The Fukushima nuclear plant has us on tenterhooks as we watch the ongoing efforts to contain the radioactive contamination from the damaged reactors. Insight into what is happening there and and what might happen there depend on understanding how the accident happened and how it unfolded in the early days. A recent summary* of the incident put forth by the energy company Areva puts together in one concise statement those early events. What I read there clarified this sequence for me and I thought it might help others too. If you would like to follow a timeline join me below the swirl of orange.
*The Areva pdf is available at many sites but I could not find the original Areva link. This new link is from a neutral site and the most recent (April 15) but there are many others across the spectrum.
14:46 - Earthquake - Magnitude 9
The power grid in northern Japan fails but the reactors are mainly undamaged. The SCRAM was successful and shut down the power generation due to chain reactions. Heat generation from radioactive decay of fission products continues at a much lower rate:
· After Scram ~6%
· After 1 Day ~1%
· After 5 Days ~0.5%
Additional automatic responses to the earthquake include closing all non-safety related penetrations of the containment and starting the diesel generators. Emergency Core cooling systems were supplied and the plant was in a stable save state.
15:01(?) Tsunami hits plant
About fifteen minutes after the earthquake the tsunami hits. Because it exceeds the plant design, there is flooding of some or all of these: diesel; switchgear building; fuel tanks; essential service water buildings.
15:41 Station Blackout
About an hour after the earthquake and SCRAM, there is a common cause failure of the power supply. This leaves only batteries for power and leads to failure of all but one Emergency core cooling system.
16:36 Unit 1
Nearly two hours after the SCRAM Unit 1, the oldest of the reactors, has its isolation condenser stop, perhaps because the tank that feeds it is now empty.
2:44 Unit 3
About a day and a half after the SCRAM in Unit 3 the reactor isolation pump stops because the batteries that powered it are drained.
13:25 Unit 2
About two days after the SCRAM the last of the three reactors that were powered up before the earth quake has pump failure and the reactor isolation pump stops.
At this point all three reactors have no means to remove heat from the cores (now putting out about ~1% the power that was generated during full power).
This residual decay heat still produces steam in the reactor pressure vessel. Rising pressure opens the steam relieve valves. Steam discharges into the Wet-Well (Torus). As the reactor water turns into steam that is vented into the torus, the liquid level in the reactor pressure vessel (RPV) falls.
As the liquid level falls, at some point ~50% of the core is exposed. The residual heat causes the cladding temperatures to rise, but at this level of exposure no significant core damage happens.
When the liquid level falls further, ~2/3 of the core is exposed and the cladding temperature exceeds ~900°C. This temperature leads to balooning or breaking of the cladding. Without this covering the fuel rods release fission products.
When the liquid level falls further still and ~3/4 of the core is exposed, the temperature of the cladding exceeds ~1200°C. At this high temperature, the zirconium in the cladding starts to burn under the steam atmosphere in a reaction with the steam:
Zr + 2H20 ->ZrO2 + 2H2
This is an exothermal reaction (chemical reaction that releases heat). The oxidation of the zirconium further heats the core and produces hydrogen gas.
Effect of high temperatures on the core
It is expected that all three units reached ~1800°C. This temperature causes melting of the cladding and of steel structures.
It is expected that Units 1 and 2 reached ~2500°C which would cause breaking of the fuel rods and could leave a debris bed inside the core.
Maybe Unit 1, the oldest unit and the one that lost cooling first, reached ~2700°C. If it did, this temperature is high enough to cause significant melting of Uranium-Zirconium-oxides.
Restoring the water supply brings the temperatures down and stops zirconium oxidation in all 3 Units:
Unit 1: 12.3. 20:20 (27h w.o. water)
Unit 2: 14.3. 20:33 (7h w.o. water)
Unit 3: 13.3. 9:38 (7h w.o. water)
The Areva report goes on in much detail into aspects such as how the hydrogen produced by the zirconium fire reached the upper story where it exploded and reconstructions of the spent fuel pool events. The diagrams throughout are very helpful; I am hoping that by confining myself to a partial summary of the narrative that this is still fair use.
For ongoing coverage of the Fukushima incident, check out and follow Japan Nuclear Incident Liveblogs - the ROVs and individual diaries.