30 Apr 2022

On the track of safety: simulating events and accidents in nuclear power plants

© istockphoto.com/alvarez  ​

GRS publishes new version of its AC² code package.

GRS now offers an extended and revised version of its AC² code package which is used worldwide by supervisory authorities, expert organisations and universities as well as research centres to review the safety of nuclear power plants and other nuclear facilities. The updated version allows i. a. the better simulation of technical features of so-called SMRs (Small Modular Reactors). 

Use of simulation codes in reactor safety

Simulation codes are an indispensable tool for assessing the safety of nuclear power plants and other nuclear facilities. Among other things, the codes are used to check whether the so-called nuclear protection goals - i.e. the control of reactivity, the cooling of the fuel assemblies and the containment of radioactive materials - are met in all relevant events. 

GRS has been using numerous simulation codes for this purpose for decades. With the help of the codes, the entire spectrum of events to be assumed (operational processes as well as incidents and accidents) in nuclear facilities can be simulated - starting with the neutron-physical processes in a single fuel rod and the flow and heat transfer in the reactor up to the stability and integrity of the containment and building structures in the event of an accident or external hazards, such as a strong earthquake.

GRS develops essential codes from this so-called calculation chain itself within the scope of reactor safety research and makes them available to scientific institutions, authorities and expert organisations free of charge. Supervisory authorities and their experts use the GRS calculation chain i. a. for the examination of verifications submitted by the plant operators within the scope of nuclear licensing and supervisory procedures. Furthermore, the codes are used to reconstruct and better understand accident sequences such as the one at Fukushima. Based on these calculations, the salvaging of the nuclear fuel and the dismantling can then be planned.

The allrounder – what‘s behind the AC² code package

The three simulation codes ATHLET, ATHLET-CD and COCOSYS, which were developed by GRS in cooperation 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 integrated simulation of the thermohydraulic behaviour in the cooling system and in the containment during operation, in case of disturbances as well as in case of incidents and accidents in nuclear facilities.

ATHLET can be used to analyse the reactions of the plant to different events (leaks, failures or malfunctions of various systems) in nuclear power plants as well as research reactors. The ATHLET-CD extension allows the calculation of the destruction of fuel assemblies during an accident and the release of radioactivity inside the plant. It can be used, for example, to model and calculate the formation and dislocation of melt in the core area as well as in underlying areas of the reactor building. COCOSYS can be used to simulate the processes in the containment during operation and disturbances as well as during incidents and accidents involving the release of radioactive substances. Another component of AC² is the visualisation tool ATLAS. Currently, about 50 scientific organisations and regulatory authorities worldwide use AC² for research and safety assessments. 

„The further development of AC² is an ongoing process. When we look at a phenomenon in nuclear engineering, we want to prove that the code can also calculate it - or find out why it is not capable of doing so. In this way, AC² always becomes a little more accurate and thus better for the users.“

Dr. Fabian Weyermann, AC² developer

This is what the new AC² version can do 

For the new version of AC², improvements and extensions have been made to the individual 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, and do not depend on their own power supply (e.g. electricity), which - as in Fukushima Daiichi - may fail. While the European Pressurised Water Reactor (EPR) under construction in France, the UK and Finland has both active and passive systems, the AP1000 operating in China is supposed to be able to handle accidents with passive systems alone. This is true to an even greater extent of the currently often-discussed SMR concepts, such as the NuScale concept (USA) and the NUWARD concept (France). The current improvements in ATHLET also allow the simulation of SMR concepts with other cooling media (gas, molten salt or liquid metals) as well as the lead-bismuth-cooled accelerator-driven MYRRHA system, which is currently being planned in Belgium.

With the revision of ATHLET-CD, users are now able 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 destruction of the fuel assemblies in the spent fuel pool with the new code version. Finally, with the help of the AIDA module, the retention of a molten core in the reactor pressure vessel by cooling the reactor pressure vessel from the outside - an important emergency measure provided by many reactor concepts to minimise radioactive releases - can be evaluated more precisely.

The updated version of COCOSYS contains i. a. 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 all phenomena relevant in this process into account, such as the interaction between the melt and other materials or the stratification of the melt. In addition, the code includes the newly developed NewAFP (New Aerosol and Fission Products) module for a more realistic simulation of aerosols and fission products and their behaviour inside the containment and the reactor building.

The extended and revised version of the AC² code package is now available free of charge to supervisory authorities, expert organisations, universities and research centres.


Fabian Weyermann