Friday, May 20, 2011

Seawater Mix-up at #Hamaoka Nuke Plant: Pipes Broke in the Condenser When the End Cap Fell Off

Chubu Electric Power Company released the one-page announcement (in Japanese) with colorful diagrams and photos of the condenser unit of its Reactor 5, where 500 tons of seawater entered the Reactor Pressure Vessel the other day when the reactor was being shut down.

"Multiple" small-diameter (3 centimeters) pipes out of "about 21,000" that carry seawater to cool the steam that drives the turbine broke, probably having been hit by the end cap of the 20-centimeter diameter pipe for recirculating the water.

Chubu's press release also says that a "small amount" of cobalt-60 was detected in the water chamber of the condenser. Chubu doesn't say how much, and but says cobalt-60 is one of the nuclides regularly found in the water in the RPV. Rest assured it wasn't released into the ocean, says Chubu.

Why can't they say exact numbers, instead of "multiple", "about 21,000", and "small amount"?

The number of these small pipes that broke was "about 20", according to Yomiuri (in Japanese; 5/20/2011).

Yomiuri also says the end cap was welded, not threaded. During the regular inspection in February this year, there was no problem with either the small pipes or the end cap, according to Yomiuri. The Reactor 5 at Hamaoka started to operate in 2005.

Other than bad weld, what could make the end cap to drop off like that?


Anonymous said...

Water hammer from a valve closing when it shouldn't have.

Anonymous said...

Water hammer could indeed cause a weak weld to fail, but seldom all the way around a fitting. (cap, in this case)It looks like a pressure surge of greater duration would have been required.

While I can't read all of the text in the photo, it is hard to see how a falling cap could have caused the damage shown. Nuclear-grade heat exchangers are built to exacting standards, and all welds are x-ray inspected- at least here in the US. That cap would have had to be traveling at one hell of a velocity, and I don't see how it could have had access to the tube bundle.

All that points to an over-pressure situation, likely a pressure surge. I suspect that the cap and tube damage are coincidental to some other event.

Which leads us to the question of how sea water entered the reactor pressure vessel. I do not know the specified pressure for either the steam side or the cooling (sea) water side, but somehow the RPV was at a lower pressure than the cooling circuit. This seems odd- but as I understand it, they were in the process of taking this unit off-line, is that right?


Anonymous said...

Wow! I've dealt with heat exchangers but 21000 tubes? Nuclear grade, mil spec, whatever you want to call it give it 21,000 points of failure and, as our host points out, Murphy's going to find it.

Anonymous said...

Yes, they were shutting it down. I've wondered about the pressures myself. Best I can do is say that the pressure "downstream" (wrt steam flow) from the condenser was indeed smaller.

But then, it would mean that steam from the reactor should have entered the seawater from "upstream" also.

Look at those condenser pipes. There is difference in coloring near the break. What does it mean? Has the rust been boiled off?

Anonymous said...

Looks like a mechanical cap not a weld cap.

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