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
Choir wins public vote, while judges pick dancers ahead of 12-year-old singer-songwriter Henry Gallagher.
A cloudy start to the morning but this should clear in most places leaving a dry and bright afternoon.
The live semi-finals kick off tonight on Britain's Got Talent - and three Welsh acts will be among those battling it out this week.