Of all subatomic particles, the neutrino is the least understood. It is the second most abundant particle in the universe but it has minimal interaction with the particles in normal matter. Neutrinos come in three varieties known as the electron neutrino, the muon neutrino and the tau neutrino. To make matter more complex and confusing, each of these three types can change into another type in a process called neutrino oscillation. There are hints in nuclear experiments that there may be a forth type called a sterile neutrino that has even less interaction with normal matter. Some recent experiments have raised doubts that such sterile neutrinos exist, but other recent experiments seem to confirm their existence.
Experiments at nuclear reactors and particle accelerators have verified that neutrinos have the ability to switch types in neutrino oscillation. An experiment at the Los Alamos National Laboratory reveals that there were extra electron antineutrinos. In the current theory of particle physics, every particle should have an anti-particle twin that behaves in the same way. If a particle and its anti-particle collide, they will annihilate each other. Another experiment also showed extra particles.
One possible explanation for the extra electron neutrinos is that there might be a forth type of neutrino which was named the sterile neutrino. This type cannot be detected by the experimental apparatus but the other three known types can change into this type. There are four fundamental forces in nature that govern particle interaction; gravity, electromagnetism, the weak nuclear force and the strong nuclear force in order of the decreasing distance at which interaction can take place. The three known types of neutrinos interact with the particles of normal matter via the weak nuclear force which can be detected in current experiments and the force of gravity which cannot. It is theorized that sterile neutrinos interact with other particles of matter via gravity alone which places their interactions beyond current experimental detection.
At the Daya Bay nuclear power plant in China, near Hong Kong, neutrino detectors have been set up around the nuclear reactor to study neutrinos being ejected. The detectors contain giant pools of a special liquid that generates a burst of light when a neutrino passes through. The light bursts are amplified and signals are generated by the detectors. This experimental setup has shown something that is being referred to as the “reactor anomaly.” The detectors are only reporting ninety-four percent of the neutrinos predicted by theory. A possible explanation is that after the reactor spits out one of the three verified types of neutrinos, that neutrino changes into a sterile neutrino via the oscillation process before reaching the detector. The detection of the neutrinos in this experiment relies on the weak nuclear force and so it would not be able to see any sterile neutrino.
New data from the detector array in April now suggest that sterile neutrinos might not be the reason for the six percent shortage of neutrinos in the earlier experiments. The researchers found that as the nuclear fuel burned and changed in composition over time, the neutrino shortage also changes. The detectors may be misreporting the neutrinos being generated by uranium-235, the amount of which changes as the fuel burns. Perhaps sterile neutrinos are not involved in the anomaly.
Further analysis of the new data reduces the evidence for sterile neutrinos but does not eliminated it altogether. A study of data from the nuclear power industry and other sources about experiments in the past showed that while they did not show a neutrino shortfall of six percent, they still showed missing neutrinos and those observations were not related a change in fuel composition.
A new experiment called PROSPECT is being developed at the Oak Ridge National Laboratory which will utilize pure uranium-235 instead of a mixture of isotopes such as present experiment at Daya Bay. One major change in the experimental set up is that the much smaller reactor at Oak Ridge is only about twenty feet from the detectors as opposed to a gap of hundreds of feet between the reactor and detectors at Daya Bay.
This research into the generation and transformation of neutrinos is important to the nuclear industry. Neutrinos are generated by beta decay in nuclear fission. This beta decay produces a lot of the heat generated in a nuclear reactor. If neutrinos are not being properly accounted for in the operation of a nuclear reactor, then the calculations of the heat being generated by the fission process may not be correct. If this is the case, it could mean that there might be problems with current nuclear reactor safety provisions.
