Where There’s Steam, There’s … a Steam Generator

Kenneth Karwoski
Senior Advisor for Steam Generators

News articles recently brought the phrase “steam generators” into the national conversation, but we’re not talking about teakettles. pwr[1]Steam generators provide vital technical and safety functions at many U.S. nuclear power plants.

In the United States, steam generators are only found in 65 pressurized-water reactors, one of the two types of U.S. reactors. There can be two to four steam generators for each reactor unit. The generators mark the spot where two closed loops of piping meet. The first loop sends water past the reactor core to carry away heat, and this loop is at such high pressure that the water never boils. The second loop is at a lower pressure, so the water in this loop turns to steam and runs the plant’s turbine to generate electricity.

The steam generator’s main technical job is to let the first loop pass its heat to the second loop as easily as possible. To do this, a steam generator packs thousands of small tubes closely together, allowing the maximum area for heat to pass through the tubes and into the second loop’s water.

At the same time, the steam generators provide an important safety barrier – the first loop can contain radioactive material, so the tubes must keep the two loops of water separate. NRC rules require plants to closely monitor the second loop and immediately shut the reactor down if a tube leak exceeds very strict limits.

The NRC’s rules for inspections, maintenance and repair of steam generator tubes help ensure the tubes continue providing the safety barrier. If an inspection shows a tube is starting to get too thin, the plant will repair or even plug a tube to maintain safety.

Steam generator tube material has improved over time. The first steam generators had tubes made from a type of stainless steel that experience showed could be corroded by the chemicals, temperatures and pressures in the first and second loop. Over time, plants have replaced those steam generators with ones using more advanced alloys that are less likely to corrode. Steam generator replacement only happens when the reactor is shut down for refueling, and plant owners bring in hundreds of specialized workers to safely remove the old generators and install the new ones. The old generators have to be safely disposed of as low-level radioactive waste.

Author: Moderator

Public Affairs Officer for the U.S. Nuclear Regulatory Commission

17 thoughts on “Where There’s Steam, There’s … a Steam Generator”

  1. Design basis accidents include design basis earthquakes of course. With on-going, multiple degraded steam generator tubes how can you ensure degraded tubes will still meet seismic qualification requirements? In an earthquake operators may then have to deal not only with multiple steam generator tube failures in one steam generator but with multiple tube failures in more than one steam generator. Prompt operator action is required within 30 minutes or so of a single tube rupture in a single steam generator to cool down the reactor coolant system enough to allow isolation of the steam generator with the tube rupture. All the while this cool-down is taking place radioactivity is being released to the public. If more than one steam generator has multiple ruptured tubes there is no intact steam generator with which to cool-down. Huge amounts of radioactivity would be released as a result.

    Operating nuclear power plants with degraded & seismically unqualified steam generator tubes is irresponsible.

  2. PPB just posted this here. Before it was posted on the Open Forum

    Originally submitted on 2015/10/23 at 2:31 am
    Thanks to the NRC for providing this forum, it is obvious that many outside the NRC are not only “informed” but have issues with the “wonderful” job the NRC is doing. Hopefully the NRC will reconsider the Radiation Study and start helping everyone better understand the risks of being near or downwind from a nuclear reactor!
    Since developing better communications with the public is a major goal of the NRC, I expect to see much more discussion by NRC professionals with those making comments, since this will led to more understanding by everyone, since so many cannot afford to travel to Rockville Pike to ask questions in person.
    I would suggest that the NRC also enable Blog topics be posted by those outside the NRC, so that the public can help the NRC answer those questions that are of top priority!
    If I could post a topic, this is the one that I would choose:
    Since San Onofre proved that more than one steam generator tube can fail at a time, why does the NRC (still) deny considering multiple (or cascade of) steam generator tube failures?
    Arnie Gundersen said, “Eight replacement steam generator tubes failed their pressure tests in 2012 and more than 1,000 others have been plugged. Therefore, a review of the evidence makes it clear that the San Onofre Replacement Steam Generator tube damage discovered in 2012 was so severe and extensive that both reactors [Uint 2 and Unit 3] have been operating in violation of their NRC FSAR license design basis as defined in their Technical Specifications. The Main Steam Line Break with radiological leakage through the steam generator tubes is one of the bounding conditions in emergency plan evaluation and the extent of steam generator tube failures directly impacts the FSAR analysis. The Replacement Steam Generator (RSG) modifications at San Onofre increased both the likelihood of equipment failure and the radiological consequence of such failure and therefore directly affect the FSAR Current Design Basis.”
    Dr. Joram Hopenfeld, who retired from the NRC staff, raised the following concerns about the integrity of
    steam generator tube ruptures to NRC for years. These concerns were ignored until an accident occurred in 2000 at Indian Point 2. The NRC asked its Advisory Committee on Reactor Safeguards (ACRS) to evaluate the decade-old concerns. The ACRS issued a report in February 2001. The ACRS substantiated many of Dr. Hopenfeld’s concerns. For example, the ACRS concluded: “The techniques used to look for cracked steam generator tubes are not nearly so reliable for determining the depth of a crack, and in particular, whether a crack penetrates through 40% of the tube wall thickness. Note: NRC’s regulations do not allow a nuclear plant to start up with any steam generator tube cracked more than 40 percent of its wall thickness, but the methods used to inspect the tubes for cracks cannot reliably determine the depth of cracks. The NRC staff acknowledged that there would be some possibility that cracks of objectionable depth might be overlooked and left in the steam generator for an additional operating cycle! The design of the present generation of nuclear plants was based upon the
    assumption that steam generator tubes maintained their original strength during design basis accidents, but SONGS has proven that assumption to be false, because the failures at SONGS Unit 3, where not one but eight tubes failed and thousands of other tubes were damaged, has demonstrated that those assumptions are no longer valid for RSG’s. A design basis steam line break accident, which assumes that the tubes are in their “as received” condition could result in not only a single tube rupture, but it could also quickly lead to a multi-tube leakage if the tubes are aged, not well designed and or contain fatigue cracks. In the first case, the consequences to the public are minor, while in the second case they would be catastrophic. The level of risk of operating degraded steam generators such as those at SONGS Unit 2 is patently unacceptable.”
    Add to the above that Unit 2 (which was shut down for a scheduled refueling) had a SG tube that had 90% wear and the operator (SCE) never even had a clue that there was “unprecedented” tube wear in both Unit 2 and Unit 3’s almost new RSGs. It was later discovered, after the tube leak on 01/31//12, that Unit 2 and Unit 3 had more tube wear than the rest of the US nuclear fleet combined!

    Click to access steamgeneratortubesplugged1.pdf

    If Unit 3 had not leaked when it did, then both units would have been put back into service with possible catastrophic consequences, yet the NRC still fails to consider multiple SG tube failures. What will it take for the NRC to change its position?


  3. Please resubmit the comment. I cannot find one submitted to this post that was moved or not approved. Sorry for the inconvenience.


  4. The NRC moderator buried additional very significant safety comments relative to steam generator tubes on the NRC Open Forum site. I have requested that this moderator repost them here.

  5. No, I was asking questions about *your* professional expertise. If you have any, then we know that it might possibly be worth listening to what you say. However, I very much doubt it since you have repeatedly demonstrated your inability to put the supposed risks in any kind of factual and numerate engineering context. Likening the SONGS events to a Fukushima near-miss is so over-the-top ridiculous to any practising engineer who knows anything about the case that it quite gives you away.

  6. Lets be clear, the replacement steam generator design debacle was responsible for closing down San Onofre, so if you want to ask expertise questions I suggest that you ask who at SCE approved the designs and then used their CA licenses “chop” to sign off on the designs as meeting approved steam generator standards! They are the folks that caused the debacle, so I suggest that you question them, and the other professionals at the NRC that allowed the “like for like” replacement.

    BTW: Good luck finding out, since SCE has redacted its documentation to eliminate their employee’s names, to prevent them from being called to testify at the CA Public Utilities Commission (CPUC) investigation that is ongoing!

  7. And who are you, CaptD? What professional expertise, what actual standing do you have, in presuming to lecture all these hardworking professional staff who’ve just lost their jobs due in no small part to the relentless flood of disinformation from antinukes like you?

  8. I was referring to the picture in the original post and “it:s” color coding. The color coding shows primary & secondary fluids mixing! Your experts evidently need a refresher course in PWR primary and secondary systems.

  9. What I’m saying is, that, while mistakes were most certainly made, a lot of good hard working professional people got hosed by this administration, which has speaks a good nuclear out of one side of their face, but sticks the shiv when the oportunity at every oportunity and continues on like nothing happened. The only positive thing that this administration has done for nuclear was to guarantee loans – which they have not made good on. As per my previous post – the Obama nuclear score card: Barbara Boxer now oversees the NRC, and the NRC is now chaired by the second concurrent avowed anti-nuke that this administration has appointed. Several decades worth of research and development on used nuclear fuel storage have been abandoned and Yucca closed for purely political reasons. A moratorium has been placed on both new licenses and license extensions based on the lack of waste confidence. The shrinking budget for new license reviews has been claimed under the guise of austerity for an agency that bills the recipients of their reviews at a rate of $274 per bureaucrat hour. And, you can add the premature closure of San Onofre to the list…I’m sure the administration has.

  10. There is an ongoing NRC adjudicatory proceeding to consider whether the matter of steam generator replacement should have been subject to a license amendment rather than “self-approved” under 10 CFR 50.59. For laymen, this means, we’re going to shut you down because you think you should have asked us to change your steam generators, but didn’t. Also, while the NRC historically has no legal standing in a utility restarting a nuke following a tube leak (and Jaczko confirmed so in the case of San Onofre), the NRC got San Onofre to voluntarily commit to NRC approval prior to restart and then issued a CAL, turning it into an obligation. Then the NRC tied the restart to public hearings via an Atomic Safety and Licensing Board. In the mean time there was a full press being applied by Barbara Boxer (chair of the Senate Environment and Public Works Committee, which has oversight of the NRC) who was pursuing criminal investigations of SCE leaders, and who held up Macfarlane’s conformation until San Onofre was closed in a delaying tactic designed to extend the time that the NRC had to make decisions on the restart. Notably, Jaczko wrote to Boxer on March 13 2012, that SCE did not need NRC approval to restart San Onofre, then issued the CAL on March 27 requiring NRC approval of the restart, and suddenly resigned on May 21.

  11. No snip, your fears are erroneous. Faulty tubes can be plugged and the plant run at powers proportional to the heat transfer lost. PWR steam generator technology has come a long way over the years and the criterion for continued operation and the technology for detecting and preventing leaks is light years beyond what it was years ago. Back in the eighties, many of the older plants that used single pass strait tube generators were fitted with many tubes, (30,000 for example), and would plug hundreds of leaking tubes at each outage as they failed until they had 10,000 plugged and then would change out steam generators. No activity was ever released to the public. Note, as per your own provided data that technology (eddy current testing) now allows the industry to detect failing tubes even before they leak. Even today, it is not uncommon for older plants to plug or “stake” dozens of tubes during an outage. Yes, there were mistakes made at San Onofre – very much so based on the expectations for new steam generators NOT to have failing tubes right out of the gate. But the presumption that San Onofre is too dangerous to run is just flat wrong. Keep in mind that Barbara Boxer now oversees the NRC, and that the NRC is now chaired by the second concurrent avowed anti-nuke that this administration has appointed, and that several decades worth of research and development on used nuclear fuel storage have been abandoned, and that a moratorium has been placed on both new licenses and license extensions based on the lack of waste confidence, and that the shrinking budget for new license reviews has been claimed under the guise of austerity for an agency that bills the recipients of their reviews at a rate of $274 per bureaucrat hour and you might be getting closer to determining what really happened at San Onofre.

  12. Here is Nuclear Power Plant Basics, by the DAB SAFETY TEAM


    SCE’s claims that SONGS Unit 2 Steam Generators are Safe for Restart are Erroneous because they can create a Fukushima-type nuclear meltdown in Southern California

    Nuclear Power Accidents

    Nuclear power plant accidents include Three Mile Island (1979), Chernobyl (1986), Fukushima Daiichi (2011), and San Onofre Nuclear Generating Station (SONGS) Unit 3’s near miss nuclear disaster (2012). After the SONGS 3 Replacement Steam Generators (RSG’s) tube leakage, additional testing found that 8 tubes failed in-situ testing and could not sustain their structural integrity during a main steam line break (MSLB). Additionally, one RSG tube was discovered with 90% through wall wear in Unit 2 and the structural integrity of thousands of damaged tubes in both SONGS Units 3 and 2 RSG’s has been termed by NRC as a “very serious” safety issue. Now the safety of SCE’s RSG design is being questioned by the public because these almost new SONGS RSG’s now have more damaged and/or plugged tubes than all the rest of the US power plants combined, which is unprecedented in the history of the U.S. Operating Nuclear Fleet. Credit: sanonofresafety.org;

    • Unit 3: 1 Tube leaked core coolant/radiation, 8 Tubes then failed in-situ testing, 1600 tubes damaged, 807 tubes plugged – WORST US Record!

    • Unit 2: 1 tube found with 90% wear, (almost core coolant/radiation leak/failure), 510 tubes plugged
    all tubes still not fully examined! 2nd WORST US Record!

  13. Rod Adams, you of all people, need to get up to speed on what really was going on with San Onofre’s replacement steam generators (RSG) instead of just trying to down play what happened by saying that they should have been restarted! Expect to see SCE get ordered to pay a multi-Billion dollar rebate to ratepayers because of their debacle!

    Note: even the NRC realized that SCE’s in-house design had major flaws and proved that multiple tube failures could occur, rather than the single tube failure that up until now had been the worse case used in the NRC calculations!

    Here is a 5 part factual review (one of many) that might help you stop living in denial or spreading Profitganda*

    Part 1 First Part (Rejecting Intertek’s Operational Assessment)

    * http://www.urbandictionary.com/define.php?term=Profitganda

    Profitganda is the use of phony “feel good” information to sell an idea, product or concept to the masses.

  14. I completely disagree with CaptD.

    What people really should be told is that the operators at San Onofre correctly shut down Unit 3 when they received an indication that there was a tiny leak in one of the tubes of the generator that turned out to be just 1/2 of the “very strict limits” that the NRC imposes to require a unit shut down.

    The action was correct because tube leaks tend to grow rather rapidly, but they do not tend to spread to other tubes. The small diameter of each tube prevents leakage from ever getting very large, even in the case of a complete rupture. The nature of primary coolant in a reactor where fuel rod cladding is intact makes primary to secondary leakage more of a nuisance than a safety issue – after all, the coolant is almost pure water containing tiny quantities of radioactive material with half lives longer than a few seconds.

    The most exposed person would have received a radiation dose of about 5.2E-5 millirem (5.2E-7 mSv) in a world where the average annual dose is about 300 millirem (3 mSv) from background radiation.

    After inspecting and plugging damaged tubes, both units 2 and 3 should have been restarted, probably no later than March or April 2012. Everyone should remember that steam generators are only a design choice – 1/3 of all of the reactors in the US don’t even bother to try to separate primary from secondary water. Pressurized water reactors with isolated primary systems were the right choice for their initial application – producing power inside sealed submarines full of people.

    I lay the blame for the destruction of 1500 jobs and the capacity to produce about 14 billion kilowatt-hours of emission free electricity every year solidly on overly conservative decision-making and political action designed to force California to burn more natural gas.

  15. Your color coding of the reactor coolant & steam generator loops appear to show co-mingling of fluids?

  16. Kenneth Karwoski
    Your blog post does a disservice to all those that spent countless hours writing technical articles about the replacement steam generator (RSG) debacle at San Onofre that lead to it being shut down permanently and costing ratepayers Billions, all because SCE’s in-house RSG design team failed to get a CFR 50.59 review of their RSG design!

    Here is a listing of the DAB Safety Teams “papers” about San Onofre’s RSG and other safety issues:

    Posted 06-08-13
    N☢ San Onofre Gate
    First, a salute to all of San Onofre’s loyal workers at that have or will lose their jobs because of SCE Managements poor engineering decisions, everyone feels sorry for both you and your families, there is no good time to be laid off. We also feel sorry for all the local businesses and families that live in the neighborhoods located near San Onofre that will feel the effects of these job loses. Hopefully as many as possible of you can remain here by getting retrained by SCE, so that you can be retained or re-hired (along with many others) to decommission San Onofre a big job that we now know will last for many years and cost billions of dollars which will hopefully help jumpstart the entire southern California economy!

    To all those that are now upset, angry and/or worried about the future because San Onofre is being decommissioned, I urge all you to not focus your frustration upon those who protested by publicizing the many actual safety concerns at San Onofre but join with them and together lets all demand to learn much more about why San Onofre had to be decommissioned. We all deserve to know exactly who at SCE was responsible for their decisions to use unproven radical designed RSG’s at San Onofre that not only failed so quickly after being put into service but leaked radioactivity into the air we breathe, putting everyone in southern California at risk!

    Because of this Corporate engineering debacle*, recently laid off employees, present employees soon to be laid off and all the ratepayers are now suffering while those that are responsible are simply going to change their retirement packages and/or enjoy their golden parachutes; they are the only ones that should be held responsible! The US Government, the NRC, the CPUC and the State of California Attorney General must complete all of their investigations into San Onofre, so that those that are responsible can be identified and we all receive the maximum financial relief possible from them. SCE, SDG&E, the CPUC and MHI must be instructed ASAP not to shred any documents relating to San Onofre; the public will not accept a San Onofre Gate cover up. No longer can SCE or MHI claim that they must restrict any “proprietary information” on any company documents relating to San Onofre since they are now going to be decommissioning it, along with its new unsafe Replacement Steam Generators that contain so many major design flaws. In hindsight, Southern California had a nuclear near miss on 01/31/12 and if we were not just plain lucky we could have suffered a nuclear accident just like Fukushima, because of the dangerous Replacement Steam Generators that were in use at San Onofre.

    We now also have another major problem, which is to determine where both our Federal and State Regulatory Systems of checks and balances failed, in order to make sure that something like this never happens again because the USA cannot afford a trillion dollar Eco-disaster. To do that, it is vital that we insure that all the investigations mentioned above are done publicly and not behind closed doors so that all of us can better understand exactly how San Onofre’s debacle occurred, the names of those that are responsible for it and exactly who we should hold responsible for the restitution of all our financial losses.

    *San Onofre will be remembered as both a nuclear near miss, and as an engineering debacle of epic proportions like the Tacoma Narrows Bridge. http://www.youtube.com/watch?v=j-zczJXSxnw

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