The US Department of Energy’s Office of Nuclear Energy said on 27 October that the Idaho National Laboratory (INL) had conducted experiments at its Transient Reactor Test Facility (TREAT) using a first-of-a-kind device that can detect and study the critical heat flux of a nuclear fuel rod.

Critical heat flux is the physical phenomenon that occurs when a fuel rod first begins to overheat and can no longer transfer additional heat to the water. This leads to excessive boiling around the surface of the pin and could potentially cause excessive 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 vapour bubbles touch the surface of the fuel rod. The experiment was conducted outside of the test reactor in a specially-designed water-filled capsule that used an electrically heated fuel pin to simulate the conditions. The entire experiment lasted one second, but provided unique insights into this phenomenon.

“Critical heat flux is an important parameter that regulators use to determine the safety limits of nuclear fuel,” said Dr Colby Jensen, the Transient Testing Technical Leader. “These experiments will help us better understand fuel behaviour and to demonstrate how robust safety features of advanced fuel designs will allow more efficient use of those designs.”

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 light-water reactor fuel designs, including accident tolerant fuel tests in 2022. DOE said the goal is to ultimately improve the heat transfer from the fuel in light-water reactors in order to operate the units more efficiently to maximise their electricity production.