The video is only 14 seconds long, but it provides visual documentation of what happens to a nuclear fuel pin when it starts to overheat. A still from the video shows intense boiling around an electrically heated fuel pin. Source: INL

The slow-motion footage was captured at U.S. Idaho National Laboratory (INL) during an experiment at its Transient Reactor Test Facility that simulated what happens to a nuclear fuel pin as it reaches critical heat flux. This physical phenomenon occurs when a fuel rod first begins to overheat and can no longer transfer additional heat to the surrounding water, resulting in excessive boiling around the pin surface and potentially causing fuel damage. The video 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 water-filled capsule that used an electrically heated fuel pin to simulate extreme temperature conditions. The boiling detector device used in this and other fuel safety tests will be incorporated into future safety assessments of advanced light water reactor fuel designs, including accident tolerant fuel tests in 2022, according to INL.