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 . 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  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  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  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.  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.
 D. Butler: Nuclear power's new dawn. Nature V429, (2004), P.238-240.
 L. R. Greenwood: Neutron interactions and atomic recoil spectra. Journal of Nuclear Materials V216, (1994), P. 29-44.
 G. S. Was, Fundamentals of Radiation Materials Science. 2007, Springer, Berlin.
 Y. Guerin, G. S. Was and S. J. Zinkle: Materials Challenges for Advanced Nuclear Energy Systems. MRS Bulletin V34(1), (2009), P.10-14.