Today's scenario is an incident involving the transportation of radioactive materials.
"It's extremely unlikely that an event like this could happen for real" said a spokesman for Magnox.
"However we can't allow ourselves to be complacent and need to be prepare for such eventualities".
More top news
A mostly cloudy and murky day with widespread hill fog and outbreaks of rain.
Official figures show areas of Caerphilly, Rhyl, Wrexham, Rhondda Cynon Taf and Bridgend to be the most deprived in Wales.
A £500 million investment has been secured for a new riverside development in Cardiff, which would link the City Centre with Cardiff Bay.