[en] Stainless steels used as cladding and structural materials in nuclear reactors undergo very pronounced changes in physical and mechanical properties during irradiation at elevated temperatures, often quickly leading to an increased tendency toward embrittlement. On a somewhat longer time scale there arise very significant changes in component volume and relative dimensions due to void swelling and irradiation creep. Irradiation creep is an inherently undamaging process but once swelling exceeds the 5-10% range austenitic steels become exceptionally brittle. Other processes also contribute to embrittlement and thereby contribute to difficulty in storing and handling of spent fuel assemblies removed from decommissioned fast reactors. In light water reactors other forms of embrittlement develop prior to reaching significant levels of void swelling. A review is presented of our current understanding of the radiation-induced changes in physical and mechanical properties that contribute to embrittlement

[en] A review is presented of our current understanding of those factors related to the radiation environment that must be considered in the conduct and analysis of LWR-relevant experiments on the in-service aging of stainless steels. The majority of the review focuses on the impact of spatial and temporal details of the neutron and gamma ray spectra, which are shown to vary more strongly than usually perceived. Problems associated with predictions of LWR behavior from fast reactor or charged particle data are also explored. Finally, an assessment is made of progress on previously identified and new outstanding problems, both microscopic and macroscopic in nature, identifying those problems close to resolution and those still requiring additional research

[en] A series of annealed and aged Fe-xCr, Fe-12Cr-yC and Fe-12Cr-0.1C-zMo model alloys were irradiated in EBR-II at eight temperatures between 400 and 650C and dose levels ranging from 35 to 131 dpa. Swelling-induced density changes observed in the binary alloys generally peaked at mid-chromium levels, with the chromium and temperature dependence expressed primarily in the duration of the transient regime. The steady state swelling rate at the lower irradiation temperatures was much higher than previous estimates, reaching ∼0.2%/dpa and possibly still climbing at higher neutron exposures. The dependence of swelling on molybdenum and carbon was more complex, depending on whether the temperature was relatively low or high. At temperatures of 482oC and above the effect of carbon additions was very pronounced, with swelling of Fe-12Cr jumping dramatically from near zero at 0.002%C to 6-10% at 0.1%C. This indicates that the major determinant of the composition and temperature dependence probably lies in the duration of the nucleation-dominated transient regime of swelling and not primarily in the steady-state swelling rate as previously envisioned. This raises the possibility that significant swelling may occur earlier in fusion and spallation neutron spectra where high gas generation rates may assist void nucleation.

[en] Pressurized tubes of AISI 316 stainless steel irradiated in the P-1 experiment in the EBR-II fast reactor have been measured to determine the dependence of irradiation-induced strains resulting from plastic deformation, irradiation creep, void swelling and precipitation. It is shown that the Soderberg relation predicting no axial creep strains in biaxially-loaded tubes is correct for both plastic and creep strains. Swelling strains are shown to be isotropically distributed both for stress-free and stress-affected swelling, while precipitation strains are somewhat anisotropic in their distribution. When corrected for stress-enhancement of swelling, the derived irradiation creep strains appear to be identical for both annealed and 20% cold-worked specimens, and also for tubes strained by rise to power increases in pressure. For relatively small creep strains it is often difficult to separate the creep and non-creep components of deformation

[en] The subject of fission-fusion correlation is usually cast in terms of reactor-to-reactor differences, but recently the fusion community has become aware of the impact of differences within a given surrogate facility, especially in constant time experiments when different dose levels are attained in different positions of one reactor. For some materials, it is not safe to assume that in-reactor spectral variations are small and of no consequence. This point is illustrated using calculations for fusion-relevant materials that were irradiated in the Fast Flux Test Facility - Materials Open Test Assembly (FFTF-MOTA) over a wide range of in-core and out of core positions spanning more than two orders of magnitude in dpa rate. It is shown that although both the neutron spectrum and flux changes, the spectral effectiveness factor, dpa/10(22) superscript n/cm(2) superscript (E > 0.1 MeV), remains remarkably constant over this range. The transmutation rate per dpa varies strongly with reactor position, however

[en] Neutron-induced microstructural evolution in response to long term irradiation at very low dose rates was studied for a Russian low-nickel austenitic stainless steel designated X18H9 that is analogous to AISI 304. The irradiated samples were obtained from an out-of-core support column for the pressure vessel of the BN-600 fast reactor with doses ranging from 1.7 to 20.5 dpa generated at 3.8 x 10^-9 to 4.3 x 10^-8 dpa/s. The irradiation temperatures were in a very narrow range of 370-375 C. Microstructural observation showed that in addition to voids and dislocations, an unexpectedly high density of small G-phase precipitates was formed that are not usually observed at higher dpa rates in this temperature range. A similar behavior was observed in a Western stainless steel, namely AISI 304 stainless steel, irradiated at similar temperatures and somewhat higher dpa rates in the EBR-II fast reactor, indicating that irradiation at low dpa rates for many years can lead to a different precipitate microstructure and therefore different associated changes in matrix composition than are generated at higher dpa rates. The contribution of such radiation-induced precipitation to changes in electrical resistivity was measured in the X18H9 specimens and was shown to cause significant deviation from predictions based only on void swelling.

[en] A comprehensive experimental investigation of microstructural evolution has been conducted on Fe-15Cr-16Ni irradiated with 4.0 MeV nickel ions in the High Fluence Irradiation Facility of the University of Tokyo. Irradiations proceeded to dose levels ranging from ∼0.2 to ∼26 dpa at temperatures of 300, 400 and 500 degrees C at displacement rates of 1 x 10^-4, 4 x 10^-4 and 1 x 10^-3 dpa/sec. This experiment is one of two companion experiments directed toward the study of the dependence of void swelling on displacement rate. The other experiment proceeded at seven different but lower dpa rates in FFTF-MOTA at ∼400 degrees C. In both experiments the swelling was found at every irradiation condition studied to monotonically increase with decreases in dpa rate. The microstructural evolution under ion irradiation was found to be very sensitive to the displacement rate at all three temperatures. The earliest and most sensitive component of microstructure to both temperature and especially displacement rate was found to be the Frank loops. The second most sensitive component was found to be the void microstructure, which co-evolves with the loop and dislocation microstructure. These data support the prediction that void swelling will probably be higher in lower-flux fusion devices and PWRs at a given irradiation temperature when compared to irradiations conducted at higher dpa rates in fast reactors.

[en] Electron paramagnetic resonance (EPR) and optical absorption spectra were measured during thermal annealing for stoichiometric MgAl2O4 spinel that was previously irradiated in FFTF-MOTA at ∼405 C to ∼50 dpa. Both F and F+ centres are to persist up to very high temperatures (over 700C), suggesting the operation of an annealing mechanism based on evaporation from extended defects Using x-ray irradiation following the different annealing steps it was shown that the optical absorption band is related to a sharp EPR band at g=2.0005 and that the defect causing these effects is the F+ centre

[en] Measurements are presented of electrical resistance and elastic moduli (Young's modulus and shear modulus) of stabilized austenitic fuel pin cladding after irradiation in the BN-600 reactor. Additional data are presented on changes in electrical resistivity of another stabilized austenitic steel irradiated in the BN-350. The elastic moduli are reduced and the electrical resistance is increased as the neutron dose increases. Dependencies of these changes in physical properties on neutron irradiation dose, temperature, and swelling level are plotted and it is shown that to the first order, the property changes are dependent on the swelling level, in agreement with earlier U.S. and Russian data. It is also observed, however, that changes in electrical resistance and elastic moduli frequently differ slightly for specimens with equal swelling, but which were obtained at different combinations of temperature and dose. These second-order differences appear to arise from contributions of other radiation-induced structural changes, especially in precipitation, which depends strongly on irradiation temperature in these stabilized steels.

[en] In several recently published studies conducted on a Soviet analog of AISI 321 stainless steel irradiated in either fast reactors or light water reactors, it was shown that the void swelling phenomenon extended to temperatures as low as ∼300 C or less, when produced by neutron irradiation at dpa rates in the range 10-7 to 10-8 dpa/sec. Other studies yielded similar results for AISI 316 and the Russian analog of AISI 316. In the current study a blanket duct assembly from BN-350, constructed from the Soviet analog of AISI 321, also exhibits swelling at dpa rates on the order of 10-8 dpa/sec, with voids seen as low as 281 C and only 0.65 dpa. It appears that low-temperature swelling occurs at low dpa rates in 300 series stainless steels in general, and also occurs during irradiations conducted in either fast or mixed spectrum reactors. Therefore it is expected that a similar behavior will be observed in fusion devices as well