Japan Nuclear Reactor Disaster Update: Frantic Efforts to Cool Reactors and Fuel Pools
It’s Wednesday evening in Japan, mid-morning in US east coast.
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 fuel assemblies into and out of the reactor building, and to transfer them between the reactor vessel and the fuel storage pond.
— There is 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 melting occurred 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 large doughnut-shaped “suppression” 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, emergency valves open to allow steam to escape into the “suppression” pool. The release steam is slightly (relatively) radioactive, so maintaining the integrity of the suppression pool and structure is important. Officials believe the suppression structure in Unit 2 was damaged by 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, 2, and 4 have already experienced hydrogen explosions that have severely damaged the walls and/or roofs of these reactor buildings.
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 fully uncovered in the last 48 hours and may still be partially uncovered, suggesting that fuel and cladding melting may be continuing. [cont’d.]
At units 3 [and 4?], there was also a plan 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 radation in and around the plants, particularly Unit 3. The next plan is to use high power water cannons and fire trucks to spray water into the reactor buildings. This will be extremely dangerous for the fire crews.
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.
Yesterday, the focus was on Unit 4, which had been shutdown for maintenance, so all the rods were in the fourth floor fuel ponds.
Today, it’s also been on Unit 3, where there are reportedly 514 fuel rods in the Unit 3’s storage pond. Early Wednesday, we saw smoke or steam plumes rising from Unit 3, believed to be associated with a possible fire started at or near the fuel pond. (There may have been flammable materials, like lubrication in the area).
For a good unit by unit update from last night (US) from commenter lobster, see here.
More updates as we get them.