Fundamental Studies of Irradiation Effects in Fusion Materials
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for some time to come, particularly at the high exposures expected during the lifetime of conceptual fusion reactors. The neutron environment of D-T fusion reactors, irrespective of confinement geometry, will differ from fission environments by the presence of significant numbers of high energy neutrons. To make use of the existing data base and, using fission reactors, extend it to materials of special interest for fusion
applications requires a thorough understanding of the response of materials to high energy neutrons — in particular, how this response differs from fission environments. Neutron environments available for testing and those expected in fusion reactors differ significantly. Unlike simple D-T fusion, which produces nearly mono-energetic neutrons, fission and fusion reactors produce environments with a broad range of neutron energies. Figure 2 compares the spectrum at the "first wall" of a conceptual fusion reactor design with those of the two types of fission reactors used for high fluence materials studies. The spectrum shown for the peripheral target position in the High Flux Isotope Reactor, HFIR(PTP), is typical of thermal (mixed spectrum) reactors. The neutrons are about equally divided into three groups, a thermal neutron peak, a fast (fission) neutron peak and fully moderated neutrons with a 1/E spectrum between. The relative ratios of the three components vary from reactor to reactor and with position in a given reactor. The spectrum shown for Row 2 of the Experimental Breeder Reactor-II, EBR-II (Row 2) is typical of fast reactors. Here the fission neutrons are only slightly
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3.5 MeV
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14 MeV
Figure 1. Fusion of deuterium and tritium produce a neutron (14.1 MeV) and 3He (3.5 MeV) as products.
MRS BULLETIN/JULY 1989
Fundamental Studies of Irradiation Effects in Fusion Materials
moderated and only fast neutrons are present. The reaction is maintained by fission from fast rather than thermal neutrons. The energy at the peak and the width of the distribution will vary depending on position. The spectrum expected at the first wall of a commercial fusion reactor design (Starfire) consists of the high energy spike of 14 MeV source neutrons from the D-T reaction superimposed on a broad peak of neutrons returning from the blanket and shield. While all fusion reactor spectra will be similar, the detailed distribution of the returning neutrons will depend on blanket and shield design. Also shown for comparison is the spectrum expected outside the blanket and shield at the position of the superconducting magnets in the Starfire design. The fraction of high energy n e u t r o n s (E>5MeV) has decreased from 30% at the first wall to 5% at this position. Neutron interactions with materials can be classed into three categories which are roughly separated in energy: 1. Neutron absorbtion is the dominant m o d e of interaction at low a n d thermal energies. The (n,-y) interaction cross-section decreases as 1/En as neutron energy, E n , increases. The n e w nucleus, with its mass increased by o
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