Sensors within fuel rod assemblies: W developing with academics

20 June 2014

Nuclear power plant operators may soon be able to monitor a reactor core by using sensors within fuel rod assemblies that literally scream the type and location of a problem, Michael Heibel, technical programme manager at Westinghouse, told World Nuclear News.

Westinghouse has applied for a patent on the device it is developing with academics from the Pennsylvania State University and Idaho National Laboratory. The team is building a prototype they plan to test at the Breazeale research reactor early in January 2015 and Westinghouse expects to bring the device to market by 2019, Heibel said.

Based on the thermo-acoustic engine principle – turning heat into sound – these devices do not require electric power supply. That fact has safety and cost benefits, Heibel said.

Using a thermo-acoustic neutron sensor, or an array of them, in the reactor to monitor the core power distribution and the temperature distribution, removes the need for tubing, wiring and vessel penetrations that are required to support existing surveillance instruments. That reduces the costs associated with maintaining such equipment, Heibel said. Plant operators will be able to monitor the core much more accurately, allowing them to produce more electricity from the same amount of uranium, he said.

Asked whether the device could be seen as a potential passive safety measure developed in the wake of Fukushima, Heibel said it would indicate "when melting is imminent".

"The more heat input you have, which you get when you have lost cooling, the louder it's going to scream," he said.

The "scream" is the frequency of sound wave produced according to the length of each device's resonance chamber.

Ideally, each fuel rod would have its own particular frequency, so plant operators would be able to determine by listening outside the reactor which fuel rods have an issue, if any, Heibel said.

Although these devices could be built into the fuel rods themselves, the Westinghouse team is looking at loading a number of them into a single tube that goes into the instrument thimble located inside the centre of a fuel assembly.

"We would be able to monitor different axial positions in every fuel assembly in the core and from that we could get fission rate and temperature information. Fission rate is probably the most important for the commercial nuclear power industry outside of extreme accident conditions," Heibel said.

Each device is somewhere between five and eight inches (13-20 centimetres) long, depending on the length of its resonance chamber. "We want each one to have a slightly different frequency, so that when we're listening, we can tell which assembly contains that particular device and we can then infer what the power distribution in that assembly is doing," Heibel said. Up to seven of these devices will go into each fuel assembly, he said.

Asked whether the device is being designed specifically for Westinghouse fuel rods, Heibel said, "We do have proprietary information associated with this specific application. It is not something off the shelf," he said. In its patent search, Westinghouse did not find any comparable devices being patented at this time, he said.

The sensors require a "signal processing system" to generate the core condition information in a form an operator can work with, he said.

"This investment generates the best return if the sensors are integral to the fuel. Ideally this system will be capable of being back-fit into existing fuel assemblies and be made integral to new fuel assembly designs," he said.

Researched and written
by World Nuclear News