Defined as “Accident conditions more severe than a design basis accident” and “Involving significant core degradation”, severe accidents are beyond design accidents – low probability but high impact.
Especially after the 2011 Fukushima Daiichi accident, nuclear experts have intensified the evaluation of severe accidents with increased attention to severe accident computational codes and modelling methods.
MELCOR: MELCOR is a fully integrated, engineering-level computer code developed by Sandia National Laboratories for the U.S. Nuclear Regulatory Commission to model the progression of severe accidents in nuclear power plants.
It can simulate Loss-Of-Coolant Accident and non-LOCA transients for Probabilistic Risk Analysis applications as well as severe accident sequences, including actions taken as part of the Severe Accident Management Guidelines.
SCDAP/RELAP5: The Integral Severe Accident Analysis Code has been developed for best-estimate transient simulation of light water reactor coolant systems during a severe accident.
The code models the coupled behavior of the reactor coolant system and reactor core during severe accidents as well as large and small break loss-of-coolant accidents, operational transients such as anticipated transient without SCRAM, loss of offsite power, loss of feedwater, and loss of flow.
COCOSYS is developed for the simulation of all relevant phenomena, containment systems and conditions during the course of design basis accidents and severe accidents.