Men wearing protective suits and masks work in front of welding storage tanks for radioactive water, under construction in the J1 area at the Tokyo Electric Power Co's (TEPCO) tsunami-crippled Fukushima Daiichi nuclear power plant in Okuma in Fukushima pr

Japan on Tuesday marks the third anniversary of the massive quake-tsunami disaster that sparked the Fukushima nuclear crisis.Here are some key facts about the still-unresolved situation three years after the worst nuclear disaster in a generation. Cleaning up the shattered site is expected to take decades.Reactors 1, 2 and 3 went into meltdown after their cooling systems were knocked out as giant waves thundered into the plant. The temperature of the cores and spent fuel pools at all units is now stable and water is being used to keep them cool.Reactor 4 has an empty core but around 1,500 spent fuel rods had been in its storage pool. The outer building was damaged by fires and an explosion. About one third of the rods have been removed since operations began in November, a risky and complicated task. Plant operator Tokyo Electric Power (TEPCO) plans to remove all the fuel rods by the end of this year.TEPCO has decided to decommission reactors 5 and 6, which had been in a state of "cold shutdown" since the accident.Much of the work done to stabilise the plant has been temporary and there is no permanent solution for the water used to cool overheating reactors.Some 436,000 cubic metres of toxic water is being stored at the site, with some 70,000 cubic metres of highly contaminated water separately filling the basements of the reactors.The water is mostly stored in some 1,200 tanks at the site, and TEPCO has faced occasional leaks.Many experts say that at some point this cooling water will have to be released into the sea after being scoured of the most harmful contaminants. They say it will pose negligible risk to marine life or people, but local fishermen and neighbouring countries are fiercely opposed.Workers face a constant battle against incoming groundwater that mixes with contaminated cooling water, putting pressure on storage and resulting in seepage into the nearby Pacific Ocean, at a rate of about 400 cubic metres a day.

The next major step towards decommissioning is to start removing fuel rods from Reactors 1, 2, 3 which went into meltdown. The work requires cutting-edge technologies to block dangerous levels of radiation and to repair damaged parts of reactor containers. That is expected to start in 2020 at the earliest.The government has agreed to fund a plan to freeze the ground underneath the reactors to divert groundwater. This is part of a nearly $500 million plan that will take two years.