Plant MaterialsDOE-HDBK-1017/2-93EFFECT DUE TO NEUTRON CAPTURESwelling can also result from gases produced in materials, such as helium formed by (n,a)reactions and other gaseous impurities present in the metals. These traces of gas increase theconcentration of voids formed upon exposure to radiation. For example, the (n,a) and (n,2n)reactions between fast neutrons and beryllium form helium and tritium gases that create swelling.Under certain conditions, embrittlement can be enhanced by the presence of the helium bubbles(helium embrittlement). The accepted view is that this embrittlement is the result of stress-induced growth of helium gas bubbles at the grain boundaries. The bubbles eventually link upand cause intergranular failure.Fissionable metals suffer from radiationFigure 6(a) Growth of Uranium Rod; (b) Uranium Rod Size Dummydamage in a manner similar to thatencountered in structural alloys.Additional problems are introduced bythe high energy fission fragments and theheavy gases xenon and krypton, whichappear among the fission products. Twofragments that share 167 MeV of kineticenergy, in inverse proportion to theiratomic masses, are produced from eachfission. Each fragment will have a rangeof several hundred angstroms as itproduces a displacement spike. A coreof vacancies is surrounded by a shell ofinterstitials, producing growth anddistortion. Figure 6 shows the growth ina uranium rod upon irradiation.The gas formation produces eventualswelling of the fuel and may place thecladding under considerable pressure aswell. One of the major challenges in alloying metallic uranium is the attainment of betterstability under irradiation. Small additions of zirconium have shown marked improvement inreducing growth and distortion.The physical effects of ionizing radiation in metals is a uniform heating of the metal. Ions areproduced by the passage of gamma rays or charged particles through the metal, causingsufficient electrical interaction to remove an external (or orbital) electron from the atom. Metalswith shared electrons, which are relatively free to wander through the crystal lattice, are effectedvery little by ionization.Rev. 0Page 43MS-05
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