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What Causes Nuclear Power Plant Accidents?

As with any complex industrial system, nuclear power plants aren't foolproof. Though they have a good safety record overall, various kinds of accidents have occurred, from the mundane to the catastrophic. Employees have mishaps and equipment breaks down. When accidents occur, they usually result in a minor release of radioactive steam or water. Constant vigilance by industry, government and independent groups limits the chances of another Chernobyl or Three Mile Island type of incident.

  1. Design Flaws

    • The incident at Chernobyl was the most extreme nuclear accident in history, resulting in deaths from radiation, destruction of the plant and widely-dispersed radioactivity. Its reactor had an early, primitive design that used graphite to moderate radioactivity. Graphite, however, is both brittle and flammable, and both these problems contributed to the accident. Since Chernobyl, the few utilities still operating this type of reactor modified them for better safety.

    Poor Control Systems

    • Technicians monitor the myriad valves, pumps and other equipment in a nuclear plant from a central control room. During the Three Mile Island incident, a stuck valve registered incorrectly on a display panel, which led to mistakes by the staff. In addition, when the reactor overheated, the operators became overwhelmed by hundreds of flashing warning lights.

    Lack of Maintenance

    • The combination of heat, pressure and radiation stresses a nuclear power plant's mechanical and electronic systems. The utility can run the plant safely as long as the equipment receives the required frequent and regular inspections and maintenance. Control and cooling systems for the reactor are crucially important, as some reactors can enter a state called thermal runaway where nuclear fuel becomes increasingly hotter, melting the supporting structures around it.

      Pipes carrying radioactive water and steam are vulnerable to leaks if they're allowed to corrode or loosen. This has happened several times to nuclear power plants.

    Operator Fatigue

    • The crew running the Chernobyl reactor had already been overworked when they began a series of tests that led to a reactor meltdown. They began the tests in the early morning hours, compounding operator fatigue and inattention. The staff members were unable to conduct the tests correctly and could not effectively respond to an escalating series of problems.

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