Monday, April 26, 2010

GenIV materials, part 1: fluence and displacement

First, let me thank the various readers for some good comments on the previous posts, I always value feedback. Also thanks to Yes Vermont Yankee for the advertising. Any comments and especially suggestions are welcome!

Last time, I discussed some of the broad problems present in engineering materials for a LWR environment. This new series of posts will try to extend a few of these concepts to GenIV (and conceivably fusion) materials. Today's topic is not what materials to put in a pebble-bed or fluoride-cooled reactor; rather, today's topic is why you probably can't use the same materials that have worked so well over the last 60 years in LWRs.

Because the advanced reactor concepts will have spectacularly high neutron fluxes, neutron damage of the materials will be a major concern in GenIV. A 2004 news article in the scientific journal Nature made the point that materials research is a major barrier, if not the main barrier, to all of the DOE-GenIV concepts [1]. Let's go back to the displacement-per-atom concept. Damage inflicted on a material by neutron fluence is best quantified by dpa. A paper [2] illustrates this beautifully. Due to copyright I can't reproduce the figure here, but I will claim fair use and show a schematic adaption. Three reactors (red, green, blue) have different neutron energy spectra, and the changes in materials property, in this case the change in yield stress of stainless steel, don't really relate to one another. (Fluence is neutrons per square meter, in the way that when in a gunfight the number of wounds you have is measured by bullet fluence: bullets per square meter. Although you occupy cubic meters, the bullets hit your surface, which is square meters.) The change in materials property shows a nice trend for each reactor, but fluence in one reactor vs. fluence in another doesn't let you make any comparisons. However, through some math and knowledge of materials properties, you can convert fluence and neutron energy spectrum into dpa; on the bottom of the graph, the three reactors overlap nicely (but not perfectly) into a single trendline. Thus, a change in mechanical property as a function of dpa is a materials parameter, essentially independent of the neutron properties. (The recent text of Was [2] describes the gory details of the math.)

As dpa accumulate, a number of effects are seen. There are firstly the microstructural changes we previously discussed, such as radiation induced segregation of alloying elements to defects in the structure. Also seen are macrostructural changes, such as swelling and growth. Was [3] defines swelling as an increase in volume of a part with irradiation, and growth as a change in shape at constant volume. Because reactor parts must be precisely fitted together -- this is a precision piece of machinery, of course -- the ability to control, reduce, or predict such effects are important.

So our interest here is to relate damage, as quantified by dpa, to GenIV materials. The recent article of Guerin et al. [4] has an excellent graph. That graph contains some sub-parts from a DOE brochure, so I could probably claim "public domain" and put the whole figure here. However, let's error on the side of appeasing the copyright gods again and make a fair-use schematic summary. This chart illustrates the range of temperature and dpa anticipated to be seen in the different advanced concept reactors (VHTR, very high temperature reactor; SWCR, supercritical water cooled reactor; LFR, lead fast reactor; SFR, sodium fast reactor; GFR, gas fast reactor; MSR, molten salt reactor). What this tells us is that the closer a reactor is to the bottom-left of the graph, the easier it will be to build. The closer it is to the top-right, the more difficult. More dpa will result in more changes to the material structure and properties over time. Higher temperature and more aggressive coolants (like lead or molten salt) will result in more degradation to the materials whether a radiation field is present or not.

Amelioration of these damage effects and design of damage resistant materials are a major thrust of research in government, academic, and industrial labs at the moment: the primary showstoppers preventing advanced reactor development are likely to be the ability of the materials to withstand these temperatures and damage levels for years on end. As this series of posts continue, we'll look into materials selection and design to withstand these damage levels.

[1] D. Butler: Nuclear power's new dawn. Nature V429, (2004), P.238-240.

[2] L. R. Greenwood: Neutron interactions and atomic recoil spectra. Journal of Nuclear Materials V216, (1994), P. 29-44.

[3] G. S. Was, Fundamentals of Radiation Materials Science. 2007, Springer, Berlin.

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

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.

Sunday, April 18, 2010

Problems with CNN

A few days ago I saw a link to a CNN-dot-com video asking if nuclear powerplants were responsible for a town in Georgia having a cancer rate 51% higher than the national average.

I didn't have time to watch the video then, and I can't find it now. If you can find it, please leave a comment.

However, we can guess basically what it said:
1. There are two nuclear powerplants in this town
2. Someone worked out some numbers and found the town had more cancer than average. CNN left out the assumptions and the confidence intervals.
3. They interviewed some locals (who are normally only on TV describing what the tornado did to the trailer park).
4. The locals assume the powerplants are responsible for the cancers, including grampa the tobacco farmer's lung cancer.

Again, you don't need to see the actual CNN bit to know where the scientific fallacies are. A better method would be to compare the town to the deep-south or Georgia cancer rate (rural Georgia; excluding Atlanta. Urbanites are statistically healthier than country folk).

I imagine using a non-biased subset for comparison would remove all statistically significant differences. A rural Georgia town will have high cancer rates due to fried food, tobacco, and alcohol. A nuclear plant is hardly needed.

Some future posts

Some educational posts I'm contemplating for the next few weeks / months:

  • The tritium trick (both versions)
  • Materials science problems in GenIV & fusion
  • Experimental techniques in nuclear materials engineering
  • Will we ever control fusion?
  • Are NIF and Z good investments of taxdollars?
Comments and suggestions welcome.

Saturday, April 17, 2010

Krauthammer articles

Nuclear Posturing
Nuclear Posturing (II)

I generally like Krauthammer: his arguments are usually well thought out, and unlike the rest of the media, he's not over enthralled by Obama's formidable charisma.
Some good quotes:

Imagine the scenario: Hundreds of thousands are lying dead in the streets of Boston after a massive anthrax or nerve gas attack. The president immediately calls in the lawyers to determine whether the attacking state is in compliance with the NPT. If it turns out that the attacker is up-to-date with its latest IAEA inspections, well, it gets immunity from nuclear retaliation.... However, if the lawyers tell the president that the attacking state is NPT noncompliant, we are free to blow the bastards to nuclear kingdom come.

Well, if we thought we'd get something good out of the deal, that might be worthwhile. Normally you make concessions in exchange for something of value. But:

This under the theory that our moral example will move other countries to eschew nukes.

On the contrary. The last quarter-century -- the time of greatest superpower nuclear arms reduction -- is precisely when Iran and North Korea went hellbent into the development of nuclear weapons.

Really, Obama's foreign policy is like Carter's: Step 1, set a good example; Step 3, profit! We're collecting thousands of tons of spent nuclear fuel because of Carter's decision to stop recycling it. Because that would stop proliferation. Great plan, eh?

And, of course:
This is deeply worrying to many small nations who for half a century relied on the extended U.S. nuclear umbrella to keep them from being attacked or overrun by far more powerful neighbors. When smaller allies see the United States determined to move inexorably away from that posture -- and for them it's not posture, but existential protection -- what are they to think?
Obama's nuclear weapons policies are going from bad to worse. We're going to give away all of our best counters, while getting nothing in return. When a person acts like this in his personal life, he's called a rube. What is it called when he's the president?

Iran is about 15 months away from Day Zero,
At which point the world changes irrevocably: the regional Arab states go nuclear, the Non-Proliferation Treaty dies, the threat of nuclear transfer to terror groups grows astronomically....But not to worry. Canadian uranium is secured. A nonbinding summit communique has been issued. And a "Work Plan" has been agreed to.
Is attacking Iran necessarily the right plan? No. But telling our National Labs not to modernize our deteriorating stockpile, and telling our allies they can't depend on our umbrella anymore: that is necessarily not the right plan.

Book review: "Physics for future presidents"

"Physics for Future Presidents: the science behind the headlines," Muller.

The conceit is a little irritating, particularly because he only mentions it every 40 pages or so: You, the reader, will be president of the United States. This is what you need to know to understand the problems America faces, and how to explain them.

In reality, this book is "How to be an informed, non-wanker, contributor to modern society." His sections are (1) Terrorism (2) Energy (3) Nukes (4) Space (5) Global Warming. Why can't we require everyone to take -- and pass! -- this class before they can register to vote? Muller, a Berkley physics professor, is clearly a brilliant teacher. Let's start with a few quotes:

P.117, By the most pessimistic but credible estimates (i.e., not from Greenpeace), there may be 4,000 excess cancer deaths from Chernobyl. He then compares this to living in Denver, Colorado:
“A reasonable estimate is that the average yearly excess [radiation exposure] in Denver (compared to the US average)is about 0.1 rem per person per year. For 2.4 million people living in Denver for 50 years, this excess amounts t0 0.1 × 2,400,000 × 50 = 12 million rem, enough to cause 4800 excess cancers [using the unproven linear hypothesis]. That’s more death than is expected from the Chernobyl nuclear accident!”

P. 214, on the GRACE (Gravity Recovery and Climate Experiment) satellite, which measures the mass of the part of the Earth it is traveling directly over:
“These satellites recently found that the ice volume in Antarctica is decreasing by 36 cubic miles per year! That amazing result is important in the discussion of global warming, a topic we’ll return to….”

Then, on P. 282:
“… the melting-Antarctica results didn’t verify the global-warming predictions but actually contradicted them…. [the IPCC] had predicted that global warming would increase the Antarctic ice mass. It is easy to see why this paradoxical prediction makes sense: Global warming causes increased evaporation of the ocean waters. When this extra water vapor reaches Antarctica, it falls as snow – because even with the present 1°F of global warming, most of Antarctica remains well below freezing.”

P. 222, On the detonation of the Space Shuttle Columbia:
“…at 5 miles a second, every ounce of the shuttle, including its human cargo, carries over 10 times the energy of an equal weight of TNT.”

P. 267, on the problem with computer models of climate change:
“Amazingly, out poor understanding of cloud formation is responsible for the largest uncertainty in climate calculations.”

P. 299: Muller, who is not global warming skeptic, lays out the fallacies, cherry-picking, and outright errors in Al Gore’s movie, then:
“A colleague of mine told me that in a poll asking the public to name a living scientist, the person whose name appeared most often was Al Gore. I don’t know if this is true or apocryphal… When it is discovered [by the public] that Gore has exaggerated his case, the public may reject the truly scientific case for fossil fuel-induced global warming. To use an old cliché, I fear that the public will throw out the baby with the dirty bathwater.” [This was written and published well before climate-gate.]

P. 308, on fusion for power plants:
“Fusion will likely be the energy source of the twenty-second century.”
Fusion burns prodigious quantities of tritium, which has to be made in a reactor (fission or fusion). However, even with clever engineering, the most you could hope for, in the case of 100% efficiency, is to produce two tritiums in the fusion reactor blanket per tritium burned. I don’t believe the engineering is ever likely to hit that level, so I think controlled fusion power is doomed. (Googling "tritium trick" mostly gives hits about an unrelated materials science experimental method.)

Muller says a few things I don't like. For starters, he says a fast-breeder could blow up in a high-yield explosion. I can't prove he's wrong, but I'm going to do some more research on that topic in hopes he is -- I love the fast reactor as a concept. In his chapter on solutions to global warming, he entitles a sub-section "Safe Nukes." Okay, I call bullshit on that: the nukes we have now are safe.

But anyway, quibbles notwithstanding, I totally endorse this book to anyone who is (1) interested in the science behind the headlines and (2) doesn't mind a non-mathematical approach.