Mechanical Response of UO₂ Subjected to Transient Heating

A direct-electrical-heating apparatus was utilized to perform fundamental experiments on short, unirradiated, unclad UO₂ pellet stacks to investigate the effect of varying the energy-deposition rate and energy level on the mechanical response of the fuel to transient heating. Results show that as the rate of energy input to the UO₂ pellet stacks increases, (a) the energy failure threshold decreases and (b) the areal melt fraction at failure decreases. Two significantly different regimes of fuel-motion behavior were observed above and below a threshold designated as the threshold of gross fuel motion. Above the threshold, this motion occurs by molten fuel release. Below the threshold, fuel deforms plastically by a creep mechanism.