Cooling system breakdowns at Fukushima Daiichi Unit 4 high-level radioactive waste storage pool risk catastrophic "boil down" fire
While international concern has focused on the risk of a big earthquake draining away the cooling water from the Fukushima Daiichi Unit 4 high-level radioactive waste storage pool and sparking a catastrophic fire, a more mundane pathway to disaster keeps rearing its ugly head: the simple malfunction of the pool's ad hoc cooling system. As the Mainichi has reported, most recently, such breakdowns have occurred on both June 4th and June 30th. As soon as the cooling system breaks down, the pool water begins to heat up due to the 1 Megawatt of heat still emitted by the irradiated nuclear fuel stored in the Unit 4 pool, even though some of it has already radioactively decayed for many years or even decades. If the water begins to boil, and evaporates away until the tops of the irradiated nuclear fuel assemblies are exposed to air, they could then catch fire, spewing large quantities of hazardous radioactive cesium-137, cesium-134, and other volatile radionuclides directly into the environment.
Regarding the latest breakdown, Aileen Mioko Smith of Green Action Japan provided immediate, real time translations into English on the news breaking in Japanese language media outlets:
"The latest news (Sankei 7:55am 1 July Japan time) is that Tepco [Tokyo Electric Power Co.] will re-route the UPS and restart the cooling for the Unit 4 spent nuclear fuel pool. At this writing, about 58 hours remain before the temperature reaches the safety limit of 65 degrees [Celsius]."
Sankei News, 1 July (Sunday) 7:55, Fukushima Daiichi, Japanese language article http://headlines.yahoo.co.jp/hl?a=20120701-00000089-san-soci
"Around 6:25am on (June) 30th, The alarm of the coolant system equipment of the Unit 4 spent nuclear fuel pool rang and the equipment stopped automatically. The pool temperature is not going up rapidly and Tepco
will begin restoration measures on (July) 1st. Tepco states there is not leak [a] of radioactive materials from the water."
"According to Tepco, it appears that they connected the main electric source and the coolant system equipment, and the UPS (literal translation: electric source equipment that will not have power failure) broke down (malfunctioned). They tried to undertake cooling with the reserve (stand-by) sytem, however because it was using the same UPS, they could not get it restarted. They will re-route the UPS and restart the cooling. 60 hours remain before the temperature will attain the safety limit of 65 degrees [Celsius]."
Aileen also translated the following Jiji news coverage from Japanese into English:
JIJI, 30 June (Saturday) 23:34, Unit 4 Coolant System Equipment Stops--Irregularities in 2 pumps /
Tepco Fukushima Daiichi, http://headlines.yahoo.co.jp/hl?a=20120630-00000150-jij-soci
"Tepco announced on the 30th that the alarm system which indicates irregularity of the Fukushima Daiichi Unit 4 spent nuclear fuel coolant equipment system sounded, and the equipment shutdown automatically.
According to Tepco and NISA [the federal government's Nuclear and Industrial Safety Agency], the alarm rang around 6:25am on the 30th. Both (two) of the two coolant system equipment pumps stopped working. It is stated that there is a possibility that the emergency power source equipment has broken down (malfunctioned).
Tepco intends to begin repair work from the 1st onward. Although the pool water temperature which was 31 degrees [Celsius] when the coolant system equipment stopped went up to 36.6 degrees at 6:00PM on the 30th, NISA states, 'There is about 60 hours leeway before the temperature will reach 65 degrees which is the safety limit.'"
Aileen reported that on July 1st, "Jiji reported at 16:23 that the Fukushima Unit 4 Pool coolant system had been repaired. Coolant temperature had risen to 42.9 [degrees Celsius] when repairs were completed."