While reporting progress made in its restoration of the stricken Fukushima Daiichi nuclear power plant, Tokyo Electric Power Company (Tepco) has amended its roadmap. However, the company still expects the damaged units to be stabilized by the end of the year.
One month after releasing its original restoration roadmap, Tepco said that the objectives of its plan remain unchanged: bringing the reactors and used fuel pools to a stable cooling condition and mitigating the release of radioactive materials. However, recent discoveries about the state of the individual units and fuel pools have prompted the company to change the way it aims to meet those objectives.
Tepco still aims to achieve a steady decline in radiation levels at and around the plant by mid-July. It also aims to have the release of radioactive materials under control and for the radiation dose to be "significantly held down" in around three to six months. However, it said there will still be "various uncertainties and risks" that could affect those timelines.
The company has expanded the scope of the roadmap to include environmental improvement and countermeasures against aftershocks. In addition, it has added three more issues that need to be addressed: groundwater, tsunami reinforcement measures and improving the life/work environment for the workers at the plant.
Having now managed to send workers into the reactor building of unit 1, Tepco has been able to assess some of the damage to the unit. Measurements taken by newly installed gauges indicate that the water level in the reactor pressure vessel of unit 1 is far lower than previously thought, suggesting that most of the fuel in the reactor has melted and dropped to the bottom of the vessel. There is probably damage to the vessel as water levels have not risen despite the injection of water into the unit and water has been leaking out of it. This discovery increases the likelihood that similar damage has occurred at both units 2 and 3 as well.
The Japanese government has invited an international fact-finding mission to visit the Fukushima Daiichi site.
A group of some 20 experts from the International Atomic Energy Agency (IAEA) and other organizations will visit Japan between 24 May and 2 June. The mission - led by Mike Weightman of the UK's nuclear safety regulator - will make a preliminary assessment of the safety issues linked to the Fukushima accident. During the mission, areas that need further exploration or assessment will be identified.
In a press conference, Japan's chief cabinet secretary Yukio Edano said, 'Japan has been doing its best to ensure utmost transparency to the international community about the nuclear power plant accident. Receiving the IAEA mission is a part of such efforts and this will be a useful opportunity for us to share our experience with other countries.'
As a result of assessing the damage to unit 1, Tepco has now given priority to establishing a circulating injection cooling system at the reactor in order to achieve cold shutdown, where coolant water is at less than 100ºC. Such a system would reuse contaminated water that has accumulated in the reactor and turbine buildings, once it has been processed. "Operation of processing facilities and early establishment of circulating injection cooling to control accumulated water are key items," Tepco said. The company had previously planned to flood the primary containment vessels of units 1, 2 and 3, but has since decided this would just increase the amount of contaminated water accumulating at each unit due to the leaking pressure vessels.
With regards to the used fuel pools, Tepco reported that it has already successfully implemented remote control of the water injection hoses at units 1, 3 and 4 ahead of schedule. In addition, the installation of heat exchangers to help cool fuel in the pools is now expected to be completed by mid-July, again ahead of the previous schedule.
The threat of further aftershocks and tsunami has prompted Tepco to include the construction of temporary tidal barriers to its list of countermeasures. It has already moved its emergency power generators to higher ground and ensured that back-up water pumping vehicles are to hand.
The roadmap previously called for the construction of a support structure for the used fuel pool of unit 4 as the walls of the building supporting the pool have been damaged. The construction of this is set to begin next week. In addition, Tepco said it is now considering reinforcement work at each of the other units.
Work has already started on constructing a cover over the damaged reactor building of unit 1 to prevent the spread of radioactive materials. Similar covers for the reactor buildings of units 3 and 4 are now being designed. Unit 2 will not require a cover as the reactor building remains intact.
The company will also consider how it can continue to improve the living and working conditions of workers at the site. Tepco plans to expand the sleeping quarters and rest areas at the site, as well as increasing the amount of drinking water available.
Researched and written
by World Nuclear News