Sunday, May 30, 2010

Superalloys: the choice for GenIV?

My previous post (Metallurgy: Austenite vs. ferrite) generated some great comments. There's a lot of interest in the possibility of using nickel-based superalloys (such as the Hastelloys) for major components in GenIV fast reactors. In between the day job and some business travel, I did as much reading as I could along these lines. First, I summarize some of what I've learned from my reading, and then go out on a limb with my crystal ball regarding the use of superalloys in next-gen reactors.

But before even the first item, let's take a quick aside. One of the "new things" in the superalloy literature are cobalt-based alloys for applications such as fossil-fired turbines. We can dismiss this possibility for the simple reason that cobalt and neutrons don't mix -- 59Co activates to 60Co, and becomes a significant radiological hazard to the reactor personnel. We can rest assured than cobalt-based superalloys, and even nickel-based superalloys containing more than a trace quantity of cobalt, are ruled out.

Superalloys are the ne plus ultra for high-temperature, high-corrosion applications in non-nuclear environments. Jet engines and stationary gas turbines, as we know them today, would be utterly impossible without superalloys. Because any thermodynamic system becomes more efficient as the temperature increases, superalloys are constantly creeping up to higher and higher operating temperatures. (American Airlines, for example, would fork over big cash to a jet engine company that can cut their fuel cost per mile even a small amount.)

The fast reactor research programs in the 70s produced quite a lot of irradiation data for superalloys. The Rowcliffe et al. paper summarizes them quite readably [1]. Murty and Charit [2] and Nanstad et al. [3] also cover on the topic.

Superalloys gain their spectacular high-temperature behavior due to the presence of "gamma-prime" and "gamma-double-prime" precipitates in their structure. These are tiny particles in the nickel-based matrix that pin "dislocations" (the basis of ductile deformation) up to very high temperatures. The main problems with using nickel in a reactor environment are twofold: (1) superalloys contain large numbers of alloying elements. Not quite half the periodic table, but it seems that way. These solutes move around under the gradients of defects produced during irradiation which destroys the gamma-prime and gamma-double-prime, produces new precipitates (with names like "eta" or "laves") which can embrittle the grain boundaries, and radiation induced segregation which can induce both brittle failure and corrosion at the grain boundaries. (2) Nickel is highly susceptible to (n,alpha) reactions, in which a neutron is absorbed by a nickel nucleus, causing it to kick off an alpha particle. This transmutes the nickel into a different element, and results in the alpha becoming a helium atom in the metal. Since helium has (effectively) zero solubility in metals, helium bubbles begin to form almost immediately. (Figure)

These helium bubbles are the primary cause of the swelling and shape-change seen in irradiated superalloys, which can affect the tight tolerances needed for precision machinery as well as weakening the material. Superalloys always contain some amount of carbon; radiation can induce the formation of M6C and M23C6 precipitates, where "M" denotes "metal", mostly Cr but with some fraction of the rest of the cations. These are blamed for destroying the swelling resistance of the material and allowing the helium bubbles to begin their dirty work. Murty and Charit suggest that superalloys should certainly be useful in regions of the reactor with lower radiation flux (i.e., steam generators) but more data is needed, as well as new alloys, to truly understand to what degree the radiation damage, particularly helium and swelling, can be ameliorated.

Nanstad et al. studied a series of alloys: Nickel-chrome-iron "800H", nickel-cobalt "617", and a series of steels. The 617 was interesting from a fundamental point of view, but its cobalt content (~12%) will make it uninteresting for real applications. The 800H showed appealing properties, especially when processed differently to improve its grain boundaries, but swelling due to transmutant helium is still a threat.

OK, so that's a quick once-over of some literature. Now comes opinion. GenIV reactors are going to show spectacular temperature / dose combinations, which will result in huge amounts of nickel transmutation to helium in any alloy. My guess -- and I emphasize that this is a guess -- is that any in-core metal parts are going to have to be nickel-free in these new reactor concepts. Rowcliffe et al. discussed concepts such as mechanical alloying to produce nanoclusters in the nickel superalloy matrix to vacuum up the point defects and helium, but this approach has really only been successful for ferritic steels. The problems of GenIV materials are simply extraordinary, and as a taxpayer and voter I say we need to pursue multiple parallel tracks of research towards the goal of clean energy via GenIV, including superalloys. But my personal crystal ball says that the oxide-dispersion-strengthened iron-chrome steels are the most likely solution.

(Figure is hotlinked and may go away, change, etc.)

[1] A. F. Rowcliffe, L. K. Mansur, D. T. Hoelzer and R. K. Nanstad: Perspectives on radiation effects in nickel-base alloys for applications in advanced reactors. Journal of Nuclear Materials V392, (2009), P.341-332.

[2] K. L. Murty and I. Charit: Structural materials for Gen-IV nuclear reactors: Challenges and opportunities. Journal of Nuclear Materials V383, (2008), P.189-195.

[3] R.K. Nanstad, D.A. McClintock, D.T. Hoelzer, L. Tan and T.R. Allen: High temperature irradiation effects in selected Generation IV structural alloys. Journal of Nuclear Materials V392, (2009), P. 331-340.

Sunday, May 9, 2010

Metallurgy: austenite vs. ferrite

Everyone has heard of "stainless steel." By convention, a stainless steel is an iron based alloy containing a large addition (several percent to 20 percent) of chromium. Chromium at the surface forms a thin, adherent, nearly impermeable, and self-healing layer of chromium oxide, which slows or prevents the attack of the iron base metal by oxygen or other corrosives.

But here's a trick: stick a magnet to a piece of stainless steel. Most of the time, the magnet will stick. This is "ferritic stainless steel." Every now and then, however, a part made from more highly alloyed (and thus, more expensive) stainless steel will be "austenitic stainless steel," and will be non-magnetic. Typically austenitic stainless steels will contain even more chromium and 10 or 20 percent nickel. As a result, they're expensive, but some applications require their very high properties.

If you cooled pure liquid iron, it crystallizes into delta-ferrite, which has the crystal structure shown in the first figure. ("BCC") As it cools from white-hot to dull-red, the crystals will transform to gamma-austenite, which is the second crystal illustrated ("FCC"). Cool it down from red hot to black but still very warm, it will transform again, back to ferrite, but now referred to as "alpha." This is called an allotropic transformation, and several chemical elements (like plutonium, tin, or carbon [graphite, diamond]) undergo similar reactions.

Impurities -- called alloying elements when they're added on purpose -- change the temperatures where these changes happen, and can cause both phases to coexist in a particular temperature range. Adding chromium to iron gives ferritic stainless steel; the chromium atoms substitute for the iron atoms at random. Nickel, however, is an austenite stabilizer and at additions around 10 percent will cause the iron-chromium-nickel alloy to switch to austenite at room temperature.

The austenitic steels are often used for higher temperature applications because they can withstand a hot, corrosive environment and maintain mechanical properties to somewhat higher temperatures than the ferritic grades. Both are used in large amounts in different parts of a modern, GenII light-water reactor. A GenII reactor, or even the GenIII / III+ ones under construction, will see outlet temperatures in the low-300s Celsius. The GenIV concepts will push as far as 1000°C, and the current crop of materials would turn buttery there. The austenitic alloys show better resistance to creep at high temperature than the ferritics, but under neutron bombardment they also suffer void swelling which can compromise their mechanical strength.

There's been a large amount of work on nickel base superalloys for these types of applications; Hastelloy X, as one previous commenter mentioned, being a strong contender. A nickel superalloy is "nickel-based," but even that is more of an honorary distinction: modern superalloys can contain dozens of alloying elements, with nickel being barely more than 50%. Superalloys have nearly seven decades of proven use in jet engines and land-based gas turbines. However, even these very super alloys will be hard pressed to withstand the temperatures and neutron doses and corrosive coolants in a GenIV plant. They have good potential, particularly for the regions of a GenIV plant operating short of the highest conceived temperatures, but problems such as radiation induced segregation and radiation induced precipitation of particles such as carbides at grain boundaries can be pronounced. Nickel also suffers internal helium generation during neutron irradiation, which can form small dispersed bubbles. Undoubtedly, superalloys will have a part in GenIV reactors but it remains to be seen how large or small.

The most interesting new class of materials are the F/M "ferritic-martensitic" materials and the ODS "oxide dispersion strengthened" materials. Recall that austenite is the higher-temperature phase of iron; quench austenite in water and instead of transforming to the low-temperature ferrite phase, the rushed reactor forms martensite, which is a slight variation on the ferrite structure. Martensite, however, is spectacularly hard but brittle. Mixing ferrite and martensite into a structure gives composite properties superior to either alone. The fossil energy research programs resemble nuclear in that they are always striving for longer-life components at higher temperature. Fossil energy research has yielded a number of promising F-M steels that may show good corrosion, temperature, and neutron resistance. However, like for superalloys, the database of long-term radiation-exposure data is incomplete. A variation on this theme is ODS, where F-M materials have a fine dispersion of yttrium oxide particles less than 2 nm in diameter dispersed through the structure. This high density of nanoparticles results in a large amount of matrix-particle surface area, which vacuums up radiation-induced defects such as vacancies or helium, significantly reducing the action of radiation induced segregation and precipitation. Again, long term studies are underway, but the smart money is for a mix of superalloys and ODS alloys in the GenIV prototypes.

A. F. Rowcliffe, L. K. Mansur, D. T. Hoelzer and R. K. Nanstad: Perspectives on radiation effects in nickel-base alloys for applications in advanced reactors. Journal of Nuclear Materials V392, (2009), P.341-332.

K. L. Murty and I. Charit: Structural materials for Gen-IV nuclear reactors: Challenges and opportunities. Journal of Nuclear Materials V383, (2008), P.189-195.

T. R. Allen and J. T. Busby: Radiation damage concerns for extended light water reactor service. JOM V61(7), (2009), P.29-34.

T. Allen, H. Burlet, R. K. Nanstad, M. Samaras and S. Ukai: Advanced Structural Materials and Cladding. MRS Bulletin V34(1), (2009), P.20-27.