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July 09, 2021

Using fibre optics to better understand fuel changes during operation

Enabled by advances in 3-D printing, CNL is embarking on an innovative approach to fuel monitoring and examination:  the embedding of fibre optic sensors directly into the fuel. Over the course of the last year, Daniel Huston, a PhD student working with others in the Advanced Fuels and Reactor Physics branches at CNL, has completed a number of tests exploring this new technique.

“This is, to my knowledge, a first-of-a-kind experiment,” says Daniel. “The Fuel Development team here at CNL has been working on printed fuel for a few years, and this work is a natural ‘next step’ as we become more confident in building complex geometries.”

Fiber Bragg Gratings (FBG) inscribed on optical fibers were chosen as sensors to be tested as they allow for the simultaneous measurement of pellet strain and temperature and are able to operate under high radiation fields. These results will be used as a benchmark to compare to the results of similar tests to be later performed within an irradiation field (e.g., using irradiation sources and neutron generators at CNL or at a research reactor. While this testing was done on a surrogate alumina pellet, ultimately, the goal is to develop a process for the 3D printing of a real ThO2 or UO2 pellet with embedded temperature and strain sensors.

The ability to embed the fibre optics directly into the pellet will allow an operator to monitor ‘real-time’ behaviour of the fuel under irradiation during operation, but, it will also help reduce the time needed for the regulatory qualification of new fuel, as it simplifies or mitigates some of the more conventional testing typically done through post-irradiation examination (PIE).

“As SMR and advanced reactor designs move towards deployment, many of these will need to qualify their fuels; a technique like this can provide a lot of data, formerly only available in PIE, in a much faster time frame.”

The goal for this initial exploratory round of testing was focused on understanding how to embed the sensors themselves, evaluate the various modes of operation, and understand whether this is a viable technique.  To do, the team created a stack of four pellets.  Between each pellet a thermocouple was embedded.  These thermocouples provide data against which the readings from the embedded optical fibres could be compared. The assemblies were heated through a range of temperatures, and through the application of weights, strain was measured.

Dan and the team completed four tests, and learned quite a bit along the way.

“Our initial test used a pellet which had a helical shaped void, and threading the cables through that shape proved difficult. Each cable has five sections with FBGs inscribed; we broke three of the five sections in the process. We also realized that if the voids fit too tightly around the cables, that friction became an issue, as the pellets were heated. Subsequent tests used a pellet with two straight voids: one to measure temperature with the cable fixed only at one point, and one with the cable fixed at both ends which was used to measure the strain as the pellet expanded during heating.”

So, what did we learn?  In short, that this technique is viable. Though it took some trial and error, and there are several areas of improvement which could be addressed in subsequent tests, the results have shown the applicability of embedded Fibre Bragg Gratings for the measurement of temperature and strain properties of ceramic pellets in preliminary benchtop experiments.

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