Board zum Unfall von Fukushima vom physikBlog

Tokyo Electric Power Co. is considering changing the method of injecting water into the No. 3 reactor at its hobbled Fukushima No. 1 nuclear power plant as the current system isn't cutting it.

The No. 3 reactor is consuming nearly three times the coolant water that the No. 1 and No. 2 reactors are taking to cool down their fuel rods, as a considerable amount is missing the target.

TEPCO said that the pressure vessels in the No. 1 through No. 3 reactors, where fuel meltdowns have occurred, currently have temperatures at the bottom between about 90 and 120 degrees. In the meantime, the amount of water pumped in daily to maintain the temperatures at these levels is about 216 tons for the No. 3 reactor, as opposed to 84 tons for the No. 2 reactor, which is about the same size and contains roughly the same number of fuel rods, and 91 tons for the No. 1 reactor, which is smaller.

The question is, why is this discrepancy occurring?

TEPCO said that in all three reactors, coolant water is being injected from outside the shroud, a major component covering the core.

Analysis conducted so far has hinted at the possibility that, unlike in the No. 1 and No. 2 reactors, part of the melted fuel in the No. 3 reactor did not fall through to the bottom of the pressure vessel but has stayed on the grid-like core support plate. The current injection method cannot pump water into there, resulting in inefficient cooling and increasing the amount of radioactive water.

The new water injection method under consideration is based on the use of an emergency cooling system called a "core spray." It can pour water down like a shower above the fuel rods, resulting in more efficient cooling and the use of less coolant water, TEPCO said.

Much has been learned about the state of the cooling pipe systems since workers regained access to the reactor buildings.

On Aug. 3, TEPCO conducted tests on the operability of valves along the piping.

"We plan to make decisions in two or three weeks," a TEPCO official said.

From Asahi Shinbun (3:02AM JST 8/8/2011; not the literal translation):

Fumiya Tanabe, former head of the research at Japan Atomic Energy Agency, will present the result of his research on Reactor 3 at Fukushima I Nuclear Power Plant in the upcoming Atomic Energy Society of Japan's conference next month.

Tanabe thinks the fuel melted and dropped to the bottom of the Reactor Pressure Vessel of Reactor 3 by March 14's explosion; then the melted fuel stayed there, cooled by more than 300-tonnes/day water. However, the amount of water injected dropped to only 24 tonnes per day from March 21 to 23, and 69 tonnes per day on March 24, probably due to increased pressure within the RPV.

It caused the melted fuel to heat up again, and the fuel melted through the RPV and dropped onto the Containment Vessel (pedestal; see the diagram from Asahi).



According to Tanabe, the amount of water from March 21 to 24 was only about 11 to 32% of what was needed to remove the decay heat, and within one day the melted fuel would attain the melting temperature again.

Tanabe thinks this massive "re-melting" caused the release of a large amount of radioactive materials into the environment which caused a spike in air radiation in wide areas of Tohoku and Kanto including Tokyo, and most of the re-melted fuel dropped from the RPV to the Containment Vessel.

The article doesn't say what Tanabe thinks has happened to the melted fuel that dropped onto the Containment Vessel since.

There are a bunch of holes in the bottom of the reactor vessel for all the control rods, since boiling water reactors have a steam generator in the top that prevents the control rods from being on top. Each of these rods is sealed, I believe, with graphite. I think it is more likely that the seals failed than that the reactor vessel itself was breached. In contrast, at Three Mile Island, the reactor was a pressurized water reactor with the control rods at the top and NO holes in the bottom, so there was no breach of the reactor vessel by the complex mixture of stuff that ended up at the bottom of the reactor vessel at TMI.

dirk said:

Dazu nochmals die Frage: wie kommt das Corium aus dem Steuerstabraum ins Containment?

TEPCO began using a "spray-shower" technique to disseminate water into the No. 3 reactor at its troubled Fukushima No. 1 nuclear power plant on Sept. 1.

The method consists of spraying coolant water like a shower above the fuel rods so that water will fall evenly on all fuel rods and cool them efficiently.

Until Sept. 1, Tokyo Electric Power Co. has been using a method in which coolant water trickles down the inner walls of the pressure vessels at the No. 1 through No. 3 reactors. The No. 3 reactor, however, did not cool down as efficiently as did the No. 1 and No. 2 reactors. TEPCO officials have hinted at the possibility that part of the melted fuel in the No. 3 reactor did not fall through to the bottom, but remains on the grid-like core support plate beyond the reach of the trickling water.

In the new injection method, water is sprayed inside the shroud, a major component covering the core.

TEPCO has also begun considering a review of its plan to remove all highly radioactive water at the Fukushima No. 1 plant by the end of this year. Once the quantity of radioactive water has been reduced to such levels that heavy rains would not cause an overflow, the rate of water treatment will be adjusted to minimize the amount of waste generated.

dirk said:

Das CoreSpray System scheint zu funktionieren. Die Temperaturen gehen, nach anfänglichem leichten Steigen, runter.
Demnach sollte sich der Hauptteil des Coriums sich noch im RDB befinden. Evtl sogar nicht unten sondern auf der CorePlate.

Analysis of new Fukushima 3 photographs released last week by TEPCO substantiate Fairewinds’ claim that explosion of Unit 3 began over the spent fuel pool. Fairewinds believes that significant damage has also occurred to the containment system of Fukushima Unit 3, and that the two events (fuel pool explosion and containment breach) did not occur simultaneously. Video also includes brief discussion of tent system being constructed over Fukushima Unit 1.