Reactor Containment & Fuel Storage from UCS

Early Wednesday morning, Ma
rch 16, in Japan; 5:30 p.m. EDT
.

Japanese authorities are reporting a new fire and explosion coming from the area of the spent fuel storage pond of Unit 4 of the Fukushima Daiichi Nuclear Power Station.

That’s the same site at which fires were reported on Tuesday and an explosion that released very high levels of radiation. Wednesday’s blast and fire also resulted in increased radiation readings at the site, and according to one [unconfirmed] other report, at least two workers are missing.

About 5:40 a.m. Wednesday morning, their time, officials reported a fire and explosion that blew a hole about 8 meters wide in the walls of the Unit 4 building. The location corresponds to the 4th floor area where the spent fuel storage pools are located. (In the diagram, the storage pool area is near the top of the reactor vessel, to facilitate easier loading and removal of fuel rods.)

The spent fuel storage pools must be continuously cooled to prevent excessive heat building up from the residual radioactive decay of materials in the irradiated fuel and fuel cladding, even though most fission has ceased. Without the cooling, the fuel/cladding can continue to heat up, boiling away the cooling water and exposing the fuel rods. (By Monday, those temperatures had already doubled normal levels, reaching 84 degrees Celsius.) Melting can then release hydrogen and lead to an explosion and fire.

Plant operators have been struggling to get cooling water into the pools, but it’s not clear how that can be done. The normal cooling system is disabled. Attempts to inject water have been frustrated by high radiation levels and limited access.

There is reportedly a hole in the building roof, as a result of yesterday’s explosion and fire, and authorities considered using helicopters to dump or inject water from the roof. However the hole is apparently not near enough to the fuel rod pools to make an aerial drop effective. Radiation levels may also be complicating efforts to extinguish the fire or add water.

At this point, we’ve seen no reports the new fire has been extinguished, and reports of high radiation levels vary. We’ll try to get more details.

I should add that Units 5 and 6, which you don’t see in the usual pictures because they’re a little further away, also have cooling ponds with spent fuel rods that have been heating up. TEPCO is trying to keep those pools cooled down, too, and that fact tells you normal cooling systems are not operating — presumably because of loss of power.

We’ll add edits and more details as we get them.

Update from Govt. press conference :
Since about 8:30 a.m. JST, there has been white smoke, steam or “vapor” billowing from around Unit 3, which suggests there may be a breach in the containment structure. Radiation levels temporarily spiked at about 10:00 a.m., forcing a further, temporary evacuation at the site.

[The live tv feed periodically shows aerial views of the white smoke taken from a helicopter, but it's not clear which site is the source of the smoke/vapor.]

At unit 4, the radiation levels are currently too high to allow firefighters to continue efforts to fight the fire.

The official was ambiguous about whether and how TEPCO might be using US military or others to assist.

In a further briefing by Nuclear Energy Institute officials. . . TEPCO didn’t notify them until about 10:30, indicating reactor #3 was emitting smoke.
Readings at the Gate at that time, about 800 micro-sv. By 10:54, it was 2.9 mili-sv. and further “ups and downs.” Increase attributed to reactor damage at Unit 3, but they’re uncertain of exact cause. [Kyodo News: Radiation briefly topped 10 milisievert, but see more complete details posted by commenter powwow here.]

wrt Unit 2, the suppression structure has been damaged, so it too in contributing, but they don’t have measurements specific to it.

Kyodo News: An estimated 70 percent of the nuclear fuel rods have been damaged at the plant’s No.1 reactor and 33 percent at the No.2 reactor, the firm said. The cores of both reactors are believed to have partially melted with their cooling functions lost in the wake of Friday’s magnitude 9.0 earthquake and tsunami.


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