Preventing the proliferation of nuclear weapons is high on the agenda of many nations, but monitoring the hundreds of nuclear reactors worldwide to ensure that they are not secretly producing material for bombs requires time-consuming on-site inspections. Now, a team of scientists in the US believes that much of this monitoring could be done remotely, by detecting the antineutrinos generated during nuclear fission.

Adam Bernstein of the Lawrence Livermore National Laboratory and colleagues at Sandia National Laboratory — both in Livermore, California — have built and tested a prototype of such a detector, and have shown that it could be used to alert inspectors to possible illicit activities within a matter of hours.

Plutonium extraction

Nuclear power reactors generate electricity through the fission of uranium-235. However, the isotope that makes up most of the uranium in a reactor’s fuel, uranium-238, is not itself fissionable, but can capture neutrons and then undergo two beta decays to form plutonium-239. This plutonium is fissionable, which means that it too generates power within a reactor — but it can also be extracted to make bombs.

The fission of both uranium-235 and plutonium-239 generates antineutrinos at a rate that depends on the reactor’s total rate of fission and therefore its power. According to Bernstein and colleagues, monitoring these antineutrinos can therefore tell weapons inspectors both when a reactor has been switched off — which in most reactors is mandatory if the plutonium is to be extracted — and how much plutonium the reactor could have generated while it was running.

Liquid scintillator

In a paper to be published in the Journal of Applied Physics, Bernstein’s group describes how it was able to make these measurements using a 1 m3 detector operating inside the San Onofre Nuclear Generating Station in southern California over the past two years. Their detector was placed some 25 m from the reactor core and consists of a liquid scintillator (composed of mineral oil and trimethylbenzene), surrounded by a 0.5 m-thick water shield that screens out extraneous radioactivity.

While antineutrinos will normally pass straight though even the densest materials without interacting, they will occasionally strike protons in the scintillator causing a distinctive pair of light flashes that are registered by photomultiplier tubes. These flashes are converted into an electronic output and sent to an on-site computer, which filters out real antineutrino events from background noise, calculates the antineutrino detection rate and then sends off the results of its analysis via a modem (a similar, albeit slightly more complex, experiment had previously been carried out by researchers in the Ukraine in the early 1990s).

Hundreds per day

The researchers were able to detect about 400 antineutrinos per day, compared to an antineutrino-like background of about 100 events per day. Given the time needed to build up statistically significant results, they were able to judge with 99% confidence whether the reactor was on or off using around five hours’ worth of data. They were also able to measure the relative power output of the reactor (i.e. its power relative to a known initial value) with a precision of 3.5% using data collected over a week.

Bernstein believes that the detector would make life easier for both reactor operators and the International Atomic Energy Agency (IAEA), which is responsible for monitoring the safety and security of the world’s nuclear installations He says it may be less intrusive than the current inspection regime, since it could cut down on techniques such as video surveillance; that it would provide a continuous, independent source of data for inspectors; and that it may also lead to fewer costly and time consuming visits by inspectors