A nuclear and radiation accident is defined by the International Atomic Energy Agency (IAEA) as "an event that has led to significant consequences to people, the environment or the facility." Examples include lethal effects to individuals, large radioactivity release to the environment, or reactor core melt." The prime example of a "major nuclear accident" is one in which a reactor core is damaged and significant amounts of radioactivity are released, such as in the Chernobyl disaster in 1986. The impact of nuclear accidents has been a topic of debate practically since the first nuclear reactors were constructed in 1954. It has also been a key factor in public concern about nuclear facilities.Some technical measures to reduce the risk of accidents or to minimize the amount of radioactivity released to the environment have been adopted. Despite the use of such measures, human errorremains, and "there have been many accidents with varying impacts as well near misses and incidents". Worldwide there have been 99 accidents at nuclear power plants. Fifty-seven accidents have occurred since the Chernobyl disaster, and 57% (56 out of 99) of all nuclear-related accidents have occurred in the USA.Serious nuclear power plant accidents include theFukushima Daiichi nuclear disaster (2011), Chernobyl disaster (1986), Three Mile Island accident (1979), and the SL-1 accident (1961).
The failure occurred when the plant was hit by a tsunami that had been triggered by the magnitude 9.0 Tōhoku earthquake. The following day, 12 March, substantial amounts of radioactive material began to be released,creating the largest nuclear incident since the Chernobyl disaster in April 1986 and the only (after Chernobyl) to measure Level 7 on the International Nuclear Event Scale[9] (initially releasing an estimated 10–30% of the earlier incident's radioactivity). In August 2013, it was stated[by whom?] that the significant amount of radioactive water was among the most pressing problems affecting the cleanup process, which is expected to take decades. There have been continued spills of contaminated water at the plant, some into the sea. Plant workers are trying to contain the leaks using measures such as building chemical underground walls, but they have not yet improved the situation significantly. Nonetheless, to keep the matter in perspective, the entire release of radioactivity into the sea will add less than 0.01% to the background radiation.