Japanese Whaling Ships are mounted with powerful water cannons to fight anti-whali­ng ships from a distance. Similarly, anti-whali­ng ships are also equipped powerful water cannons. Using water cannons mounted on ships, water can be sprayed on the overheated reactors round the clock, until the reactors cool down and the radiation level goes down.

Please inform Japanese authorities, if it is a suitable solution. It can be quickly implemented, since Japan has lot of whaling ships.

Demonstration of the powerful water cannons mounted on whaling ships:
http://www­.allvoices­.com/news/­7804731-se­a-shepherd­-attacks-j­apanese-wh­aling-ship­s-in-antar­ctic-ocean­-video/ima­ge/6484029­5-the-sea-­shepherd-s­hip-steve-­irwin-l-ex­changes-wa­ter-cannon­-fire-with­-the-japan­ese-whalin­g

The two factors that had to be dealt with were pressure and heat. Regarding pressure:
1) The venting of the steam from the reactor core contained radioactive elements, but as the main uranium reaction had already been long stopped, the radioactive elements were minor byproducts which have very short half-lifes and pose no serious health risk.
2) Rather than venting the steam directly into the atmosphere, the operators must have decided to first vent it into the building containment in order to help contain the (harmless) radioactive elements locally. However, when releasing extreme pressure, hydrogen-oxygen bonds can quickly separate causing an explosive force. This is the presumed explanation for the explosion at reactor 1.
3) The reactor core and venting mechanisms were designed to prevent hydrogen-oxygen explosion from occurring from within the reactor core (which is what happened at Chernobyl), so there was no significant risk of a catastrophic explosion and release of serious radioactive material.

Regarding heat:
1) The uranium pellets will stand up to 3000°C heat (not 4000).
2) The pellets are housed in a Zircaloy containment (which makes up the fuel rod), itself capable of standing up to 2200°C.
3) A small amount of Cesium was detected in the steam, so it is assumed that the Zircaloy rods may have partially melted.
4) By the design of the outer containment vessel (to prevent any catastrophic explosion, as above), even if the fuel had melted and destroyed the reactor core, the melted elements would safely flow into the containment basin below the reactor, after which point operators would simply wait for the melted materials to cool and solidify, and then cleanup (at extreme expense, but no health risk).

Misc points:
1) Because of cooling the reactors with sea water, the reactor chambers will be contaminated with minerals (like salt) that would weaken the integrity of the structure. As a result, reactors 1 and 3 will almost certainly be decommissioned and dismantled (they are already at the end of their usable lifespan of 40 years).
2) Inspection of the remaining reactors (including those at Onagawa and Tokai) may take at least 6 months to 3 years or more. Electricity will be at 15% below capacity, and the current implementation of rolling blackouts may continue for a significant length of time.

For more detailed explanation, please see:
http://bravenewclimate.com/2011/03/13/fukushima-simple-explanation/