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Figure 5   Increase in NDT Temperatures of Steels from Irradiation Below 232C
Effect Due To Neutron Capture Summary

Material Science Volume 2 of 2
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Plant Materials DOE-HDBK-1017/2-93 EFFECT DUE TO NEUTRON CAPTURE Swelling  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 the concentration 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 up and cause intergranular failure. Fissionable  metals  suffer  from  radiation Figure 6     (a) Growth of Uranium Rod;      (b) Uranium Rod Size Dummy damage   in   a   manner   similar   to   that encountered in structural alloys. Additional  problems  are  introduced  by the high energy fission fragments and the heavy  gases  xenon  and  krypton,  which appear among the fission products.   Two fragments that share 167 MeV of kinetic energy,   in   inverse   proportion   to   their atomic  masses,  are  produced  from  each fission.   Each fragment will have a range of    several    hundred    angstroms    as    it produces  a  displacement  spike.    A  core of  vacancies  is  surrounded  by a  shell of interstitials, producing growth and distortion.   Figure 6 shows the growth in a uranium rod upon irradiation. The   gas   formation   produces   eventual swelling  of  the  fuel  and  may  place  the cladding  under  considerable  pressure  as well.    One  of  the  major  challenges  in  alloying  metallic  uranium  is  the  attainment  of  better stability under irradiation.   Small additions of zirconium have shown marked improvement in reducing growth and distortion. The physical effects of ionizing radiation in metals is a uniform heating of the metal.   Ions are produced  by  the  passage  of  gamma  rays  or  charged  particles  through  the  metal,  causing sufficient electrical interaction to remove an external (or orbital) electron from the atom.  Metals with shared electrons, which are relatively free to wander through the crystal lattice, are effected very little by ionization. Rev. 0 Page 43 MS-05







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