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Nickel forms the basis of a large class of materials called superalloys that have good mechanical strength and good creep properties at high temperatures. These alloys are not used extensively in thermal power reactors because they absorb the slow neutrons needed to maintain the nuclear chain reactions. As a result of the neutron absorption they become highly radioactive, which can impede maintenance. When nuclear reactors were first designed and built the effect of neutron irradiation on the core materials after many years of operation was largely unknown. Accelerated tests were therefore conducted in liquid metal fast reactors having radiation damage rates that were an order of magnitude higher than conventional power reactors. For nickel alloys, however, the adverse effect of neutron irradiation in a power reactor environment compared with a fast reactor environment has not been fully appreciated until recently. Thermal reactor components examined in the Chalk River Laboratories hot cells have exhibited lower strength and ductility than expected compared to most irradiation-hardened materials. In order to understand the observations one has to consider the unique situation that arises because one is dealing with: (i) a nickel-rich alloy; (ii) a reactor with a spectrum that promotes large changes in nickel. Inconel X-750 contains about 70 wt. % nickel. Of this approximately 68% is the isotope Ni-58. This isotope transforms to Ni-59 when irradiated with thermal neutrons. The Ni-58(n, γ) reaction itself creates atomic displacements that are over and above what one would normally see in a fast reactor test, but more significantly the Ni-59 that is produced from this reaction has very high cross-sections for interactions with neutrons over a wide range of neutron energies. The Ni-59 undergoes three reactions with neutrons, (n, γ), (n, p), and (n, α). The latter two reactions release very large amounts of energy and thereby cause a very high level of atomic displacement as well as producing very high levels of hydrogen and helium. Measurements of irradiated material show significant He levels and indications of some retained H. Transmission electron microscopy (TEM) has been conducted on Inconel X-750 core components removed after 9.4 and 11.15 years of full power service. These particular components are coiled springs made from wire that has a square cross section and widths of 1 and 0.7 mm respectively.

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Over the past 10 years, internally pressurized capsules made from Zr-2.5Nb tubing have been irradiated in the National Research Universal (NRU) reactor at Chalk River Laboratories at temperatures of 280, 320, and 340°C and dose rates between 3 × 1015 n · m−2 · s−1 and 2 × 1017 n · m−2 · s−1 (E > 1 MeV). Periodic gaging has been used to assess the primary and secondary (steady-state) creep behavior. The objective of this detailed and controlled experiment was to determine, for the first time, the creep and microstructure evolution in Zr-2.5Nb tubing over a wide range of irradiation conditions for fast neutron fluxes applicable to a CANDU pressure tube. Similar but “accelerated” creep experiments have been conducted in the Osiris test reactor at fast neutron fluxes of approximately 1.8 × 1018 n · m−2 · s−1 (E > 1 MeV), much greater than the neutron fluxes in the NRU reactor. Although accelerated tests in high-flux reactors such as Osiris provide information on irradiation creep, they do not represent the neutron flux conditions applicable to a power reactor. Tests covering power reactor operating conditions are needed to develop models for in-reactor creep of pressure tubes under the appropriate conditions. The data from the NRU reactor are compared with results from creep capsules with similar starting microstructures but irradiated in the Osiris reactor. The results show that the steady-state diametral and axial creep rates have a complex dependence on stress, temperature, and fast neutron flux. Data from out-reactor creep tests on unirradiated and pre-irradiated creep capsules that show the effect of prior irradiation on creep are also reported. The results are discussed in terms of a combination of creep mechanisms involving dislocation glide and mass transport.

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