in operation in China is supposed to be able to cope with accidents using passive systems alone. This applies to an even greater extent to the currently much-discussed SMR concepts, such as the one on NuScale (USA) and NUWARD (France). The current improvements in ATHLET also allow the simulation of SMR concepts with other cooling media (gas, molten salt or liquid metals).

With the revised version of ATHLET-CD, users have the opportunity to model the reactor core even more flexibly and thus better represent local effects during core destruction. This also makes it possible to adequately simulate accidents with fuel destruction in the spent fuel pool with the new code version. Finally, with the AIDA module, the retention of a molten core in the reactor pressure vessel by cooling the reactor pressure vessel from the outside can be assessed more precisely. This is an important emergency measure of many reactor concepts for minimising radioactive releases.

The updated version of COCOSYS contains, among other things, a new generic core catcher model. Core catchers are provided in some reactors from Generation 3 onwards, such as the EPR or the VVER-1200, in order to catch and cool down core melt inside the containment in the event of an accident and thus minimise the release of radioactivity. COCOSYS takes into account all phenomena that are relevant in this process, such as the interaction between the melt and other materials and the stratification of the melt. In addition, the software contains the newly developed NewAFP (New Aerosol and Fission Products) module for a more realistic simulation of aerosols and fission products and their behaviour in the containment and the reactor building.

Application: Who uses AC²?

The AC² code package is used worldwide by more than 50 supervisory authorities, expert organisations and universities as well as research centres to review and research the safety of nuclear power plants and other nuclear installations. GRS provides the software to the respective institutions free of charge.
Supervisory authorities and their experts use the GRS code chain i.a. to check verifications that have to be submitted by the operator in nuclear licensing and supervisory procedures. Furthermore, the codes are used to reconstruct and better understand accident sequences such as those at Fukushima. The results of the calculations contribute to the planning of the recovery of the nuclear fuel and of dismantling.