One of the thornier problems facing the international community is to monitor the spread of nuclear technology and prevent it spreading to rogue regimes. This task falls to the International Atomic Energy Authority based in Vienna and it is by no means easy.

Which is why the IAEA is exploring various new technologies for monitoring nuclear reactors at a distance. These technologies fall into two categories. Near-field devices must sit within a few tens of metres of a reactor to do their job. A couple of years ago we looked at a promising example of such a device being developed at the Lawrence Livermore National Laboratory in California.

By contrast, far-field technologies can do the same job from much further afield. The goal here is to spot clandestine reactors in other countries. But how might they work?

Today we get a fascinating insight thanks to Thierry Lasserre at the French Alternative Energies and Atomic Energy Commission and a few amis.

First, a little background. Fission reactors are prodigious sources of antineutrinos. A gigawatt-sized reactor produces some 10^21 antineutrinos each second. By that measure, these reactors light up like Christmas trees.

The trouble is that antineutrinos interact only very weakly with ordinary matter so spotting these particles is hard. But they can be detected given large enough volumes of matter. The standard technique is to fill a giant swimming pool with water and wait for an antineutrino to smash into a proton, generating a positron and a neutron. The positron produces Cherenkov radiation which can be picked up by light detectors around the pool.

In principle, a large enough detector could pick up the signal produced by any reactor.

But there is a caveat. In analysing the data from this detector, physicists would have to be able to screen out any background signal. This is tricky because there are numerous sources of noise. These fall into two main classes. First, there are the many legal reactors working around the world which are luminous beacons of antineutrinos themselves. These would all have to be taken into account when analysing the signal.

Then there is the Earth itself, which is full of radioactive stuff that wreaks of antineutrinos. These too would have to be subtracted from the measured signal.

But Lasserre et amis are equal to this task. Their idea is to turn a supertanker into an antineutrino detector by kitting it out with the necessary photon detectors and filling it with 10^34 protons in the form of 138,000 tons of linearalkylbenzene (C13 H30). They call this detector a SNIF (a Secret Neutrino Interactions Finder).

The plan is to sail the supertanker to the coast of a suspicious state and temporarily sink it in up to 4 kilometres of water. The supertanker would then watch for the telltale signs of undeclared antineutrino activity.

Lasserre et amis have even calculated what kind of background signal their detector is likely to see and what a suspicious signal would look like from an undeclared reactor placed in various locations, such as on an island, a peninsula or a flat shore.

“Our study attests that 138,000 ton neutrino detectors have the capability to detect and even localize clandestine reactors from across borders,” they say. However, they also acknowledge that such a detector would present formidable practical, political and technological challenges.

Will we ever see such a device in action? That’s an interesting question, which is not as easy to answer as the mind-boggling complexities of the task might suggest.

One interesting clue is that Lasserre and co say that near-field devices are already being tested in Brazil, France, Italy, Japan, Russia and the United States. That suggests a significant interest in nuclear monitoring technology.

Ref: arxiv.org/abs/1011.3850: SNIF: A Futuristic Neutrino Probe for Undeclared Nuclear Fission Reactors