Greenpeace has projected a large image onto Dungeness Nuclear Power Station in Kent, to highlight what it says is its vulnerability to terrorist attacks.
Public tours around the power station were stopped after 9/11
The environmental group shone the word "KER-POW" onto the two power stations in the early hours of Friday morning.
A Greenpeace spokesman said security had improved but it was still at risk.
The plant's operator British Energy said security was a high priority and government rules were closely followed.
A spokesman for the company added: "Security is more an issue for the government but we do take it very seriously."
He described Greenpeace's action as "little more than a stunt" and added "what we need is a mature debate about the future energy needs of the UK".
The threat of more terrorist attacks after the 11 September attacks in the United States meant tours around the working power station had to be stopped.
But Greenpeace campaigner Nathan Argent insisted: "If a terrorist wanted to fly a jumbo jet into a nuclear reactor there's nothing they can do to prevent it... it would be absolutely catastrophic."
He said cleaner technology such as renewable energy should be used as an alternative to nuclear power.