Wednesday, April 21, 2010

Where nuclear engineering meets materials science

When you're a freshman in engineering college, the different departments try to recruit you into their program, over the others. The most compelling argument the Materials Science and Engineering program can make is, "we're the branch of engineering that makes the others possible." The program I attended likened MSE to the spokes of a wheel, where physics, chemistry and math were the hub and the other engineering disciplines the tire.

In nuclear energy, this is no different. Someone -- Google fails me to find the correct attribution -- said that a reactor is just a design until the metallurgists say if it can be built or not. The (historical) cradle of the nuclear industry was the US national energy lab system. Although nuclear is now only a small part of their portfolio, the energy labs -- the full constellation of LBL, triple-L, Sandia, Oak Ridge, Idaho, Argonne, Savannah River -- are today the preeminent materials science facilities in the US, and arguably the world.

So, I'm going to write a series of short posts on materials science challenges for the nuclear renaissance. Today, let's discuss some of the fundamentals of nuclear materials in order to lay the groundwork for later posts. This is actually more difficult than I'd envisioned, because the recent journal reviews all discuss problems in GenIII+, GenIV, and fusion. I've made a trip to my local library can checked out some dead trees. We'll hold off the "advanced concept reactors" materials problems for later; today we'll discuss the simpler problems seen in GenII. I will not be using the Wikipedia definition of nuclear materials, which I would call "fissionable materials." In my context, any solid substance inside the reactor dome is a "nuclear material." The reactor vessel and many internal parts are steel, often highly alloyed with chromium and nickel. Some internal parts will be nickel-based superalloys, originally developed by the Nazis for the earliest jet engines but now omnipresent in extreme environment engineering. The fuel is often uranium oxide, sometimes doped with other elements like niobium or mixed with plutonium. The cladding and channels around the fuel will often be "zircaloy," lightly-alloyed zirconium base materials

Even in a "simple" GenII-type light water reactor, there's still an amazing amount of advanced materials science. First, operating temperatures of the in-core materials might run above 300°C, which will result in the degradation of many materials over time due to changes in their microstructure. As a similar example, in the World Trade Center, the towers fell because the steel suffered changes ("recovery") to its defect structure that were kinetically forbidden at room temperature, but allowed at burning-jet-fuel-temperature; the steel didn't come close to melting.

Second, water itself can be a very corrosive environment unless materials are well-selected and -manufactured. Third, irradiation has profound effects on materials properties and behavior over long times. Most significantly, these three effects can often combine in ways that are difficult to foresee.

Metals and ceramics, at the scale of nanometers to millimeters, are made up of "grains." (See figure.) Each grain is a single crystal, which means that the atoms are arranged in a regular order across the extent of each grain. Where two grains meet is a "grain boundary," a region of slight atomic disorder and where atoms are more loosely packed (Second figure). "Second-phase particles," an example of which might be non-metallic carbides in steel (Third figure), often preferentially form at grain boundaries. Solute and impurity atoms often prefer to move to grain boundaries because of the looser atomic packing. Sulfur and phosphorus, for example, are unavoidable impurities in commercially produced steels, and segregation of these contaminants to grain boundaries at moderate temperatures over long times are known to cause embrittlement by making the materials susceptible to mechanical failure along the grain boundaries.



How does this apply to nuclear materials? In a light water reactor the materials (often alloy steels, nickel alloys, or zirconium alloys) and simultaneously subject to high temperature, a corrosive environment (water itself) and a radiation flux. As temperature increases, the atoms in the crystals vibrate about their equilibrium positions with higher and higher amplitudes. Further, as temperature increases, "vacancies" form. A vacancy is an atomic position in a crystal that is not occupied by its atom. Vacancies disappear at absolute zero: all atomic sites are occupied (theoretically). At a metal's melting point, perhaps 1 atomic site in 100-10,000 (depending on many factors) will be a vacancy. Because atoms are wiggling around more energetically at high temperature, and the chances of an atom being next to an empty site as temperature (and vacancy fraction) increases, atoms are more likely to hop from one atomic position to an adjoining vacancy, resulting in "diffusion." Any of the atoms now surrounding the vacancy's new position have an equal chance to change places with it, so the first hopping atom is unlikely to return to its original position, which means the vacancy, statistically, moves around the crystal. Because the chances of any atom making such a jump are perfectly random, it's possible to find that atoms move, on the average and under most circumstances, from areas of high atomic concentration to low atomic concentration. This is mathematically the same as what happens when someone opens a bottle of perfume on one side of a room, and you smell it on the far side of the room a few minutes later. Even if the air in the room is perfectly still, Brownian motion moves the perfume molecules at random and they eventually fill the room evenly.

Solute atoms, perhaps the sulfur and phosphorus naturally present in an alloy, hydrogen introduced by the reaction of water (H2O) with the alloy's surface, or helium created when nickel or boron captures a neutron, will move around the metal due to diffusion at operating temperatures. These atoms often try to move to grain boundaries, or to the surfaces of second-phase particles like carbides, due to the slightly greater amount of "elbow room" at these locations. These effects (helium excepted) are all present in any high temperature materials system, such as fossil-fuel fired turbines or jet engines. Where these problems become interesting is when neutrons -- obviously present in a reactor -- bombard the material. Neutron-induced perturbation of a material is measured in "displacements per atom," or "dpa." A neutron carrying enough energy can bump an atom out of its lattice site, turning the atom into an "interstitial" and leaving a vacancy behind. At a level of 1 dpa, this means that each atom has been moved an average of one time. This would be expected to completely destroy the material's crystal structure; in semiconductor processing, ion implantation into silicon wafers amorphize the surface of the material at levels well below 1 dpa. However, primarily due to the elevated temperature operation, almost all of these atoms quickly relax to their equilibrium states in a self-healing process. However, excess vacancies or other defects can remain, and result in even faster diffusion. As a result, a commonly observed effect in steels and other materials is "radiation induced segregation," RIS. In addition to the usual suspects such as sulphur, phosphorus and helium, even the major components like nickel, copper, and chromium are induced to move around. These major alloying additions cost money, so they're only added to the base steel for a reason. An example would be chromium, which reacts to form chromium oxide on the surface of a steel -- this adherent chromia layer is what makes stainless steel stainless. Moving the solutes firstly depletes them from their intended purpose and, by driving them to the grain boundaries or other defects, can result in embrittlement of the material. Reactor pressure vessel steels are seen to form 1-5 nanometer diameter copper- and manganese-rich precipitates over time, which will both harden but embrittle the steels.

It's a tribute to the engineers and scientists working at the national labs in the 1950s, 60s and 70s that, despite all of these driving forces towards materials degradation, the GenII reactors are still going strong and have the potential to keep running for decades more [1]. Even so, a PWR or BWR might show operating temperatures in the range of 280-320°C and thermal neutron fluxes over time of perhaps 7-80 dpa. (See [2].) The concepts for high-temperature, high-thermodynamic-efficiency, high-burnup reactors of GenIII+ and GenIV will require much high temperatures (up to 1000°C), enormous doses (up to 200 dpa) of thermal and fast neutrons, and far more chemically aggressive environments such as salt-based coolants ([2], [3]). In later posts we'll explore those more advanced materials problems and some potential solutions.

(Figures are hotlinked; they may change or go away. Sorry.)

[1] R. Szilard, P. Planchon and J. Busby: The case for extended nuclear reactor operation. JOM V61(7), (2009), P.24-27.

[2] Y. Guerin, G. S. Was and S. J. Zinkle: Materials Challenges for Advanced Nuclear Energy Systems. MRS Bulletin V34(1), (2009), P.10-14.

[3] C. Cabet, J. Jang, J. Konys and P. F. Tortorelli: Environmental Degradation of Materials in Advanced Reactors. MRS Bulletin V34(1), (2009), P.35-39.

2 comments:

  1. Very insightful blog, Sterling Archer. Thanks for posting, glad to have you with us.

    ReplyDelete
  2. Right on Sterling. I'm growing tired of hearing CNN reporters and antinuke kooks talk about operating nuclear plants "beyond their intended design life." The truth is the 40 year number is a financial model. The fact is, with the exception of the reactor and the structures, almost all of a nuclear plant can be replaced. The real design life is the number of coupons inside containment containing reactor metals. Without them, we can't measure neutron embrittlement of the RPV, otherwise we could run that thing for over a century until Greenpeace is red in the face!

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It's been pointed out to me that using a cartoon character's screen name while disabling anonymous comments would be a pot//kettle//black situation. So let's try the "anonymous allowed, all moderated" paradigm. Please be aware that moderation will occur at most once per day, every other day might be more likely. Please be patient. (I also notice some FireFox related comment-leaving problems; perhaps Google is getting angry at Mozilla users?)