Serious nuclear reactor accidents
Today 436 civil nuclear power reactors operate worldwide, generating 373 GWe of electrical power, which is about 15% of total global electricity consumption. These reactors have operated for 14,000 reactor-years, the vast majority working very safely and without any notable safety incidents. Based on past nuclear accidents at Chernobyl, Three Mile Island and Windscale the probability of a reactor core meltdown is about once in every 4,600 reactor-years.
Nevertheless, historically a serious nuclear reactor accident has happened somewhere in the world about once every 20 years since the first nuclear reactor was built at the University of Chicago in 1942. The most serious were the three core meltdown accidents at Windscale in Cumbria, United Kingdom in October 1957; Three Mile Island in Pennsylvania, United States in March 1979; and Chernobyl near Kiev, Ukraine in April 1986.
These accidents were particularly serious because the reactor cores were essentially destroyed and radioactive contamination fall-out was released into the atmosphere from the Windscale and Chernobyl meltdowns, directly affecting members of the public. Historical accident statistics show that for every serious accident that occurs there are also usually a much larger number of smaller near-miss incidents.
The Davis Besse near-miss accident
The most important recent near-miss was at the Davis Besse nuclear power station at Ohio in the United States in March 2002.
During routine maintenance work a deep corrosion cavity was found in the steel reactor pressure vessel encasing the pressurised water reactor core. The corrosion had completely penetrated through 6.6 inches of carbon steel and was corroding into the thin stainless steel outer cladding of the pressure vessel. If the reactor pressure vessel had been breached, cooling water would have escaped potentially resulting in a loss of cooling-water accident (LOCA).
This could have triggered a possible reactor core meltdown if automatic shut-down safety systems had also failed. Davis Bess was an important learning incident both because the extent of corrosion was unexpected and the PWR reactor technology used is essentially the same as will be deployed for many new nuclear power stations in the future. Nuclear safety engineers have learned a great deal from these past incidents.
Technical innovations
The most important technical innovation has been the introduction of passive safety systems into the design of modern Generation III+ nuclear reactor designs. Older nuclear reactor designs rely on active safety systems, a mix of human intervention and computer-control to automatically shut-down the reactor if a major fault or emergency occurs. Passive safety systems do not require any operator action or electronic feedback in order to shut-down safely in the event of an emergency.
These passive safety systems employ simple natural fail-safes such as pressure differentials, gravity, natural convection, or the natural response of materials to high temperatures to ensure that reactors shut-down quickly without relying on humans or computers.