N15: Severe Accidents


The ultimate safety objective of a nuclear power plant is to avoid the release of radioactive materials from the fuel of the core. For LWRs, the most likely cause of this is the loss of water from the core region, leading to a loss of suitable heat sink resulting in the eventual melting of the cladding and the collapse of the core. Modern nuclear power plants are designed so that the probability of radionuclide release occurring is very low, however should this event occur, the economic, environmental and health impacts are potentially so severe that the risk has elevated “nuclear severe accidents” as a scientific research field in its own right. Consequently, “nuclear severe accidents” has attracted billions of euros of research around the world over four decades, which has the attention of every nuclear regulator.

This module offers an introduction to nuclear severe accidents for LWRs by first introducing basic safety principles and the history of severe accidents. The module principally focuses on the various phenomena associated with the severe accident transient, covering the thermal-hydraulics of core uncovery through to the chemistry of radionuclides. The consequences of a severe accident are also covered, including the release of fission products into the environment and the emergency response. The module will also include an overview of some of the tools and codes available and widely used within the industry.

On completion, students should have obtained:

  • a recognition of the nuclear safety principles and how they apply to preventative and mitigative measures on a nuclear power plant;
  • an appreciation of the history of nuclear severe accidents and how that history has directed experimental research and plant design;
  • an appreciation of computer codes used to assess severe accident transients;
  • an understanding of the important severe accidents phenomena, from accident initiation to the eventual release of radionuclides;
  • an understanding of the societal impact of a severe accident.


  • Nuclear safety principles
  • History of severe accidents
  • Regulation for a chaotic accident
  • Overview of thermal-hydraulics
  • Conventional clads and accident tolerant fuels and clads
  • Clad oxidation and failure
  • Reflood and recriticality
  • Core degradation
  • Fission product release and retention
  • Fuel-coolant interactions and debris quench
  • In-vessel retention
  • Hydrogen explosions
  • Radionuclide chemistry
  • Radiological, societal and environmental consequences