Remember the leak from SARRY in February? TEPCO released the analysis of the pipe that leaked highly radioactive water from the Toshiba's cesium absorption system, and it sure looks like a bad weld, from the photos of inside the pipe.
It is particularly bad on the left side of the rust. The leak occurred BELOW the weld, but the bad weld may have contributed to the rust and corrosion of the pipe.
From TEPCO's Photos for Press, 3/12/2012 (click for bigger image):
Strangely, TEPCO does not blame the weld. The company blames sodium hypochlorite (bleach) which is injected in the water to "prevent the microbially caused clog" (from their document, page 3) to have caused corrosion.
If you're wondering "microbe??", it's because TEPCO mixes the treated water with the filtered river water and inject the water back into the reactor.
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5 comments:
As one can see, the crack developed at the region of blue discoloring of the steel.
Depending on the metal alloy these colors ("Anlauffarben" in German, don't know the english translation) indicate the temperature which the metal reached when heated up at welding.
To me it looks like that pit corrosion happened at some micro-crack which developed at weld time at the start/end point of the weld which is exposed to much more heat stress than the rest of the weld.
Don't worry, Tepco was lucky that it was only a lowly low-pressure waste water pipe.
Such massive corrosion often happens even at reactor pressure vessels without operators or NRC taking note.
A (in-)famous example is the Davis Besse nuclear plant which came near to a loss of coolant disaster.
This disaster was only averted because when preparing for restart some worker wondered why he could wiggle the control rods freely, which should be seated firmly.
Nobody cared of the large, well visible rust deposits, neither NRC nor the operator, which both were eager to get the reactor restarted asap.
If the worker hadn't complained, the Davis Besse NPP would have passed the "revision checks" without problems and eventually would have blown up some time later.
Look at this page to see yourself: http://www.nirs.org/photogallery/davisbesseimages.htm
Anlauffarben = tempering colors
The Blue discoloration is normal when welding stainless. Looks like when the pipe was welded, the welder didn't sweep the torch at the end of the weld job, that left a pin hole. Notice that they that the welded surface near the hole is different from the reset of the weld that looks like a stack of dimes.
Generally after completing a weld, a pressure test is performed. Soapy water can be applied to the outside on the weld to check for leaks. My understanding for critical welds (boilers, nuclear, etc) welds are inspected either with x-rays or ultrasound to check for flawed welds. In this case it appears no test were performed. Perhaps they were in a rush to get it done because they had no choice, and skipped testing to save time.
If the weld is rusting, then it may also be because they used a non-stainless steel filer rod. But I since rust only appears where the leak is, I suspect they the water contains disolved rust that is being depositing near the leak.
It also looks like they didn't passivate the pipe after welding.
it looks like it is rusting because they cut the pipe with a carbon contaminated band saw blade and the coolant water settled there on the bottom
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