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Pebble Bed Reactor - a fuel nightmare?

  1. Jan 12, 2007 #1

    Andrew Mason

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    I am not sure why the Pebble Bed modular reactor (PBMR) is being touted as the great solution to problems with nuclear power. It uses a once through, virtually non-reprocessible fuel. It produces a high volume of waste consisting not only of the fuel but the casing/moderator which has about 50 times the volume of the fuel and 20 times its mass. It is essentially a uranium hogging once-through thermal reactor, but with a bigger waste problem. The graphite moderator in each pebble could also be a fire hazard. If the graphite moderator in the pebbles should catch fire, there could be a catastrophic release of radiation.

    Its chief advantages seem to be the high efficiency due to its ability to use the helium coolant that is in the core to drive turbines directly. Supposedly, there will be no radioactivity in the helium coolant. I am not so sure about that.

    It seems to me that the PBMR is a vastly overrated reactor.

  2. jcsd
  3. Jan 13, 2007 #2
    Well, I was talking specifically about propulsion applications for it.
  4. Jan 13, 2007 #3


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    I don't think those numbers are correct. Here is an example of a TRISO fuel particle - http://en.wikipedia.org/wiki/Nuclear_fuel#TRISO_fuel


    The pyrolytic carbon and SiC coatings have some volume similar (maybe 3-4x) to the fuel kernel, and the mass IIRC is slightly less.

    However, clearly there is still a waste issue.

    Reprocessing is difficult, but not impossible.
  5. Jan 13, 2007 #4

    Andrew Mason

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    According to this article there is 9 g. of U in a 210 g. 60 mm diameter ball (volume:113 cm^3) .

    The density of U is 18.7 g/cm^3 so the U would occupy .5 cm^2. So this fuel pebble occupies over 200 times the volume of the U. I was being kind at saying 50 times.

    Not only the waste in terms of handling and storing it, but in terms of making efficient use of uranium. Although U is a plentifiul element, economic deposits are not abundant. There is a serious world shortage of U at the moment, as the 700% increase on price since 2003 indicates.

  6. Jan 13, 2007 #5


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    The Wikipedia article - Pebble_bed_reactor#Containment - may be incorrect. I checked the Idaho site and a report on the calculation of Dancoff factors for a typical pebble has the typical diameter of a TRISO particle at 25 mm. I know several people involved with the fuel development, so I can get better data. Petti et al reported about some of the fuel development work at the ANS meeting last June in Reno, NV. I'll be seeing these guys later this year.
  7. Jan 14, 2007 #6

    Andrew Mason

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    There are a number of sites that provide the same information as the Wikipedia article:

    These papers describe in detail the fuel pebbles:

    This was on the World Nuclear.org site:

    See also this abstract:

    The U fuel is in the form of UO2 which has a density of about 10 g/cm3. It appears from the above that the UO2 consists of tiny .5 mm diameter grains each of which are coated with various layers of different carbon material and then mixed in a resin to form a ball and then coated with a final ceramic carbon layer. 9 grams of UO2 in a 210 gram pebble with a 60 mm diameter.

  8. Jan 17, 2007 #7
    The fuel is not "once through", if that is meant in the literal sense. Each "pebble" will be analyzed when it is ejected from the core and diverted either for re-use or to a waste stream.

    Helium has a relatively low activation cross section so if it is maintained pure then it will not activate from neutron irradiation.

    One of the main PBMR advantages is its inherent safety feature of a negative temperature co-efficient of reactivity and the fact the fuel design is less attractive for potential proliferation activities.
  9. Jan 17, 2007 #8

    Andrew Mason

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    "Once-through" refers to the lack of reprocessing. Once the U235 is down to levels that will not sustain a reaction, the pebble, and the uranium in it, is treated as waste.

    I am having difficulty imagining how you can run the gas repeatedly through a turbine and not contaminate it with matter that will capture neutrons. But I take your point.

    I think that can be achieved with much better fuel usage using other designs.

  10. Jan 17, 2007 #9


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    The alpha particle (nuclear of He-4) is extremely stable. If it absorbed a neutron to become He-5, He-5 decays to He-4 (alpha) and a neutron. Basically the He-4 does not become activated.

    However, if cracks develop in the carbon or SiC coatings, then fission products, e.g. noble gases and volatile radionuclides like I-131, I-135, can escape and deposit on the fuel or in the primary system.
  11. Jan 18, 2007 #10
    Yes, the coolant itself will not become active itself, certainly much less so than other coolant materials such as carbon dioxide, water, sodium, etc As well as any escaped fission products, there will always be impurities present in any reactor cooling system, such as activated trace metals from metal components and grease, lubricants, etc used to construct and maintain the containment. These are virtually impossible to eliminate entirely, even with good scrubbing systems, although they can be reduced by appropriate choice of materials. It is difficult to avoid steel though, & the associated activated Co-60. Not perhaps a contender for long term waste consideration but certainly for radiation protection considerations.

    This could be an American term? I have not heard it. I will conduct a discrete investigation amongst my colleagues.

    PBMR-type design technology was dropped in the UK & Germany. It would be interesting to know if this was purely from an economic standpoint or whether there were issues with it.
  12. Jan 20, 2007 #11
    All reactor design was dropped in the UK when the future for electricity generation was seen as gas-fuelled. The South African PBMR does however draw on German expertise but it's seen as a Generation III technology. Six different reactor technologies have been chosen for the Generation IV programme.

    http://www.world-nuclear.org/ is a good source of information, albeit not unbiased.
  13. Jan 20, 2007 #12


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    I'm sorry to hear that the UK dropped their work on nuclear reactors in favor of gas.

    While gas may be the "cleanest" of the fossil fuels, it probably has the shortest
    longevity of any fossil fuel, as well as continuing to be a green house gas [CO2]
    emitter; as are all fossil fuels.

    Dr. Gregory Greenman
  14. Jan 21, 2007 #13
    Oh, while the public posture is still anti-nuclear, the technological illiterates in Westminster seem to have realised that they have no option but nuclear power if they want to:

    a) provide the capacity for the whole country to make a cup of tea when the six-o'clock news starts

    b) guarantee power in the face of the increasingly-aggressive Gasprom, who supply the gas from Russia.

    So, we've signed up for the Generation IV programme ( http://www.gen-4.org ). The problem is to re-establish expertise, particularly in fast reactor technology, where the UK once led the world. Partly to that end, there's a new nuclear research facility at Manchester Uni ( http://www.dalton.manchester.ac.uk/ ). But it might have been easier if British Nuclear Fuels, who owned Westinghouse Electric, hadn't sold it to Toshiba.

    However, while the UK government may have woken up, our local parliament here in Scotland is still hiding its head in the sand - in spite of the fact that two of our major nuclear plants (Hunterston and Torness) are due for decommissioning soon. Reality will fall on them when the power-cuts start.
  15. Mar 14, 2008 #14


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    Nuclear Fuel Performance Milestone Achieved [with TRISO carbide fuel]

    That's close to 90 GWd/tHM, which is a significant burnup level.

    There will probably be some papers and discussion at the upcoming ANS Summer Meeting in Anaheim, CA this June. I'm planning on being there.
  16. Mar 14, 2008 #15


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    When the electricity market was privatised it was done in such a way as to make time to market the most important factor. Gas fired are the quickest to build - although many of them have had to be rebuilt only 5years later.

    We don't have to worry about gas supplies, although the north sea has run out we can get gas from Russia and Iran so supplies are perfectly secure.
  17. Mar 14, 2008 #16
    So that's for TRISO fuel.
    But what's the highest burnup achieved for any type of fuel?


    Is the previous record 60GWd/tHM?
    In which case, that would make this a 50% improvement. Wow.
    And they said they expect to achieve 14% burnup by yearend? Wow, so more than a doubling of nuclear fuel efficiency.
    Last edited: Mar 14, 2008
  18. Oct 21, 2010 #17
    Can this technology (massive amounts of very low grade spent fuel) place us closer to the old Jimmy Carter dream for long term storage of spent fuel, ie., by diluting in ceramic blocks at "natural" (3%) concentrations in locations like the Marianas trench?
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