What is nuclear accidents . give examples?

If you mean accident with nuclear weapons, the US codename for that is Broken Arrow.

One example is the incident at Palomares Spain, where a US Bomber carrying 4 hydrogen bombs collided with a refueling tanker. Both planes crashed and the unarmed bombes fell on the Spanish coast. 2 landed inland with physical damage but no explosion, 1 landed inland with chemical explosion in primary on impact, and 1 landed in the water and remained lost for a few months until located by divers.

Another example iis the Thule Greenland accident, where a B-52 with unarmed hydrogen bombs had an onboard electrical fire and the crew bailed out. The plane then crashed on the ice and at least one bomb had a chemical explosion in its primary.