Special Reports

"IRAN is not frightened by the threat of any country and it will continue the path of production of nuclear energy."

With those words from president Mahmoud Ahmadinejad, Iran's engineers ripped off the seals on their uranium-enrichment equipment and fired up the Natanz plant in central Iran. The move came in defiance of the governments of the European Union and the US, which fear that Iran will use the findings of its research to produce material to make a nuclear bomb.

But the issue of nuclear proliferation goes much wider than Iran. The number of nuclear reactors around the world is set to rise as nations look for ways to cut their greenhouse gas emissions, and all reactors can potentially be used to make plutonium for nuclear weapons. Because of this possibility, it might be useful for countries to be able to monitor each other to make sure weapons-grade plutonium is not being made on the sly.

At the moment there is no means of doing this, but researchers believe they have the beginnings of an answer. They are building devices they claim can detect whether a facility is producing radioactive material that could be used to produce a nuclear weapon. Their work is preliminary, and it won't solve the current political crisis involving Iran. Nor for that matter will it shed any light on North Korea's nuclear activities. But the surveillance device they are working on may prove invaluable to the nuclear police of the future.

If all new reactors were required to be fitted with such detectors, it would be very difficult for a country to produce weapons-grade plutonium in secret. It could also allow nuclear powers that have reached an agreement to halt the production of new fissile material to ensure that each country is keeping to the deal, says Cliff Singer, a physicist at the Center for Science, Technology and Security Policy in Washington DC, which advises the US government on science-related security issues. Discussions are under way between India, Pakistan and Israel on drawing up a treaty that could lead to such a moratorium.

At the moment, the International Atomic Energy Agency, the world's nuclear inspectorate, calculates the amount of plutonium produced by each of the reactors it safeguards by monitoring the amount of fuel entering the core, the total amount of time the reactor is on and its power output. The agency monitors the reactors in almost all countries that have them, with the exception of North Korea and some reactors in India and Pakistan. Iran has agreed to allow the IAEA to monitor any reactors that it builds.

The operators of each reactor also carry out their own calculations, and both sets of figures are verified by a mixture of planned and unannounced visits by inspectors, sealed surveillance cameras and satellite images of the steam and heat the reactor emits. But there is no way to directly measure the amount of plutonium a reactor is producing, says IAEA inspector Philip Durst, who is based at the agency's headquarters in Vienna, Austria.

Plutonium is produced as a by-product of the fission of uranium, and can also be used as fuel alongside uranium. By altering the type of fuel rods used in a reactor, or the rate at which neutrons permeate the reactor, it is possible to vary the amount of plutonium produced. This extra plutonium could be diverted to build a bomb without appearing as missing on the IAEA's books. "It is a potential route for proliferators to make plutonium for bombs," says Charles Ferguson, a nuclear proliferation specialist at the Council on Foreign Relations, a think tank based in Washington DC.

This is where antineutrino detectors come in. The devices would be installed near reactors to detect these "ghostly" chargeless subatomic particles, which are generated as a by-product of the fission of both uranium-235 and plutonium. But the antineutrinos produced by uranium fission have more energy than those from plutonium fission, so a detector can determine their origin. As long as you know the amount of fuel going into the reactor, the rate at which the particles stream from the core can be used as a measure of the ratio of uranium and plutonium present.

Over the course of a year-long fuel cycle, as the uranium fissions and plutonium is produced, there is a gradual and predictable reduction in the number of high-energy antineutrinos released. This is because as the uranium is spent, a growing portion of the power produced is a result of plutonium fission, which produces fewer neutrinos at the higher energies necessary to be detected. If something untoward is going on and more plutonium is being produced than expected, the number of particles will fall at a faster rate. Because stepping up the rate of uranium fission to create more power would also have this effect, all antineutrino measurements must also be compared with the power output at the time they were released. This can be measured by comparing the temperature of the water going into the reactor with that of the water flowing out.

It should then be possible to calculate exactly how much plutonium is being produced, says Adam Bernstein, a researcher at Lawrence Livermore National Laboratory in California. His team has built the first prototype detector at the San Onofre nuclear power plant in San Clemente, California, and is already using it to monitor the reactor (see "Doing the headcount").

Using information about the fuel used and the reactor's power output, he predicted how much plutonium would be produced. He then calculated by how much this would cause the rate of antineutrino production to decrease between June and September 2005, and showed that this was almost identical to what the detector observed.

Bernstein cautions that the team's results are preliminary, but they intend to present their results to the IAEA within the next six months. "My guess is they will be enthusiastic," he says.

The IAEA is aware of the research, and has held meetings to discuss the potential of anti-neutrino detectors. One thing that needs improvement, says Durst, is the sensitivity of the detectors, as spotting very small increases in plutonium levels is beyond their capability at present.

To this end, Michel Cribier, a neutrino specialist at the French Atomic Energy Commission, is building a pair of antineutrino detectors, known as Double Chooz, which will be installed near two reactors at Chooz in north-east France. The detectors will begin operating in 2008, and will be used to study antineutrinos in more detail in the hope that a greater understanding of the particles will help to improve detectors.

With the same goal, João Anjos and his team at the Brazilian Centre for Research in Physics in Rio de Janeiro is building a detector at the Angra 2 reactor near the city.

But while installing such detectors in the reactors of compliant nations would allow the IAEA to keep a more accurate record of global plutonium production, it would not help measure the output of those countries that refuse to have their facilities monitored, or which simply do not declare their reactors.

To do this, you would need detectors capable of remotely monitoring antineutrino levels, something that is not yet possible. Computer simulations carried out by Eugene Guillian, a particle physicist at the University of Hawaii in Manoa, indicate that detectors sensitive enough to pick up distant signals would weigh about 10 million tonnes and cost around $100 million to build, putting them well out of the reach of the IAEA.

But a few strategically placed detectors close to countries of interest, such as Iran and North Korea, which would be smaller and cheaper than those needed for global monitoring, would be a more realistic goal, Guillian says. "Targeted monitoring might happen," he says.