It’s Wednesday evening in Japan, mid-morning in US east coast. [Evening (US) Update below.]
First, many thanks to commenter lobster, for the annotated diagram. In particular note the various levels of containment and the location of the fuel storage ponds on the fourth floor. The orange crane above is used to move fuel assemblies into and out of the reactor building, and to transfer them between the reactor vessel and the fuel storage pond.
— There is a storage pond on the fourth floor of each of the reactors, and all units had an inventory of fuel rods at varying degrees of “spent.” Only a small percentage is actually “spent” in this type of reactor, so there is still substantial energy/heat potential in so-called “spent” fuel rods. In addition to the storage pond in each reactor building, there is also a larger common spent fuel pond in another building that is used by all reactors at the Fukushima Daiichi station.
–The fuel is contained in zircaloy tubes, sometimes called “cladding.” When the reactor is shutdown, as occurred as soon as the plant detected the first earthquake, control rods rise into the core between the fuel rods to stop most of the fission reactions by absording neutrons. That happened at Units 1, 2, and 3; Units 4, 5, and 6 were already shut down for maintenance. As far as we know, control rods were fully inserted into Units 1-3 reactors as soon as the quake occurred. However, as cooling systems failed and melting of fuel occurs inside the reactor, that control protection is becoming less and less important.
– The reactor vessel holds the core — the fuel — and is a critical containment system. If the fuel melts from loss of continuous water cooling, it may eventually damage and melt through the reactor vessel.
– The reactor vessel is surrounded by another steel and concrete containment structure. This is the next line of containment if the reactor vessel is breached.
– At the bottom is a large doughnut-shaped “suppression” pool structure with tons of water to be used for emergency cooling. We’re beyond that stage in some units. But the idea is that if the core is overheating, and there is too much steam pressure building up inside the reactor, emergency valves open to allow steam to escape into the “suppression” pool, as water is reinjected into the vessel The released steam is slightly (relatively) radioactive, and hydrogen gas is created, so maintaining the integrity of the suppression pool and structure is important to avoid possible explosions. Officials believe the suppression pool structure in Unit 2 was damaged in conjunction with the explosion there, but we haven’t yet seen steam coming from here.
– Finally, there is the outer building housing all of the above and more. Units 1, 3, and 4 have already experienced hydrogen explosions that have severely damaged the walls and/or roofs of these reactor buildings. The explosion at Unit 2 has left most of the building intact, but it reportedly damaged the critical suppression pool.
Update as of Wednesday night 11:00 p.m. JST; mid-morning US EDT:
Frantic efforts continue to get cooling water into all six units at Fukushima Daiichi Nuclear Generation Station. At units 1-3, they are attempting to inject sea water into or around the reactor vessel. Authorities say the core in Units 1 and 2 were [twice] fully uncovered in the last 48 hours and may still be partially uncovered, suggesting that fuel and cladding melting may be continuing.
In the last 24 hours, there has been increasing focus on the fuel ponds at each of the reactors, where years of partially “spent” fuel rods are stored. For a good explanation of how the stored fuel rods, if not continuously cooled, can lead to fuel and cladding melting, hydrogen and fire or explosion, see this from the Union of Concerned Scientists.
Yesterday, the focus was on Unit 4, which had been shutdown for maintenance, so all the rods were in the fourth floor fuel pond. With power out and no backup generation, the pool heated, there was some melting and a fire created in the area, possibly from the cladding itself.
Today, that concern is also focused on Unit 3, where there are reportedly 514 fuel rods in the Unit 3′s storage pond. Early Wednesday (JST) we saw white smoke or steam plumes, some with high radiation levels, rising from Unit 3. This is believed to be associated with a possible fire started by fuel in the storage pond, [but reportedly not breach of the containment structure]. The storage ponds do not have power for cooling, so the fuel rods will continue to heat up, unless somehow cooled with more water, and eventually lead to melting. The zirconium cladding can catch fire. (There may also have been flammable materials, like lubrication, in the fuel storage area which may also have caught fire).
At units 3 and 4, authorities hoped to drop water from helicopters onto the area above the fuel storage ponds. The helicopter drop was called off or delayed because of dangerous levels of radiation.
Indeed last-ditch efforts to inject water into reactors or storage bonds have been hampered by excessive radiation in and around the plants, particularly Unit 3. The next plan is to use high power water hoses (“cannons”) from enclosed fire trucks to spray water into the reactor buildings. This will be extremely dangerous for the fire crews.
Units 5 and 6, which have been shut down for maintenance, also have spent fuel ponds that bear watching. Authorities reports rising temperatures in Unit 5, but it is not regarded as serious yet, relative to Units 3 and 4. Importantly, they still have power to run the pumps for unit 6 and it’s being used to help at Unit 5. No reports so far on the condition of the common fuel pond located in a separate building on the Fukushima site.
For a good unit by unit update as of last night (US) from commenter lobster, see here.
More updates as we get them.
Update, 6:00 p.m. EDT.
Japanese focus continues to be on getting water into the fuel storage pools at Units 3 and 4. Two things worth noting. First, US NRC Chairman Jaczko, testifying in Congress, claimed US officials believe all the water in the fuel storage pond at Unit 4 is gone. Given the number of hours elapsed, that is possible unless the Japanese managed to inject more water. Japanese officials deny Unit 4 water is gone. He also said radiation levels are extreme high and that he would recommend a much larger evacuation radius — out to 50 miles. Jaczko did not address why/how US officials would have more credible measurements or Unit 4 water level information than the Japanese.
Second, Japanese officials are racing to complete a new transmission connection to the grid to restore on-site power to the Fukushima station. That presumes they have an undamaged switch yard to or other point of connection at the site.
This picture at UCS shows some of the damage to the reactor buildings. (h/t SFRob)