Neutrino detector will be installed at Angra 2 nuclear reactor in Rio de Janeiro

Facility will monitor level of reactor activity in real time and serve as additional safeguard to protect nuclear waste.

By Peter Moon  |  Agência FAPESP – If there are no further unforeseen obstacles, a neutrino detector will finally be installed on the outside wall of the Angra 2 nuclear reactor at Angra dos Reis in Rio de Janeiro State, Brazil.

The detector is designed to perform real-time monitoring of the level of activity in the reactor. It will serve as an additional safeguard to prevent the undeclared removal of spent uranium, the reactor’s fuel, and plutonium, the nuclear plant’s main waste product, which could potentially be used to make nuclear weapons.

“Eletronuclear, the company that operates Angra 2, wants to check its technical feasibility,” said Ernesto Kemp, a researcher affiliated with the Physics Institute at the University of Campinas (UNICAMP) in São Paulo State.

The neutrino detector has been entirely designed and built in Brazil, with the backing of the International Atomic Energy Agency (IAEA).

Its final design is described in the paper “Using Neutrinos to Monitor Nuclear Reactors: the Angra Neutrino Experiment, Simulation and Detector Status”, published by the journal Nuclear and Particle Physics Proceedings. The work is funded by several Brazilian research funding agencies, including FAPESP.

Fission of enriched uranium in a nuclear reactor produces steam to generate power, as well as a number of fission products as radioactive waste, such as plutonium. It also generates neutrinos, the smallest and most numerous known subatomic particles. They carry no electric charge (hence the name), and until the 1990s they were thought to have no mass. Scientists have since discovered that neutrinos do have mass, albeit much less than electrons. However, although their mass is almost negligible, they are produced in astounding quantities by thermonuclear reactions in stars.

The Sun bathes the Earth in floods of neutrinos, which penetrate the atmosphere, sweep around the entire planet, and continue barreling on their way as if they had crossed a void. Neutrinos are so tiny that 60 million of them pass through every cubic centimeter of your body each second, as if all the atoms and cells of which you are made did not exist.

It is precisely this ghost-like property of neutrinos that makes them suitable for use in monitoring the activity of Angra 2’s nuclear reactor. Although the reactor is shielded by meters-thick barriers of lead, steel and concrete, the neutrinos generated by fission traverse all these as if they did not exist. The detector will be located on the outside wall of the power plant at a distance of 30 m from the reactor core. It will measure the flow of neutrinos to monitor the level of activity in the plant.

“To steal plutonium, you have to switch off the reactor. If the reactor’s output falls, fewer neutrinos are generated. There’s a linear correlation between reactor output and the quantity of neutrinos generated,” explained João dos Anjos, co-leader of the project, a researcher at the Brazilian Center for Research in Physics (CBPF) and director of the National Observatory. “We expect to detect some 5,000 interactions per day.”

New technique

A device capable of detecting 5,000 neutrinos per day must be highly sensitive. The neutrino detector consists of a tank containing a metric ton of water purified by ultrafiltration, with 32 photomultiplier tubes on all sides. Every time a furtive neutrino collides with the atoms in the detector’s water (a highly unlikely yet possible occurrence), it will emit light in the form of photons with sufficient intensity to be captured by the phototubes.

Every second the Angra 2 reactor, whose rated thermal output is 4 gigawatts, produces 100 billion trillion (10²²) neutrinos. At a distance of 30 m, the detector will be swamped every second by 1 trillion neutrinos (10¹²). Theoretically, no more than 5,000 neutrinos should be reported each day. In practice, however, the scientists expect to detect approximately 2,500 neutrinos, “for two reasons: the detector isn’t 100% efficient, and filtering out the noise caused by natural radiation will eliminate a substantial number of genuine events,” Kemp said.

According to Anjos, “Japan and France have plenty of experience with neutrino detectors in nuclear reactors. They’re much bigger, weighing 20-80 tons and buried in mines or tunnels. Ours weighs a ton. The main challenge has been creating a small mobile detector for use on the surface. This is a new technique that has to be tested.”

Testing must prove that the hardware and analytical software can distinguish between the interactions generated by neutrinos from the reactor and background noise caused by cosmic rays, the torrent of solar neutrinos and the natural radiation that is all around us. Noise is minimized when the detector is buried and shielded. “But we’re going to operate on the surface, so we’ll have zero shielding,” Kemp said. “Signals from cosmic rays may mimic or distort the signals from reactor neutrinos. We can’t afford to make mistakes.”

When the neutrino detector starts up, the data from all interactions will be sent over a dedicated line to CBPF in Rio de Janeiro City for real-time filtering and analysis. This is a major advantage of the Brazilian detector compared with the control devices currently required by the IAEA for nuclear power plants. Plutonium theft is prevented by monitoring with cameras and temperature sensors. The doors are locked with special seals. “These are invasive methods that operators of nuclear power plants don’t like because they interfere with operations,” Anjos said. “Our detector is external to the plant and doesn’t have any impact on operations.”

The neutrino detector is now being tested at CBPF. In May, it will be reassembled inside of a 12 m container that is already at Angra 2. “We’ll have to put it back together again as quickly as possible,” Anjos said. “We can’t move freely around the facility: we’re obliged to have an escort. The health and safety rules in a nuclear power plant are extremely strict, as they should be, of course, but it makes our work harder. We have to comply with all the rules and controls.”

A long scientific saga

The project team held their first meetings in 2005, eleven years ago. The original idea was to build a small detector as well as bury a second, much larger, detector weighing 50 tons under Morro do Frade, a 700-m-high ridge located about a mile from Angra 2.

The cost of the large detector was estimated at US$50 million. This preliminary design focused on the study of neutrino oscillation. At the time, there were several groups with similar proposals around the world. For cost reasons, various groups joined forces, and the Brazilian group joined the Double Chooz neutrino oscillation experiment, which was built in France.

“The Brazilian group had already organized themselves with the aim of building a neutrino detector for our own nuclear power plant,” Kemp said, “so we decided to go ahead with this project with a different aim, which was monitoring reactor activity.”

The smaller mobile detector cost R$1 million, most of which came from FINEP, the Brazilian Innovation Agency.

“The design and construction of the detector was a long process full of unforeseen hurdles and complications,” he added.

Initially, the design called for a detector filled with liquid scintillator, which is more precise than ultrapure water. “We told the Angra 2 operator that we planned to use liquid scintillator. They had no objection, but when the design was ready they changed their minds, arguing that it was unsafe, with a very small but real risk of combustion. So, we then opted for a water detector, which is less precise but safer. All the work we did between 2006 and 2008 was thrown away,” Kemp said.

Before construction began, the design was tested by computer simulation under the responsibility of physicist Pietro Chimenti, then at the Federal University of the ABC.

“The point of a simulation is that it should be the best possible representation of the experiment,” Chimenti said. “As the results come in, you keep refining the simulation until you reach a satisfactory outcome.”

The simulation had to be refined and completely revamped many times when water replaced liquid scintillator.

This will be the first neutrino detector entirely built and operated in Brazil. “We Brazilian physicists have already participated significantly in particle experiments conducted at the world’s leading labs,” Kemp said. “Building this detector has taught us important lessons on how to do science here at home. If we get positive results, we’ll be contributing to the IAEA’s mission of monitoring the safe disposal of nuclear waste.”

The article “Using Neutrinos to Monitor Nuclear Reactors: the Angra Neutrino Experiment, Simulation and Detector Status” (doi:10.1016/j.nuclphysbps.2015.10.090) by João dos Anjos, Kemp, Chimenti et al., published in Nuclear and Particle Physics Proceedings 267-269: 108-115, can be read at www.sciencedirect.com/science/article/pii/S2405601415010494.