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Physicists working with the International Atomic Energy Agency (IAEA) have been developing a way for nuclear weapons inspectors to use neutrinos to monitor reactors remotely and in real time. With more development, it should be possible to detect if any of the reactor’s spent fuel has been removed for possibly nefarious purposes.
Spent nuclear fuel has long been a major source of concern for anti-proliferation experts, who fear that the plutonium produced in reactions could be diverted for building weapons. This new system could augment the IAEA’s inspection regime and help to better keep track of dangerous fissile material.
“Current safeguard systems are effective, but they require some cooperation and they also don’t provide real time monitoring. We think we can improve this with antineutrino detectors,” said Fangfei Shen of MIT, speaking at the APS April Meeting in Anaheim.
All fission reactions produce antineutrinos when the bonds holding the nuclei of uranium together are broken. When an isotope’s nucleus splits, it releases antineutrinos with a distinct signature, which the team’s detectors can monitor. Neutrinos make the perfect indicators because they can’t be blocked or suppressed.
“Antineutrinos are very independent particles, they go through everything. You can’t stop them. You can’t fake them. What comes out of the reactor is what’s there,” Shen said.
As uranium-235 is used, some of it is converted to plutonium, the material of choice for most modern nuclear weapons. In order to retrieve this plutonium in most nuclear reactors, typically the reactor has to be shut down and the spent fuel rods removed for reprocessing while fresh uranium is added.
The detectors that the team has developed can register when a nuclear reactor has been shut down, and more importantly can detect if plutonium has been removed and replaced with fresh uranium. If the monitors detect an unexpected and sudden spike in the energy of the neutrinos, plutonium has been replaced with uranium.
“What we are providing for the IAEA is a means to determine analytically the amount of uranium and plutonium in a reactor core in a real-time environment,” said Gregory Keefer of Lawrence Livermore National Laboratory.
The two prototypes developed by the team have shown promising results so far. The first, a one-ton water Cherenkov light detector, has been in place at the San Onofre Nuclear Generating Station in Southern California since July of last year. Located about 50 meters from one of its pressurized water reactors, the detector has been reading the neutrino signatures of what’s happening inside the reactor in almost real time.
“In the first prototype, we’ve shown that we are capable of determining if the reactor has been turned off within five hours. We can determine over the time of one week what the absolute thermal power of the reactor is to three percent. During these refueling outages this was a chance for us to really determine what the amount of plutonium difference would be,” Keefer said. “We’re capable of saying with a 95% confidence level that during refueling there was a 70 kilogram removal of plutonium.”
The team also installed a second, more advanced prototype at San Onofre in December. This one is a prototype segmented liquid scintillator, which can identify several kinds of events including neutrons and gamma rays. This feature comes into play because these neutrino detectors are located above ground, so they need to differentiate between normal background signals from cosmic rays, and neutrinos generated in the reactor. The team is still sifting through the data from this second detector, and should be finished with the analysis by the end of the year.
The IAEA also asked the team to develop a way to monitor another kind of reactor more difficult to keep tabs on: bulk process reactors. In order to remove the plutonium from pressurized water reactors like the ones at San Onofre, they have to be shut down, which is easily detected by a drop off in neutrino production. The fuel in bulk process reactors moves through it on a kind of conveyor belt. Fresh fuel goes in one end and spent fuel can come out the other, without ever shutting down. The team has developed another prototype detector, this one to be installed at the Point Lepreau Nuclear Generating Station in New Brunswick, Canada next year to start developing a profile of the neutrino flux in these reactors.