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Hafnium neutron Absorber [Nuclear reactor design]

  1. Mar 21, 2012 #1
    I was reading about PWR control system and I saw that control rods are stainless steel tubes encapsulating a Hafnium absorber material , it has Excellent mechanical properties and exceptional corrosion-resistance properties allow its use in the harsh environment of a pressurized water reactors and The German research reactor FRM II uses hafnium as a neutron absorber.

    so the question is if I'm designing a nuclear reactor based on what I should choose the control rods' material ?

    and as a Hafnium absorber why its better than Gadolinium [Gd] as control rods while Gd has much higher absorption cross section..?

    is Hf used as pure or in some composition ?

    and what's the Hf interactions with neutron?
  2. jcsd
  3. Mar 21, 2012 #2

    jim hardy

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    From memory of last year about this time --

    Daichi 3 had fifteen hafnium control rods.

    A search will take you to several papers on Hafnium rods, some of them by Tepco.
    I read them last year while we were all following events so closely. Will see if i wrote down any links.
    TEPCO has them in several reactors as best i recall.
    It would be interesting to know where in U-3's core they were.
    Like boron hafnium is nearly transparent to fast neutrons.
    But it has one nice property - its melting point is similar to Zirconium. That means hafnium control rods shouldn't leave the core so early in a meltdown.
    And it has another nice property, it can absorb several neutrons before losing its absorbtion cross section (affinity for neutrons) . So it doesn't "burn out" quickly.
    It has one isomer 178 that's claimed to be pretty explosive.

    Search on hafnium absorbtion cross section

    Are you familiar with "Chart of the Nuclides" ?

    Here's a PDF to get you started

    search on this phrase
    and Google returns this link.

    it's 10 meg, an 8 minute download here.
    Last edited: Mar 21, 2012
  4. Mar 21, 2012 #3


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    Hf has 5 stable isotopes (Hf-176, 177, 178, 179, 180) and one quasi-stable isotope, i.e., an isotope with a long half-life, Hf-174.

    Hf-174 has a thermal σ(n,γ) cross-section of 561.7 b, where σ(n,γ) = thermal neutron capture cross section at 300 K, in barns.

    Hf-176 13.76 b
    Hf-177 373.6 b
    Hf-178 84.05 b
    Hf-179 43.59 b
    Hf-180 13.01 b

    http://www.nndc.bnl.gov/chart/reCenter.jsp?z=72&n=106 select σ(n,γ) and Zoom 1. http://www.nndc.bnl.gov/chart/reCenter.jsp?z=64&n=91

    A subsequent daughter from transmutation, Ta-182, has a cross-section of 8289 b.

    Gd by comparison has a much higher cross-section for thermal neutrons
    Gd-155 60740 b
    Gd-157 253700 b

    However, one must look at the isotopic vector for natural Gd.

    One also has to consider the temperature or 1/v effects, because PWR control rods are sitting above the reactor, they start at about average core exit coolant temperature, usually in excess to 600K, because the tips get some shine from the core. One also has to look at resonance absorption. So one has to look at the PWR neutron spectrum and use that with the σn,γ(E)

    Gd density 7900 kg/m3 and a melting point of 1312 °C
    Hf density 13310 kg/m3 and a melting point of 2233 °C

    Also, one has to look at the cost. Gadolinium is a relatively expensive rare-earth element. Hf is a by-product of Zr production. Hf is present at about 3-4% of zircon sand, and to get nuclear grade Zr, one has to remove the Hf.

    The biggest problem with Hf is its tendency to absorb hydrogen. The formation of hafnium hydrides HfHx, x ~ 1.6 to 2, causes Hf to swell and this can be a problem for RCCA insertion. In some cases, the stainless steel cracks.

    Traditionally, PWR absorber has been 80%Ag-15%In-5%Cd (AIC), and in some cases, B4C has been used, often with B enriched in B-10. Hybrid control rods may use B4C with AIC tips. And traditionally, the neutron absorbing material is sealed in a tube of SS-304 or 316, and stainless steel is somewhat permeable with respect to hydrogen.

    If Hf has a nice adherent oxide, the oxide can prevent uptake of hydrogen, so ideally, Hf is either oxidized initially, or is allowed to have contact with the coolant, such that it always maintains an oxide coating. Then one has to determine the degree of oxidation over the lifetime of the control rod, which is usually ideally 15 years. If it can go 20 years, that's better.

    Alternatively, the form of Hf could be a rolled foil that allows for some hydrogen uptake without significant swelling. Another alternative could be a Hf tube containing B4C.

    The Russians have looked at dysprosium titanate as an absorber material.

    Most BWR control rods use B4C with some Hf slugs or rods in high exposure locations, such as the tips. Hf swelling due to H-update is also a concern.

    The stainless steel fingers (rodlets) are attached via endplugs with long shanks to spiders, the spiders are attached to the hub, and the hub forms a housing which interacts/couples with the control rod drive mechanism. The spider is often cast, but could be manufactured from forged plate which is then welded to the hub.

    Attached Files:

    Last edited: Mar 21, 2012
  5. Mar 24, 2012 #4
    Thank you very much "jim hardy" and thank you for the pdf file , No I'm not familiar with "Chart of the Nuclides" but I saw the link "Astronuc" posed below and I'm getting familiar with its very amazing such data are hard to find through Google.
    Thanks "Astronuc" that made many thing much cleaner , and thank you very much for the links this is the first time I hear about "Chart of the Nuclides" :) really thanks a lot :)
  6. Mar 24, 2012 #5


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    One can also find more data here for different cross-sections.


    Click on an element then select which isotope/nuclide, then which cross-section. Gd-155 has a benefit below 1 eV. One could do an annulus of Gd sandwiched between an inner and outer tube of Hf. The center could be hollow for a flux trap. Any fast or epi-thermal neutron slowing down in the water filled interior would be readily absorbed in the Gd, or Hf.

    Attached Files:

    Last edited: Mar 24, 2012
  7. Mar 24, 2012 #6

    jim hardy

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    I found a couple of my old links from last year.




    http://www.jstage.jst.go.jp/article/jnst/47/2/160/_pdf [Broken]


    http://www2.jnes.go.jp/atom-db/en/trouble/individ/power/f/f20060201/index.html [Broken]
    (doesn't seem to work anymore - was an industry paper by TEPCO outlning their R&D and operating experience with Hafnium rods.)


    I'm an old maintenance guy not a reactor engineer so cant advise you on the neutronics or design.
    I hope the links are interesting to you. (And that they still work)

    Have Fun !

    old jim
    Last edited by a moderator: May 5, 2017
  8. Mar 24, 2012 #7
    Thank you again "Astronuc" with you're amazing links , and that idea sounds great , but this hollow in the control rod how it would effect the cosine shape of flux distribution in the core?
    Thanks a lot Jim for those links, I found some useful pdf files among them could help a lot , really thanks a lot for your concern... :)
  9. Mar 25, 2012 #8


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    Is one planning a PWR control rod design? Or BWR?

    And is one planning to use the control rods for reactivity control, which is not typical in PWRs, although some PWRs use 'grey' control rods for power shaping purposes.

    Typically for PWRs, control rods are used to shutdown the reactor. When entering (from the top) the core slowly, they will suppress the neutron flux strongly at the top of the core. They may be driven in quickly in symmetric groups or dropped (scrammed) into the core in a couple of seconds. Fully inserted, they will have a uniform effect on the axial shape of the flux, but they will strongly affect the radial power distribution through local power suppression. Basically, their job is the shutdown the reactor.

    Grey rods are often part length and they are adjusted axially in the reactor to balance the burnup accumulation. They are used in sets, usually of 4 or 8 so as to balance the effect on the core radially, i.e., maintain a symmetric power distribution.

    PWRs use soluble boron in the form of boric acid with LiOH as pH buffer. These days, the boron is enriched in B-10, a strong neutron absorber.

    With respect to axail power shapes, power reactors never have a cosine distribution. Their power distributions are as flat as possible, although there is some peaking toward the center of the core, or toward the ends of the core (about 0.3-0.5 m from each end). But the particular shape depends on several factors.

    I have seen some near-cosine shaped flux distribution in test reactors, such as in Halden or DR-3 reactors.
  10. Mar 25, 2012 #9
    I have this class project to design a full reactor with complete calculations excepts the safety systems , its PWR similar to AP-600 , and the control rods are just a stage , I'm following the steps in Chapter.11 " nuclear reactor analysis" for "Deuderstadt & Hamilton" if you're familiar with...
    Thank you very much for these information, things are little more complicated than I thought, and if you have any links about the design of AP-600 reactor , Fuel , clad , control rods and geometry I would appreciate it...
  11. Mar 25, 2012 #10


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    This paper gives a good description of the AP600 and AP1000.


    Basically, the AP600 uses a typical 17x17 fuel assembly with a 3.66 m (12 ft) active fuel region, although the fuel may use enriched axial blankets of 150 to 200 mm (6 to 8 inches) in length. The assembly is commonly used in many current 17x17 plants. The AP1000 uses a longer 4.27 m (14 ft) active fuel length - similar to South Texas Project reactors, or the advanced 1300/1450 MWe reactors of France.

    The RCCA for the AP600 would be compatible with one use in current W-plants that use 17x17 fuel with a core height (active fuel zone) of 3.66 m (12 ft).

    I have Duderstadt and Hamilton, and I taught a class in reactor physics from it - many years ago.
  12. Mar 31, 2012 #11
    Yeah , I've read this paper before...
    Thank you very much , I wish you the best :)
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