Battle to Stabilise Earthquake Reactors (With Cutaway Graphic)

[tekisui]

Source: World Nuclear News

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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.

Explosion

Television cameras trained on the plant captured a dramatic explosion surrounding unit 1 at around 6pm. Amid a visible pressure release and a cloud of dust it was not possible to immediately know the extent of any damage. Later television shots showed a naked steel frame remaining at the top of the reactor building. The external building structure does not act as the containment, which is an airtight engineered boundary within.

Chief cabinet secretary Yukio Edano appeared on television to clarify that the explosion had damaged the walls and roof of the reactor building but had not compromised the containment.



This cutaway diagram shows the central reactor vessel and
thick concrete containment in a typical boiling water reactor
of the same era as Fukushima Daiichi 1


more: http://www.world-nuclear-news.org/RS_Battle_to_stabilise_earthquake_reactors_1203111.html