San Onofre reactor hosts experimental monitoring scheme that measures antineutrino production
A Sandia National Laboratories/Lawrence Livermore National Laboratory collaboration is exploring a new way to safeguard fissionable material, by building an antineutrino detector a few dozen yards away from the core of a reactor at the San Onofre Nuclear Generating Station in San Clemente, Calif.
Antineutrinos are created in fission reactions in which atoms split into lighter elements, which themselves decay, emitting pairs of electrons and antineutrinos in the process. The latter are subatomic particles so insubstantial that they pass through most matter without interacting. However, room-sized detectors like the one coming online at the San Onofre plant can detect a small fraction of the huge number of antineutrinos released from the reactor core. The detector, a pool of liquid laced with massive gadolinium atoms, emits two bursts of light generated when an antineutrino hits a proton. Photomultiplier tubes above the scintillation fluid detect the light.
Equipping the plant for the field trial has been an ongoing project for the past year. John Estrada (8120) joined the team as project lead at Sandia when he came here in April. He completed his doctorate in physics at MIT, where he researched antimatter.
"Reactors make lots of plutonium while making electricity," he says. "The purpose of this project is to measure antineutrinos to show the material has not been removed, or plutonium is not being produced at an abnormally high rate by modifying the operating parameters of the reactor."
Unlike standard monitoring practices of visits by inspectors, record logs, and video cameras mounted on fuel rods, the detector provides a continuous, direct, remote measurement of events inside the reactor core.
The tests will explore whether measurements can be done well enough to become a facet of safeguarding and surveillance. Factors being explored include detector volume, background shielding, the amount of photomultiplier tubes, and sophistication of electronics.
Rather than spotting diversion of material, John anticipates it would have a deterrent effect. "It’s like a metal detector at an airport," he says. "People don’t bring anything through because they know they’d get caught."
The device can also be used to independently measure the plutonium content in a spent reactor core destined for reprocessing or storage. In contrast, the international regime that currently safeguards civilian plutonium production relies on operator declarations and plant operating history to estimate plutonium content.
In the reactor, uranium in the fuel rods releases six antineutrinos per atom during radioactive decay. As electricity is produced, some plutonium is also created, which releases fewer antineutrinos per atom. By measuring the energy and rate of antineutrinos released, the researchers can watch for anomalies, such as an unexpected change in spectrum after a shutdown.
Although they are hard to detect, plenty of antineutrinos are produced — 100,000,000,000,000,000,000 per second. "It’s a bigger number than anyone can imagine," John says. Of these, about 5,000 per day collide with protons in the detector volume, producing a neutron and a positron simultaneously. These final state particles each generate a flash of light in the liquid scintillator. The flash is detected by a photomultiplier tube.
The overall detector measures about 15 feet per side and is located between the inner and outer walls of the reactor, about 25 feet from the core, in a ring-shaped room known as the "tendon gallery." It was partly due to the convenience and availability of this space that led research team member Adam Bernstein, a former Sandian who is now the project lead at Lawrence Livermore National Laboratory, to arrange for the project to take place at this reactor near San Clemente.
The project grew from an early study of antineutrino rates and spectrums from bombs, Bernstein said.
At the center of the detector is a cubic meter of scintillator fluid, surrounded by a water shield that screens out gamma and neutron backgrounds. Five sides have an additional shield that reduces the effect of muon particles, which can create antineutrino-like events.
The central core was recently redesigned. Final installation is being completed now. In May, the detector should be turned on to collect data for about a year to evaluate this potential approach to monitoring.
The technology has the potential of supporting verification of the Nuclear Nonproliferation Treaty, John said. "It can be used to help determine that countries aren’t making more plutonium than they have agreed to make in a civilian reactor, and that this plutonium is not being diverted into weapons programs," he said.
Team members on the project include Mike Greaves (8120), Steve Haney (8731), Jim Lund (8233), Duanne Sunnarborg (8358), Mark Zimmerman (8731), and Dept. 8120 Manager Carolyn Pura.