Japanese prime minister Yoshihiko Noda has declared that the Fukushima Daiichi reactors have attained cold shutdown, eight months after the onset of the tsunami that damaged them.

Reactor bottom and containment vessel temperatures are reliably lower than 100°C, and emitted radiation has been reduced to below 1 mSv/year at the site boundary, according to a report from Japanese broadcaster NHK, as published on the Japan Atomic Industrial Forum’s Atoms in Japan service.

Although the change in designation does not reflect any new activity on site over the past few weeks, it does indicate a change in approach from Tepco and the government – from an emergency management project to a decommissioning project structure.

First, the government has announced that it will take over supervision of Tepco’s decommissioning work, JAIF reports. Also, during decommissioning, a laboratory will be built near the site to analyse the ‘condition and behaviour’ of debris and radiation waste. A mock-up facility will also be built. The plan was approved by the Japan Atomic Energy Commission.

NHK reports that the decommissioning project may take 40 years to complete. An early task would be the removal of spent fuel from the unit 4 reactor. Melted and unmelted fuel in units 1, 2 and 3 would be removed within 25 years, and then demolition could begin. Holes in containment vessels would need to be repaired while highly-radioactive water is flowing inside it, NHK said.

Second, the government is reviewing the evacuation area around the site into three groups based on radiation levels: areas less than 20 mSv/year, areas greater than 50 mSv/year and areas in between. In the first category, residents will be allowed to return, and be given governmental help for cleanup. In the second category, residents will be told to stay away long-term; the government is considering buying the houses of evacuees. In the middle category, residents will need to stay away while government agencies carry out decontamination and rebuilding work so that evacuees can return, perhaps after several years.

On site, Tepco reports that it has set up a gas filtration system to reduce levels of radioactive caesium in unit 1, following the installation of a similar system at unit 2 in October. The system uses a high-efficiency particulate air (HEPA) filter and activated carbon filter to remove radioactivity, and then monitors the flow of radioactive dust and H2 and O2 concentration before exhaust. A new pipe takes air from the existing PCV cooling system, passes it through a dehumidification step using an air conditioner, and then through an electric heater, two filters, and then an extraction fan. Heater, filters and fan are duplicated for safety. Tepco says that the filters will remove 99% of radioactive compounds. A similar system will be installed at unit 3 in February 2012.

In addition Tepco has published the results of an investigation of the radioactive solid waste storage buildings. Despite the collapse of 32 drums from their stacks (0.7% of the total amount), probably during the earthquake, radioactive materials were not released because they were stored in plastic bags. Radiation dose (ranging from 0.3-30 microSv/hr) was equal to or lower than the rate around the exterior of the building. No significant building damage was discovered during a visual inspection.

A month-long project to decontaminate the water in the unit 2 spent fuel pond has been completed. It has the effect of reducing the density of radioactivity, principally from caesium, by 1000 times, to 1000 Bq/cc. The next step is desalination to reduce the risk of corrosion. Seawater was injected into the pool as an emergency cooling measure.

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