Nuclear reactors are designed so that core damage events (e.g. meltdowns) are very very rare. Newer designs also have internal protection such as chemical reactions designed to scrub hydrogen gas out of the containment structure in the event of a meltdown. Basically, in order for a steam or hydrogen explosion to happen, the reactor core has to have lost all cooling and become exposed to air. Air is a much worse coolant and water acts as a neutron moderator, so when the water is removed, fission can take off and the reactor can't be cooled, so it starts to melt itself. When the big slug of molten metal hits water again, it flashes a LOT of steam instantly, causing a huge pressure wave and a steam explosion. This is what happened at Chernobyl.
The reactor core at Fukushima survived the earthquake and tsunami. External power and on-site generators were knocked out (because the gens and their fuel were not adequately protected) and the reactor lost all cooling. It likely boiled off all the water in the core and then began to melt down. Then, some of the steam began to split into hydrogen and eventually caused a hydrogen explosion, which is what ultimately caused the damage to secondary containment and the radioactive release. If they could have kept the reactor cooled, the core would not have melted.
Fun fact - there was another nuclear plant closer to the earthquake epicenter and hit by the same tsunami. It survived just fine because its seawall was taller and its backup generators were protected. The reactor was totally undamaged.
Sent from my SCH-I535 using Tapatalk