Dark days for San Onofre

23 January 2013



Operators of the San Onofre nuclear station in California have begun to get to the bottom of the problems plaguing their steam generators, and have come up with a plan to restart unit 2 at reduced power.


Replacement power costs are continuing to stack up at San Onofre Generating Station (SONGS) 2 and 3, which have been out of service since January 2012 and remain so pending resolution of steam generator tube wear problems.

More positively, though, the operator has now determined the cause of the problems, and a restart plan for the least-damaged unit, SONGS 2, was submitted to the US nuclear regulator for approval on 4 October. As of yet no timeframe has been given for the resumption of power generation at SONGS 2. The Nuclear Regulatory Commission said in December that it could take 'many months' to finish evaluating the licensee's analysis of the excessive tube wear and its plan to restart.

For unit 3, where further investigations and analyses are needed, it will be the summer of 2013 before operator Southern California Edison (SCE) is in the position to answer the NRC confirmatory action letter (CAL), and begin thinking about restart.
SCE has stressed that it places the highest priority on safety before a restart of any unit at SONGS.

The CE-designed SONGS 2 and 3 entered commercial operation in 1983 and 1984 (SONGS 1 was shut down in 1992). The plant is majority (78.21%) owned by Southern California Edison (SCE), with San Diego Gas & Electric having a 20% share and the city of Riverside 1.79%. Each unit is rated at 1180 MW, but they only have two loops, that is, two SGs apiece, instead of the more usual four -- so the steam generators are very large indeed, in fact the biggest in the nuclear business.

The original steam generators employed Alloy 600 Mill Annealed tubing material, and like many other steam generators, were prone to the all too familiar degradation problems of intergranular attack (IGA) and primary water stress corrosion cracking (PWSCC), as well as tube wear in the U-bend region. Extensive plugging was required. The plant owner decided to replace the steam generators in each unit after 16 planned refuelling outage cycles. Steam generator replacements have been carried out at around 50 US nuclear plants since the early 1980s.

A contract was placed with Mitsubishi Heavy Industries for the replacement steam generators. Those in unit 2 were replaced in 2009 and those in unit 3 in 2010. The basic strategy was to do the replacement under US regulation 10CFR50.59. This means essentially a straight replacement of what was there before in terms of form, fit, function and therefore safety implications - so that prior approval from the Nuclear Regulatory Commission is not required. But at the same time the aim was to incorporate improvements in the new units based on steam generator operating experience, avoiding tube wear through careful design of tube supports, and employing tube materials (in particular Inconel 690) that eliminate the risk of inter-granular attack (IGA) and primary water stress corrosion cracking (PWSCC). An article published in the January 2012 issue of NEI looks at the design and fabrication process for the SONGS replacement steam generators (NRC has publicly stated that SCE followed its guidance in procuring the RSGs and that it followed regulations by fully informing NRC of the design changes, such as the additional number of tubes in the RSGs).

Unfortunately, things have not gone according to plan. In the newly-installed steam generators at San Onofre, excessive tube degradation occurred in unit 2 and unit 3 after 22 months and 11 months of operation, respectively.
Once both reactors were shut down in January 2012, extensive inspection, testing, and analysis of SG tubes was performed in both the unit 2 and unit 3 steam generators (four in total). Tube-to-tube wear (TTW) was identified as the cause of the leak in the SONGS 3 steam generators. 'Minor signs' of tube-to-tube wear were also found in SONGS 2 steam generator, but only in a single pair of adjacent tubes, SCE said in an October press conference.

The SONGS steam generators, each with 9727 tubes, have been built with additional tubes so that up to 8% of them (778) can be taken out of service ('plugged') if necessary. In total, 807 tubes were plugged in unit 3 (420 in one steam generator, and 387 in the other). The total plugging for unit 2 was 205 tubes in one steam generator, and 305 in the other. As indicated in the table (p. 19), TTW was only found in a single pair of adjacent tubes in unit 2, compared with more than 300 tubes in the unit 3 steam generators.

Investigations

Over the past 10 months, SCE has assembled leading nuclear steam generator experts from around the world to work on the issue. In addition to MHI, among the organisations supporting SCE in its efforts, which have so-far involved 170,000 individual inspections, are AREVA, Westinghouse, Babcock & Wilcox Canada, MPR Associates and EPRI.

The analyses identified two types of wear in the steam generators. The first is tube-to-support structure wear, which is said to be similar to wear found at other replacement steam generators after one cycle of operation. Most of the tube wall thinning is less than 20%, which is well below the 35% wall-thinning limit that requires a tube to be plugged. According to SCE "the nature of the support structure wear is not unusual in new steam generators and is part of the equipment settling in."

The second type of wear, tube-to-tube wear, is unexpected. SCE said that it "understands that the tube-to-tube contact arises from excessive vibration of the tubes in certain areas of the steam generators."

Investigations and analysis have enabled it to determine the mechanistic cause of the TTW as a phenomenon called 'fluid elastic instability.' Fluid elastic instability, or FEI, results from a "combination of factors, including steam velocity, moisture content of the steam and effectiveness of the supports in the areas where the vibration occurs."

Most of the excessive wear has been found to be in a relatively limited area of the unit 3 SGs, attributable to high steam flow velocities, very dry steam and an "inadequate tube support structure in the U bend region."

Dry steam increases the potential for FEI, as it is less effective at damping vibrations than wet steam. Damping is the result of energy dissipation and delays the onset of FEI. Damping is greater for a tube surrounded by liquid compared to a tube surrounded by gas. Thus, a tube surrounded by gas or dryer steam has less damping, which increases the potential for FEI.

So why is the problem worse in the unit 3 steam generators? "Although all of the SG were procured to the same design, there were slight differences in the manufacturing and assembly of the unit 2 and unit 3 steam generators, that when added up caused the tubes in the unit 3 steam generators to be much more susceptible," said Pete Dietrich, SCE senior vice president and chief nuclear officer during an October press conference.

In its report to the NRC, SCE stated: "MHI concluded that the reduced manufacturing dispersion in the Unit 3 SGs resulted in smaller average tube-to-AVB [anti-vibration bar] contact force than in the Unit 2 SGs. Due to the smaller average tube-to-AVB contact force, Unit-3 was more susceptible to in-plane vibration."

Deitrich added that "computer modeling used in the design phase by the manufacturer MHI under-predicted the thermal-hydraulic conditions in the steam generators, which allowed unstable tube vibration to occur."

But there is a solution. Detailed vibration analyses by experts have concluded operating unit 2 at 70% power will eliminate the thermal-hydraulic conditions that caused TTW in unit 3. And it will also minimize the tube to support structure wear.


Outlook

As previously stated, SCE submitted its response to the CAL and its restart plan for SONGS 2 to the NRC in October; a further review meeting took place in early December. It is seeking NRC permission to operate unit 2 at 70% power for approximately five months, after which it would shut down for inspection to examine the steam generators again. After that, it would plan to operate for 15 months before another inspection and shut-down.

NRC said in October that it expected to spend 'several months' reviewing documents submitted by the licensee and performing a thorough and independent assessment prior to making any decisions about restart.

SCE said in its CAL response to the NRC that it has taken precautions to ensure that tube integrity will be maintained during any future operation.

"SCE has evaluated the causes of TTW in the Unit 3 SGs and...has completed corrective and compensatory actions in Unit 2 to prevent loss of tube integrity due to these causes."

"Tubes within regions of the Unit 2 SGs that might be susceptible to FEI have been plugged. In addition...SCE has established operational limits that eliminate the thermal-hydraulic conditions associated with FEI from the SONGS unit 2 SGs."

Within 150 days of operation at or above 15% power, but below 70% power, unit 2 will be shut down for inspection to confirm the condition of the SG tubes.

"The shorter, five month operating period we are proposing is designed to provide an additional margin of safety," SCE said.
SCE stated in its CAL response that: "The analyses and operational assessments performed by SCE and independent industry experts demonstrate that under these conditions, tube integrity will be maintained. On this basis, SCE concludes that unit 2 will operate safely."

It is not clear how long SONGS 2 will have to operate at less than full load. In a teleconference on 1 November, Ted Craver, chairman and chief executive officer of Edison International said that SCE continues to work with its outside experts and the steam generator designer and manufacturer, MHI, on what it will take to restore both units to full load.

"It is not clear at this time if the units can be repaired, and it appears complete replacement of the steam generators would take some years," he said.

Unit 3 is not expected to return to service until beyond the summer of 2013. SCE said that it is still studying the SGs there, and that it may need to do a series of mock-ups and tests to fully understand the situation.


Costs

The replacement steam generators are still under warranty, but the extended outage is clearly having serious financial implications for Southern California Edison.

In its quarter 3 earnings statement, SCE revealed that so far its share of inspection and repair costs related to the SG outage total $96 million. On top of this it has incurred $221 million in replacement power costs through 30 September 2012. This represents about 7% of SCE's total fuel and purchased power expense incurred to that date.

SCE said that the steam generators have a 20-year warranty with Mitsubishi Heavy Industries, which covers the repair or replacement of defective items and has specified damage limits for some repairs. There is a $138 million liability limit, according to SCE, and this excludes consequential damages, such as replacement power. In September SCE submitted a $45 million invoice to MHI for some repair costs through June 2012.

An August press release announced SCE's intention to downsize the SONGS' organization to 1500 - a reduction of approximately 730 employees, beginning fourth quarter of 2012 - although it was emphasised that this was as a result of a benchmarking exercise initiated more than two years ago to "align SCE's processes and staffing levels with the top performing nuclear operating plants in the industry." SCE said it has concluded that SONGS' staffing and costs are significantly higher than other similar dual unit, non-fleet nuclear power plants and that "there are opportunities to reduce operating and maintenance costs by improving plant processes while fully maintaining all safety commitments."

San Onofre nuclear generating station
SONGS schematic SONGS schematic
Steam generator assembly
U-bend section
U-bend section diagram
Steam generator diagram
Steam quality simulation
Interstitial velocity simulation


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