Fukushima Daiichi’s wUnit 4 Spent-Fuel Pool Up Close
Wall Street Journal
Phred Dvorak
May 27, 2012

[...] the pool, an 11-meter deep well [...] stretches between floors three and four [...] 

Radiation spiked on the second floor at 500 microsieverts per hour [...]

By the fourth floor, near the top of the building, where an explosion during the nuclear accident a year earlier had blown off the ceiling and bits of walls, readings were down to 300 microsieverts per hour.