San Onofre steam generator tubes leaking - why?

jim hardy

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

Ahhh,, Turbine blades - another of those fascinating industry "niches" .
Rotor dynamics is fascinating.

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

Tubes will rattle.
I suppose it's quite a calculation to get the natural frequency and vibration modes of a long hollow tube that's pressurized with the fluid inside having considerable velocity.
When i read how a Coriolis Flowmeter works , i just felt like saluting the entire Mechanical Engineering community.
http://en.wikipedia.org/wiki/Mass_fl...lis_flow_meter
it somewhat resembles the u-tubes in steam generator, see this graphic
http://en.wikipedia.org/wiki/File:Co...ow_512x512.gif

I'm admitting my abysmal ignorance here. I know just enough to not cast stones, and that wasn't intent of previous post...

Astronuc

NRC Region IV Administrator Elmo E. Collins said. “Until we are satisfied that has been done, the plant will not be permitted to restart.”

On Jan. 31, operators performed a rapid shutdown of the Unit 3 reactor after indications of a steam generator tube leak. Unit 2 has been shut down since Jan. 9 for a planned refueling and maintenance outage. Subsequent inspections at both units have identified unusual wear in many tubes of the steam generators, which were replaced in January 2010 at Unit 2 and January 2011 in Unit 3.

SCE has identified two causes of the unusual wear: tubes are vibrating and rubbing against adjacent tubes and against support structures inside the steam generators. They are still working to determine why this is occurring.

Only one tube required pressure testing on Unit 2. However, six other tubes required plugging, and 186 additional tubes were plugged as a precautionary measure. Eight tubes failed pressure testing at Unit 3, indicating that these tubes could have failed under some accident conditions. Evaluation for additional plugging or other corrective actions are continuing for Unit 2, based on ongoing evaluations of Unit 3 test results.

CAL 4-12-001 - http://www.nrc.gov/reading-rm/doc-co.../12-011.iv.pdf
CONFIRMATORY ACTION LETTER – SAN ONOFRE NUCLEAR GENERATING STATION, UNITS 2 AND 3, COMMITMENTS TO ADDRESS STEAM GENERATOR TUBE DEGRADATION

For both Units 2 and 3, this was the first cycle of operation with new replacement steam generators. Unit 2 replaced its steam generators in January 2010, and Unit 3 in January 2011. Each steam generator has 9,727 steam generator tubes.

NUCENG

Story on San Onofre Steam Generator Leakage.

http://www.power-eng.com/news/2012/0...-troubles.html

Story above is based on Arnie Gunderson Report prepared for the Friends of the Earth environmental and anti-nuclear group.

http://fairewinds.com/content/foe-re...res-san-onofre

Biggest error is that the quote from NRC chairman Jaczco that NRC approval is not required for restart is not current. The Confirmatory Action Letter issued to SCE requires NRC approval. I am still looking for a copy of the CAL itself. I haven’t found it on ADAMS yet.

I do like the list of changes implemented in the new steam generators. Arnie is correct that the increased number of tubes, change in tube alloy, changes in tube support structure (egg crate - implying fragility???) and increased coolant flow are potential causes.

I think pulling in the issue of BWR Dryer Cracking is a stretch though. Anyway this is a potential for a good discussion here on PF.

Astronuc

The CAL is in my previous post.

I don't think switching to 690 from 600 (?) is an issue, as that is the industry practice over the last two decades. Smaller tubes and increased flow rates might play a role, but I'd have expected a CFD analysis would have caught that - but maybe not if not done right.

BWR dryer is different material in a different environment over a longer period. That's mixing apples with oranges, but it is an interesting topic nevertheless.

NUCENG

Quote by Astronuc

The CAL is in my previous post.

I don't think switching to 690 from 600 (?) is an issue, as that is the industry practice over the last two decades. Smaller tubes and increased flow rates might play a role, but I'd have expected a CFD analysis would have caught that - but maybe not if not done right.

BWR dryer is different material in a different environment over a longer period. That's mixing apples with oranges, but it is an interesting topic nevertheless.

Thanks, I was not paging down far enough to see the CAL - operator error!

You expressed interest in the divider plate defect in the rplacement S/G for unit 3. Here are the references I found. (I am not a weld engineer and won't even try to comment.)

IN 2010-07
http://pbadupws.nrc.gov/docs/ML1000/ML100070106.pdf

Slide Presentation on root cause:
http://pbadupws.nrc.gov/docs/ML0925/ML092590470.pdf

Non Proprietary Root Cause Report:

http://pbadupws.nrc.gov/docs/ML0926/ML092600513.pdf
http://pbadupws.nrc.gov/docs/ML0926/ML092600515.pdf
http://pbadupws.nrc.gov/docs/ML0926/ML092600516.pdf

Astronuc

Somewhat relevant - Effects of Alloy Chemistry, Cold Work, and Water Chemistry on Corrosion Fatigue and Stress Corrosion Cracking of Nickel Alloys and Welds
http://www.nrc.gov/reading-rm/doc-co...721/cr6721.pdf

They process used low carbon Alloy 152 in butter welds as expected.

They used gouging to remove the SS cladding in Unit 3 RSGs rather than the machining used in Unit 2 RSGs.

Hologram0110

I came across this news article that may be of interest.

It sounds as though anti-vibration supports were removed to increase the number of tubes in the generator. This has led to more vibration and mechanical wear as a result.

NUCENG

Quote by Hologram0110

I came across this news article that may be of interest.

It sounds as though anti-vibration supports were removed to increase the number of tubes in the generator. This has led to more vibration and mechanical wear as a result.

The source of that position is Arne Gunderson speculation on a cause. In previous posts we have listed other possible causes. Clearly the NRC recognizes the need to determine a root cause (as evidenced by the CAL). I urge you to reserve judgment until the facts are determined. I am certain that the root cause evaluation will be released (although some proprietary information may be withheld). I am also certain that NRC staff, ACRS, and every "nuclear watchdog" organization will subject the root cause to independent review. It is possible that Arnie is right, just not certain based on his previous record.

Hologram0110

You're absolutely right. From the article:

The report on San Onofre by Fairewinds Associates, a Vermont-based consultant that has worked with groups critical of nuclear power, suggests that "imprudent design and fabrication decisions" may be to blame for accelerated wear on generator steam tubes. Friends of the Earth commissioned the analysis.

The article just popped up in my news feed this morning and I remembered there was a thread about it on Physics Forums. I wasn't following the tread so I assumed that this was a 'new' analysis. Seems I missed that the tread was dead. Sorry about that.

NUCENG

Quote by Hologram0110

You're absolutely right. From the article:

The article just popped up in my news feed this morning and I remembered there was a thread about it on Physics Forums. I wasn't following the tread so I assumed that this was a 'new' analysis. Seems I missed that the tread was dead. Sorry about that.

Here is the latest - a Part 21 report from MHI.

http://www.nrc.gov/reading-rm/doc-co...0120416en.html

Part 21 Event Number: 47833
Rep Org: MITSUBISHI NUCLEAR ENERGY SYSTEMS
Licensee: MITSUBISHI HEAVY INDUSTRIES, LTD
Region: 1
City: ARLINGTON State: VA
County:
License #:
Agreement: Y
Docket:
NRC Notified By: EI KADOKAMI
HQ OPS Officer: JOHN KNOKE Notification Date: 04/13/2012
Notification Time: 15:58 [ET]
Event Date: 04/13/2012
Event Time: [EDT]
Last Update Date: 04/16/2012
Emergency Class: NON EMERGENCY
10 CFR Section:
21.21(a)(2) - INTERIM EVAL OF DEVIATION
Person (Organization):
BLAKE WELLING (R1DO)
KATHLEEN O'DONOHUE (R2DO)
DAVID HILLS (R3DO)
VINCENT GADDY (R4DO)
PART 21 GROUP (EMAI)

Event Text

PART 21 INTERIM REPORT - STEAM GENERATOR TUBE WEAR

This interim Part 21 is in regard to San Onofre Nuclear Generating Station, Unit 2, Steam Generator replacement.

"During the first refueling outage following steam generator replacement, eddy current testing identified ten total tubes with depths of 90 to 28 percent of the tube wall thickness. Some of the affected tubes were located adjacent to retainer bars. The retainer bars are part of the floating anti-vibration bar (AVB) structure that stabilizes the u-bend region of the tubes.

"Other tubes in the two steam generators had detectable wear associated with support points elsewhere in the AVB structure. Each steam generator has 9727 tubes with an 8 percent (778 tubes) design margin for tube plugging.

"Discovery Date: February 13, 2012

"Evaluation completion schedule date: May 31, 2012"

"Those Mitsubishi Heavy Industries customers potentially affected by this issue have been notified and will receive a copy of this interim report."

Reference Document: UET-20120089
Interim Report No: U21-018-IR (0)

Astronuc

More than 1300 steam generator tubes have now been plugged at Southern California Edison's (SCE's) San Onofre Nuclear Generating Station (SONGS) in California as the utility continues to investigate the cause of excessive wear in some of the tubes. It is not yet known when the two-unit plant will resume operation.

. . . .

http://www.world-nuclear-news.org/C-...S-0905124.html

NUCENG

Quote by Astronuc

http://www.world-nuclear-news.org/C-...S-0905124.html

The news discusses a potential that SONGS may be able to restart, but be limited to operate at a lower power rating to avoid the possibility that flow induced vibration is a cause. I would love to have a peek at the performance guarantees in the contract for those new steam generators!

jim hardy

i'm sure curious why the new tubes fail .

....limited to operate at a lower power rating to avoid the possibility that flow induced vibration is a cause.

Vibration can be excited from either inside or outside a tube.
Flow inside those tubes barely changes with power.
Seems to me a microphone on the steam generator could hear tubes clattering.
I'd instrument a steam generator and listen. If they clatter at zero power then excitation is from primary flow not secondary.
Most plants have loose parts monitors that are basically microphones at natural collection points like reactor vessel bottom and steam generator inlet side tubesheet. Move one up to vicinity of the tube wear region.

old jim

Astronuc

Quote by jim hardy

i'm sure curious why the new tubes fail .

Vibration can be excited from either inside or outside a tube.
Flow inside those tubes barely changes with power.

If the tubes are contacting each other, then that's some relatively large amplitude vibration, which means the tubes are not sufficiently stiff, or there is some pretty substatial excitation mechanism.

Flow might have increased because of the reduced pressure drop, and perhaps flow was increased slightly, on the primary and/or secondary side in order to increase power output. Increased flow in the primary circuit always an issue when replacing steam generators.

I'm puzzled about what kind of analysis was performed concerning the design. In this day and age, we have pretty advanced CFD capability. I'm left wondering - what did they miss, or not consider, in the design and the analysis.

Seems to me a microphone on the steam generator could hear tubes clattering.
I'd instrument a steam generator and listen. If they clatter at zero power then excitation is from primary flow not secondary.

Most plants have loose parts monitors that are basically microphones at natural collection points like reactor vessel bottom and steam generator inlet side tubesheet. Move one up to vicinity of the tube wear region.

Acoustic emissions (noise) analysis would be appropriate, but I'm not sure it if is done on SGs.

jim hardy

If the tubes are contacting each other, then that's some relatively large amplitude vibration, which means the tubes are not sufficiently stiff, or there is some pretty substatial excitation mechanism.

iirc the tube diameter was decreased and bending moment is in proportion to moment of inertia of cross section, i think 3_rd or 4_th power of diameter ?
http://en.wikipedia.org/wiki/List_of...nts_of_inertia
(Pardon me i'm no mechanical engineer) so reducing diameter will reduce stiffness ? Surely they calculated that. The tubes get additional stiffness due to internal-external Δp and i dont know how to calculate that. That Δp is not constant as main steam pressure changes from ~ 1000 psi to ~ 800 with power.
As you said surely they couldn't have missed that.

It gets curioser and curioser.
They'll figure it out. They have my genuine sympathy .

Acoustic emissions (noise) analysis would be appropriate, but I'm not sure it if is done on SGs.

we had loose parts sensors at entry point of feedwater line to steam generator. You could hear internals of check valve tinkling at low flow. That'd be the closest point i know of. Sound telegraphs pretty well through steel , so one might hear something at primary tube sheet.

old jim

gmax137

Installing such acoustic monitors might have been possible as part of the RSG startup, but I don't think they will be heating the unit up / running the RCPs just so they can listen in and try to identify the problem. Too late for that. They will have to figure it out with inspections, whatever operating data is available now, and some fancy analysis.

Astronuc

Vendor singled out by SONGS findings
http://www.world-nuclear-news.org/RS...s_200712a.html

Nevertheless - the licensee is responsible for oversight and quality of whatever component or system is installed in the plant.

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Astronuc Admin	Somewhat relevant - Effects of Alloy Chemistry, Cold Work, and Water Chemistry on Corrosion Fatigue and Stress Corrosion Cracking of Nickel Alloys and Welds http://www.nrc.gov/reading-rm/doc-co...721/cr6721.pdf They process used low carbon Alloy 152 in butter welds as expected. They used gouging to remove the SS cladding in Unit 3 RSGs rather than the machining used in Unit 2 RSGs.

Hologram0110	I came across this news article that may be of interest. It sounds as though anti-vibration supports were removed to increase the number of tubes in the generator. This has led to more vibration and mechanical wear as a result.

gmax137	Installing such acoustic monitors might have been possible as part of the RSG startup, but I don't think they will be heating the unit up / running the RCPs just so they can listen in and try to identify the problem. Too late for that. They will have to figure it out with inspections, whatever operating data is available now, and some fancy analysis.