Antineutrino Monitoring Technique Shows Potential for Current and Future Nuclear Reactors

Technology to determine the flow of subatomic particles, called antineutrinos from nuclear reactors, could enable constant remote monitoring engineered to identify fueling variations that may denote the diversion of nuclear materials.

It is possible to do the monitoring from beyond the reactor vessel, and the technology may be sufficiently sensitive to identify the replacement of one fuel assembly. The method, which can be utilized with current pressurized water reactors and also upcoming designs anticipated to need less frequent refueling, can possibly supplement other monitoring methods, such as the presence of human inspectors.

The possible utility of the above-ground antineutrino monitoring method for both future and current reactors was validated via widespread simulations performed by scientists at the Georgia Institute of Technology.

Antineutrino detectors offer a solution for continuous, real-time verification of what is going on within a nuclear reactor without actually having to be in the reactor core. You cannot shield antineutrinos, so if the state running a reactor decides to use it for nefarious purposes, they can’t prevent us from seeing that there was a change in reactor operations.

Anna Erickson, Associate Professor, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology

The study, which will be published the journal Nature Communications on August 6^th 2019, was partly supported by a grant from the Nuclear Regulatory Commission (NRC). The study assessed two kinds of reactors, and antineutrino detection technology predicated on a PROSPECT detector now installed at the High Flux Isotope Reactor (HFIR) of Oak Ridge National Laboratory.

Defined as elementary subatomic particles, antineutrinos do not have any electrical charge and possess an extremely small mass. They can also travel through shielding around the core of a nuclear reactor, where they are generated as part of the nuclear fission process. Moreover, the flux of antineutrinos created in a nuclear reactor relies on the power level at which the reactor is used and on the type of fission materials.

Traditional nuclear reactors slowly build up plutonium 239 in their cores as a consequence of uranium 238 absorption of neutrons, shifting the fission reaction from uranium 235 to plutonium 239 during the fuel cycle. We can see that in the signature of antineutrino emission changes over time.

Anna Erickson, Associate Professor, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology

Erickson continued, "if the fuel is changed by a rogue nation attempting to divert plutonium for weapons by replacing fuel assemblies, we should be able to see that with a detector capable of measuring even small changes in the signatures."

The fuel’s antineutrino signature can be as special as a retinal scan, and the way the signature alters over time can be estimated with the help of simulations, she stated. “We could then verify that what we see with the antineutrino detector matches what we would expect to see.”

In the study, high-fidelity computer simulations were used by Erickson and recent PhD graduates Abdalla Abou-Jaoude and Christopher Stewart to evaluate the potential of near-field antineutrino detectors that would be positioned close to—but not within—reactor containment vessels.

Differentiating between particles produced by fission and also those from the natural background are some of the difficulties faced.

“We would measure the energy, position and timing to determine whether a detection was an antineutrino from the reactor or something else,” she stated. “Antineutrinos are difficult to detect and we cannot do that directly. These particles have a very small chance of interacting with a hydrogen nucleus, so we rely on those protons to convert the antineutrinos into positrons and neutrons.”

At present, nuclear reactors utilized for power generation need to be refueled on a frequent basis. While that operation offers a change for human inspection, upcoming generations of nuclear reactors are predicted to work as long as three decades without refueling.

The simulation demonstrated that antineutrino detectors could be used for monitoring sodium-cooled reactors although their signatures will not be same from those of the present range of pressurized water reactors.

Among the difficulties ahead is miniaturizing the antineutrino detectors to make them sufficiently portable to fit inside a vehicle that can be driven past a nuclear reactor.

In addition, scientists wish to enhance the detectors’ directionality to make sure that these remain focused on emissions from the core of the reactor to increase their potential to identify even slight variations.

Furthermore, the detection principle is analogous in concept to that of retinal scans utilized for verifying identity. In the case of retinal scans, an infrared beam passes a person’s retina as well as the blood vessels, which can be identified by their higher light absorption in relation to other tissues.

This mapping data is subsequently derived and compared to a retinal scan that was taken previously and saved in a database. If the mapping data and the retinal scan match, the identity of the person can be verified.

In a similar way, a nuclear reactor constantly produces antineutrinos that differ in spectrum and flux with the specific fuel isotopes experiencing fission. A few antineutrinos interact in an adjacent detector through inverse beta decay.

The signal determined by that detector is then compared to a reference copy saved in a database for the pertinent reactor, initial fuel and burnup; a signal that adequately corresponds with the reference copy would denote that the core inventory has not been secretly changed. Conversely, if the antineutrino fluxes of a perturbed reactor are adequately different from what would be anticipated, that would imply that a diversion has occurred.

In addition, the emission rates of antineutrino particles at varying energies are seen to differ with operating lifetime as reactors move from burning uranium to plutonium.

While the signal produced from a pressurized water reactor includes a repeated 18-month operating cycle along with a three-month refueling interval, the signal produced from an ultra-long cycle fast reactor, or UCFR, would represent a nonstop operation, except for maintenance interruptions.

Inhibiting the proliferation of exclusive nuclear materials appropriate for weapons is a long-term concern of scientists from several different organizations and agencies, stated Erickson.

“It goes all the way from mining of nuclear material to disposition of nuclear material, and at every step of that process, we have to be concerned about who's handling it and whether it might get into the wrong hands,” Erickson explained. “The picture is more complicated because we don't want to prevent the use of nuclear materials for power generation because nuclear is a big contributor to non-carbon energy.”

The study demonstrates the viability of the method and should promote the continued advancement of detector technologies, she stated.

One of the highlights of the research is a detailed analysis of assembly-level diversion that is critical to our understanding of the limitations on antineutrino detectors and the potential implications for policy that could be implemented. I think the paper will encourage people to look into future systems in more detail.

Anna Erickson, Associate Professor, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology

Source: https://www.gatech.edu/