The Nuclear Non-Proliferation Treaty has been ratified by more countries than any other arms limitation or disarmament treaty (187 at the last count). Its goal is to prevent the spread of nuclear weapons and weapons technology.

The task of monitoring compliance of the treaty is the job of the International Atomic Energy Authority and one area of particular concern is the spread of fissile material, particularly of weapons grade. But how to monitor this?

Ideally, the IAEA would like a non-invasive device that can be placed in the vicinity of a nuclear reactor that monitors its power output and its fissile isotopic content.

Now Adam Bernstein from the Lawrence Livermore National Laboratory in California and colleagues have developed and tested just such a device which works by measuring the reactor’s rate of antineutrino production, which in turn depends on the reactor’s fissile isotopic content and its power output.

The detector is about a cubic metre in volume and consists essentially of a liquid core doped with 0.1% gadolinium. This core is surrounded by various shields to screen out unwanted signals and various detectors to pick up the interesting ones. The detectors are looking for two pulses of energy (from a positron-electron annihilation followed by a neutron capture by a gadolinium nucleus) which are the characteristic signature of an antineutrino reaction.

Bernstein says he and his team have been testing the device for the last two years at unit 2 of San Onofre Nuclear Generating Station in Southern California and all the evidence is that it works well. They say the prototype can determine whether a reactor is on or off with a time resolution of 5 hours with greater than 99% confidence, which is handy because down times are when fuel is removed. It also directly measures power levels over month long periods. The isotopic content is a little more tricky to measure because it changes as fuel is used up within the reactor but should be within reach of the design.

Obviously such a device would also need to be able to operate remotely and record information securely but that’s for development further down the line

The detector’s biggest advantage, however, is its simplicity, says the group: “Our experience is that the simplicity of the detector design will play a key, even decisive role in determining whether this technology is adopted by the IAEA or other safeguards regimes.”

Interesting stuff.

Ref: arxiv.org/abs/0804.4723: Monitoring the Thermal Power of Nuclear Reactors with a Prototype Cubic Meter Antineutrino Detector

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