CNL to develop ‘finned’ Candu fuel

13 July 2021

Canadian Nuclear Laboratories (CNL) said on 9 July that the use of 3-D printing had opened a new range of possibilities for fuel, with new geometries, new materials, new fuel blends, and the ability to embed other materials into the fuel itself.

Over the past year, CNL’s Fuel Development team has been exploring through modelling a concept which would see the embedding of metallic fins into a conventional fuel pellet. These fins would help transfer the heat from the centre of the fuel to the exterior of the fuel, and into the reactor coolant.

“In nuclear fuel for power generation purposes, one of the limiting factors is known as centreline temperature,” said Andrew Bergeron, a chemist in CNL’s Fuel Development branch. “Just as it sounds, the centreline temperature refers to the temperature of the fuel in the dead centre, horizontally, in the fuel pellet. Too hot in the centre could cause premature fuel failure; too cool and your fuel isn’t being used efficiently.”

Before the fuel is created, and certainly before it is put into a reactor for testing, it will be thoroughly examined through modelling. Understanding how the fuel will behave, and how it will affect heat transfer is really important. CNL noted that, while building the fuel may seem like the end goal, a lot of work needs to be done in advance. “For this, we turn to CNL’s modelling expertise, well developed over many years of modelling in support of the Candu reactor system.”

“The models are based on conventional heat transfer models,” said computational materials scientist Nana Ofori-Opoku. “The really interesting part is the implementing the specific design of the inserts and matching the materials specific properties across the different materials. This is accomplished by building the modelling framework in COMSOL, a modelling software.”

The modelling results to date show that the temperature across the fuel can be greatly reduced through the embedding of these metallic fins. This can result in a reduction of up to 35% of the peak temperature with respect to a high linear power density of 500 W/cm (standard operating condition for a Candu power reactor). The modelling is currently looking at conventional UO2 and ThO2-UO2 pellets. The metals chosen for the fins – molybdenum and zirconium – were selected because they have good thermal conductivity and have been shown to be compatible with UO2 fuel.

“The initial modelling results are on a single element. We suspect that based on symmetry conditions, our results should be representative of a more general behaviour of a bundle. The more effectively we can get the heat out of that bundle, the more power we can safely generate from within the reactor.”

So, with the modelling well advanced, the next step is to begin fabricating the fuel, a project which will be undertaken in the coming year. For this, CNL will call on the 3D printing expertise within two groups. The fuel portion will be printed in the fuel development laboratories, and the metallic fins will be printed using a metal printer operated by the Mechanical Equipment Development team.

“The current approach – which we are still developing – is that during the printing process, we will put in a polymer resin where the fins will go,” explains Bergeron. “We will then sinter out the resin afterwards, leaving an annulus space where we are able to insert the fins.” CNL said that, although this work focuses on a Candu pellet, the outcome of this project has applications well beyond the heavy-water fleet. In principle, this could be expanded to include research on PWR and BWR reactor fuels as well.

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