FUKUSHIMA PREFECTURE, Japan -- One of the reactor buildings at Japan's damaged Fukushima Daiichi nuclear plant looks far different from the others. Once just a squat rectangle, the No. 4 unit is now wrapped inside a fortress of iron beams, rising as high as a 10-story apartment.
Working from platforms and catwalks atop this new steel tower, Japanese engineers will soon begin extracting the No. 4 unit's fuel rods, in what the plant operator calls a delicate but groundbreaking step in a cleanup expected to last for decades. The operation could begin in the next week or two, officials say, marking the first time since the March 2011 meltdowns that Japan has taken fuel from any of its wrecked reactors.
If it goes smoothly, the rods' extraction will reduce one of several risks that plant workers are contending with, including sustained ocean leaks of contaminated water. Japan's nuclear regulator last month gave approval to the plant operator, the Tokyo Electric Power Co., or Tepco, to begin the rod-removal process.
The operation amounts to a nuclear high-wire act, carried out with cranes and other machinery operating on scaffolding above the No. 4 unit -- an area accessible via several hundred stairs or a skeletally built elevator.
As workers wearing protective white suits prepared some equipment Thursday, Tepco led a foreign media tour that included an up-close look at the fuel rods inside the unit. For 2 1/2 years, the rods have been a source of sharp fear and, later, apprehension.
At the time of the massive earthquake and tsunami, the No. 4 unit wasn't in operation; it was shut down for maintenance, its fuel rods tucked away. The problem was the storage location; the rods were stowed in a pool of water on a high shelf, about 100 feet above the ground.
As nuclear fuel in Units 1, 2 and 3 melted down, U.S. and Japanese government officials feared that cracks may have formed in the concrete basin that held the water of the No. 4 pool. The fear was unfounded, but there was no immediate way to know that, because radiation levels were high enough to prevent access to the pool. Had the water drained out of the pool, the thousands of unprotected uranium fuel rods would have begun melting within eight hours.
Plant workers have since reinforced the pool against aftershocks. But experts say the rods could be stored far more safely at ground level. Once the extraction is complete, they will be.
From the scaffolding above the No. 4 reactor, one can see a pool of murky blue water, as wide and long as a backyard swimming pool but several times the depth. The 13-foot-long fuel rods are submerged, bunched into 1,533 clusters of 80 rods each.
The clusters will be removed by a green crane that one Tepco spokesman described as a "robot arm" and carefully placed into a cask that has been lowered into the water. Filling one cask will take a week. Each cask will be capped, then raised out of the water, lowered to a waiting truck and transported to a storage building about 100 feet away.
During the tour Thursday, several Tepco spokesmen and plant superintendent Akira Ono took pains to say the removal process would not be -- in Mr. Ono's words -- "particularly dangerous." They said the crane and machinery uses redundant wires and brakes, and even in the event of an earthquake or tsunami, the crane won't drop any fuel.
But there are a few potential complications. Small bits of debris, the result of a March 2011 hydrogen explosion, remain in the pool and could make it hard for engineers to pull out the rods cleanly. Some rods could also be damaged and therefore leak radiation. This fuel removal is a "totally different operation" from the usual, Shunichi Tanaka, chairman of Japan's nuclear regulatory agency, told reporters last month.
After the No. 4 unit fuel is removed, Tepco will extract spent fuel from other, far-less-crowded pools in Units 1, 2 and 3. The trickier step, removing fuel thought to be in melted lumps at the bottom of the 1, 2 and 3 reactor vessels, won't begin until 2020.
Tepco hopes to remove all the fuel within 20 to 25 years.