APR: your source for nuclear news and analysis since April 16, 2010

Tuesday, March 27, 2012

San Onofre S/G update 3/27

Thanks to a tip off from Dan Yurman, we have the following release from the NRC:

NRC Confirmatory Letter / San Onofre restart preparations

The information contained in this release from the NRC answers several of the questions we've been asking about the exact nature of the failures in the new Mitsubishi steam generators installed in the two operating San Onofre Nuclear Generating Station plants (originally manufactured by Combustion Engineering.) The briefest possible version is as follows:

-Tubes in the steam generators are vibrating.
-The vibration is causing rubbing against support and alignment structures for the tubes, and rubbing against adjacent tubes.

One might wonder whether the vibration is caused by water and steam flow on the secondary side (steam side) of the U-tubes, or if it is caused by flow not within desired parameters on the primary side (inside) or both. Clearly the exact cause is not known as I write this.

What's clear is that all the appropriate actions for S/G tube leaks have been taken up until this point, and that proper investigation continues. There is nothing particularly alarming or disturbing in the NRC's report. As I have mentioned before, steam generators have thousands of tubes (these have 9727 each) so that there is always substantial reserve to have tubes plugged - and many steam generators are in full, safe, daily operation with hundreds of tubes having been plugged over their lifetimes.

Permit a clumsy historical semi-analogy here. Many tens of thousands of steam locomotives used to travel the world's railways - and these often experienced leaking flue tubes. These were often hand rolled in place or else power rolled in the shop to stop leakage... but since locomotive boilers aren't that large, the flues aren't that large or heavy, and since the locomotive boilers weren't contaminated, and weren't inside a massive containment structure, replacement of flues was always done. With nuclear plant steam generators, there is no other option -- and never has been -- other than to permanently plug the U-tubes in place if they leak. This set of facts is not a surprise whatsoever, and excess capacity has been designed into plants from the very beginning.

Some media outlets are carrying interviews or pieces that make this issue sound catastrophically dangerous. I can assure you that it is not. What is more important is the allegation now being floated that massive U-tube rupture is possible, leading to a possible meltdown.

Would the designers of these plants, knowing the possibility (since the beginning, in the early 1950's, that is) of primary to secondary leakage NOT prepare for rupture of the tubes? The answer is yes ... but frankly, the also designed for fast primary coolant leak would lead to a loss of coolant far in excess, in terms of rate, than you'd see with even a major primary to secondary leak.. and that loss of coolant rate is also designed for by emergency makeup systems which can flood coolant (borated, in most plants) into the primary to keep the core covered and cooled. So, then, "experts" saying that the potential for primary to secondary leaks that can lead to meltdown on this plant are not telling the truth.

To be fair, primary to secondary leakage is a problem. It's one of the problems you expect you might have to deal with in a pressurized water plant. That's why detectors on the condenser air ejector vents are there in the first place! That's also why they alarm at a detected level far below that which would correspond to a serious primary to secondary leak. Further, monitoring of primary coolant inventory isn't that hard - compensated pressurizer level is a good clue. The point here is that you know if you're losing primary coolant to the secondary pretty quickly after it starts -- even with a small leak.

Closing for now, I'd like to point out that the NRC is clearly confident that the owner operator (Southern California Edison) is on top of the plan to make sure the plant is safe before restart. The communication between the NRC and SoCal Ed is clear, concise, accurate and realistic. The plan to make sure the causes of the U-tube vibrations are known and corrected before restart is clearly in progress. Knowing these things we can all be assured more now than ever that the real fundamental causes of this difficulty will be found and corrected prior to San Onofre 2 and 3 being restarted.

9:25 PM Eastern Tuesday March 27, 2012


  1. Good article. Be interested to know the percent reduction of power output per tubes plugged. Is there an alternative method of heat transfer not prone to these issues?

    Too bad you're not on the upcoming "Freeze Pilgrim" panel opposite Arnie!

    James Greenidge

  2. @James: Thank you. There really isn't any better way to do this. The steam generators of today are amazing compared to the very first ones; perhaps I'll publish some of the data I have about plant outages of early plants due to steam generator problems to give some perspective. These problems are nothing compared with, say, the problems that sodium or other liquid metal cooled plants had with heat exchangers; it's all relative. Only in today's super-rarified nuclear atmosphere would this be getting this much press. Had Fukushima never happened, this wouldn't be on the radar screen.

  3. Now that this problem has showed up, have you looked back at reports from a few years ago -- 'whistleblower' complaints about the plant's management of the installation of these heat exchangers?

    Earlier I'd read about this as likely a corrosion issue peculiar to this individual plant, but I gather the tube hangers and mounts have had problems at other plants as well.

    Can vibration be distinguished from corrosion, or do you always get corrosion once vibration damages something, and vice versa?

  4. @Hank: I am not getting any sort of feeling, nor any good information yet, as to whether this is a fabrication problem, an installation problem, both, a design problem, all three of those, or something else. Right now we know that there are U-tube vibrations and these could be caused by .. or allowed by .. any number of things. We do have the first data, but we need more data. Have not looked into whistleblowing at the plant.

  5. Would it not be possible to have a couple of steam generators on hand already manufactured so that if something like this happens, they could be switched out and then the broken SG repaired? Kind of like apartment buildings have extra air conditioners they can switch out?(I know a steam generator is much larger but maybe this could reduce downtime). For example, with new Westinghouse plants, there should be a couple of extra steam generators somewhere.

    Meanwhile, what about boiling water reactors? Are they less safe because of what happend in Japan and would the steam have radioactivity and why isn't that a problem?

  6. @Robert: These components are too large and expensive, in the present corporate environment, to manufacture "as stock" or for standby service.

    Boiling water reactors are really a wholly different sort of animal. In fact, it's a safe thing to say generally that other than the fact that they derive heat from fission and have a few parts whose functions correlate they're really totally different animals. There is some radioactivity to the steam in BWR's- a very tiny amount. Making sure that no water is entrained in the steam, which has been improved steadily ever since the first BWR plants, is a great way to reduce exposure and nowadays this is no longer considered a major problem or, should I say, a barrier to construction of BWR plants like it was when this was just a concept. What happened in Japan has so many implications that it's tough to sort out what applies most generally and what applies just to BWR plants in just this little comments section!