One of the main challenges
in monitoring nuclear power plants is determining quickly and accurately how much uranium is present in the fuel rods of the reactors. Keeping track of the fuel in reactors is crucial to determining whether any fissile material is being diverted from the power plants and one possible way of doing this is to measure antineutrino emissions from nuclear power plant. Antineutrino detectors are already used for other purposes, but with this new prospective use on the horizon, many groups are now researching ways to produce cost efficient antineutrino detectors specifically designed to monitor nuclear power plants.
What are antineutrinos? Antineutrinos are the antimatter counterparts to neutrinos. They are particles that have practically no mass and no charge, and can therefore pass through matter virtually undetected, rarely interacting with other particles.
How the detectors work: As uranium atoms inside the nuclear power plant reactor absorb neutrons and undergo fission, they produce antineutrinos in relatively large quantities. Some Uranium 238 atoms, when undergoing fission, decay into Plutonium, which is also fissionable.
However, when plutonium undergoes fission, it releases only about 40% of the antineutrinos emitted by uranium. Over the course of the fuel cycle the quantity of uranium and plutonium should change in a predictable manner and the number of antineutrino emissions should also change. Therefore, by observing these emissions it could be possible to keep track fuel in a power plant. According to Vera Bulaevskaya and Adam Bernstein, antineutrino detectors could recognize if approximately 73 kg of Plutonium were removed from a power plant and replaced with Uranium.
Another use on the antineutrino detectors is to monitor when the shutting off of the reactors, which would necessarily happen before fuel could be removed. When reactors are shut down, the number of antineutrinos emitted immediately decrease and the change would be detected within a matter of hours. According to recent studies, a detector that was placed 24 meters from a reactor was able to detect that the reactor had been turned off within 5 hours of the event with 99% certainty. This same detector was also able to determine the reactor’s power with 3% accuracy.
Types of detectors: The models of detectors that are currently being developed are either scintillators or water detectors. The scintillators are detectors with mediums, such as oil or plastic, which emits light when antineutrinos interact with particles within the material. When antineutrinos collide with protons, an inverse beta decay process occurs and this leads to two separate, though almost simultaneous, flashes of light which can be easily identified. The other technique, which involves using water as a detection medium, looks for a Cerenkov radiation. This type of radiation is emitted when a particle moves through a medium faster than the light does in that medium.
While the water detectors have the advantage of using a cheap medium and possibly needing less shielding from cosmic radiation, they are far less sensitive than the scintillator detectors. However, some of the scintillator detectors that use oil as a detection medium are also problematic because the liquids can be a health hazard. An alternative for scintillator detectors is the use of plastic materials instead of liquids, which make them safer.
However, even with the scintillator detectors, only a very small portion of the antineutrinos emitted is actually detected. Of all the antineutrinos that reach detectors, only a tiny percentage reacts with the material and is detected. Therefore, research is still being done to find other materials, which could provide higher interaction rates with antineutrinos per volume of detector.
Most of the current prototypes were designed to be placed underground near the nuclear reactors. This has the advantage of shielding the detectors from cosmic rays and keeps them from being an inconvenience to workers at the plant. However, placing the detectors underground might not be an easy task at all power plants, so a new area of research is also trying to develop prototypes that could work above ground, taking into account the cosmic background radiation.
It is very likely that various types of detectors will need to be developed and used together. For example, while scintillator detectors are preferred for close range observations, water detectors are the more viable option for far-field observations due to their size. In this case, the sheer size of the detector makes the use of any other material economically unviable.
At the moment, none of these detectors are ready for widespread use. But prototypes are being tested and researchers seem confident that they will soon be a valuable addition to nuclear safeguard systems. Their use may even go beyond merely physicalizing power plants, for they could be used to detect unknown reactors and even covert nuclear explosions.
- Meissner's article from the Science and Technology Review on Lawrence Livermore National Laboratory's study of antineutrino detectors.
- Vera Bulaevskaya and Adam Bernstein's paper on detecting irregular reactor activity with antineutrinos.
- Bernstein et al, a paper on the current capabilities and future potential of antineutrino detectors.
Bernstein, A. et al. “Nuclear Security Applications of Antineutrino Detectors: Current Capabilities and Future Prospects”. Science & Global Security. 2010, Dec 10; 18(3): 127-192.
Bulaevskaya, Vera & Adam Bernstein. “Detection of Anomalous Reactor Activity Using Antineutrino Count Rate Evolution Over the Course of a Reactor Cycle”. The European Physical Journal. 2011.
Meissner, Caryn. “Antineutrino Detectors Improve Reactor Safeguards”. Science and Technology Review, July/August 2008, Web. July 2012.