What is the difference between sealed and unsealed radioactive source?

Sealed radioactive material is that which is contained or encapsulated in some way so that the radioactive material cannot move within the environment around it. This is sometimes known as a sealed or closed radioactive source. Such sources can only present an external radiation hazard, walking away from the source will reduced radiation exposure to negligible levels. Unsealed radioactive material (or source) is where the material is not contained, and depending on circumstances, can move around the environment. An example could be a radioactive liquid that has been spilled from an open container. This unsealed material is called contamination and could be taken into the body by ingestion, inhalation or through cuts in the skin. Walking away from the container will not necessarily reduce the radiation exposure hazard to zero since this person may already be contaminated.

Reality is merely an illusion, albeit a very persistent one

– Albert Einstein -