Did any accidents happened on a nuclear plant?

There have been a number of accidents at nuclear plants, of a number of different types.
For most people, the most feared accident is a nuclear meltdown, in which some part of the fuel in the reactor overheats and melts. There have been two large scale meltdowns, and at least nine smaller ones.
The worst meltdown was the Chernobyl Disaster of 1986, in the former Soviet Union. The reactor exploded and large amounts of radioactive material were released into the environment. This accident caused the evacuation of 200,000 people and rendered about 20,000 square miles (50,000 square kilometers) of land unusable for a period of years. Damage estimates have been as high as a trillion 1995 US dollars.
The other was the Three Mile Island accident in Pennsylvania in 1979, in which radioactive krypton gas and iodine-131 were released, causing (according to at least one article in a peer-reviewed medical journal) a statistically significant increase of cancer in the area downwind for ten miles (16 km).
There have been other partial meltdowns that did less damage. Four of these were in the United States, and two in the UK. Others were in Canada, Czechoslovakia, and Switzerland.
In addition to meltdowns, there have been a large number of releases, leaks, and spills of radioactive contaminants, which were unplanned. The number of these spills and leaks is very high. They probably have happened at all operational nuclear facilities; over a quarter of nuclear plants in the United States are leaking radioactive tritiated water into groundwater at their sites.
Waste materials also pose a hazard, and there have been some relatively minor accidents involving waste. A worst case spill of waste could be as destructive as a meltdown, and though no accident close to such a scale has happened, the length of time during which such an event could take place (over 10,000 years) makes storage of waste problematical.