Plant MaterialsDOE-HDBK-1017/2-93EFFECT DUE TO NEUTRON CAPTUREImpurity atoms are produced by nuclear transmutations. Neutron capture in a reactor producesan isotope that may be unstable and produce an entirely new atom as it decays. For mostmetallic materials, long irradiations at high flux levels are necessary to produce significantproperty changes due to the building of impurities. However, a semiconductor such asgermanium (Ge) may have large changes in conductivity due to the gallium and arsenic atomsthat are introduced as the activated Ge isotopes decay. In stainless steel, trace amounts of boronundergo a (n,a) reaction that generates helium bubbles which lead to the deterioration ofmechanical properties.PhysicalEffectsof RadiationThe general physical and mechanical effects of the irradiation of metals by fast neutrons andother high-energy particles are summarized in Table 1.TABLE 1General Effects of Fast-Neutron Irradiation on MetalsPropertyIncreasesPropertyDecreasesYield strengthDuctilityTensile strengthStress-rupture strengthNDT temperatureDensityYoung's Modulus (slight)Impact strengthHardnessThermal conductivityHigh-temperature creep rate(during irradiation)For fast neutrons, the changes are usually undetectable below certain radiation levels (fluencesbelow 1022neutrons/m2). With increasing radiation levels, the magnitude of the effects increasesand may reach a limit at very large fluences. The effects listed in Table 1 are generally lesssignificant at elevated temperatures for a given fluence and some defects can be removed byheating (annealing).Both the yield strength and the tensile strength of a metal are increased by irradiation, as shownin Table 2, but the increase in yield strength is generally greater than the increase in tensilestrength. At the same time, ductility is decreased by irradiation as shown in Figure 4, whichis representative of the behavior of many metals, including steel and zircaloy. The accelerateddecrease in the ductility of reactor vessels is due to the residual copper (Cu), phosphorous (P),and nickel (Ni) content in the vessel steel.Rev. 0Page 39MS-05
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business