日本福島第一核電廠的經營者東京電力公司(東電)在5月12日發布訊息，確認其中一台受損反應爐的核燃料「已熔毀」。
東電官員說，1號機組的燃料棒大部分都已經熔毀，並且落至反應器底部。他們表示，該反應器的冷卻水位已經下降，將使仍然完好的燃料棒再暴露出來。
熔毀的核燃料在反應爐壓力艙管線底部產生了裂口，讓電力公司先前灌入的大量冷卻水溢散出去。東京電力公司說，幸而反應爐壓力容器底部殘留的一些水讓熔毀的燃料保持了低溫。
東京電力公司發言人松本純一(Junichi Matsumoto)表示，水位偏低表示1號機組受損情形比預期的更加嚴重。不過1號機組的溫度仍在危險值以下，因為東京電力公司不停地注入淡水冷卻核燃料，雖然許多的水都由壓力容器底部的破洞漏掉了。
水錶一度修復 燃料棒至少裸露1公尺
在3月12日反應爐發生氫氣爆炸之後，維修人員在5月10日首次進入了反應爐建築內對水錶進行維修，得到了關於核燃料最新的狀態：根據一度修復的數據顯示，反應爐內的水位在燃料棒以下至少有1米深。
根據一個月前通過的搶救策略，東電計畫將污染的反應艙注滿水，並透過熱交換器回收輻射水。
不過，原子能保安院官員西山英彥（Hidehiko Nishiyama）13日表示，東電的注水量不必將反應爐淹沒，可以調整計畫，達到可供系統運轉的水準就可以了。
艙底破洞 冷卻水多數外漏
日本經濟產業省大臣海江田萬里（Banri Kaieda）則表示，注入1號機的水實際上已經由熔毀造成的破洞漏出，因此東電的計畫必須再加以檢討。
目前東電已在1號機組注入超過1萬噸的水，不過推斷只有一半不到的量留在反應爐或壓力艙內。
工作人員表示，洩漏的水應該是在反應爐的地下室內，但是由於高量的輻射使得工作人員無法進入該區域。
機器人也許可以替代工人，但是東電表示，無線遙控系統可能無法延伸到地下室之內。
2、3號爐冷卻水也外洩
東電也對2號及3號機組噴灑了成噸的淡水，而這些在反應爐心受污染的水也有滲漏的情形。為了防止大約7萬噸的污水進入太平洋，工人們將這些水抽到一處已經安裝淨水裝置的建築內。
儘管如此，部份高放射性的污水仍然跨過了東京電力公司為了防止污水入海所設的黏土堤防而流入海中。
東電13日宣佈，該公司正在對4月份所發布的一項計畫進行準備工作，將設置一個將1號機組建物完全蓋住的屏障裝置，預計在6月份開始安裝。東電表示，在「中長期輻射擴散防堵做法尚未出爐前的緊急替代措施」。
福島核災等級達最嚴重的7級
回顧事故歷史，這座日本太平洋岸、東京北方220公里處的福島第一核電廠，在3月11日規模9的地震發生時，因為外部供電中斷而使冷卻系統失效。而隨後而來的海嘯，也使得原先設計用來給冷卻系統緊急供電的發電機，也失去了作用。
地震發生5天之後，核電廠6個反應爐中的3個發生了氫氣爆炸，其中包含了1號機，而1，2與3號機組也發生了部份燃料棒熔毀的事故。
這起事故被評定為聯合國國際核事件分級中最嚴重的7級，而且被認為是繼車諾比核災變後，全球最嚴重的核災變。
Tokyo Electric Power Company, operator of the damaged Fukushima Daiichi nuclear power plant, revealed Thursday that nuclear fuel at one of the reactors is in a state of "meltdown."
TEPCO officials said that most of the fuel rods in the Unit 1 reactor have melted and fallen to the bottom of the reactor. They said cooling water in this reactor has fallen to a level that would expose those fuel rods that are still intact.
Holes have been created by melted nuclear fuel in piping at the bottom of the reactor's pressure vessel, allowing tons of water the company has been spraying into the reactor to cool the fuel to escape.
Still, TEPCO said that some water remains at the bottom of the reactor vessel and is keeping the melted fuel cool.
TEPCO spokesman Junichi Matsumoto said the low level of water indicates that the core of Unit 1 is more damaged than previously believed.
But temperatures in Unit 1 are still below dangerous levels because TEPCO continues to inject fresh water to cool the fuel, although much of the injected water is leaking through holes at the bottom of the vessel.
The new information on the state of the fuel in Unit 1 emerged after workers repaired a water gauge Tuesday after entering the reactor building for the first time since a March 12 hydrogen explosion at the unit. Once fixed, the gauge showed that water in the reactor was at least one meter (39 inches) below the fuel rods.
Under a strategy approved last month, TEPCO planned to fill the reactor containment vessel with water and circulate the water through a heat exchanger.
But today, an official of Japan's nuclear safety agency told reporters that flooding the containment vessel may not be necessary and the company should change its strategy.
Hidehiko Nishiyama said TEPCO only needs to inject water to a height that would allow the system to work.
Economy, Trade and Industry Minister Banri Kaieda said today that the water injected into the Unit 1 reactor has in fact leaked out through holes created by the meltdown. For this reason TEPCO's plan must be reviewed, he said.
TEPCO has injected more than 10,000 tons of water into the Unit 1 reactor but less than half that amount is thought to remain in the reactor or the containment vessel.
The utility says the leaked water is likely in the basement of the reactor building, but high radiation levels prevent workers from entering the area.
Robots could be used instead, but TEPCO says their wireless remote control system may not extend to the basement.
TEPCO also has been spraying tons of fresh water onto Units 2 and 3, and contaminated cooling water from these reactor cores has been leaking. To prevent an estimated 70,000 tons of contaminated water from entering the Pacific Ocean, workers have been pumping it into a waste processing building while a system to purify the water is installed.
Still, some highly-radioactive water has leaked into the sea past the silt fence that TEPCO has set up to contain it.
Today, TEPCO announced it has begun preparation work to install a cover that will completely encase the Unit 1 reactor building, in accordance with its plan announced in April.
The cover will be installed "as an emergency measure to prevent the diffusion of radioactive substances until mid-to-long term measures including radiation shielding are implemented," the company said.
Installation of the cover is scheduled to begin in June.
The power plant 220 km (140 miles) north of Tokyo on Japan's Pacific coast lost cooling functions on March 11 when the magnitude 9.0 earthquake cut off the outside power supply. The resulting tsunami disabled generators that were supposed to operate the reactors' cooling systems in case of emergency.
In the five days following the quake, there were hydrogen gas explosions in three of the plant's six reactors, including Unit 1, and partial meltdown of nuclear fuel rods in Units 1, 2, and 3.
The accident is rated at Level 7, the highest on the UN's International Nuclear Event Scale, and is considered the world's second most serious nuclear disaster, after Chernobyl.
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