Update at 6pm EST Friday 3/11/11:
The Japanese Nuclear and Industrial Safety Agency is now saying the containment pressure at Unit 1—not Unit 2, whose core cooling was said to have failed—has risen to about double its normal value.
The Tokyo Electric Power Company (TEPCO) has announced it will “implement measures to reduce the pressure of the reactor containment vessel for those units that cannot confirm certain level of water injection by the Reactor Core Isolation Cooling System, in order to fully secure safety.” It is not clear if this refers just to Unit 1, or to the other two affected units as well.
The increase in containment pressure resulted from the loss of alternating-current (AC) power to the reactors, which stopped the containment cooling system. There are large water-cooled air conditioning units inside containment. Motor-driven pumps send cool water to the units. Motor-driven fans blow air inside the containment across the metal tubes containing the cool water. But without AC power, the pumps and fans don’t work and can’t provide cooling. The heat radiating off the hot reactor vessel (over 500F) and the hot piping heats up the air in the containment building very rapidly, which causes an increase in pressure.
The rising pressure reduces the ability of the containment to absorb the energy released from a pipe rupture, should one occur. The volume of air in the containment building and its wall thickness are designed to contain a specified level of energy being dumped into containment. If the pressure gets too high, then an energy release like a broken pipe, should it occur, could over-pressurize the containment and cause it to fail. So emergency procedures call for venting air from the containment to reduce the pressure if it gets too high.
If the containment structure was weakened by the earthquake, then what pressure it could withstand is not known.
The reactors have a containment ventilation system that can be used to vent air from the containment building. In this situation, the vented air would be routed through a high-efficiency particulate air (HEPA) filter, charcoal beds, and another HEPA filter to remove as much radioactivity as possible before being released from a very tall stack to dilute the flow as much as possible.
If there has been no appreciable reactor core damage, the air vented from containment will contain minute but detectable amounts of radiation. The filtration systems are designed to lower that radioactivity release by nearly a factor of 100.
The latest news is that evacuation around the plants is being expanded from a 3 km to a 10 km radius, which suggests the crisis isn’t over yet.
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