Top secret security arrangements for nuclear plants like Sizewell B will be examined by world leaders as they discuss how to prevent terrorists getting hold of radioactive material.

The UK is set to play a leading role in protecting nuclear facilities from cyber attack, and the UK and United States will take part in a joint exercise later this year to prepare for any online attack against nuclear power plants and waste storage facilities.

Prime Minister David Cameron will also offer British expertise to other countries to safeguard their own civil nuclear installations amid fears Islamic State jihadists could attempt to create a dirty bomb.

Security at Sizewell B – where EDF Energy say its highest priority is the safety and security of the public, its staff, buildings and installations – was tested only a few months ago with a mock terrorist attack.

The exercise to test the response of the security and emergency services was instigated by the Office for Nuclear Regulation (ONR), the UK nuclear safety and security agency, in conjunction with the Civil Nuclear Constabulary (CNC), which has a team of armed officers working on and off the site to deter or detect and respond to terrorist activity.

The ONR would not reveal the scenario for the “terrorist attack” but described it as “credible, challenging and well-planned”.

An ONR report said: “It was considered that the exercise was an adequate demonstration of the arrangements. The exercise generated a number of learning points. In addition the ONR observers also noted several good practices.”

Yesterday an EDF Energy spokeswoman at Sizewell B said: “We maintain regular contact with the police and other appropriate agencies and regularly review and adjust our security procedures across our sites.

“For obvious reasons we can not discuss the detail of what security precautions we have in place.

“With respect to our nuclear sites in particular, we are in close contact with our safety and security regulator, the ONR. They are the independent body responsible for approving the level of security on our sites and we will continue to work with them and will act upon any instructions or recommendations.

“In order to maintain the highest standards of security, we do not discuss operational details, but like all civil nuclear facilities in the UK, we have detailed security arrangements in place.”

American president Barack Obama will host the Nuclear Security Summit in Washington when heads of government will consider their response to a nightmare possibility – a “scenario-based session” focusing on the “threat from nuclear terrorism”.