This is a good comparison i guess

Steam explosions in boilers and reactors occur when hot surfaces or molten materials suddenly contact cooler water, causing rapid vaporization and a pressure surge. This violent phase change from liquid to steam generates a shock wave capable of structural damage.

Boiler Mechanism

In steam boilers, low water levels expose heated tubes or the furnace to air, overheating metal to extreme temperatures (often over 750°F). When cold feedwater refills the boiler, it flashes to steam instantly due to thermal shock, creating immense pressure that ruptures the vessel. Faulty level sensors or controls exacerbate this by failing to maintain water, while overpressure from stuck safety valves or excessive fuel adds risk.[bimakskimya +3]

Reactor Mechanism

Nuclear reactors face fuel-coolant interactions (FCI) during meltdowns, where molten corium contacts water, fragmenting into fine particles that accelerate heat transfer and steam generation. This premixing phase leads to explosive boiling if steam content stays below ~70% in the mixture. Unlike boilers, confinement in reactor vessels amplifies the blast, as seen in SL-1 (1961) where water hammer from steam slugs destroyed the core.[wikipedia]