San Onofre steam generator tubes leaking - why?

jensjakob

Hi,

San Onofre, menioned durring the Japan crisis, is leaking and corroding pipes fast:
http://www.scpr.org/blogs/news/2012/...-indefinitely/

http://sciencedude.ocregister.com/20...-units/167503/

What is going on- any ideas?

Take care

Jens Jakob

jim hardy

Steam generator tubes are leaking thinning .
This was a big problem stateside 1n 1970's.

Chemistry of water on secondary side is extremely important.
We measured impurities in parts-per--billion. One cup of tap water was enough to cause a shutdown to flush the steam generators, i know because one of our technicians used a cup of tapwater to top off a level instrument..... once.

Metallurgy was important also. Copper contributed to corrosion.
We replaced the admiralty brass tubes in condenser with titanium, and feedwater heaters with stainless steel.

These lessons were learned almost forty years ago. Our replacement generators from Westinghouse-Tampa are doing fine.
So - what's going on now? That's REAL good question.
First question pops to mind is "Where did Mitsubishi procure the metal for the tubes in those replacement steam generators?"

Second is "How's the plant's water chemistry ?"

It'll be interesting to follow this one.

Edited first line. sorry.

wizwom

http://www.clipsyndicate.com/video/p...clips&wpid=752

The San Onofre plant replaced its steam generators in 2010; it is almost certain this is a manufacturing defect.

NUCENG

NRC discussion for background. We don't know the cause yet, so I will not speculate.

http://www.nrc.gov/reading-rm/doc-co...steam-gen.html

Astronuc

The fact that these are replacement SG's and the tubes failed during the first cycle of operation would implicate 1) a manufacturing defect or 2) a problem with installation.

SCE replaced SGs in SONGS2 during 2009.
http://www.world-nuclear-news.org/ne....aspx?id=24719

http://www.world-nuclear-news.org/C-...3-0410105.html

'Why' is the question - indeed! There is an ongoing investigation. It must be determined if any corrosion (intergranular stress-corrosion) and/or cracking is occurring, and if it is initiated on the primary or secondary side.

I believe the material is Inconel 690, which is supposed to be superior to Inconel 600. However, Inconels are notoriously tricky alloy systems.

jensjakob

"The company has received overseas orders for 31 units, mainly from North America and Europe. "

Hmmm - could be interesting to track those and compare corrosion.

NUCENG

Updated Daily Event Report:

http://www.nrc.gov/reading-rm/doc-co...0120319en.html

Preliminary Notification and Update

http://pbadupws.nrc.gov/docs/ML1203/ML12032A243.pdf

http://pbadupws.nrc.gov/docs/ML1207/ML12075A219.pdf

NRC press release for AIT:

http://pbadupws.nrc.gov/docs/ML1208/ML120800460.pdf

NUCENG

More info at the SONGS website:

http://www.songscommunity.com/news.asp

Confirms Unit 2 S/G replacement in 2009 and Unit 3 in 2010. The following link is good info on the testing methods being used:

http://www.songscommunity.com/docs/Test_Inspections.pdf

NUCENG

Picture of Steam Generator (MHI):

http://www.mhi.co.jp/en/products/det...generator.html

Unit 3 S/G had leakage before delivery. Note that this was not tube leakage so current problems may not be directly related.

http://mdn.mainichi.jp/mdnnews/news/...dm058000c.html

If there is a manufacturing problem, Mitsubishi has already delivered over 100 steam generators around the world.I am less worried about the 31 on order.

Astronuc

The article states If that is the replacement S/G, that's rather troubling. In 2009, that SG would have been at the Mitsubishi shop - ostensibly before shipment. Or did Mitsubishi inspect the older in-service SG, which was replaced in 2010?

A crack in the divider plate is not bad as long as it doesn't propagate. A breach in the divider plate would allow leakage from the hot leg to the cold leg, thus by-passing the SG tube bundle. It is still within the primary system.

From the description, it sounds like the crack was at the edge of the divider plate where it joins the vessel (channel) head.

NUCENG

I understand the crack was on the Unit 3 replacement S/G and was repaired before shipment and installation in 2010.

Astronuc

That would seem to be what the article implicates. I was hoping for confirmation.

Nevertheless, I'm puzzled about the crack, and also about welding dissimilar metals. I would expect the vessel shell to be line with stainless steel, and the divider plate to be made of the same stainless steel. Certainly if one welds a low carbon stainless steel to a high carbon steel, cracking can be an issue.

I'm curious about their process and procedures, since the procedures should be such that cracking is prevented/avoided.

I'd like to know if the Inconel tubes are cracking (which would imply either poor material and/or poor fabrication practice) or leaking about the fitup at the tube sheet (which would imply a poor process).

Nevertheless, it is very troubling that such failures occur in something that is designed to last 20 to 30 years.

Original SGs were supposed to last the life of the plant (40) years, and if possible now 60 years. They represent a substantial capital cost, and the economic models, which I studied at university, never included SG replacement. After I finished by undergrad, I learned about how Inconel 600 components (and certainly welding materials) were failing prematurely.

Primary water chemistry, and in some cases, secondary water chemistry are certainly factors.

NUCENG

And anytime you open a system there is a chance for loose parts or materials to enter a system. Flow induced vibration can cause wear and tear. A few years ago replacement condensate pumps for a nuclear plant were exposed to road grime and sludge during shipment because penetration seals were not properly installed. I too am concerned this has developed so soon after S/G replacement and that is likely why NRC sent the AIT. I am not a welding or S/G expert, but if I find additional information I will post it. We should learn more when the NRC AIT has their exit meeting.

jim hardy

From Nuceng's http://pbadupws.nrc.gov/docs/ML1207/ML12075A219.pdf

and http://www.nrc.gov/reading-rm/doc-co...0120319en.html

Chemistry problems can deposit solids in the support to tube annulus and squeeze the tubes.
But tube-to-tube wear sounds more like a vibration issue arising from mechanical design.
It's hard to believe mechanical vibration trouble after this many years experience making Steam Generators.

As you said, it'll be interesting to see what they find.

Astronuc

The chemistry practices are pretty standard these days. There could be an issue with commissioning a fresh surface.

Replacement generators may have higher flow rates. I can't remember if there was a plant uprate with the steam generator replacement.

Tube wear after one cycle of operation would be troubling. Despite experience, designer make 'improvements' that sometime may introduce performance problems. There was a case of two BWRs* in which new advanced turbines developed cracks in one of the late stages in the LP turbine. Subsequent CFD reveal a design flaw. The CFD analysis (which is very mature these days) should have been part of the initial design process.

*Hamaoka 5 and Shika 2 off line after turbine vane failures
http://www.neimagazine.com/story.asp?storyCode=2038314


High cycle fatigue (either mechanical (FIV) or thermo-mechanical) is a possibility if the frequency is in the acoustic range (10-1000s Hz) with 3.156E7 s/yr.

NUCENG

San Onofre Units 2 and 3 have both got approved Margin Uncertainty Recovery Power Uprates of 1.4% in 2001.

Astronuc

That's enough time to incorporate into the current replacement design. In terms of uprate, I was thinking more along the lines of an extended or stretch uprate with 5+% increase in reactor/plant output.

Mitsubishi is a Westinghouse licensee, and they have probably replaced more W-SG than CE SGs. The large CE plants (mostly 16x16 fueled) typically use 2 steam generators - with one hot leg and two cold legs. They are therefore typically larger than W-SGs. Could that be a factor?