In order to scan nuclear reactors for forbidden uses such as weapon-making, researchers are now working on remotely monitoring nuclear activity by focusing on ethereal particles known as antineutrinos.
Nuclear reactors supply the planet with much of its electricity, providing France alone with more than three-quarters of its power. However, the uranium and plutonium that serve as their fuel can be diverted from reactors for use in weapons.
The International Atomic Energy Agency has installed nuclear safeguard systems to monitor these reactors. Although effective, these systems cannot accurately determine in real time how much plutonium or uranium is present in the fuel rods of operating reactors. Some of these systems also interfere with reactor operations.
Now researchers are investigating devices known as antineutrino detectors as a continuous, real-time and less intrusive technique than prior safeguard systems. And the International Atomic Energy Agency has started to consider the potential of these detectors to keep tabs on reactors by flagging excess plutonium and uranium being used beyond what its operators declare it is making. Such a detector could be placed by safeguard agencies on the reactor site a few dozen yards away from the reactor core.
Detecting odd particles
Here's how they'd work: Nuclear reactions and radioactive decay emit particles known as neutrinos and their antimatter counterparts, antineutrinos. These particles can zip through matter almost unaffected.
"Nuclear fission reactors are the most intense man-made source of antineutrinos," said physicist Nathaniel Bowden at Lawrence Livermore National Laboratory. "They don't produce neutrinos, only antineutrinos."
Extraordinarily infrequently, an antineutrino reacts with a proton to produce a neutron and a positron, the antimatter counterpart of an electron. Positrons quickly annihilate electrons, generating gamma rays.
The detectors that researchers are building contain instruments that spot both the neutrons and gamma rays that result from antineutrino collisions, as well as a material with a lot of protons in it. The number of antineutrinos that nuclear reactors emit is so large that a detector just a cubic yard or so is large enough to record hundreds or thousands of them per day.
"Two of the detection mediums we typically use are called scintillators — these are made of either a solid plastic or a kind of oil, materials that contain a lot of hydrogen or essentially protons," Bowden explained. (A hydrogen atom consists of one proton and one electron.) "You also introduce something that makes the detection material scintillate — give off light when charged particles interact in it."
Another detector system type relies on water as the detection material. There, researchers look for Cerenkov radiation instead, a type of light emitted when a charged particle travels through a material faster than light does. (This is possible because rays of light each possess a certain wavelength; if a particle is smaller than that wavelength, it can zip through certain materials faster than light can.)
"This Cerenkov radiation is about 10 times less than that produced with the scintillators, so it makes our job harder, but of course, water is very inexpensive, so we're trading affordability with performance there," Bowden said.
Ideally, antineutrino detectors are placed underground, as the overlying material helps shield against cosmic rays that might be confused as antineutrinos. Still, not all reactors have underground spaces where such detectors might be housed, so researchers are also developing devices that can work above ground and take the extra noise from cosmic rays into consideration.
"If this technology was to be accepted broadly, I'd expect an antineutrino detector to cost $100,000, comparable to other reactor safeguard systems in use, with greater capability," Bowden said.
Scientists at Lawrence Livermore and Sandia National Laboratories have performed proof-of-principle tests that show antineutrino detectors can monitor nuclear reactors, using a liquid scintillator detector about 30 feet below ground. Now researchers are testing two prototype above-ground antineutrino detectors at the San Onofre Nuclear Generating Station in California, one using a solid plastic scintillator, the other using water.
"We have encouraging indications that we may have above-ground capability in the future," Bowden told LiveScience.
The scientists will detail their findings May 2 at the American Physical Society meeting in Anaheim, Calif.
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