San Onofre (SONGS) owner updates public on cause of steam generator tube wear
The US Nuclear Regulatory Commission’s Augmented Inspection Team (AIT) held a public information meeting on June 18, 2012 to present findings from their investigation into the cause of the excessive steam generator tube degradation found in units 2 and 3 of the San Onofre Nuclear Generating Station (SONGS). Southern California Edison (SCE), SONGS owner and operator, participated in the meeting and presented the results from their own internal investigations. Here is an SCE produced video that provides the highlights of their presentations created so that the public can have easy access to the best available information that the company has.
I hope that you get the flavor from rather short excerpt from a four hour meeting that the company takes its responsibilities seriously and that it has spared no expense in performing tests and analysis to determine the root cause of a the disappointing performance of their expensive, recently installed steam generators.
Tom Palmisano, VP Nuclear Engineering SCE, provided the following interesting bit of technical information.
As has been pointed out by the NRC, it is significant to note that there are differenced between the two units. Unit 3, which experienced the tube leak had 326 tubes damaged by this tube to tube wear. Unit 2 had only two tubes which showed minor indications of tube to tube wear, so small it was almost undetectable. It was our rigorous retesting that identified two tubes that had minor indications. So Unit 2 is in much better condition than Unit 3.
Interestingly enough, the Unit 2 steam generators have been in service for a longer period of time; they were replaced in 2009 while the steam generators in Unit 3 were replaced about 18 months later in late 2010.
It sounds like the company ended up with a couple of lemons. It is not entirely clear why the generators in Unit 2 are different enough from those in Unit 3 that they did not experience the same level of damage. It is pretty clear to me, however, that the Unit 3 generators will not be able to operate at their designed power level again. Based my understanding of the reports that the company provided in the above video, it might be possible for the generators to operate safely at a reduced power level.
The already damaged tubes have been plugged so that they will not carry any primary coolant. Since a part of wear mechanism is the specific conditions of the steam and water during power operations it might be possible to change those conditions enough by operating at a reduced power to avoid the vibration. Carefully monitored operations at a reduced power for a period of time would allow the company to supply needed electricity while waiting for replacement steam generators to be manufactured and the replacement job to be planned. These are not items that are kept in inventory or that require simple replacement evolutions.
Steam generator tubes have been a challenge for pressurized water reactors since the early days of the technology’s implementation. Many of the early issues have been overcome or mitigated by design refinements and water chemistry improvements, but it is not surprising that failures still occur occasionally. Pressurized water reactor steam generators are large, complex, precision components operating in a challenging environment. They include thousands of parts built to exacting tolerances. It is not hard to imagine how small variations can lead to unexpected performance problems.
It is worth noting in passing that at least one type of steam generator is available that could not possibly experience the exact problem experienced by the Mitsubishi steam generators installed in SONGS because once through steam generators (OTSG) do not have any bends in the tube and do not have a complex tube bend area needing intricate support structures. That is not to say that OTSGs are a panacea; they just have different challenges that must be addressed.
As a curious observer, I appreciate SCE’s forthright presentation and effort to make sure that it is published in an accessible format for others like me. YouTube is a wonderful part of the communications revolution that has shifted some of the power from the established network owners to the rest of us, including those businesses that take the time to learn to use the tools. I hope more nuclear companies continue to learn and apply communications best practices.
Dan Yurman at Idaho Samizdat has provided some additional details about the rest of the meeting in his post titled No easy answers at SONGS – Update
Does this kind of hardware come with any warranty? Since they were just installed not too long ago, I would hope so.
From what I understood, the SGs in Unit 3 were from a different manufacturer than the ones in Unit 2.
The OTSGs like those used in the B&W PWR design also have the advantage that you get a significant amount of superheating of the exit steam, which helps on thermal efficiency. The downside is that they are more sensitive to feedwater transients, as we have learned. Improvements in the ICS for PWRs may help on that score. I am working on a system for in-situ monitoring of flow-accelerated corrosion, which may provide prognostic capability for tube wear and eventual replacement and/or plugging.
Don’t all steam generator tubes have the possibility of failure through improper or un- intended operation, regardless of the source of heat?
Paul W — EVERYTHING has a possibility of failure through improper or unintended operation. “Possibility” is a very vague word. My car’s radiator has a “possibility of failure” if I don’t use a corrosion-inhibiting anti-freeze and clog it up.
It has been almost 30 years since I was involved with reactor technology, but I seem to remember that the once-through design of Babcock and Wilcox steam generators supposedly contributed to the dynamics of the Three Mile Island reactor plant during the 1979 disaster. I guess that relates to Wayne SW’s comment.
Northcoast, yes, they are more vulnerable to dry-out and thus the introduction of thermal transients on the primary side. There is nothing inherently wrong with the design, its just you have to be more aware of the possibility of this kind of scenario and understand the basic physics a little more. In TMI they took care of the steam generator problem fairly quickly but by then they had the stuck-open PORV and didn’t recognize right away that they were losing primary coolant through that pathway. They were so fixated on the pressurizer level that they didn’t know the RV level was dropping. If not for that, it would have been a pretty minor incident.
Are steam generators routinely instrumented with vibration monitors?
Digital signal processing can be used to recognize nominal vibration signatures and provide additional alarms when conditions are abnormal. Metal-to-metal contact should be discernable.