Monitoring Nuclear Fuel With Sound

Photo: Toby Talbot/AP Photo

Nuclear reactors are packed with a host of sensing and control systems. But at their innermost core, where nuclear fuel burns inside metal rods, conditions are so extreme that placing any kind of sensor has been a huge challenge. As a result, reactor operators haven't been able to get a complete picture of how their nuclear cores are performing.

Now researchers have developed sensors that convert the heat differences inside the reactors into whistle-like sounds that reveal the condition of the fuel. These thermoacoustic sensors could provide operators with valuable data like temperature, gas composition, and pressure inside the fuel rods, as well as fuel burn-up rate and swelling of the rods.

“These parameters aren’t measured now because it’s cost-prohibitive to do so,” says James Smith, an Idaho National Lab researcher involved in the project. Thermocouple sensors that have been tested in reactors fail too often to be worth the time and money it takes to put them in a reactor, he explains.

Yet the need for sensors that operate and transmit data without requiring electrical power is clear after the Fukushima Dai-ichi nuclear accident, he says. A loss of power at that plant left operators unable to monitor the fuel rods in the reactor and the spent fuel in storage ponds. And monitoring isn’t just for emergencies, but also for research and development. “We would like to develop sensors to monitor the fuel real time so that we can understand how it performs in the reactor,” Smith says.

The sensor that Smith and his colleagues at Penn State University and Westinghouse Nuclear Services have developed is a thermoacoustic heat engine, a device that converts a heat gradient to a high-amplitude sound. A nuclear reactor is an ideal environment for the device, Smith says. “You have hot fuel giving off heat, then cooling water providing gradient to accept all the heat.” 

A thermoacoustic heat engine consists of a resonator, which is a metal tube, and a stack, which is a structure containing parallel channels. The researchers made their stack from a ceramic material containing 1100 parallel square channels per square inch. Depending on what is being monitored, the fuel rods themselves could be used as resonators or fuel-rod-sized resonators could be inserted into  empty spaces in the fuel stack between fuel rods.

A temperature difference across the stack induces a sound wave inside the resonator. The sound vibrates back and forth inside, resonating at certain frequencies. Each resonator would be tuned to a different resonance frequency by, say, changing its length or adding some weight to it, Smith says.

Changes in temperature, pressure and gas composition inside the fuel rod, as well as radiation-induced changes in the rod's dimensions all affect the resonance frequency, changing the sound coming out of the resonator. In a reactor, the sound would propagate from the resonating rod through the surrounding water or other cooling liquid to be detected with a hydrophone outside the reactor vessel. “Basically we’re making a whistle out of fuel rods,” Smith says. “It’s a novel, simple idea, and a viable option for monitoring nuclear fuel.”

The researchers have already demonstrated that the sensor can measure temperature and changes in the gas mix in a fuel rod. They presented the research at the Acoustical Society of America meeting in Providence, RI this week.

They plan to test the sensor at a test reactor on the Penn State campus next summer to see if they can measure the density and velocity of neutrons in the reactor core based on the changes in the amplitude of the sound output. These neutron measurements are used to control the reaction inside a nuclear reactor.

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