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Safety of Nuclear Reactors : Design level Safety

The basic design philosophy followed world-over for assuring nuclear power plant safety is called "defence-in-depth" approach with multiple safety systems supplementing the natural features of the reactor core. Key aspects of this approach which can be summed up as Prevention, Monitoring, and Action (to mitigate consequences of failures) are

  • High-quality design and construction so that the reactor operates with a high degree of reliability. The prevention of accidents is through intrinsic design features and stresses on quality control, redundancy, testing, inspection and fail-safe design.
  • Equipment which prevents operational disturbances or human failures and errors developing into problems,
  • Comprehensive monitoring and regular testing to detect equipment or operator failures,
  • Redundant and diverse systems to control damage to the fuel and prevent significant radioactive releases,
  • Provision to confine the effects of severe fuel damage (or any other problem) to the plant itself.

The safety provisions include a series of physical barriers between the radioactive reactor core and the environment, the provision of multiple safety systems, each with backup and designed to accommodate human error. The barriers in a typical plant are: the fuel is in the form of solid ceramic (UO2) pellets, and radioactive fission products remain largely bound inside these pellets as the fuel is burnt. The pellets are packed inside sealed zirconium alloy tubes to form fuel rods. These are confined inside a large steel pressure vessel (for a light water reactor, LWR) or a pressure tube (for a pressurized heavy water reactor, PHWR) with walls up to 30 cm thick. The associated Primary Heat Transport system removes the heat substantially. All this, in turn, is enclosed inside a robust pre-stressed or reinforced concrete containment structure with walls at least 1 m thick. This amounts to three significant barriers around the fuel, which itself is stable up to very high temperatures.

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Barriers to Radioactivity Release (Courtesy IAEA)

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Relation between Physical Barriers and Levels of Defence in Depth (Courtesy IAEA)

Despite efforts to prevent accidents, it may be anticipated that one might occur. Therefore reliable protection devices are provided to prevent or minimise the effects of an incident. Such devices include an emergency core cooling system (ECCS) to provide adequate core cooling in event of a loss of coolant accident, engineered limits on the rate of power increase, a fast reactor shutdown system activated by redundant and independent instrument channels, an independent supply of off-site power etc.

An added level of safety is ensured by evaluating the design concept under conditions of severe hypothetical accidents. This adds design margin by assuring protection of the public even if seemingly remote and unlikely events occur. In this respect, several Design Basis Accidents (DBAs) are considered, such as the loss-of-coolant accident (LOCA) where a large pipe rupture is assumed to abruptly occur. Other design features include protection against seismic events, tsunamis, cyclones, floods, and component failures.

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