Japan's TEPCO faces obstacles in its plan to shut down the crippled Fukushima Daiichi nuclear reactors in less than a year.
Delays could hold back the operation and even the Japanese government has tried to lower expectations.
The time frame could be achieved if "everything goes smoothly", said Chief Cabinet Secretary Yukio Edano.
Difficulties, including the dumping of contaminated water into the ocean, have sparked an outcry in South Korea and expressions of concern from China.
Tokyo offered apologies for a failure to give sufficient warning and explanation.
Eight risk factors could get in the way of shutting down the nuclear reactors, including hydrogen explosions and power outages.
Aftershocks still jolt the area, and the upcoming rainy season and typhoons pose further threats.
TEPCO's plan calls for continued injection of water into the reactors and spent pool fuels, while workers remove contaminated water so that they can access the reactors' cooling systems.
Workers will simultaneously build an external cooling system while preparing for "water entombment", a last-ditch step in which containment vessels are flooded with water.
But each step and contingency plan listed by TEPCO not only faces formidable obstacles but also entails new risks.
"Japan may very well have to weigh speed against the health of the workers on site and the amount of radiation that it lets out," said Osaka University Professor Kenji Sumita.
Reactors No.1, No.2 and No.3 at the Fukushima Daiichi complex are all flooded with dangerous water, while no one can get near enough to the reactors to find where it is coming from.
Tepco has spent over a month on its Plan A, injecting water into the three reactors while also moving some of the contaminated water flooding the turbine building in reactor No.2 into condensors to gain access to essential cooling pumps.
Coolant levels are not rising as much as would be expected inside the reactors, while the contaminated water levels refuse to go down, suggesting leaks.
Fuel rods in the No. 4 reactor's spent fuel pool could also possibly be damaged, according to specialists.
"There are many unknowns, including the risk of further earthquakes, and we are talking about people working in highly contaminated areas," said Kazuhiko Kudo, a professor at Kyushu University who specialises in nuclear engineering.
"It's not going to be easy."
It was difficult, Kudo said, to predict how technical setbacks might affect the proposed timetable.
"If another disaster hits, you could easily see a couple of months in which all work stalls," he said.
The biggest challenge is bringing under control reactor No.2, flooded with the most highly contaminated water.
Its nuclear pressure vessel and containment structure are likely to have been damaged.
The big pre-requisite for water entombment is that the containment vessels are sound and that workers find and stop all leaks, otherwise there will be a build-up of contaminated water.
Radiation seeps out in the meantime, and the longer the current uncertainties persist, the more contaminated water TEPCO creates through its stopgap cooling measures.
TEPCO dumped 10,393 tonnes of water with "low levels of contamination" into the ocean earlier this month to make room for storing water with higher levels of radiation.
While it has worked to stop the main leaks into the ocean, some experts said TEPCO has not yet turned its attention to leaks into the water table.
Once radiation levels are low enough for workers to gain more access to the reactor buildings, TEPCO plans to cover the reactors and spread the grounds with inhibitors to absorb radioactive particles.
"This is the first time that TEPCO has come out clearly and presented their plan and some contingencies, so I have hopes that this will work," said Takashi Sawada, vice chairman at the Atomic Energy Society of Japan.
"But I also have doubts."