The Mark I boiling water reactor was designed by GE to save money with a smaller containment vessel and by allowing the hot reactor core to directly boil the water.
When a fire erupted in a spent fuel pond at Fukshima Dai-ichi next to reactor No. 4, at first the nuclear fire fighters ignored it, since they were trying to prevent a meltdown. Unfortunately, it only takes 110 pounds of melted uranium to cause a nuclear explosion--called a criticality accident because the explosion only lasts microseconds before blowing apart the critical mass. That's what happened in spent fuel pond No. 4, releasing a vast pulse of radiation that drove the humans into their armored vehicles and safe rooms. When they came out, they had voted to multiple the maximum allowed dose of radiation by 2-1/2-times, and had since been spraying sea water into fuel pond No. 4 to prevent any further nuclear explosions. Look for extensive contamination of water and food supplies as the nuclear fires expell radioactive isotopes into the open air and seawater around Fukishima Dai-ichi.
Further Reading: http://bit.ly/NextGenLog-fL2M
