16 December 2011 Last updated at 08:12

Viewpoint: How Fukushima was stabilised

By Malcolm Grimston Chatham House

Remarkable progress has been made, but the work is not yet over

On 17 April, some five weeks after the earthquake and tsunami wrecked the Fukushima Daiichi nuclear plant, the energy firm in charge of the facility announced a roadmap for recovery.

Tepco stated its aim as: "By bringing the reactors and spent fuel pools to a stable cooling condition and mitigating the release of radioactive materials, we will make every effort to enable evacuees to return to their homes and for all citizens to be able to secure a sound life."

The plan envisaged two phases:

• "radiation dose in steady decline", to be achieved within three months,
• "release of radioactive material under control and radiation dose being significantly held down", to be achieved within the following six months.

Underlying the ambition was the need to get the three stricken reactors into "cold shutdown condition" - a state defined by Tepco where the temperature inside the core of the reactors is stably below 100C (allowing water to be collected and reused rather than boiling away and taking radioactive material with it) and releases of radioactive materials are low.

The problems at Fukushima are not yet solved. That task will take decades, not months. Nonetheless, a great deal of progress has been made since the roadmap was published, both in terms of developing a better understanding of what actually happened and in developing responses.

The challenges can perhaps be grouped into four issues: the state of the reactor cores; contamination on and off-site; the state of the spent fuel ponds; and ongoing radioactive releases.

'Impressive' improvisation

In response to the spectacular explosions at the facility, a new cover has been built over reactor 1 to withstand snow and high winds, and others are being planned for reactors 2 and 4. Nitrogen has been pumped into the reactors to make sure there are no more hydrogen explosions and air filters introduced to allow workers to get into the buildings. Although there is evidence that some nuclear activity may have started up in reactor 2 recently, if this is the case it has been very small scale and has been addressed.

The danger of occasional leaks near the plant remains

The latest analysis of the reactors themselves suggests that the core of reactor 1 was by far the most severely damaged. It seems that the fuel melted through the steel pressure vessel into the primary containment, there melting its way through about 70cm of the 10.2m-thick concrete of that containment. In reactors 2 and 3 (which are more modern in some key respects, dating from the early 1970s rather than the 1960s) it seems fuel did melt but remained largely, if not completely, within the pressure vessel, although some small holes may have melted in the pressure vessels.

The main progress here has been to move on from the initial response, whereby seawater was pumped in from fire trucks through an emergency line into the reactors, to using fresh water through the circulating circuits designed for such situations, coupled in the case of reactors 2 and 3 with use of "core spray" apparatus. These normal circuits were unavailable until September, when power was restored.

These developments have had two enormous benefits. First, the original circuits are much more efficient at removing heat than the improvised solutions of the early months, imaginative and impressive though those were. So the temperatures of the cores, which would have reached nearly 2,000C at the height of the crisis, have been brought well below 100C - the key parameter in cold shutdown condition. As of mid-November the core temperatures of the three reactors were 39C, 70C and 59C respectively.

Still vulnerable

The second advantage is that cooling is now done by water flowing into the cores, warming up as it removed heat, then being cooled and recycled through the core, rather than being passed through once and then accumulating in the reactor basement.

Nuclear crisis

• 11 March: Fukushima Daiichi nuclear plant struck by huge earthquake and tsunami
• 16 March: 20km (11-mile) evacuation zone declared around plant
• 17 April: Plant owner Tepco says crisis will be under control by end of the year
• 20 May: Tepco President Masataka Shimizu resigns as firm posts losses of 1.25tn yen (£9.4bn; $15.3bn) for the past financial year
• 26 August: Naoto Kan quits as prime minister amid fierce criticism of his handling of quake and nuclear crises

The main issue of on-site contamination involves very large amounts of contaminated water (about 100,000 tonnes) made up of tsunami water with dissolved radioactive material in it, and water that was passed into the reactors in the early days but accumulated in the basement of the reactors. A great deal of radioactive water escaped into the sea in the early days, but a lot of work has now been completed in decontaminating the water, plugging leaks and building more storage facilities.

The site is still vulnerable to further undetected cracks in the concrete (as happened in early December for example) or very heavy rain (though the plant withstood Typhoon Songda in late May) but there is now storage space for about 170,000 tonnes of contaminated water and the decontamination process is running at a maximum of about 12,000 tonnes per day. Work has also started on deep walls to prevent contamination getting into groundwater and then into water supplies.

Offsite, decontamination has begun by using pressure hoses on roads and buildings, clearing drains and guttering, cutting back vegetation and, in cases such as school play areas, removing topsoil to reduce doses. Some 220,000 people were screened for radiation without any cause for concern being uncovered. Two workers on site did receive doses above 600 millisieverts (natural radiation gives us typically about 2.5mSv per year; more than 1,000mSv is needed to induce non-fatal radiation sickness).

It now seems that the spent fuel ponds never boiled dry as was feared at the time, and circulating cooling circuits have been restored. The pond in reactor 4 has been buttressed to make sure it would not collapse during a serious aftershock or other challenge. Routine radioactive emissions practically stopped in July - current releases are at less than one millionth the level they were at height of the crisis. However, the danger of occasional leaks remains.

It is realistic to expect, then, that the authorities may start to allow some of the 80,000 evacuees to return permanently at least to some of the less contaminated areas of the 20km exclusion zone; people have been allowed to make short visits to their homes for some months.

The immediate challenges are to find any more cracks that might lead to leakage and to make sure the site is robust against a major aftershock and the forthcoming winter. Site clearance will take decades. But progress has been remarkable over the last months, especially given the backdrop of the devastation caused by the earthquake and tsunami.