The device could greatly help combat proliferation of weapons-grade plutonium among states that turn to nuclear power in the coming decades to meet their energy needs, the report in next Saturday's New Scientist says.

But it is still being tested and thus cannot be used to defuse the present row embroiling Iran, which Western countries suspect is trying to build a bomb.

And if it works, it still could not thwart states that refuse deals with nuclear inspectors or hide their reactors, it says.

Plutonium is a man-made element created by the fission of uranium fuel. It is itself fissile, capable of producing energy either for generating electricity or for a bomb.

At present, the International Atomic Energy Agency (IAEA) calculates the amount of plutonium produced by a reactor by monitoring the amount of uranium fuel entering the core, the total amount of time the reactor is operating and its power output.

But it is possible for a plant operator to produce more plutonium than appears on the IAEA's books by altering the type of fuel rods used in a reactor or the rate at which neutrons permeate the vessel, New Scientist says.

The detector built by researchers at Lawrence Livermore National Laboratory in California counts antineutrinos -- "ghostly" sub-atomic particles that are generated by fission.

Antineutrinos produced by uranium are distinctive because they carry a higher energy than ones produced by plutonium.

Over the course of a typical year-long fuel cycle, the pattern of antineutrinos is predictable. Production of high-energy antineutrinos falls off as uranium fuel runs down.

If more plutonium is being produced than expected, the number of these high-energy particles will fall at a higher rate as more uranium is burned up.

This telltale can be verified by measuring the temperature of the water going into the reactor with that of the water going out at that point in the fuel cycle.

If those two sets of data do not tally with the IAEA's books, something untoward has been going on -- excess plutonium has been produced and has been diverted.

The first prototype detector is being used to detect antineutrinos at the San Onofre nuclear power plant at San Clemente, California.

Scientists in France and Brazil are also building their own prototypes, the British scientific weekly says.

For the detectors to work, they would have to be installed close to the reactor.

They would help the IAEA to keep a much closer eye on plutonium, one of the most feared substances around.

Plutonium is highly toxic as well as fissile, and could be used to make a "dirty" bomb, in which radioactive particles are dispersed around a city by conventional explosives.

However, the invention would not help measure the output of countries that refuse to have their facilities monitored, do not declare their reactors or press ahead with building a nuclear bomb using enriched uranium rather than plutonium.

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