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Non-nuclear energy

Severe accident phenomenology and management

Fission and radiation protection

All major engineering projects, such as nuclear power plants, have a potential to cause hazards to people and the environment. Safety relies on the interaction of three factors: Man, Technology and Organisation (MTO). Therefore sound strategies for safety need not only ‘technical defences’ but need to extend to incorporate man – machine interaction and best practice in management.

The strategy consists of three logical steps:

  • Understanding the aging effects and evaluate the safety margins
  • Understanding the most risk-relevant phenomena of design-basis and severe accidents
  • Developing accident management measures and evaluate their effectiveness

Should an accident occur in a reactor, safety systems should stop the fission process as fast as possible, cool the core and remove residual heat. Dedicated barriers are built in to contain radioactive materials and radioactivity and prevent a release to the environment.

A defence in depth concept has been developed to contain radioactive material. As an example for a light water reactor (LWR), this defence in depth consists of:

  • The fuel-cladding system contains the fissile material (barrier #1)
  • The reactor pressure vessel is a sealed container for the core and the cooling water (barrier #2)
  • Concrete containment designed to contain any radiological release in the unlikely case of a ‘beyond design basis’ accident (barrier #3)

The release of fission products constitute the principle health hazard to the public resulting from a severe accident. There are two main risk issues which need to be assessed in order to construct effective barriers against releases and develop appropriate safety strategies:

  • The radioactivity source term: this is a figure that represents the total radioactive inventory of a reactor. Although the reactor core is well protected and confined by design in extreme hypothetical severe accident scenarios, the highest possible risk needs to be evaluated.
  • The energy source term: after a severe accident, the persistent heat production, such as decay heat generated by fission products, can pressurise and melt down the various barriers and might give rise to further material interactions or chemical reactions.

Calculation of the amounts and physico-chemical forms of those materials that could potentially be released from the reactor (the source term) are of great safety significance. To better understand the source term behaviour and to develop appropriate prevention and mitigation measures research is needed which combines experimental investigations and numerical modelling activities. A large number of projects are examining the development of mitigating measures for defence-in-depth in the very remote case of severe accidents. The following key issues have been identified:

  • Core degradation, corium formation in the reactor pressure vessel and its behaviour inside and outside the vessel in particular its interaction with a ‘core-catcher’. Corium is the term for the result of a core meltdown and consists of the extremely hot mix of fuel, core matrix, local pipework etc. Understanding how the meltdown material can be cooled and ensuring containment integrity are important topics. In addition research to establish the limits for explosive detonation and related processes in the mixtures containing hydrogen, air, steam and dust that might arise after an accident is required. Finally, understanding the release of radioactive materials from a degrading core into the cooling circuits and the containment area will enable optimisation of mitigation measures and better prediction of the source terms.
  • Improved methods and tools for severe accident management and operator training that make use of modern information and control systems and can handle the uncertainties associated with man-machine interfaces in a structured way. Research is needed to develop preventive safety systems for present and future reactors, which enable operators to extend the period during a severe accident when no active intervention is needed.

Generally, the results from Severe Accident experimental investigations and analytical studies contribute to improving our understanding of these phenomena (e.g. corium behaviour, hydrogen explosions or radiological releases) and help to validate models and integral codes, which have an impact on the quality of safety assessments, reduce uncertainties in the quantification of safety margins and maintain readiness to respond to emerging issues. Furthermore, the risks associated with those phenomena can be reduced through appropriate severe accident management measures that could be implemented through improvements in the design of new plants (e.g. ex-vessel core catchers) and the development for existing plants of engineered systems and retrofitted measures (e.g. techniques for removing the hydrogen risk in the containment area). Development of specific operating emergency procedures is another anticipated outcome that can be applied to current and future nuclear power plants.