You might heard of the accident which occured in Paks, Hungary this year (damage of fuel assemblies occurred on 10 11 April 2003 inside a cleaning tank).
Following flash animation shows how it happened... (detailed description of the accident (EVENT INVESTIGATION REPORT)."8.5. Chronological sequence of the events.")
Shortly: during the chemical cleaning of partially irradiated fuel assemblies, the cleaning device got overheated, and noble gas escaped.
I have always thought mankind is not prepared to control nuclear power...