What you’ll see: Critical heat flux is a physical phenomenon that occurs when a fuel rod begins to overheat and can no longer transfer additional heat to the water, leading to excessive boiling around the surface of the pin that could cause fuel damage.
The slow-motion video by INL shows the progression of boiling leading up to the point where critical heat flux is reached, when large quantities of water vapor bubbles surround the surface of the fuel pin.
The experiment was conducted outside of the test reactor in a capsule that used an electrically heated fuel pin to simulate extreme temperature conditions. The ultimate goal is to improve heat transfer and efficiency in light water reactors. The INL research team has adapted and demonstrated its boiling detector device in fuel safety tests in TREAT. The boiling detector will be incorporated into future safety tests of advanced LWR fuel designs, including accident tolerant fuel tests in 2022, according to INL.
“Critical heat flux is an important parameter that regulators use to determine the safety limits of nuclear fuel,” said Colby Jensen, the transient testing technical leader, in a statement released by INL. “These experiments will help us better understand fuel behavior and demonstrate how robust safety features of advanced fuel designs will allow more efficient use of those designs.”