A study on recovery kinetics of radiation damage in graphite

Through the measurements of galvanomagnetic properties, an annealing study has been made of a natural graphite compact irradiated with fast neutrons to a total dose of $\text{2.3 × 10}{}^{18}\text{n/cm}{}^2$. The isothermal annealing has revealed that the recovery taking place near 200°C consists of two atomic processes having different values of activation energy, 0.9 eV in the first stage and 0.45 eV in the second. The physical significance of those energy values is discussed with reference to the past experimental work. Discussions are also given of the isochronal annealing conducted over a temperature range up to 1000°C. However, the definite solution is still prevented of the mechanism, because of a discrepancy lying between theory and experiment on the migration energy of interstitial carbon atoms.     

Intergranular damage in AISI 316L(N) austenitic stainless steel at 600 °C: Pre-strain and multiaxial effects

This study is devoted to the effect of a multiaxial stress state and of pre-straining on the creep properties of an austenitic stainless steel. Creep tests on both smooth and notched specimens have been carried out on type 316L(N) steel at 600 °C. In comparison to the annealed state, pre-straining caused a substantial increase in creep lifetime but also a dramatic drop in intergranular damage resistance. The effect of a pre-strain on creep ductility was so strong that compact tension specimens in pre-strained state tested under relaxation conditions cracked, whereas specimens in annealed state were not prone to cracking. A model taking into account both pre-strain and multiaxial effects was developed and identified on the basis of local intergranular micro-cracks measurements on notched specimens. It satisfactorily predicts the results of relaxation crack propagation tests. This model may also provide a useful estimation of the relaxation cracking risk of 316L(N) as a function of pre-strain level and stress triaxiality ratio.     

The boronizing effect on the radiation shielding and magnetization properties of AISI 316L austenitic stainless steel

The boronizing effect on the radiation shielding properties and magnetization of AISI 316L austenitic stainless steel has been investigated. For this purpose the linear attenuation coefficients of steel have been measured at the photon energies of 662, 1170 and 1332 keV and the results were compared with the calculation at the photon energy of 1-10⁸ keV. It was clearly seen from this work that both the magnetization and radiation shielding properties of the steel have been improved by boronizing process.     

Swelling and radiation creep of austenitic corrosion-resistant steel irradiated by neutrons in wide dose and temperature ranges

The results of a study of the swelling and in-reactor creep of EI-847, EP-172, and ChS-68 austenitic steel after irradiation in materials science assemblies in the range 330–700°C and damaging dose 20–96 dpa are presented. The temperature dependences of the volume change of steel were obtained from measurements of the diameter of unloaded ampuls. It is shown that the swelling of the steel increases linearly with increasing tangential stress. The modulus of in-reactor creep in the interval 410–630°C for the steel investigated in the cold-deformed state varies in the range (0.5–3)·10^–6 MPa^–1·dpa^–1. For lower and higher temperatures, the creep modulus increases to (5–8)·10^–6 MPa^–1·dpa^–1.     

The role of niobium carbide in radiation induced segregation behaviour of type 347 austenitic stainless steel

The effect of niobium carbide precipitates on radiation induced segregation (RIS) behaviour in type 347 stainless steel was investigated. The material in the as-received condition was irradiated using double-loop 4.8 MeV protons at 300 °C for 0.43 dpa (displacement per atom). The RIS in the proton irradiated specimen was characterized using double-loop electrochemical potentiokinetic reactivation (DL-EPR) test followed by atomic force microscopic examination. The nature of variation of DL-EPR values with the depth matched with the variation of the calculated irradiation damage (dpa) with the depth. The attack on grain boundaries during EPR tests was negligible indicating absence of chromium depletion zones. The interface between niobium carbide and the matrix acts as a sink for point defects generated during irradiation and this had reduced point defect flux toward grain boundaries. The attack was noticed at a few large cluster of niobium carbide after the DL-EPR test at the depth of maximum attack for the irradiated specimen. Pit-like features were not observed within the matrix indicating the absence of chromium depletion regions within the matrix.     

The development of validation methodology of multi-axial creep damage constitutive equations and its application to 0.5Cr0.5Mo0.25V ferritic steel at 590 °C

Research progress on the development of validation methodology for multi-axial creep damage constitutive equations and its specific application to 0.5Cr0.5Mo0.25V ferritic steel at 590 °C is presented. A set of new phenomenological multi-axial creep damage constitutive equations was proposed aiming at overcoming the deficiency of inconsistency between predicted rupture strains and observed ones. Based on these explicit consistent requirements, an improved validation methodology is proposed and applied to 0.5Cr0.5Mo0.25V ferritic steel at 590 °C. It shows that the predictions of this new set of constitutive equations are consistent with experimental observations. It also reveals a significant difference in creep curves between different sets of constitutive equations and the need for experimental data so that the coupling of damage and creep deformation can be further examined.     

Cracking in stabilized austenitic stainless steel piping of German boiling water reactors—characteristic features and root cause

Cracks have been found in the welds of piping systems made from stabilized austenitic stainless steels in German boiling water reactors (BWR). In the course of the intensive failure analysis metallographic examinations, microstructural investigations by electron microscopy, corrosion experiments and welding tests have been performed. The results show that cracking under the given medium conditions is due to intergranular stress corrosion cracking (IGSCC) in those parts of the heat affected zone (HAZ) which are overheated during welding and where solution of titanium carbides and subsequent precipitation of chromium carbides and depletion of chromium along the affected grain boundaries could occur.     

Effect of aging at 700 °C on precipitation and toughness of AISI 321 and AISI 347 austenitic stainless steel welds

A detailed knowledge of changes in microstructures and mechanical behaviour that occur in austenitic stainless steels with or without Nb/Ti-stabilized weld during heat treatment is of great interest, since the ductility and toughness of the material may change drastically after long aging times. Two kinds of materials, i.e. AISI 321 base and without Ti-stabilized weld steel and AISI 347 base with Nb-stabilized weld steel, were compared during aging at 700 °C up to 6000 h. Both materials present increased amount of precipitate and decreased impact energy as the aging time increases. The decreased extent of impact energy with aging is almost the same for both base materials. However, it presents differences for 347 and 321 weld samples. The latter shows a more drastic decrease of impact energy than the former due to the different amount of precipitates. 321 weld sample precipitates more numerously than 347 weld sample due to the absence of stabilized Ti/Nb on the former. Large amount of carbides is formed on 321 weld sample immediately after welding. The carbides are transformed to sigma phase, which is mainly responsible for the much more sigma phase precipitation compared with other samples, after high-temperature aging. The fractographs showed, in general, brittle fracture mode in 321 weld impact-fractured specimens after aging at 700 °C for 6000 h. However, other samples show ductile fracture mode in general. Several approaches should be employed to control sigma phase precipitation in weld material. These approaches include: decreasing content of ferrite and M₂₃C₆ carbide in weld and selecting Nb added weld wire during welding.     

Annealing of neutron damage in graphite irradiated and stored at room temperature

Nimonic PE16, a gamma-prime Ni₃(Al,Ti) precipitate-strengthened alloy under consideration for fast reactor structural applications, has been neutron irradiated in three heat treatment conditions: solution treated, aged, and overaged. After irradiation at 600° C to 5.4 × 10²²n/cm² (E > 0.1 MeV), or 27 dpa, specimens were characterized for gamma-prime precipitate stability by transmission electron microscopy. The precipitate microstructures after irradiation reflected the influence of the preirradiation heat treatment; and indeed the precipitate particles present prior to irradiation remained stable. However, additional precipitation occurred during irradiation in each of the specimens examined. The in-reactor gamma-prime precipitation process decorated such microstructural features as voids, dislocations and carbide precipitates. Examples were found in the solution-treated condition where gamma prime in the form of Archimedes' screws had precipitated on climbing screw dislocations. The precipitation behavior observed is compared with predictions from existing models. It is concluded that models for solute diffusion to point-defect sinks and for Ostwald coarsening can account for the observations, but that the models for precipitate stability controlled by cascade dissolution during neutron irradiation do not.     

Creep–fatigue interaction damage model and its application in modified 9Cr–1Mo steel

Creep–fatigue interaction damage evolution of the nuclear engineering materials modified 9Cr–1Mo steel is studied with Continuum Damage Mechanics (CDM) theory. Based on the Norton creep damage and fatigue dissipate potential theory, an effective stress controlled creep–fatigue interaction damage model has been developed in this paper, in which the creep and fatigue damage function are both considered as nonlinear variables. The damage evolution function consists of the stress amplitude and the range of mean strain. The damage parameters in the model have clear physical meaning and can be determined easily. A stress controlled creep–fatigue interaction experiment has been performed with the P91 steel to obtain the damage model. The experimental results indicated that the damage model is applicable to describe the damage evolution for creep–fatigue interaction.     

Effect of electron excitation on radiation damage in fce metals

Defect production, radiation annealing and defect recovery are studied in several fcc metals (Al, Cu, Ni, Ag and Pt) irradiated with low-energy ( 1 MeV) and high-energy ( 100 MeV) ions. Irradiation of the metals with strong electron-lattice interaction (Al, Ni and Pt) by 100 MeV ions causes an anomalous reduction, or even a complete disappearance of stage-I recovery. This experimental result shows that the energy transferred from excited electrons to lattice atoms through the electron-lattice interaction contributes to the annihilation of single interstitials. This effect is also observed in Ni as a large cross section for radiation annealing, and a decrease of the damage efficiency. On the other hand, in Cu and Ag thin foils, we find that lattice defects are produced not only through elastic interactions, but also through a process strongly associated with electron excitation. In the latter process, the defect production cross section is proportional to S_e^1.7 in Cu and S_e^1.5 in Ag. The nearly quadratic dependence of the cross section on S_e suggests that the mutual Coulomb repulsion of ions positively charged by electron excitation causes the defect production.     

Influence of Zn as a spallation product on the behaviour of martensitic steel T91 and austenitic steel 316L in liquid Pb-Bi

The liquid Pb-Bi alloy is proposed as material for the spallation target in hybrid systems. During the spallation process, several chemical elements are produced in the target which could generate specific liquid metal embrittlement phenomena. Among these species, zinc is known as an element which can promote LME (liquid metal embrittlement). Corrosion tests were carried out in liquid Pb-Bi in isothermal static conditions without and with 80 wppm of zinc at 150 °C, 350 °C and 600 °C up to 6000 h. No modification of the corrosion kinetics of T91 martensitic and 316L austenitic steels was observed for either unstressed or U-bend specimens with zinc in Pb-Bi. Moreover, no sign of embrittlement was observed for any of the samples with and without zinc.     

Thermal expansion characteristics of a titanium modified austenitic stainless steel: measurement by high-temperature X-ray diffraction and modelling using Grüneisen formalism

The thermal expansion of a titanium modified, swelling resistant austenitic stainless steel designated as D9 is studied by measuring the lattice parameter as a function of temperature in the range 300-1300 K by high-temperature X-ray diffraction technique. The thermal expansion data thus obtained is in reasonable agreement with the typical thermal expansion values reported for similar nuclear grade austenitic stainless steels. However, at temperatures exceeding 900 K, the measured thermal expansivity exhibits a pronounced non-linear increase due partly to the precipitation of complex carbide and intermetallic phases. The high-temperature thermal expansion data obtained in the present study are augmented by modelling the low-temperature thermal expansion behaviour by Grüneisen formalism.     

The effect of Cr concentration on radiation damage in Fe-Cr alloys

Displacement cascades in Fe-Cr alloys were studied using molecular dynamics computer simulations. We considered random Fe-5Cr and Fe-15Cr alloys, as well as Fe-10Cr alloys with and without Cr-rich precipitates. In the simulations two versions of a two-band embedded atom method potential were used, and the cascades were induced by recoils with energies up to 20 keV. We found that the average number of surviving Frenkel pairs and the fraction of vacancies and self-interstitials in clusters was approximately the same in pure Fe and random Fe-Cr alloys (regardless of Cr concentration). A noticeable effect of the presence of Cr in the Fe matrix was only observed in the enrichment of self-interstitials by Cr in Fe-5Cr. The calculated change in the short range order parameter showed that Fe-5Cr tends towards ordering (negative short range order parameter) and Fe-15Cr towards segregation (positive short range order parameter) of Cr atoms. In simulations with the Cr-rich precipitate, enhanced cascade splitting and segregation of self-interstitial defects created inside the precipitates towards the precipitate-matrix interface region was observed. The number of Frenkel pairs and their clustered fraction was not affected by the presence of the precipitate.     

Influence of silicon on swelling and microstructure in Russian austenitic stainless steel EI-847 irradiated to high neutron doses

Void swelling and microstructural development of niobium-stabilized EI-847 austenitic stainless steel with a range of silicon levels were investigated by destructive examination of fuel pin cladding irradiated in three fast reactors located in either Russia or Kazakhstan. The tendency of void swelling to be progressively reduced by increasing silicon concentration appears to be a very general phenomenon in this steel, whether observed in simple, single-variable experiments on well-defined materials or when observed in multivariable, time-dependent irradiations conducted on commercially produced steels over a wide range of irradiation temperatures, neutron spectra and dpa rates. The role of silicon on microstructural development is expressed both in the solid solution via its influence on dislocation and void microstructure and via its influence on formation of radiation-induced phases that in turn alter the matrix composition. Surprisingly, increases in silicon level in this study do not accelerate the formation of silicon-rich G-phase, but act to increase the formation of Nb (C,N) precipitates. Such precipitates are known to be associated with delayed void swelling.