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".
The emergency services will be tested today during an exercise at Wylfa nuclear power station on Anglesey.
The test will check the response of all the emergency services in the event of an incident involving the transport of radioactive material.