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Specific Question about PWR Fission Reactors

  1. Oct 24, 2011 #1
    One fission reaction, of a U-235 nucleouse produces 2.4 Neutrons. In a pressurized water reactor, those uranium fuel rods are clad and sitting in water, giving off these neutrons. They are fast. The water slows them down, but does not absorb them and this generates heat. The inside of that reactor is protected by a stainless steel jacket several inches thick. That jacket also gets bombarded with these neutrons. The question is the following:

    Roughly what rate does that jacket get bombarded? [neutrons / time]

    How fast does that stainless steel jacket degrade, under such bombardment? [~20 years?]

    EPRI should have this info, but you need to be a member to get stuff off their website.
  2. jcsd
  3. Oct 24, 2011 #2


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    You may want to read the information from NRC Generic Safety Issue 164 summarized here:

    There is no single value to answer your question. Neutron fluence is analyzed for several different parts of the reactor vessel and may vary by as much as 5 orders of magnitude. BWRs have lower neutron fluence than PWRs because of the shroud and downcomer region which provide a greater standoff distance between the core and the vessel wall. Test coupons are installed in reactor vessels and are periodically removed and tested for shifts in Nil Ductility Temperature. This testing confirms analytic results. Vessel Pressure and Temperature Limit curves are derived for each of the potentially limiting locations and the limiting cases are combined to ensure the vessel is heated above its brittle fracture limits before it is pressurized.

    As to the length of time a vessel can last, analysis and relicensing are clear that the lifetime is greater than 60 years.
    Last edited by a moderator: Apr 26, 2017
  4. Oct 24, 2011 #3


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    Is it economically feasible to replace a pressure reactor vessel in the interest of extending lifetime past 60 years, or does the containment structure prevent this? Has it ever been done? Granted the process would be expensive, but surely not more so than that of a new plant?
    Last edited: Oct 24, 2011
  5. Oct 24, 2011 #4
    The stainless steel clad on the inside of the vessel is not that thick, it is closer to 1/2 inch. It it there to protect the steel vessel from corrosion. The vessel is made from high quality carbon steel. It's thickness varies but most of it is between 6 & 8 inches.

    I don't think a reactor vessel has ever been replaced. I seem to recall hearing that the navy has annealed a vessel in place, but I don't have any details on that.
  6. Oct 24, 2011 #5


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    Figure 1 of this document provides a schematic showing the PWR RPV and internals, and the expected irradation in terms of dpa. I believe it is for a 40 year lifetime, so one could increase it by a factor of 1.5 for a 60 year lifetime. However, it may reflect a core design strategy that allows for higher fluences on the core internals, so use the values with caution.


    There are three major steel components located radially outside the core: 1) the baffle, which is formed with SS304L plates (including the horizontally-oriented former plates), 2) the barrel (also 304L), which provides lateral support to the baffle, and forms the inner surface of the downcomer region, and 3) the reactor pressure vessel (RPV), which contains the reactor and all its structural support. The RPC is much thicker, ~6 to 8 inches, and is composed of SA508 cl 3 carbon steel with stainless steel liner. The RPV is part of the primary system which much maintain the primary coolant pressure, which is typically a min of ~2235 psia (~15.6 MPa).

    The RPV gets a very low fluence compared to the barrel and baffle as seen in Figure 1 of the cited document. I think the value shown is rather low, but it probably shouldn't be more the 1 dpa.

    A rule of thumb for fluence to dpa is 7 x 1020 n/cm2 (E > 1 Mev) = 1 dpa. Intergranular stress corrosion cracking (IGSCC) or irradiation-assisted stress corrosion cracking (IASCC) can start to become a problem right around 1 dpa, depending on the steel or nickel-bearing alloy, and its environment (coolant, temperature, water chemistry, stress, . . . )

    As for removing internals and the RPV -

    http://www.westinghousenuclear.com/Products_&_Services/docs/flysheets/NS-IMS-0004.pdf [Broken]


    I believe the process of the RPV is to fill it and ship it for burial.

    Reactor vessel heads have been replaced, but no RPVs have been replaced in an operating reactor. There has been consideration of vessel annealing which the Russians claim has been done.
    Last edited by a moderator: May 5, 2017
  7. Oct 24, 2011 #6


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    Quite simply, if a plant can live with the more restrictive pressure temperature curves over time they could operate safely for much more than 60 years.. PWR plants have replaced reactor vessel heads and steam generators which are very large and require opening large holes in the containment and shield buildings. Reactor vessels are larger and normally are in place before the containment is poured. But If the economics justified the cost of replacement, I believe that a vessel replacement could be possible.
  8. Oct 25, 2011 #7
    Considering that 60 yr old plants are of substandard design wrt safety, I think their use should not be extended.

    Then newer reactor designs, IIRC, usually have removable shield at the reactor walls, which reduces neutron flux to the wall itself.
  9. Oct 25, 2011 #8


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    They are only substandard because the standards have increased over time, not because their safety has decreased.
  10. Oct 25, 2011 #9


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    Please substantiate this claim. Substandard with respect to what standard?

    Some small or single units were shutdown well before their 40 year design life, and the designs had margins. Now with experience, we are extending the operations of RPVs, provided that the users can demonstrate that material integrity is maintained. Some materials corrode or wear out, and they are replaced. In the case of steam generators, they are not even in the neutron radiation field, and the problem was one of material chemistry. In some cases, the original material did not meet its 40 year design life. That material has been replaced.

    In the US, the NRC, INPO and the industry have separate, independent monitoring programs on material reliability.
  11. Oct 25, 2011 #10
    Today we know much more about material behavior under neutron flux than 30 years ago. Old reactor vessels may be built with non-optimal materials.
    Older reactor vessel designs have more penetrations than newer ones (more pipe rupture failure modes).
    Today we want to have a core catcher.
    Today we want 100% passive shutdown cooling.

    #1, #2 and #3 can't be fixed without removing reactor vessel, which is about the same as dismantling existing plant and building a new one.
    In some cases, #4 also might require the same.
  12. Oct 25, 2011 #11

    jim hardy

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    Behavior of alloy steel under neutron fluence is affected by the trace elements present.

    to that end the vessels are provided with removable samples that are pulled and tested after various numbers of years. This lets designers estimate material condition of vessel metal from real measurements not computer simulations.

    the reactor does quite well thank you, it was designed by geniuses shortly after WW2.

    i worry more about complacency in management and cost-cutting in maintenance.
    it takes abject honesty to do things right.
    gotta keep all those support systems tip-top.
  13. Oct 26, 2011 #12


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    But that doesn't make current reactor substandard, nor are the older plants necessarily less safe. Old and new plants still meet the same General Design Requirements.

    For more information on irradiation effects on reactor core and reactor vessel internals:

    Determination of Baffle Bolt Operating Parmaters
    http://books.google.com/books?id=Fk...33#v=onepage&q=Fast fluence PWR baffle&f=true


    http://www.isrd13.jrc.nl/safelife/presentations/12_Gerard_R.pdf [Broken]




    Last edited by a moderator: May 5, 2017
  14. Oct 27, 2011 #13
    It does. Because safety standards evolved, and what was considered to be safe enough back then is not considered to be safe enough today.

    Your statement is meaningless without clarification: *compared to what* older plants are less safe (or not)?

    Or maybe "General Design Requirements are made to meet old plants"...
  15. Oct 27, 2011 #14


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    Nonsense. As we learn, older plants undergo retrofit, or in some cases replacement of major components.

    In the context of the discussion, it should be obvious that the comparison is to the newer proposed plants.

    Again with the nonsense. The same General Design Requirements apply to modern (GenIII+) plants.

    One has failed to substantiate one's claims regarding safety of older plants.
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