This discussion has used neutron irradiations for illustration purposes. Reactors provide an effective instrument for achieving high neutron exposures under conditions relevant for most nuclear applica­tions. However, reactor irradiations suffer from many difficult-to-control and, sometimes, uncontrolled variables. The neutron energy spectrum is responsi­ble for large differences in irradiation effects between different reactors.

The mechanism of atomic displacement is well understood.15 With a known neutron energy spec­trum, neutron atomic displacements can be calcu­lated as a function of fluence for a given reactor. Transmutation of elements in the material under study, which is a strong function ofneutron spectrum, results in wide variation in some mechanical proper­ties. This is of particular importance in applying fission reactor results to fusion. In a fusion device, helium and hydrogen will be generated through (n, a) and (n, p) reactions in nearly all common structural materials. Hydrogen has a very high diffusivity in metals so that an equilibrium concentration will be
established at a level that is believed to be benign.16 By contrast, helium is insoluble in metals, segregating at grain boundaries and other internal surfaces and discontinuities.

Although helium is produced in all nuclear reac­tors, the thermal spectrum is responsible for the highest concentrations. The largest contributors to helium in a thermal reactor are boron and nickel by the following reactions:

10B(n, a)7Li

58Ni(n, g)59Ni 59Ni(n, a)56Fe

Boron is present as a trace element in most alloy­ing elements but only at ppm levels. Nickel is a major constituent of many alloys and a minor con­stituent of still others. The two nickel reactions constitute a two-step generation process for helium, which starts slowly and accelerates as 59Ni builds up in the alloy, limited only by the supply of 58Ni, which for practical purposes is often unlimited. In austenitic alloys, the high flux isotope reactor (HFIR) has generated over 4000 appm He in austenitic stainless steels. The generation rate is so high that multistep absorber experiments have been conducted to reduce the helium generation rate to that characteristic of fusion reactors, 12 appm He per dpa in austenitic stainless steels.17 (see Chapter 1.06, The Effects of Helium in Irradiated Structural Alloys).

Other transmutation products may also compli­cate reactor irradiation studies. Examples are the transmutation of manganese to iron by the follow­ing reaction: 55Mn (n, g) 56Mn! 56Fe and the trans­mutation of chromium to vanadium by 50Cr (n, g) 51Cr! 51V. The first reaction leads to loss of an alloy constituent, and the second leads to doping with an extraneous element. However, neither of these reactions has been shown to significantly affect mechanical properties of steels.18

Helium remains the most studied transmutation product, and it can have profound effects on tensile properties, especially at high temperatures. Experi­ments have been conducted in various reactors throughout the world to assess the effects of helium on mechanical properties of alloys.19 An interesting result is that helium has little effect on strength. This is illustrated in Figure 5 where a comparison has been made between austenitic steels irradiated in Rapsodie, a fast spectrum reactor, and steels irradiated in HFIR, a mixed-spectrum reactor with a very high thermal flux. The saturation yield strength of all alloys remains within a single scatter band.20,21

The tramp impurity elements sulfur and phos­phorus have significantly high (n, a) cross sections at high energies, as shown in Figure 6. Although the cross section for phosphorus is large only at energies characteristic of fusion, a boiling water reactor pro­duces 500 appm He from sulfur and 40 appm He from phosphorus in eight years of operation. An Liquid Metal Fast Breeder Reactor (LMFBR) can produce 100 times these concentrations. All these elements are expected to enhance embrittlement when seg­regated to grain boundaries, but it remains to be determined which is more detrimental, helium, sul­fur, or phosphorus.