3. Why does Nuclear or Radiological Accident occur?

Nuclear reactors (used for power generation, military or research purposes) are the main sources of radiation. The radioactivity of a nuclear reactor core is millions of times higher than any other man-made source of radiation. Although the construction and operation of nuclear power plants are closely monitored and regulated, an accident, though unlikely, is possible. 
The potential danger from an accident at a nuclear reactor is exposure to radiation. This exposure could come from the release of radioactive material from the reactor into the environment, usually characterised by a plume (cloud-like) formation. The size of the area affected is determined by the amount of radioactive material released from the plant, wind direction and speed, and weather conditions (eg rain, snow) which would quickly drive the radioactive material to the ground, causing increased deposition of radionuclides. Significant contamination could affect areas up to 30 kilometres from the accident site. 

Radiological accidents can occur wherever radioactive materials are used, stored or transported. In addition to nuclear power plants, hospitals, universities, research laboratories, industries, major highways, railways and shipping yards could be the site of a radiological accident. Radioactive sources are frequently used in industrial gauges (eg moisture and density gauges). If these gauges or other radiation-containing equipment are disposed of incorrectly or sent for recycling as scrap metal, the sealed source may be “lost” and people may be exposed as a result. They are among the most frequently reported radioactive contaminants in shipments received by scrap metal facilities. If a steel mill melts a source, it contaminates the entire batch of metal, the processing equipment and the facility. More importantly, it can result in the exposure of workers or users to radiation.

There have also been incidents in which unsuspecting individuals find these sources and, not knowing what they are, keep them or even open them and suffer serious exposure. Some satellites use radioactive materials as a power source during long space flights. During the launch or re-entry of satellites there is potential for an accident that would disperse radioactive materials.