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View Diary: New Nukes: Obama will name Steven Chu his choice for Energy secretary (124 comments)

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  •  Thanks (2+ / 0-)
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    Plan9, LookingUp

    Finally someone who has some experience with graphite.

    Yeah, water will do the trick. More importantly, an ingress of water in such a high temperature environment will lead to oxidation of the graphite, which is not desirable.

    Graphite will burn if it contains a significant amount of impurities. Nuclear-grade graphite is quite pure (for other reasons, of course). As you have mentioned, you can take a blow torch to graphite, heat it up until it is white hot, and it still will not ignite, nor sustain combustion on its own.

    And you are correct in explaining that it is the Doppler broadening that reduces the reactivity as the temperature increases. This, of course, means that the reactor shuts itself down in the event that the temperature increases, such as when coolant is lost.

    Good job!

    Blessed is the man who, having nothing to say, abstains from giving wordy evidence of the fact.
    -- George Eliot

    by bryfry on Thu Dec 11, 2008 at 12:30:30 PM PST

    [ Parent ]

    •  Can you honestly say with a straight face (0+ / 0-)

      That a pebble that's made many passes through the reactor core is going to have few impurities in its graphite moderator?  Nuclear grade graphite is quite pure when it starts out, but that's only a temporary situation.  SiC is not an impervious boundary layer, and constant bombardment by radiation is not harmless.

      •  Yeah, I'm sure (2+ / 0-)
        Recommended by:
        Plan9, LookingUp

        That a pebble that's made many passes through the reactor core is going to have few impurities in its graphite moderator? Nuclear grade graphite is quite pure when it starts out, but that's only a temporary situation.

        Ugh .. please explain exactly where these impurities will come from?

        Anyone who knows anything about graphite knows that the impurities come from the manufacturing process.

        SiC is not an impervious boundary layer, and constant bombardment by radiation is not harmless.

        That's why researchers study this kind of stuff, and they've been studying this boundary layer for decades now. There is a potential problem with silver migration across the boundary at high temperatures, but other than that ... yeah ... SiC is pretty much an impervious boundary layer ... that is ... up to well above 2000 deg. C.

        Blessed is the man who, having nothing to say, abstains from giving wordy evidence of the fact.
        -- George Eliot

        by bryfry on Thu Dec 11, 2008 at 01:53:24 PM PST

        [ Parent ]

        •  Silicon carbide (0+ / 0-)

          Paper

          High temperature reactors (HTRs) are among the candidates of the possible next generation nuclear plant. HTRs are expected to offer attractive features such as inherent safety, low cost of electricity generation, and short construction period. The safety performance of high temperature gas cooled reactors mainly relies on the quality and integrity of the coated fuel particles. One of the possible failure mechanisms for TRISO coated fuel particles is kernel migration, in which the fuel kernel migrates through the buffer layer due to the overall high temperature gradient and the carbon monoxide formation. In this study, thermal and mechanical performances of a coated fuel particle with a migrated kernel are analyzed by the finite element technique. Calculations are performed for two different operating conditions represented by two different surface temperatures. Similar analyses are also carried out for a nominal particle without kernel migration for comparison. Temperatures and stress distributions are calculated and failure probabilities of the coated fuel particle are obtained based on the Weibull statistics. Further comparison is made in terms of the failure probability considering a coated fuel particle whose inner pyrolitic carbon layer is defective or already failed. Furthermore, stress distributions for the particle with kernel migration through the inner pyrolytic carbon layer has been obtained. Calculated temperature distributions, maximum stress values, and failure probabilities are reported to assess the influence of kernel migration on coated fuel particle behavior. Results show that high temperature operation, high burnup, and excessive temperature gradient on the fuel particle can lead to fuel failure. The pressure vessel failure is generally observed well before the failure by the kernel migration. In fact, these failure modes are interrelated and affect each other.

          Paper

          To optimize the SiC for use as a coating material in the nuclear reactor fuel design, it is important to elucidate the chemical bonding and interface formation of metal fission products (Pd, Ag, Cs, etc.) with SiC coating layers and to study the diffusion behavior of fission products into TRISO coating materials. It is known in the TRISO community thatPd is able to corrode the SiC layer. However, the detailed nature of this corrosion is still unknown.

          ...

          Thus, in summary, all the three experimental series shows an upward shift of all electronic levels indicating that there is band bending induced in the SiC substrate when Pd is deposited. Furthermore, there is significant charge transfer between Pd and SiC on various length scales. The interface species of all the species in all the experimental series is getting dominant as a function of Pd coverage, which is interpreted as a significant intermixing at the interface. Hence one can conclude that Pd interacts with SiC in several different ways, inducing local chemical bonding (corrosion), intermixing, and electronic structure modification. This, in turn, will impact the degradation of the SiC layer by Pd, potentially leading to coating failure in TRISO nuclear fuel.

          Paper

          Our mechanistic model for coated-particle fuel will consider both the structural and physico-chemical behavior of a particle-coated fuel system during irradiation. The following important phenomena will be included:

          * Fission gas release from the kernel as a function of burnup, temperature and kernel type (oxide, carbide, oxycarbide); * Anisotropic response of the pyrolytic carbon layers to irradiation (shrinkage, swelling, and creep that are functions of temperature, fluence, and orientation/direction in the carbon); * Failure of the pyrolytic carbon and SiC layers based on the classic Weibull formulation for a brittle material either by traditional pressure vessel failure criteria or by mechanisms such as asphericity, layer debonding, or cracking; * Fission product inventory generation as a function of burnup and enrichment of the particle; * Chemical changes of the fuel kernel during irradiation (changes in carbon/oxygen, carbon/metal and/or oxygen/metal ratio depending on the kernel fuel type, production of CO/CO2 gas) and its influence on fission product and/or kernel attack on the particle coatings; * Kernel migration; * Fission product diffusion, migration and segregation; * Statistical variations of key properties of the particle associated with the production process,requiring Monte Carlo analysis of a very large number of particles to understand the aggregate behavior.

          Due to space limitations, this paper will focus only on the structural aspects of the model.

          A typical TRISO-coated particle is shown in Figure 1. Fission gas pressure builds up in the kernel and buffer regions, while the IPyC, SiC, and OPyC act as structural layers to retain this pressure. The basic behavior modeled in PARFUME is shown schematically in Figure 2. The IPyC and OPyC layers both shrink and creep during irradiation of the particle while the SiC exhibits only elastic response. A portion of the gas pressure is transmitted through the IPyC layer to the SiC. This pressure continually increases as irradiation of the particle progresses, thereby contributing to a tensile hoop stress in the SiC layer. Countering the effect of the pressure load is the shrinkage of the IPyC during irradiation, which pulls inward on the SiC. Likewise, shrinkage of the OPyC causes it to push inward on the SiC. Failure of the particle is expected to occur if the stress anywhere in the SiC layer reaches the fracture strength of the SiC. Failure of the SiC results in an instantaneous release of elastic energy that should be sufficient to cause simultaneous failure of the pyrocarbon layers.

          This is just a handfull.  Have you noticed that I'm the only one presenting papers here?  Oh, and:

          Anyone who knows anything about graphite knows that the impurities come from the manufacturing process.

          Anyone who knows anything about nuclear reactors knows that long-term exposure to a high neutron flux changes the lattice structure of just about everything, and that completely keeping corrosive decay products inside the fuel is nigh impossible, but that their release tends to make the situation worse.

          •  Another thing... (0+ / 0-)

            Recently there was a HTR conference where the latest paper on this in July 2008 was presented with a report from Dr. Rainer
            Moormann of the Jülich Research Center.

            This paper has been used by ideologically motivated anti-nuclear activists to try to squash S. Africa's development of their PBMR. Moormann himself is much less anti-nuclear. His report here:
            http://www.hindawi.com/...

            But, the idea, specifically, of 'containment' has to do with releases of helium, which is inert, with fission products. Eskom suggests, and has designed filters to use for any related release of He into the air. The idea of catastrophic failure is simply waiting for someone to show HOW you can have one, period. Without that, there is no argument for containment beyond the the containment vessel the reactors sits in anyway. Even different generations of TRISO are being researched to address issues raised in both papers.

            I think there are issues as you point out. And there are others too. But clearly this is a away to go for a large segment of on demand base load and it's good that papers are being written so as make it better and address the problems.

            David

    •  To elaborate (1+ / 0-)
      Recommended by:
      Joy Busey

      A full size reactor has several billion fuel particles, any one of which can readily contaminate and weaken its respective sphere.  You can have a manufacturing defect -- a hole in the SiC layer, for example.  You can have a particle rupture from internal pressure from the decay products.  If a localized region of a sphere gets too hot, you can have SiC break down into silicon and carbon.  When one particle ruptures, it contaminates all of the others.  

      Great writeup from someone who's worked with HTRs here

      Go on -- argue that this justifies no containment structure.

      •  Oh man, this is getting tedious (2+ / 0-)
        Recommended by:
        Plan9, LookingUp

        A full size reactor has several billion fuel particles, ... You can have a manufacturing defect -- a hole in the SiC layer, for example. You can have a particle rupture from internal pressure from the decay products.

        Yes, defective fuel particles can be a problem, which is why good quality control standards are important. With billions of particles, some defects are to be expected. This doesn't mean that it is that important. The question is how many defects? The particle coating is just the first boundary between the fuel and the environment. If something gets outside the SiC layer, then it has to get out of the pebble, then out of the vessel, then out of the silo, then out of the building. Then, the amount that gets out has to be significant. If it involves less radiation than a person would receive from a banana tree, then who cares?!

        If a localized region of a sphere gets too hot, you can have SiC break down into silicon and carbon.

        I have no idea where you are getting this information, but nothing like this occurs below temperatures of about 2300 degrees C, which is over 500 deg C hotter than any temperatures that would ever be experienced in a pebble-bed reactor, even during an accident when all of the helium coolant is lost.

        When one particle ruptures, it contaminates all of the others.

        This is just not true and has no physical basis. Where are you getting this stuff?!!

        Blessed is the man who, having nothing to say, abstains from giving wordy evidence of the fact.
        -- George Eliot

        by bryfry on Thu Dec 11, 2008 at 01:54:31 PM PST

        [ Parent ]

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