So what caused the Earth to briefly bathe in high levels of radioactivity?

FORBES: A Gamma Ray Burst May Have Irradiated 8th Century Earth

High burnup fuel rods stay hotter longer, emit more radioactivity, and may become brittle, scientists suspect, as the cladding that holds them together endures greater stresses.

FORBES: Fancy New Lids for Nuclear Waste Casks, As Contents Get Hotter