TOKYO—Japanese authorities are set to announce Friday that they have brought the Fukushima Daiichi complex's devastated reactors to a state called cold shutdown, a milestone in stabilizing the site of the world's worst nuclear accident since Chernobyl in 1986.  A crane operator moved a prefab panel into place for a cover on Fukushima Daiichi's reactor No. 1 in October.  The declaration, falling some nine months after nuclear fuel in the plant's stricken reactors reached meltdown temperatures, would mark a triumph over the chaos loosed by Japan's March 11 earthquake and tsunami. Companies involved in bringing the plant under control describe, with previously undisclosed details, their work in a science-fiction landscape where jury-rigged robots survey forbidden zones and hazardous pools, and workers shrouded a blown-out reactor building with a covering that they maneuvered into place using electric fans and then fit together like Lego blocks.  But these advances were halting, incremental and perilous, underscoring a grim reality: Problems at Fukushima Daiichi remain immense. The looming cleanup effort, people on the ground say, is enormous. "We're taking a step, but it's a big step," Toshio Nishizawa, the president of plant operator Tokyo Electric Power Co., said in an interview Wednesday.  See a timeline of how the Fukushima nuclear incident compares to other nuclear incidents.  Eight months after the March 11 tsunami, the government opened the complex to the press on Nov. 12 for the first time. Read an account of the visit and see photos.  Part of Reactor No. 3 building is seen from a bus window on Nov 12.  The tsunami-stricken nuclear-power complex came closer to a catastrophic meltdown than previously indicated by its operator, Tepco. (More: Reactor Core Melted Fully, Japan Says.)  Mr. Nishizawa's company is set to announce Friday that the fuel in the complex's reactors has cooled to a temperature at which no nuclear reaction is taking place and little radiation is being emitted into the atmosphere. This cold-shutdown announcement would signal an end to crisis control and the start of a new phase of cleanup and plant decommissioning.  Mr. Nishizawa said the plant has stopped emitting levels of radiation that would be harmful to human health, and that Tepco had taken steps to strengthen the crippled reactors to withstand another natural disaster. "We've done everything necessary" to guard against another accident at Daiichi, he said. "But that doesn't mean it's foolproof," he said.  Indeed, there can be few firm declarations about the plant's status. Daiichi's reactors are littered with debris. Many measurement and control systems are on the blink. Radiation levels are too high for people to get close to the reactors, leaving engineers and scientists to make important judgments using computer simulations, scattered bits of data and guesses.  Video of the Quince robot during testing. One Quince became stranded during its deployment in a zone of Japan's Fukushima Daiichi nuclear plant deemed too hazardous for workers.  This modeling has led to dire assessments, such as Tepco's announcement late last month that fuel in the complex's No. 1 reactor likely melted completely through its pressure vessel and into a cement floor of the surrounding containment vessel. Hard information is so scarce that Tepco officials say they still aren't sure how the meltdowns unfolded and about the current state of the nuclear fuel.  Even if cold shutdown is attained, it will take decades for isotopes to decay to safe levels, and to remove the last fuel and completely dismantle the plant. For now, the unknowns are so great that authorities aren't even sure how to start tackling some of the biggest problems, which include locating and stopping the flow of toxic water and removing the melted nuclear fuel.  "We don't know what we should do," says Hajimu Yamana, a professor of nuclear engineering at Kyoto University who heads a government committee studying how to decommission Daiichi. "After all, we don't even know what's happening inside the plant."  The tsunami-crippled No. 4 reactor of Tokyo Electric Power's Fukushima Daiichi Nuclear Power Station in Fukushima prefecture is seen in this photo taken Sept. 24.  Fukushima Daiichi is hemorrhaging enough radiated water each month to fill four Olympic-size swimming pools. To help keep it from getting to the ocean, as happened a few weeks ago, engineers in October started building a 2,500-foot(760-meter), underground wall, using 700 steel pipes, weighing 10 tons each.  Another urgent challenge is keeping the complex's vulnerable reactors protected and separated from the outside world. Bits of highly radioactive debris and dust around the compound could still be scattered by wind and rain.In March, Tepco commissioned a vinyl covering for Unit No. 1, where a hydrogen explosion had blown the roof and walls off its reactor building, exposing much of the fuel containers and plumbing inside.  But radiation levels were too high to send people in to do the work, says Masahiro Indo, a general manager in Shimizu Corp.'s construction technology division, who oversaw the job.  Shimizu wanted to have the cover stretched just 20 inches (50 centimeters) from the walls of No. 1, to minimize the structure's size and weight. That would mean exposing the welders and scaffolding workers normally used for such jobs to radiation levels that ranged in some spots from 35 to 150 millisieverts per hour, well over the 1 to 10 millisieverts companies commonly set as the maximum exposure for one mission. "We would have had to arrange the work in shifts that lasted minutes, which would have meant a huge number of workers,'' Mr. Indo says.  So Shimizu preassembled the pieces of covering in modules, attached to steel beams. Typically, workers would join such beams with welds and bolts. Shimizu outfitted these to snap together. To avoid using workers to guide the modules into place, Mr. Indo says Shimizu had each one rigged with as many as eight electric fans, which could be turned on or off to rotate the pieces as they dangled from a crane.  To minimize crane operators' exposure to radiation, Shimizu covered crane windows with lead and outfitted the cabin with video screens that the operators monitored as they worked the controls.  They finished the job in October. "People were cheering, 'we finally did it,' " says Mr. Indo.But the structure will likely need to be replaced with something more robust in a few years, Mr. Indo said. Tepco is also hoping to cover the damaged building at reactor No. 3, which is as much as 20% larger than the one at No. 1, posing a tougher engineering task.  A bigger challenge: Stricken reactors continue to be cooled by pumping in hundreds of tons of water a day, and somewhere, Tepco has concluded, some of that water is leaking out. Workers are finding highly radioactive water in basements and drains, and detecting hot spots along routes where pipes are laid. Also, cracks in the reactor buildings can let groundwater in or, if water levels aren't strictly controlled, let contaminated liquid flow out. Nobody knows where the holes or cracks are. Nuclear-equipment maker Hitachi Ltd. estimates there are several hundred miles worth of pipes in the reactor complexes alone. In all, thousands of tons of this radioactive water has flooded basements and seeped into drains. Storing it is Hiroyuki Shinohara's job. A Tepco employee who specializes in water-related engineering, Mr. Shinohara says he had never stepped foot inside Fukushima Daiichi before he was called back from a project overseas in March.  Mr. Shinohara started by "buying up every storage tank in eastern Japan''—944 in all, he says—with capacities from 40 to 1,000 cubic meters. To make room for them on the site, he ordered some 41 acres (17 hectares) of forests, an area the size of 31 football fields, to be leveled.  He says he raced to get enough tanks set up before contaminated water started spilling out of Daiichi's basements toward the sea. He says his team didn't have enough time to test the soil for sturdiness.  At one point, he says, a few tanks he had buried in the earth floated out after a heavy typhoon. At another, three of the biggest containers started to tilt under the weight of the water Mr. Shinohara was putting in during a test.  Now, with 90,000 tons of contaminated water stored away and enough tanks for an additional 80,000 tons, Mr. Shinohara has some breathing room. But between leakage and groundwater seepage, Daiichi is generating radioactive fluids at the rate of 10,000 tons a month, with no end in sight.  "To be honest," he says, "there's not enough land'' to store the radioactive water inside the plant compound.  Eiji Koyanagi's job is to help figure out what's going on in buildings where radiation levels are too high for humans to venture. The Chiba Institute of Technology researcher is the designer of one of a handful of robots deployed inside the Fukushima Daiichi reactor buildings, and the only one, says Tepco, that can negotiate steep stairs to check out conditions on upper floors.  The Quince, Mr. Koyanagi's robot, looks like a little cart on caterpillar treads, with a slim camera-mounted stalk on top. The ¥12 million ($154,000) robot can't weld pipes or seal cracks, but it can look for the spots where radiation is lowest, so people can be sent in to perform such tasks.  Mr. Koyanagi's team found that the wireless signal they normally use to control the Quince wouldn't penetrate into a reactor building. So the engineer sent the robot trailing a cable stretching nearly one-third of a mile, up and down stairs and around hallways, rewinding as it came back.  On Oct. 20, on the way back from the fifth floor of Unit No. 2, Quince's cable caught on something. Communications broke off. The robot was stranded on the third floor, Mr. Koyanagi said, where radiation levels remain too high to send a worker to retrieve it.  His team is readying another Quince—he had six total—to send back up to the fifth floor of No. 2. There, it will send readings from the area around the spent fuel pools, a repository for fuel rods that aren't in use but could still be dangerous if they get too hot.  As for surveying the buildings' leaky basements, that will take a future generation of robot—probably one that can navigate underwater like a submarine. Even then, Mr. Koyanagi says, there are challenges. The muddy water down there means the robots will need sonar to spot cracks. After cracks are found, engineers will have to figure out how to seal them with glue that doesn't dissolve in radioactive liquid. Adds Mr. Koyanagi: "Even if a submarine robot is developed, who is going to take it to the basement and put it into the water? The water is so radioactive that workers can't even put a water-level gauge there."  It will take thousands of people, and as much as 30 years, experts have estimated, until the last bit of fuel is removed from Fukushima Daiichi and the plant completely dismantled.  Write to Phred Dvorak at phred.dvorak@wsj.com and Mitsuru Obe at mitsuru.obe@dowjones.com
Japanese officials detail how they brought the Fukushima Daiichi nuclear complex to cold shutdown and outline the challenges ahead – a science-fiction tale of Lego-like pieces and robots.