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Centrifuge Sentries

By Craig Tyler|August 01, 2020
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Inspectors need robust and reliable instrumentation to ensure that nuclear enrichment facilities are used for peaceful purposes.

What is happening inside the world’s uranium enrichment facilities?

It is the job of the International Atomic Energy Agency (IAEA) to verify the peaceful use of special nuclear material. The IAEA deploys teams of inspectors, partially trained at Los Alamos, to gas centrifuge enrichment plants (GCEPs) to ensure that the plants are only producing low enriched uranium (LEU, suitable for nuclear power production but not weapons)—and in declared quantities—not highly enriched uranium (HEU). They use a suite of instrumentation, including an on-line enrichment monitor (OLEM), to verify the production of LEU for domestic nuclear power. These inspectors may be okay with a proliferation of acronyms, but not a proliferation of weapons-grade nuclear material.

A GCEP, working with uranium in the form of uranium hexafluoride gas (UF6), separates the fissionable isotope uranium-235 from other isotopes in natural uranium, which contains less than 1 percent U-235. Standard enrichment results in a few percent U-235—that’s LEU. But with certain illicit modifications, the enrichment can exceed the 20 percent threshold for HEU, and such undeclared enrichment could take place in a small corner of the GCEP. To determine the relative U-235 enrichment, an OLEM combines gamma-ray spectrometry with indirect gas density measurements. However, those density measurements rely on temperature and pressure measurements, the latter of which generally come from separate instrumentation controlled by the plant operators, not the OLEM itself. Furthermore, since almost everything happening inside of a GCEP is considered commercial proprietary or sensitive information, the IAEA and the plant operators have to agree on a monitoring regime that allows the IAEA to verify an operator’s declaration while also protecting its sensitive technology.

An illustration of a pipe showing the hoop strain, which is measured by the pipe's circumference, and the axial strain, which is measured by the length of the pipe.
Sensing gas pressure inside a pipe from the outside: With sufficiently sensitive sensors, the measured hoop and axial strain—expansion or contraction in the circumference and along the length of the pipe, respectively—reveal subtle changes in the pressure difference between the interior and exterior of the pipe. (Simultaneous measurements of strain along the circumference and length of the pipe capture the confounding effects of thermal expansion, allowing them to be subtracted out.)

“GCEP monitoring is a complex task, especially in less-cooperative environments,” says Rollin Lakis, a Los Alamos nuclear safeguards scientist. “To monitor a centrifuge plant with confidence—and what’s the point otherwise?—requires independent and trusted measurements of different process variables, including the pressure inside the UF6-carrying pipe. A new method to measure the mass-flow rate at many different locations in a GCEP would enable significant, near real-time design verification against facility misuse scenarios.”

Lakis teamed up with Los Alamos colleague Alessandro Cattaneo, a mechanical engineer with expertise in heat transport, modeling, and complex sensor systems. The devices Lakis and Cattaneo have in mind must be noninvasive, mounting onto an existing pipe rather than being built directly into the gas flow, if they are to be used at many locations within a GCEP. They have to be self-reliant, obtaining the pressure and mass flow inside the pipe using only the data they collect from the outside. They have to be connected to other devices around the plant and to IAEA inspectors’ information stream. And they have to be easy for inspectors to install and maintain.

So Cattaneo, Lakis, and other collaborators designed two different devices aiming at measuring the flow pressure and the flow rate from the exterior of a UF6-carrying pipe.

One collaboration, with Marcelo Jaime of the Materials Physics and Applications division at Los Alamos, resulted in a device that determines the internal pipe pressure (relative to the external atmospheric pressure) based upon strain measurements taken on the outer surface of the pipe—i.e., the utterly minuscule amount by which the metal pipe itself expands or contracts in response to the pressure difference. To obtain a sensitive enough strain measurement, they integrated an ultra-sophisticated infrared laser-based fiber-Bragg-grating (FBG) interferometer, which detects stretching in the pipe metal at the level of tens of parts per billion, or equivalently the length of approximately ten iron atoms along the circumference of tested pipes.

“Our simulations and feasibility studies showed it was possible,” says Cattaneo. “So we built a mockup device to try it out. For pressures of interest in appropriately stiff aluminum and steel pipes, we have already obtained about 5 percent internal pressure-measurement sensitivity.” That’s good but not good enough. Lakis, Cattaneo, and Jaime are currently working to get the sensitivity down to 1 percent or better.

A graph above a pipe showing how the temperature of the middle of the pipe causes less flow.
Sensing mass flow inside a pipe from the outside: The effect of a localized heat source on pipe temperature up- and downstream depends on the rate of mass flow inside the pipe; more flow lowers the temperature on the outer surface of the pipe. In real-world operation, both devices would be self-contained inside an insulating, tamper-proof casing.

“A strategy to improve resolution by at least a factor of ten, possibly even 100, in the FBG interrogation has been identified in a new technology currently under consideration,” reassures Jaime. 

The second collaboration, carried out with Robert Goldston of the Princeton Plasma Physics Laboratory, led to the creation of a noninvasive and operator-independent thermal mass-flow meter. The prototype applies a temperature gradient along the length of the outer surface of a gas-carrying pipe. The device correlates the internal mass-flow rate with external temperature and heat power measurements. In its simplest embodiment, with a single heater wrapped around the pipe, the more gas is flowing, the greater the cooling effect on the pipe. “Our team showed that a 1 percent mass-flow-rate accuracy is within reach.” says Cattaneo.

“We have a little farther to go yet,” says Lakis, “but the good news is, we’ve already demonstrated that these methods work effectively. We have confidence that we can get the rest of the way there.”

We have confidence: and that’s exactly the point. LDRD

Top image credit: U.S. Department of Energy