On Monday, by far the most dangerous nuclear operation attempted in human history was set to begin in the crippled Fukushima Daiichi power plant in Japan, the removal of more than 1,300 spent fuel rods and some 200 unused rods from a reservoir on top of Unit 4.
While the undertaking is necessary, the worst-case scenario would pale in comparison the triple meltdowns of 2011 and necessitate the evacuation of the capital Tokyo.
Experts are unanimous that the engineering challenges are on a scale unseen to date, given that the fuel pool was damaged in a fire caused by a cooling failure and a subsequent explosion during the meltdowns. If the fuel rods, some of which may be damaged, come too close to each other, there is a chance that the nuclear
chain reaction would resume, which would be catastrophic in the presence of so much fissile material, as well as extremely difficult to stop.
If, on the other hand, a fuel rod breaks or is exposed to air and ignites, this would release into the atmosphere a massive amount of radiation, likely necessitating the evacuation of the plant. The total amount of radiation present in the pool is estimated at 14,000 times that released by the atomic bomb dropped at Hiroshima, or about the same as in the combined cores of the three reactors that melted down.
"[F]ull release from the Unit-4 spent fuel pool, without any containment or control, could cause by far the most serious radiological disaster to date," states The World Nuclear Industry Status Report 2013, compiled by two independent nuclear energy consultants. 
In several recent interviews with different media, Arnie Gundersen, a former nuclear industry executive and chief engineer of the Fairewinds Energy Education non-profit, cautioned that there was no system to stop a nuclear chain reaction, if one should occur, at the pool, and recommended that the operators "throw all sorts of boron into the water" (boron captures neutrons and slows down chain reactions) before they start pulling the rods out.
"I ran a division that built fuel racks, and these high density fuel racks like they have at Fuksuhima are very close to going critical anyway. ... Normally its 0.95, and it can get as high as 0.99; that means thereís a 1% margin before a self-sustaining chain reaction can occur." 
Gundersen said in a separate interview with Radio Ecoshock, expressing his opinion that the Japanese government rather than Tokyo Electric Power Company (TEPCO), the plantís operator, should take charge of the operation: "I suspect come November-December-January we are going to hear that the building has been evacuated, they broke a fuel rod, the fuel rod is off-gassing, we have to wait a couple of days and then go back in." 
But even the most vocal critics of TEPCOís and Japanís response to the crisis so far acknowledge that the fuel has to be removed because the danger of doing nothing far outweighs the dangers of doing something wrong.
"If there is another earthquake and building four collapses ... I am going to evacuate my family from Boston," Dr Helen Caldicott, an influential Australian anti-nuclear advocate, said during a recent conference.
While the other exploded buildings hold less nuclear material than Unit 4, moreover, the challenge of removing molten and spent fuel from them is far greater. At least some of the reactor cores are believed to have melted through the containment vessels, and possibly into the ground, contaminating groundwater with unprecedented levels of hot particles.
Some of the buildings are off limits to workers due to the deadly levels of radiation inside, and TEPCO does not even plan to start working there until a few years to a decade from now. Engineers say the present undertaking will be a learning experience and a practice test for that work.
The effort to secure and decontaminate the site has run into numerous snags recently, with critics claiming mismanagement (a story about how the Japanese mafia runs many of the low-paid workers at the plant recently made headlines ) and attempts to cover-up the real severity of the situation. Over the summer, it emerged that the Pacific Ocean was being continuously contaminated with highly radioactive groundwater and that some of the hundreds of make-shift water tanks on site were leaking.
Workers are pumping out some 400 tons of water a day from the reactor basements and the ground nearby, to a total of almost 500,000 tons at present stored at the plant, while another 300 tons a day are running into the ocean. The three molten cores require constant cooling with water, most of which escapes the breached reactor vessels. To make matters worse, Fukushima Daiichi is near an ancient river bed at the base of a hill at the ocean shore, and it is constantly being flooded with groundwater.
To stem the water flow, TEPCO has announced that it will attempt to freeze the ground near the plant for up to 100 years in a project worth hundreds of millions of dollars, but critics say that this will take years to implement and may not be as reliable as expected. Efforts to filter the radioactive water in the tanks have also stalled. 
There has been a lot of speculation and few hard facts recently about ocean contamination, with one of the more esoteric dangers identified by scientists being that "buckyballs" of uranium fuel could drift all the way to North America in the next year or so.  But though simulations suggest that radiation from Fukushima would spread across the entire Pacific in the next few years, scientists also say that it will be so diluted that no panic is warranted.
Still, there is little ground for optimism either. Even in the best-case scenario, a major nuclear catastrophe would be averted, but Fukushima Daiichi would continue to create problems and to pose deadly threats for decades.