NRC representation of San Onofre outage contested

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Over the course of the last year while the San Onofre reactors have been shut down by steam generator failures, readers may have visited and read the NRC’s representation of the steam generator issues in Unit 2 as described on their website.[1] 

They too may have felt that the description as is, stands in direct contrast to the actual sequence and severity of events, and provides only a partial description of the outage and subsequent investigations.

Specifically, the contention with the NRC’s representation lies in the following selected portions of their description: “In accordance with the plant’s Technical Specifications, a 100-percent inspection of all steam generator tubes was conducted. Mechanical wear was observed at various locations along the tube lengths, similar to what has been observed in comparable steam generators at other plants.”

As shown below from Southern California Edison documents relating to the inspection of the outage, the bobbin tools used to perform the 100% inspection were not capable of, and did not detect, note, or make public statement relating to any wear discovered in steam generator tubes stemming from the 100-percent inspection.  The notes from the SONGS 2C17 Steam Generator Condition Monitoring Report, “These inspections did not detect tube-to-tube wear or any other conditions that may be precursor signals to tube-to-tube wear, such as AVB wear indications extending outside of the intersection between the tube and the AVB.”

SONGS SONGS 2C17 Steam Generator Condition Monitoring Report_Page_01

Based on the severity of tube-to-tube wear found in Unit 3, the licensee completed additional rotating coil eddy current testing, which is much more capable of detecting the damage, in a limited portion (1,375 of the 9,727 total tubes in each steam generator) of the tubes in the Unit 2 steam generator.

Therefore, it is hard to agree with the NRC’s subsequent statement that “Based on the initial 100-percent tube inspections, the licensee completed tube plugging and staking (internal cable support of select plugged tubes) of 192 tubes total: 98 in steam generator 2E088 and 94 in steam generator 2E089.”

Based on evidence collected from investigation documentation and public announcements made by the licensee, the plugging and staking of tubes was in response to the rotating coil eddy current testing of only 1300 tubes in each of the two steam generators, which performed after the initial 100% tube inspections.

The NRC description also fails to disclose or acknowledge the amount of tubes which have not been plugged or staked but which have shown signs of wear.  This includes the 4,348 indications detected at AVB contact points, and the additional 236 indications of wear at tube support plates.

SONGS – Summary of Degraded Tubes in the Unit 2 Steam Generators High Wear Region

Long-Term Solutions Unavailable

One of the main problems with the investigation at Unit 2 is that it was limited to areas where problems were thought to be most likely to be found, called the “High Wear Region”, and not all areas that have potential susceptibility to damage.  What conclusions can be made about the fear of asking questions except for that we should be ashamed to learn the answer?

The problem is that leakage predictions for wear-related degradations are not very accurate, and can vary significantly from simple slits to large holes.  Small differences in initial assumptions or conditions can produce very great differences in final actual results.

Edison, free of being forced to perform a root cause analysis by the NRC, was able to supply forward-looking operational assessments to bolster their arguments and muddy the water.

Edison used Unit 3 tube-to-tube wear index data to define the likelihood of initiating TTW and the potential growth rates.  TTW is based on the change in wear index during Cycle 17 as a result of both further AVB and further TSP wear.

However, the wear index profiles are not the same between the units.  Many more TSP locations were found to show wear in Unit 3 than in Unit 2.  Unit 2 only had 2 total TTW indications in both the 2SG89 and 2SG88 steam generators, but significantly more AVB wear.  Essentially, while both reactors are being affected by TTW, they are from different modes of wear.

SONGS - Tube to Tube Wear Profiles

Next, they used the forecasted operation at 70% power to scale the model data from Unit 3.  They combined this data with Monte Carlo modeling to identify tubes in high-wear regions of the Unit 2 steam generators and plug them.

It should be noted that the operating conditions are not that much different between 100% power and 70% power in many areas outside of dynamic pressure.

SONGS - Unit 2 Cycle Conditions

The plan that Edison currently proposes is to operate Unit 2 for a short 5 month run at 70% power in order to provide additional margins to request to restart after inspecting the tube wear.[2]

It is important for the licensee to get consumers and regulators to agree that reducing power to 70% will “significantly reduce” the damage which has kept the reactor shuttered for so long, and allow them to continue reaping the benefits of operating a nuclear power plant without any need for the additional electrical capacity.

Standard refueling cycles last between 18 and 22 months, but SONGs own investigations and independent analysis have confirmed that SIPC margins will only be met for a cycle of 1.48 years at 70% power level.

SONGS - SIPC Margins


SONGS - Cycle Lengths

SONGS - OA Analysis of SIPC Margin Requirements

The root question to be asked of Edison is what is the business basis for the decision to restart?

Simply, why does Edison feel that restarting the San Onofre Nuclear Generating Station is a good business decision if they know that there is no long-term effective solution, and the existing problems will soon force the plant to retire?

[2] Operational Assessment for SONGS Unit 2 Steam Generators for Upper Bundle Tube-to-Tube Wear Degradation at End of Cycle 16

ML12285A269 – SONGS – AREVA Executive Report by Enformable

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