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".
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Foggy and misty overnight with temperatures below freezing
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David and Angela Southwood, from Newport, say they plan to live in a caravan over the border after learning of the decision.