Driving on with Fukushima roadmap
17 November 2011
.jpg "Fukushima devastation (Tepco)") |
Power plant buildings were among the few left standing after the tsunami
of 11 March hit the coastline of Fukushima prefecture |
Six of ten stabilisation goals at Fukushima Daiichi are complete, with official recognition of cold shutdown still outstanding despite low core temperatures.
The latest update on roadmap activities from Tokyo Electric Power Company (Tepco) and the Ministry of Economy, Trade and Industry detailed the tasks completed as well as those that lie ahead for the ruined power plant.
Stable cooling of damaged reactors has been improved and the release of radioactivity further reduced. The accumulation of water used for cooling is being managed and the site has improving margins to avoid this water overflowing, in case of typhoon for example.
Steady cooling
Temperatures recorded at the bottoms of the reactor vessels for units 2 and 3 are below 70ºC, while unit 1 is cooler still at 37ºC. Being below the landmark 100ºC, these basically fulfil the conditions for the declaration of cold shutdown although this has not been officially recognised by the government.
A complicating factor is the uncertainty over the melted core. Water leaking from holes in the bottoms of the reactor vessels, has lead to concerns that corium may have followed. But this theory is not supported by radiation readings from the drywell below, with the exception of unit 1 where a damaged sensor is fluctuating wildly. The drywells contain large pools of water at below 50ºC.
Amid this uncertainty, the Japanese government seems unready to declare the end-of-year goal of cold shutdown as having been achieved. Nevertheless, Tepco's document today shows it has notified the government that it has achieved and then 'maintained' cold shutdown - part of a subsequent set of goals.
Tepco said that if corium was indeed present in the drywell, "steam generation would be suppressed due to sufficient cooling, thus the release of radioactive materials from [containment] has been kept under control." The rate of emission of radioactivity is currently around 13 million times less than at the height of the accident on 15 March.
Noted among the recently completed sub-tasks of stabilisation were the expansion of water processing and decontamination facilities as well as water storage.
Water storage
Water is constantly injected to the reactor vessels and becomes radioactive on contact with the melted cores before being fully decontaminated. Despite the acknowledged safety of this water - a government minister even drank some on live television - Tepco is not currently allowed to release this to sea. Instead it is building up on site in a growing number of large tanks now totalling 106,000 cubic metres in capacity. A further 20,000 cubic metres of capacity is slated for addition every month in an arrangement that seems bound for revision.
However, the process of cleaning the water concentrates the contaminants in sludges that do require long-term storage and proper disposal. Work on a temporary tank for this was started on site by Mitsubishi Heavy Industries this week.
Apart from cold shutdown, Tepco's goals for the next six weeks cover ongoing safety topics relating to its workforce: the improvement of living and working environment; improvement of radiation control and medical facilities; and establishment of personnel training and allocation routines.
Wreckage from hydrogen explosions is being cleared from the tops of units 3 and 4 to reduce the spread of dust and enable the construction of an airtight cover like the one already complete at unit 1. Reactor unit 2 does not need a cover, with its containment breach having taken place in the torus suppression chamber in its foundations.
Tepco and the Nuclear and Industrial Safety Agency have now begun to set out a long-term roadmap, aiming at completely decommissioning units 1-4, the ultimate end point of which would not come for many years.
Researched and written
by World Nuclear News