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 dollars of research around the world over four decades, which has the attention of every nuclear regulator.

This course unit offers an introduction to nuclear severe accidents for light water reactors. The course begins by introducing the basic safety principles and the history of severe accidents before providing a grounding in thermal-hydraulics and thermodynamics. This grounding will be relevant to understanding severe accident phenomenology, covering a breadth of physics and chemistry, which is the principal focus of the course. A sound understanding of the phenomenology will equip the student, in their career, to deduce the possible consequences of a hypothesized severe accident or of an emergent real accident. The teaching of the phenomenology follows a chronological order and is broad enough to take an accident from the initiating event, through to the environmental consequences.

The course unit will also include an overview of some of the tools and codes available and widely used within the nuclear sector and will enable the student to join industry with a solid background in severe accident phenomenology and safety approach.

On completion, students should have obtained:

  • An understanding of the physics and chemistry phenomena that are expected to occur in the nuclear power plant if sustainable cooling of the core cannot be re-established;
  • Insight to balance the positive and negative aspects of some severe accident phenomena in terms of the different risk outcomes.
  • An ability to solve practical problems through calculation and / or deduction in order to estimate or bound plant responses or consequences.
  • An understanding of phase diagrams and be able to apply a basis in material science in order to provide insight into material behaviours in a degraded core environment.
  • An understanding of thermal-hydraulics and fluid mechanics in order to provide insight into the thermal behaviour of a degraded core.
  • Appreciation of computer codes used to assess severe accident transients.
  • 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