The Kyodo News has reported that the Fukushima Daiichi Unit #5 atomic reactor, and presumably its high-level radioactive waste storage pool as well, lost cooling for 15 hours yesterday. The cooling water surrounding the nuclear fuel heated to close to the boiling point. Cooling was restored before boiling began. If enough cooling water had boiled away, fuel damage could have resulted. As with Units 1, 2, and 3 reactors -- as well as one or more high-level radioactive waste storage pools at the site, including at Unit 4 -- irradiated nuclear fuel exposed to the air due to cooling water boiling and/or drain down, whether it be in a reactor pressure vessel core or high-level radioactive waste storage pool -- can lead to overheating, damage, explosive hydrogen gas generation, and even full-scale meltdown. Molten nuclear fuel burning through primary containment risks release of catastrophic amounts of hazardous radiation into the environment. So does combustion of irradiated nuclear fuel in storage pools, which are not even surrounded by a primary containment structure; the damage at Unit 2, and utter destruction at Units 1, 3, and 4, of the secondary containment buildings have exposed the high-level radioactive waste storage pools to the open sky. Unit 5 -- which was shut down for maintenance when the earthquake and tsunami struck on March 11th -- had previously been declared stable, in a state of "cold shutdown," before the 15 hour lapse in cooling raised the risk level.