INL-developed device opens door for advanced fuel safety studies

The researchers have developed a first-of-a-kind device that can detect and study the critical heat flux of a nuclear fuel rod. Critical heat flux 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 device developed by the researchers detects and measures boiling in a water-filled capsule designed for fuel safety tests.

The tests which have been carried out at INL's Transient Reactor Test Facility (TREAT) will ultimately help researchers better understand the safety limits of nuclear fuel, the DOE's Office of Nuclear Energy said.

"Critical heat flux is an important parameter that regulators use to determine the safety limits of nuclear fuel," Transient Testing Technical Leader Colby Jensen said. "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 boiling detector will be incorporated into future safety tests of advanced light-water reactor fuel designs, including accident tolerant fuel tests due to take place in 2022, DOE said. The tests aim ultimately to improve heat transfer from fuel in light-water reactors, helping units to operate more efficiently to maximise electricity production.

The laboratory has released a slow-motion video showing the progression of boiling leading up to the point where critical heat flux is reached, when large quantities of water vapor bubbles touch the surface of the fuel rod. The video shows an experiment conducted outside of the test reactor in a specially-designed water-filled capsule using an electrically heated fuel pin to simulate the conditions.

TREAT is an air-cooled, thermal-spectrum test facility designed to evaluate the response of reactor fuels and structural materials to accident conditions. The reactor, which reached its first criticality in 1959, was originally built to test fast reactor fuels, but its flexible design means it can also be used to test light-water reactor and other nuclear fuels and materials under extreme conditions. The facility was placed on standby in 1994, but the DOE in 2013 proposed re-establishing the capability to conduct transient testing of nuclear to aid in the development of new, advanced, safer and more efficient reactor fuels leading to a decision to restart the facility. TREAT restarted in 2017 and resumed operations in 2018.