Plant Materials
DOE-HDBK-1017/2-93
EFFECT DUE TO NEUTRON CAPTURE
Impurity atoms are produced by nuclear transmutations. Neutron capture in a reactor produces
an isotope that may be unstable and produce an entirely new atom as it decays. For most
metallic materials, long irradiations at high flux levels are necessary to produce significant
property changes due to the building of impurities. However, a semiconductor such as
germanium (Ge) may have large changes in conductivity due to the gallium and arsenic atoms
that are introduced as the activated Ge isotopes decay. In stainless steel, trace amounts of boron
undergo a (n,a) reaction that generates helium bubbles which lead to the deterioration of
mechanical properties.
Physical Effects of Radiation
The general physical and mechanical effects of the irradiation of metals by fast neutrons and
other high-energy particles are summarized in Table 1.
TABLE 1
General Effects of Fast-Neutron Irradiation on Metals
Property Increases
Property Decreases
Yield strength
Ductility
Tensile strength
Stress-rupture strength
NDT temperature
Density
Young's Modulus (slight)
Impact strength
Hardness
Thermal conductivity
High-temperature creep rate
(during irradiation)
For fast neutrons, the changes are usually undetectable below certain radiation levels (fluences
below 1022 neutrons/m2). With increasing radiation levels, the magnitude of the effects increases
and may reach a limit at very large fluences. The effects listed in Table 1 are generally less
significant at elevated temperatures for a given fluence and some defects can be removed by
heating (annealing).
Both the yield strength and the tensile strength of a metal are increased by irradiation, as shown
in Table 2, but the increase in yield strength is generally greater than the increase in tensile
strength. At the same time, ductility is decreased by irradiation as shown in Figure 4, which
is representative of the behavior of many metals, including steel and zircaloy. The accelerated
decrease in the ductility of reactor vessels is due to the residual copper (Cu), phosphorous (P),
and nickel (Ni) content in the vessel steel.
Rev. 0
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MS-05
