TEPCO’s Risky Operation at Fukushima - The Nation

[Sirveri]

60. All reactor criticality accidents of that nature were typically steam explosions

SL-1 killed 3 people due to a criticality accident caused by excess rod removal of the most reactive rod, this caused a thermal spike, flashing the coolant to steam and blowing the control rod out of the reactor. I heard it impaled the grunt working on it at the time and lodged in the ceiling, but that might just have been BS to make the story sound cooler.

Chernobyl also caused a criticality accident due to failure to account for decay reactivity from the Xenon transient. This caused the explosion of at least 4 reactor pressure vessels (Chernobyl RBMK design had each rod in a seperate pressure vessel, this allowed for in operation refueling, while exponentially increasing the number of potential failure points due to bad welds). The vessel surrounding those vessels was only rated to handle a single failure, and subsequently failed. The resulting steam explosion blew the lid off and through the sheet metal roof.

Both of those the criticality caused steam explosions, the rods themselves didn't actually explode in the traditional sense, though they did thermo-mechanically disassemble themselves, though not with the level of violence many would expect. Flashing water to steam however...