I was playing around with fuel categories and burner inserters to learn how things are handled in regards to their handling of the burnt fuel results. In particular, I set the burner inserter to accept both chemical and nuclear fuel, set the burnt_inventory size to 1, and waited for the burner inserter to finish burning a uranium-fuel-cell. As expected, the burnt result had a used-up-uranium-fuel-cell in its inventory. However, unlike other entities that have burnt results, you can't actually extract the burnt result from a burner inserter automatically.
As can be seen in this image, the burner inserter has a used-up-uranium-fuel-cell in its burnt inventory, and I do have another inserter set up to extract the fuel cell, however, that is NOT happening. I can extract the burnt result manually, but that defeats the purpose of doing it automatically.