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Micro-cracks in Belgian RPVs

  1. Feb 23, 2015 #1
    Anyone given thought to the mechanism(s) of the cracks in the Belgian RPVs? In normal circumstances, I personally have clues based upon experience. The material neutron bombarded for a number of years gives that all a twist. So...Anybody?
     
  2. jcsd
  3. Feb 23, 2015 #2
    I realize this morning that my phrasing of the question may seem loaded, but honestly I'm only looking for some opinion. I do understand that any answers are opinion. In my working days I was a machinist, welder, general fabricator who's worked with a vast variety of materials in corrosion resistant materials for the chemical process industry. That required that I learn and understand corrosion mechanisms. My experience in these high-strength steels is limited, while I have only general idea of the environment in this instance. I am familiar with the effects of hydrogen in steel. I'm simply wondering for my own curiosity hoping I might be led to some literature that might satisfy given the specific parameters in this use.
     
  4. Feb 23, 2015 #3

    Astronuc

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    The phenomenon is irradiation-induced embrittlement, irradiation-assisted stress corrosion cracking (IASCC) and has been a subject of study for the last 4 decades.

    If the cracks are in the stainless steel, then it's IASCC. Stainless steel can be sensitized and carbon steels are sensitive to irradiation due to microstructural changes, including radiation-induced segregation and void coalescence. Inconel 718 and SS-316 bolts have also cracked in various LWRs.

    One conference, Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors, ANS/TMS/NACE has covered the embrittlement of RPV steels and IASCC of reactor structural components.
     
  5. Feb 23, 2015 #4
    Thank you, Astronuc.
     
  6. Feb 24, 2015 #5
    Embrittlement due to neutrons, make a RPV less tough at normal reactor operating temperature so a pressurized thermal shock due to rapid cooling or pump mis-operation is of concern. How much of concern? I think according to this document NUREG-1806, not of very much concern. As far as I understood from this document.
     
  7. Feb 24, 2015 #6

    Astronuc

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    Is this question related to the cracks discovered in Doel 3 and possible concerns at Tihange 2?

    Cracks found at Doel 3

    http://www.fanc.fgov.be/GED/00000000/3700/3751.pdf

    http://fanc.fgov.be/GED/00000000/3300/3393.pdf

    From the second (earlier) report:

    Synopsis of the claims by the licensee

    "The inspection program carried out in 2012 was intended to determine if underclad
    cracks existed in the Doel 3 and Tihange 2 RPV's. While no underclad cracks were
    detected, numerous quasi-laminar indications were observed, these being
    concentrated in the upper and lower core shells of the vessels with a total of about
    8500 and 2000 for the Doel 3 and Tihange 2 RPVs, respectively. The UT inspection
    reports conclude that the indications are quasi-laminar, are located within the first
    120 mm of the vessel starting from the interface of the RPV with the austinitic
    cladding and having an increasing concentration between 10 and 50 mm depth; their
    typical dimension is 10 mm. A root cause analysis has been carried out by the
    licensee and its partners, complemented with a literature survey, to identify the origin
    and nature of these indications. According to these investigations, the observed
    indications are hydrogen flakes that were created during manufacturing. The
    literature survey ruled out almost all other types of known defects because their
    characteristics do not match those found in the two subject RPV's. For those defect
    types that were not completely ruled out, it is argued that hydrogen flaking represents
    the worst case with respect to the impact on potential crack propagation, and is
    therefore conservative.

    Several arguments are provided to explain why the indications can be characterized
    as being hydrogen flakes. The shape of the indications and the fact that these are
    quasi-laminar are characteristics of hydrogen flakes that have been seen in heavy
    section steel forgings used in other industries. The location of the indications can be
    attributed to the existence of macro-segregations, which are typical of forgings made
    from large ingots. The case for hydrogen flaking being the root-cause of the quasilaminar
    indications is further strengthened since the manufacturing archives show no
    evidence of a dedicated dehydrogenation process and the concentration of hydrogen
    in the base material before forging was shown to be sufficient to cause flaking.
    The licensee has conducted a set of tests and measurements on a block extracted
    from a steam generator shell owned by AREVA known to contain numerous
    hydrogen flakes (block designation: VB395/1). One of the objectives of these tests
    was to confirm the capability and performance of the UT inspection techniques used
    in Doel 3 and Tihange 2 to correctly detect and size hydrogen flakes. This step was
    viewed as necessary because the NDE (Non-Destructive Evaluation) inspection
    technique used on Doel 3 and Tihange 2, while representing best- and sound inspection
    practices, are not yet formally qualified for quasi-laminar indications.

    Destructive tests validated the licensee's claim that the defects can be detected and
    sized accurately. The destructive evaluation of block VB395/1 showed that the size of
    the defects is over-estimated by NDE in most cases, while the size of the corresponding
    ligaments between the defects is consequently under-estimated by NDE. Both of these
    factors constitute conservatisms when they are included in the structural integrity analysis,
    which is discussed in a later section of this report."

    Assessment by the Board and recommendations to the FANC

    "The discrepancy between the indications reported in the acceptance reports of the
    rings from the 1970s and in the 2012 inspection in the core shells of the two plants
    remains unresolved, since the UT technology available at that time should have had
    the capacity to detect the indications found. Furthermore, it is documented that some
    other parts, like the transition rings, were rejected exactly because of these hydrogen
    flakes.

    Despite these questions, the Board is convinced that the indications found are most
    likely related to hydrogen flakes that were created during manufacturing of the vessel."

    This is an interesting study, particularly as it concerns the evolution of the anomalies and the potential of the anomalies (flaws) being present in the as-manufactured vessel.
     
    Last edited: Feb 24, 2015
  8. Feb 25, 2015 #7
    Do you know who manufactured the Doel & Tihange reactor vessels? I'm just curious, as to which units may have "sister" vessels.
     
  9. Feb 25, 2015 #8

    Astronuc

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    From the NEI article:
    RDM forged the vessel for Tihange 2 at the same time and under the same contract for Doel 3.

    Importantly, "despite sharing the same manufacturer, the vessels often used different materials or fabrication techniques, so are not necessarily likely to be affected by similar phenomenon, regulators have said." However, I would expect heightened surveillance.
     
  10. Feb 25, 2015 #9
    Thanks AstroNuc! And FWIW I agree that the vessels manufactured by any given vendor may be quite different even if they share the same basic design and manufacturing methods - seemingly small differences in the details of material specs can be quite significant.
     
  11. Feb 25, 2015 #10

    Astronuc

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    The RPV for Doel 3 and Tricastin 2 are SA 508 CL 3, and Tihange 2 is SA 508 B CL 3. I'll look up the others later.

    There could be differences in the raw material or ingot depending on how much recycle was in the melts.

    Although they might come from the manufacturer, there can be variances in the process. Doel 3 and Tihange 2 vessels were made pretty close to one another.
     
  12. Mar 23, 2015 #11
    The steel Ingots for the Doel 3 and Tihange 2 forgings came from Krupp in Essen. It would be interesting to know whether RDM only used Krupp ingots for their forgings or whether they sourced ingots from other steel manufacturers. Likewise it would be interesting to know whether any other RPV manufacturers sourced ingots from Krupp.
     
  13. Mar 23, 2015 #12
    Personally, I'm not so sure that material manufacture is the problem. I'm a metal-worker by trade, and sure, one sees material variances, but it's generally on the order of mechanical differences. Those can't really be helped and should be part of the consideration at design. I'm suspect of the heat-treat, not so much because it doesn't seem the paperwork lives up to expectations in that regard, but the welds themselves likely would have had to go through their own heat-treat and maybe even full-scale stress relief with the rest of the fabrication as a whole.

    I understand that minor differences can be catastrophic, on the order of hundreths of one percent can make a big difference in such demanding environments, and don't discount base material being the source. Seems a lot of effort is going into this, while the first thing I would have done is to find somewhere to gather enough from the vessel to check the analysis. I'm disappointed that here in the US we seem to be resisting the same tests for at least one of the RPVs from the same manufacturer. I haven't compared lists of manufacturer/user of those vessels here, but, as it seems they're rather old. I do wonder if there's a decommissioned plant here in the US one could at least check.
    So, let me ask another question...I realize these go through cycles where temperature differences may vary and that there are portions of the vessel that may experience differences related to specific place in the working vessel itself, but what might be the average working temperature the vessel? Not over the max steam temp except in what might be unusual circumstance?
     
  14. Mar 23, 2015 #13

    Astronuc

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    The RPV temperature depends on location. For PWRs, the cooling water enters through an inlet nozzle (2, 3 or 4 nozzles) and then flows down the annular region between the RPV and core barrel and supporting structure. The temperature is basically the core inlet temperature which is on the order of 285-294°C, which covers most of the RPV. The coolant passes though the core where it is heated and then exits through the exit/outlet nozzle (2, 3 or 4 nozzles) at temperatures on the order of 315 to 330°C. There is about a 30 to 36°C temperature rise across the core depending on flow rate and power density. Some of the older plants run cooler, whereas the more modern units run hotter. Both sets of nozzles are located above the core exit elevation.

    BWRs are a bit more complicated since saturate liquid (from core bypass and moisture/steam separator) is combined with feedwater in the annular region between core barrel and RPV. Maximum coolant temperature in a BWR are slightly lower than the inlet temperatures of a typical PWR.
     
  15. Mar 23, 2015 #14
    Thank you, Astronuc.
    I believe it will be interesting once it's figured out.
     
  16. Mar 23, 2015 #15
    Thanks Shaker1. If the flaking is attributable to hydrogen and macro segregation then the local composition of the reactor shell (in the inner half of the wall thickness) would be good to check. If the hydrogen does not come from the ingot production as you suggest, then the only process that could then introduce the hydrogen would be the welding. As the defects are found throughout the shell the circumferential weld joints can be discounted. This leaves the corrosion resistant overlay weld which is applied to the inside surface of the reactor shell.

    I agree with you about testing other reactor material from the same manufacturer but my limited information suggests that all US reactor vessels made by RDM are still in service. I could, however, be wrong and if anybody has more accurate information to the contrary then please let me know. This brings me to another issue. With the great emphasis within the nuclear power industry for first class QA/QC and record keeping, you would think that details of the manufacturers of all RPVS should be readily available. I have been able to identify some by trawling through the internet but this has still produced only a limited number.
     
  17. Mar 24, 2015 #16
    Thnks for your input, OzzyPete.
    "I agree with you about testing other reactor material from the same manufacturer..."
    I admit that the reason for the original question is seeming resistence to testing here in the US.
    "This leaves the corrosion resistant overlay weld which is applied to the inside surface of the reactor shell."
    I can't imagine that overlay is very thick and there was some kind of weld-schedule that assured those welds have just local effects. If that's the case, it may be sloppy prep?
    The problem I see with attributing the flaking to the original material is that there are subsequent heat-treats after that point. Personally, I learned that the biggest mistake one can make in heat-treat is to try to push the process along, cutting soak times, etc. It's my understanding that the material will outgas over time naturally. Were they in a hurry? I don't have experience with such large and heavy vessels, but can't imagine that the mechanics of the heat-treat process are anything more than a scaled-up version of what I know with some accomodations for mass.
    As for the records, I believe during construction of one of the vessels the contractor went bankrupt, the vessel being completed by another outfit. At the time it was paperwork, too. Cutting expenses in that situation may have made Q/A suffer in its own quality and competence. But that's only one vessel, which explains little if the records for the other are no better.
     
  18. Mar 24, 2015 #17
    I'm not sure what information you're looking for, but I suspect the vessel manufacturing QC paperwork (which originally occupied numerous file cabinets) is all on microfilm stored in the manufacturer's vaults somewhere (with a second copy stored somewhere else). It most certainly isn't digitized on the internet. In the US, all of this stuff goes to "Quality Records" for "lifetime" retention. But just because it is stored doesn't mean it is easy to find. For one thing, just the huge volume of stored material makes finding any specific item difficult (trust me). These vessels were made in the late 1970's, right? Everything was done with paper then -- the letter logs, the microfilm logs, etc., etc.
     
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