The employee was taken to hospital as a precaution, a spokeswoman for the German subsidiary of British nuclear firm Urenco said.
None of the radiation escaped, the company added.
"There is no danger to the public. The necessary authorities were informed straightaway," Urenco said in a statement.
Joachim Neuser, a spokesman for the local government in the state of North Rhine-Westphalia where the accident occurred, said authorities were studying a report into the incident and would draw the appropriate conclusions.
The plant, near Germany's border with the Netherlands, has been in operation since 1985, according to the Urenco website.