How do you dismantle a nuclear bomb? And how do you verify another country is genuinely disarming without compromising sensitive national security material?
BBC security correspondent Gordon Corera was given exclusive access to a unique exercise run by the UK and Norway to find out.
The nuclear weapon is carefully lifted out of a large container and moved onto the floor.
Two engineers use an electric screwdriver to open up a side compartment and remove the "physics package" containing the sensitive parts of the bomb.
A scientist with a radiation detector beckons me forward as he points his machine towards the box.
It begins to emit an accelerating beeping noise. "The measurement is approximately a hundred times normal background radiation," he tells me.
"But it is not dangerous, I promise," he adds with a smile.
NUCLEAR DISARMAMENT PROCESS
1: Nuclear weapon transported to disarmament facility
2: Weapon is hauled by crane into storeroom and dismantled
3: Physics package is removed and placed in separate container
4: Inspectors use device to confirm radioactive material is present in container
5: Container is then sealed in a side-room overnight with CCTV
6: Next day physics package is transported to a hot cell for dismantling
7: Radioactive material is removed safely and put into storage
The lack of danger is because the bomb is not real. To inject an element of realism into this experiment, a weak radioactive material - Cobalt 60 - is used.
The dismantlement experiment is a joint exercise between the UK and Norway - the first of its kind - and was held a few miles from Oslo.
The five-day exercise has been keenly anticipated internationally as a way of building trust between nuclear weapons states and non-nuclear weapons states.
It is designed to see if one country can verify the disarmament of another country's nuclear weapon, but without any sensitive information about national security and weapon design being compromised.
In a role reversal, the Norwegians play a nuclear weapons state (called Torland) and the UK team play inspectors from Luvania, a non-nuclear weapons state.
There is currently a new push for global nuclear disarmament. Russia and the US announced in Moscow in early July that they would reduce their stockpiles and the UK has said it might be willing to reduce further its nuclear deterrent as part of any global disarmament talks. The non-nuclear weapons states have been pressing for more active disarmament and if there were further moves then allowing non-nuclear states to verify the disarmament would help increase confidence between the two sides.
The 10 inspectors from UK/Luvania remain in character as soon as they enter the gates of the nuclear facility. During meal breaks they are kept separate from both the Norwegian/Torland team and the joint planning group.
A huge amount of work goes in to making the exercise as realistic as possible.
A large, white binder contains briefing packs with fake Torland letters inviting the team to verify dismantlement of one of their Odin gravity bombs.
Stamped "secret", the Torland brief states that all details about the size, shape, composition, etc, "must be kept outside the knowledge of inspectors at all costs".
To complicate matters, inspectors are given a printout from a fake website which features what is alleged to be leaked pictures of the weapon.
"The aim is to develop methodologies we could use in inspections of a real nuclear facility but in an environment in which can do trial and error," explains Andreas Persbo of Vertic, which helped organise the event.
It is not an exercise in which the nuclear state is trying to clandestinely divert nuclear material or the inspecting side search for a covert facility.
The main aim instead is to try to look for practical lessons and solutions to build confidence between the haves and have-nots in the nuclear world.
A weak radioactive substance is used in the fake bomb
Even so, the British/Luvania team push the boundaries during the long negotiating sessions that begin and end each day, at one point submitting 15 questions, some of which the Norway/Torland team refuse to answer.
There is even an early disagreement over the question of what type of warning - if any - the guards would give before firing their weapons.
The guards, who follow the inspectors everywhere, are real Norwegian soldiers but armed with non-lethal weapons, similar to paintball guns.
The key task for the inspectors is to establish a chain of custody and ensure that at no point is any sensitive material diverted.
But this has to be done without ever actually seeing the sensitive material itself.
Initially, a truck takes a container carrying the device to the disarmament facility.
It is a very choreographed process, almost like a ballet
From the start inspectors watch, photograph, seal and tag key items. They cover entry and exit points to the disarmament chamber, sweeping all those going in and out to ensure no radioactive material is smuggled away.
"It is a very choreographed process, almost like a ballet," says Mr Persbo. "Timings are very precise."
The amount of fissile material in a nuclear bomb is itself classified, so a number of techniques have to be employed by the inspectors to ensure nothing is diverted when they are not able to measure it in detail themselves.
Each country's scientists have separately designed and built their own prototype devices known as "information barriers", which can confirm that an agreed amount of radioactive material is present in any container.
The machines provide a green light if the contents match the last reading but the actual contents are not revealed.
Inspectors are tricked with fake information about the bomb
There is genuine relief from the scientists when both come out with an agreed result of what is inside the container.
The other means for assuring the chain of custody are tags and seals.
Tags and seals
A tag is any form of identifying label, while a seal is used to ensure a room or box is not tampered with during times inspectors are not physically watching it.
These are surprisingly low-tech. A purple strip of adhesive goes across a door hinge. If it is moved then the colour changes and a warning appears on it.
Additionally, the seal has a blob of glue with multi-coloured glitter inside. This is photographed close-up by the inspectors once it is in place and then again when inspectors return.
The unique pattern would be almost impossible to replicate perfectly in a relatively short space of time. More high-tech variants are available involving fibre-optics and the next stage of the project may involve looking at ways of designing the most effective seals.
After the "physics package" is removed from the bomb and placed in a container, the inspectors are allowed to return into the room and watch it being placed in a storage room for the night.
A tampered seal provoked a crisis on one day of the experiment
The next morning, in the pouring rain, inspectors follow the container as it is moved by a cart to another part of the facility where the radioactive material is - at least notionally - removed in a hot cell using robotics arms.
Finally it is moved to a storage site.
"This is about having an understanding of what it means to take some material from A to B without really knowing what it is," explains Norwegian official Ole Reistad.
"Under other verification arrangements, it might be special types of fuel, it might be commercial secrets or it might be other security interests that you have to protect in some way."
In practice no nuclear weapons state has ever allowed a non-nuclear weapons state to verify disarmament. But if there was to be multilateral disarmament in the future, it may well be important to provide such states with confidence over its actions.
Officials on both sides hope that this and any future events will lead to better understanding between nuclear weapons states and non-nuclear weapons states and more collaborations, allowing trust and confidence to be increased.
This project in a way shows our commitment to try and find good practical ways of making sure we have nuclear disarmament
Gry Larsen, State Secretary for Foreign Affairs, Norway
"Norway is very much committed on the disarmament agenda," explains Gry Larsen, Norway's State Secretary for Foreign Affairs.
"This project in a way shows our commitment to try and find good practical ways of making sure we have nuclear disarmament."
UK inspectors and observers say they learnt about the challenges of being a non-nuclear weapons state and providing confidence, as well as ways of ensuring their own sensitive material is protected.
The Norwegians say they garnered a first-hand perspective of the sensitivities of nuclear states in protecting classified information.
The UK has talked of acting as a "disarmament laboratory" and being part of the process allows the UK to say that it is living up to its obligations under the Non-Proliferation Treaty for disarmament, although the emphasis is on developing the technical aspects of verification.
"It was lots of hard work but there's opportunity for more progress in the future," said one UK Ministry of Defence official.
Other countries are also said to have shown interest in the work, including the US, Canada, Russia, Australia and Japan.
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