APR: your source for nuclear news and analysis since April 16, 2010

Tuesday, March 15, 2011

Fukushima Daiichi No. 4 update

We have a little better information about the condition of No. 4 plant when the quake and tsunami occurred now, and can offer these facts and some background.

It appears that the plant was in a refuelling status, with all the fuel cells removed from inside the reactor and stored in the spent fuel pool near the top of the reactor building. This is the area exposed by the explosions at No. 1 and No. 3 plants. This fuel is in racks inside this water filled pool; the water keeps the fuel cool. Sources on the wires are indicating that a reactor vessel shroud replacement was the actual step in the refuelling which was in progress or in preparation when the earthquake and tsunami hit, and since there was a report that foreign objects had fallen into the reactor it's likely that items suspended from overhead cranes or tools or toolboxes.. or any such thing... fell into the open reactor vessel, with the vessel head of course removed and moved aside as is perfectly normal in this operation.

This fuel cannot .. repeat can NOT.. achieve criticality so long as it remains in the racks. It is not close enough (is not in a critical configuration) and the racks contain boron to absorb neutrons, which stops fission induction. However, it now appears that the cooling water was not cooled, and has evaporated away somewhat which allowed hydrogen, again generated radiolytically, to build up in the upper areas and explode.. although with nowhere near the force of No. 1 and No. 3 plants. The concentration surely was not as high, and although aerial photos we've seen show smoke coming from No. 4 reactor building the external damage is not visible, so slight is it when compared to No. 1 and No. 3.

Item: The fuel storage pools, with only a third of the number of elements, at No. 5 and No. 6 plants appear to be heating up as well.

ATOMIC POWER REVIEW 9:30 AM Eastern, Tuesday 3/15

UPDATE: Right now on the wire is a report, very fresh and minutes old, that TEPCO says it's unable to get water into the spent fuel pool at No. 4 plant. DEVELOPING NEWS at 9:35 AM Eastern Tuesday 3/15.

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