Attention is focused on the Fukushima Daiichi and Daini nuclear power plants as Japan struggles to cope in the aftermath of its worst earthquake in recorded history. An explosion has been seen at the site.
Three of Fukushima Daiichi’s six reactors were in operation when yesterday’s quake hit, at which point they shut down automatically and commenced removal of residual heat with the help of emergency diesel generators. These suddenly stopped about an hour later, and this has been put down to tsunami flooding by the International Atomic Energy Agency (IAEA).
The loss of the diesels led the plant owners Tokyo Electric Power Company (Tepco) to immediately notify the government of a technical emergency situation, which allows officials to take additional precautionary measures.
Even now, the primary focus of work at the site remains to connect enough portable power modules to fully replace the diesels and enable the full operation of cooling systems.
Pressure and releases
Without enough power for cooling systems, decay heat from the reactor cores of units 1, 2 and 3 has gradually reduced coolant water levels through evaporation. The consequent increase in pressure in the coolant circuit can be managed via pressure release valves. However, this leads to an increase in pressure within the reactor building containment. Tepco has said that the pressure within the containment of Fukushima Daiichi 1 has reached around 840 kPa, compared to reference levels of 400 kPa.
The company has decided to manage this “for those units that cannot confirm certain levels of water injection” by means of a controlled release of air and water vapour to the atmosphere. Because this water has been through the reactor core, this would inevitably mean a certain release of radiation. The IAEA said this would be filtered to retain radiation within the containment. Tepco has confirmed it was in the process of relieving pressure at unit 1 while preparing to do the same for units 2 and 3
Television cameras trained on the plant caputured a dramatic explosion surrounding unit 1 at around 6pm. Amid a visible pressure release and a cloud of dust it was not possible to know the extent of the damage. The external building structure does not act as the containment, which is an airtight engineered boundary within. The status of the containment is not yet known.
Monitoring of Fukushima Daiichi 1 had previously shown an increase in radiation levels detected emerging from the plant via routes such as the exhaust stack and the discharge canal.
Over the last several hours evacuation orders for local residents have been incrementally increased and now cover people living within ten kilometres of the power plant.
Meanwhile at adjacent Fukushima Daini, where four reactors have been shut down safely since the earthquake hit, Tepco has notified government of another emergency status.
Unit 1’s reactor core isolation cooling system had been operating normally, and this was later supplemented by a separate make-up water condensate system. However, the latter was lost at 5.32am local time when its suppression chamber reached 100ºC. This led Tepco to notify government of another technical emergency situation.
Tepco has announced it has decided to prepare for controlled releases to ease pressure in the containments of all four units at Fukushima Daini.
A three kilometre evacuation is in progress, with residents in a zone out to ten kilometres given notice of potential expansion.
A seriously injured worker was trapped within unit 1 in the crane operating console of the exhaust stack. Breathing and a pulse could not be confirmed, Tepco said as it considered a rescue. At unit 3 one worker is known to have received a radiation dose of 106 mSv.
Researched and written by World Nuclear News