Theory of pulsed irradiation microstructures

Flux pulsing is predicted to influence microstructures because of pulse annealing effects on aggregate annealing and on point defect annealing. Specifically void annealing and vacancy annealing are analyzed. A high dislocation density enhances the void annealing effect but inhibits the point defect annealing effect. Furthermore, pulsing effects are greater for higher damage rates and smaller defect aggregates. It is concluded that pulsed ion irradiations can simulate pulsed fusion irradiations if it is recognized that the ions exaggerate the expected neutron effect.     

The need for improved temperature control during reactor irradiation

The irradiation conditions of materials irradiated in fission reactors are nearly always insufficiently described in the literature. A carful inspection of temperature-reactor power histories reveals a deficiency in the conventional control of irradiation temperatures. In particular, a short transient irradiation at a lower temperature commonly occurs during the startup of the reactor. A large difference in the final defect structure is expected to be caused by this transient irradiation from the mechanism of the defect structure development. An electron irradiation with a similar transient irradiation leads to a remarkably different defect microstructure. The defect structures introduced by a fission neutron irradiation with this transient was compared with those produced by a fusion neutron irradiation with perfect temperatrue control without transient. The difference in structures was found to be much greater than what is normally expected between fission and fusion neutron irradiations. An essential improvement in the control of reactor irradiations is proposed.     

Color center annealing and ageing in electron and ion-irradiated yttria-stabilized zirconia

We have used X-band electron paramagnetic resonance (EPR) measurements at room-temperature (RT) to study the thermal annealing and RT ageing of color centers induced in yttria-stabilized zirconia (YSZ), i.e. ZrO₂:Y with 9.5 mol% Y₂O₃, by swift electron and ion-irradiations. YSZ single crystals with the 1 0 0 orientation were irradiated with 2.5 MeV electrons, and implanted with 100 MeV ¹³C ions. Electron and ion beams produce the same two color centers, namely an F⁺-type center (singly ionized oxygen vacancy) and the so-called T-center (Zr³⁺ in a trigonal oxygen local environment) which is also produced by X-ray irradiations. Isochronal annealing was performed in air up to 973 K. For both electron and ion irradiations, the defect densities are plotted versus temperature or time at various fluences. The influence of a thermal treatment at 1373 K of the YSZ single crystals under vacuum prior to the irradiations was also investigated. In these reduced samples, color centers are found to be more stable than in as-received samples. Two kinds of recovery processes are observed depending on fluence and heat treatment.     

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.     

The effect of lattice and grain boundary diffusion on the redistribution of Xe in metallic nuclear fuels: Implications for the use of ion implantation to study fission-gas-bubble nucleation mechanisms

A multi-atom gas bubble-nucleation mechanism has been proposed as part of a predictive fission-gas release model for metallic nuclear fuels. Validation of this mechanism requires experimental measurement of fission-gas bubble-size distributions at well-controlled gas concentrations and temperatures. There are advantages to carrying out such a study using ion implantation as the source of gas atoms compared with neutron irradiations. In spite of previous successes using ion implantation to study fission-gas behavior in oxide fuels, there is significant uncertainty about the efficacy of using ion beams for metallic fuel studies. To address the question of the applicability of ion beams in experiments designed to study fission-gas behavior in metallic fuels, we developed and applied an exact model for the redistribution of implanted ions under annealing conditions. The conclusion is that, given the assumptions, the results from implantations at 1 MeV or less may be overwhelmed by the surface effects at all relevant temperatures. Implanting at 10 or 80 MeV can significantly diminish the influence of the surfaces and the steep concentration gradients. At 80 MeV, the location of the peak concentration profile remains stable with annealing time. Thus, it appears that ion implantation can be an appropriate tool to study the size distribution of Xe bubbles in metallic fuels. Of the conditions investigated, the best for the study are to implant at 80 MeV and carry out anneals at 773 K, 873 K, and 973 K for times less than 10,000 s.     

Thermal and Injection Annealing of Neutron-Irradiated p-Type Silicon between 76°K and 300°K

Measurements of minority carrier lifetime damage constant and divacancy growth following neutron irradiation at 76°K have been used to characterize further the annealing of neutron damage in silicon below 300°K. It has been shown that electron injection into p-type silicon at 76°K causes recovery of the neutron induced defect clusters with the simultaneous appearance of divacancies. Comparison of isochronal annealing curves of damage constant taken with and without prior injection at 76°K illustrates the nature of cluster annealing below 300°K. The thermal annealing results are shown to agree with previous annealing measurements of the carrier removal rate.     

The effect of primary recoil spectrum on radiation induced segregation in nickel-silicon alloys

Segregation of silicon to the surface of Ni-12.7 at% Si alloys during 2.0-MeV He and 3.25-MeV Kr irradiations was measured using Rutherford backscattering spectrometry. For equal calculated defect production rates the Kr irradiation was < 3 % as efficient as the He irradiation for promoting segregation in the temperature range, 450 °C–580 °C. It was further observed that Kr preirradiation of specimens dramatically reduced segregation during subsequent He irradiation. A model for cascade annealing in Ni-Si alloys is presented which qualitatively explains the segregation results. The model assumes that small interstitial-atom-clusters form in individual cascades and that these clusters become trapped at silicon solute atoms. The vacancy thereby becomes the more mobile defect. The model should also have relevance for the observation that void swelling in nickel is suppressed by the addition of silicon solute.     

DLTS Studies of Neutron Camage in P-Type Silicon

Damage produced in p-type silicon by neutron irradiation at room temperature was studied by deep-level transient spectroscopy (DLTS). The production of three defects (Ev + 0.15, Ev + 0.34 and Ev + 0.45 eV) by neutron irradiation and the formation of two defects (Ev + 0.25 and Ev + 0.21 eV) during annealing were reported. It was found that many properties of the neutron damage were similar to those of the electron damage obtained by other works as far as the DLTS measurements are concerned. The DLTS measuements indicated no evidence for the production of defect clusters in the neutron damage. By comparing with the results previously obtained by the Hall effect measurements, it was found that the DLTS measurements mainly evaluated the properties of the point defects outside the clusters. The fact that the DLTS measurements did nto reflect the properties fo the defect clusters was ascribed to the reduction of majority carrier capture by defects inside the clusters due to the potential barrier formed by the cluster-space-charge regions.     

Dislocation loop evolution under ion irradiation in austenitic stainless steels

A solution annealed 304 and a cold worked 316 austenitic stainless steels were irradiated from 0.36 to 5 dpa at 350 °C using 160 keV Fe ions. Irradiated microstructures were characterized by transmission electron microscopy (TEM). Observations after irradiation revealed the presence of a high number density of Frank loops. Size and number density of Frank loops have been measured. Results are in good agreement with those observed in the literature and show that ion irradiation is able to simulate dislocation loop microstructure obtained after neutron irradiation.Experimental results and data from literature were compared with predictions from the cluster dynamic model, MFVIC (Mean Field Vacancy and Interstitial Clustering). It is able to reproduce dislocation loop population for neutron irradiation. Effects of dose rate and temperature on the loop number density are simulated by the model. Calculations for ion irradiations show that simulation results are consistent with experimental observations. However, results also show the model limitations due to the lack of accurate parameters.     

Correlation of neutron and heavy-ion damage: I. The influence of dose rate and injected helium on swelling in pure nickel

Displacement damage structures in pure nickel at the 1 dpa level are compared for two widely disparate damage rates, 10⁻⁷ dpa/s for neutron irradiations and 3 X 10⁻³ dpa/s for self-ion bombardments over a range of temperatures spanning those for void formation. Peak swelling at about 0.7% is found at 400° and 600°C, respectively. At equivalent swelling temperatures, voids in the ion-bombarded material are larger and fewer than those from neutron irradiation, especially at temperatures above the peak swelling temperature.Additions of 20 appm He, matching that generated in the neutron irradiations, were made to the ion-bombarded nickel either prior to ion bombardment (preinjection) or during ion bombardment (simultaneous injection). This helium caused increased swelling at the upper and lower temperature extremes. Simultaneously implanted helium did not otherwise significantly affect microstructures, whereas preinjected helium increased the dislocation density and caused more but smaller voids over the full temperature range of swelling.     

Synthesis of atom probe experiments on irradiation-induced solute segregation in French ferritic pressure vessel steels

Microstructural changes due to neutron irradiation cause an evolution of the mechanical properties of reactor pressure vessels (RPV) steels. This paper aims at identifying and characterising the microstructural changes which have been found to be responsible in part for the observed embrittlement. This intensive work relies principally on an atom probe (AP) study of a low Cu-level French RPV steel (Chooz A). This material has been irradiated in in-service conditions for 0–16 years in the frame of the surveillance program. Under this aging condition, solute clustering occurs (Cu, Ni, Mn, Si, P, …). In order to identify the role of copper, experiments were also carried out on Fe–Cu model alloys submitted to different types of irradiations (neutron, electron, ion). Cu-cluster nucleation appears to be directly related to the presence of displacement cascades during neutron (ion) irradiation. The operating basic physical process is not clearly identified yet. A recovery of the mechanical properties of the irradiated material can be achieved by annealing treatments (20 h at 450°C in the case of the RPV steel under study, following microhardness measurements). It has been shown that the corresponding microstructural evolution was a rapid dissolution of the high number density of irradiation-induced solute clusters and the precipitation of a very low number density of Cu-rich particles.     

熔盐/辐照环境中细结构等静压石墨的结构和化学稳定性研究

熔盐堆以石墨作为反射体和慢化体,熔盐与石墨直接接触,石墨在熔盐中的腐蚀反应和辐照损伤是值得研究的问题。本文采用自主研发的细结构石墨,阻隔熔盐浸渗,采用30 MeV He+模拟中子辐照,研究不同温度及熔盐环境下石墨微观形貌、微结构和化学结构的变化。研究结果表明,高温环境下,由于高温的退火效应,石墨缺陷密度的增加及形貌的变化都远小于室温环境。辐照后的石墨与熔盐接触,其缺陷密度略微降低。这种微结构的改善与高温熔盐环境中的退火效应及熔盐固化引起内部微裂纹的闭合有关。辐照后的熔盐浸泡可在石墨C—C键结构中引入C—F键,且C—F键的形成与缺陷密度及缺陷类型密切相关。稳定的空位簇及间隙原子的迁移均会影响层间化合物的形成,从而产生限制C—F键形成的环境,进而降低由层间化合物的形成对石墨表面结构的破坏。     

Annealing Characteristics of Neutron Irradiated Silicon Transistors

When a transistor is subjected to neutron irradiation, a component of base current proportional to neutron fluence is induced. From the effects of annealing on the base and collector currents, the conclusion was drawn that there is an apparent difference in the annealing characteristics between the neutral and the space-charge regions of the semiconductor device. This study of the anomalous annealing indicates that the neutron-induced component of base current is a result of one, or a combination, of the following mechanisms: a quasi-tunneling recombination phenomena in the emitter-base space-charge region, or an influence of the p-n junction electric field on the formation, annealing, and electronic behavior of the neutron-induced defect centers. A field dependence of the formation and annealing of the neutron-induced defects appears to be present both during the introduction and annealing of the neutron-induced defect centers. It could not be finally determined whether or not the quasitunneling phenomena occurred although it can be shown on theoretical grounds that it is possible for such phenomena to occur. The annealing characteristics of the defects, as represented by changes in the collector and base currents, have been obtained. Three sets of devices were irradiated and then annealed, with one set having a forward bias during annealing, one set having no bias, and one set having a reverse bias. The dependence of the field on annealing is present but appears quite complex.     

Irradiation temperature, flux and spectrum effects

Although great progress has been made in understanding the irradiation behaviour of reactor pressure vessel (RPV) steels, many aspects are still not fully understood. A large amount of data has been generated for understanding the effects of different irradiation conditions on material properties. The data needed for the long term operation of RPVs is almost always created by accelerated irradiations in test reactors, and due to insufficient knowledge on the damage interaction between the material and the high energy neutrons the potential bias of the conclusions on material properties in non-accelerated irradiation conditions can not be excluded. Important parameters for the extrapolation of results from accelerated irradiations to typical power irradiation conditions are the irradiation temperature, the neutron flux and the neutron spectrum. In particular, the effect of neutron flux on embrittlement behaviour is considered a complex phenomenon, and it seems to be dependent on the alloy composition, the neutron fluence range and the irradiation temperature. This paper will present the current knowledge on temperature, flux and spectrum effects, based on a recent literature survey and other relevant publications on the subject. It will explore the implications these effects may have for the safety evaluation of aged RPVs, especially for those exposed to long irradiation periods.     

Neutron Irradiation Effect on Magnetic Properties of Cr-, Mo- and Si-Doped FeNi-Alloys

Sample of FeNi, FeNi: Cr, FeNi: Mo, and FeNi: Si alloys were irradiated by neutron beams in the vicinity of a 2 MW reactor core and their magnetic hysteresis curves, magnetic after effects as well as variations of magnetic permeability with temperature were determined. From these curves Curie temperature shift, spectrum widthk, and activation energy for self-diffusion of Fe and Ni atoms were obtained.

The presence of Si impurities in the FeNi alloy produces a considerable attenuation in the vacancy supersaturation and the analysis of the spectrum width in this sample indicates that the defect structure due to neutron irradiations is more complex than that due to thermal effect.     

Electrical Properties of Neutron-Irradiated Silicon at 76°K: Hall Effect and Electrical Conductivtty

Defects produced in p-type silicon by neutron irradiation have been investigated using electrical conductivity and Hall effect measurements at 76°K. Samples from crucible-grown (1, 10, and 50 ohm-cm) and float-zone (10 ohm-cm) boron-doped silicon were irradiated at 76°K with nearly fission spectrum neutrons and annealed isochronally between 76° and 700°K. The electrical properties of neutron-irradiated p-type silicon exhibit an illumination dependence similar to that observed previously in n-type silicon and attributed to the presence of defect clusters. Therefore, neutron-produced changes in the electrical properties of p-type silicon also are attributed to defect clusters. The sensitivity to illumination was observed for all resistivities and for both crucible and float-zone p-type silicon. The illumination-induced conductivity decays slowly with time at 76°K and influences the thermally produced changes in the electrical properties upon annealing for temperatures up to 150°K. Neutron-produced changes in the electrical properties measured after annealing to 150° K are found to be qualitatively consistent with an insulating void model for cluster-space-charge regions. The best modeling of the experimentally observed changes is for the 50 ohm-cm (large void volume) silicon. The annealing loss of the light sensitive defects occurs in diffuse stages between 150° and 550°K with the largest stage between 150° and 240°K. A major fraction of the hole mobility annealing parallels that for the light sensitive defects and suggests that the mobility change is caused primarily by defect clusters.     

Defect production in simulated cascades: Cascade quenching and short-term annealing

Defect production in displacement cascades in copper has been modeled using the MARLOWE code to generate cascades and the stochastic annealing code ALSOME to simulate cascade quenching and short-term annealing of isolated cascades. Quenching is accomplished by using exaggerated values for defect mobilities and for critical reaction distances in ALSOME for a very short time. The quenched cascades are then short-term annealed with normal parameter values. The quenching parameter values were empirically determined by comparison with results of resistivity measurements. Throughout the collisional, quenching and short-term annealing phases of cascade development, the high energy cascades continue to behave as a collection of independent lower energy lobes. For recoils above about 30 keV the total number of defects and the numbers of free defects scale with the damage energy. As the energy decreases from 30 keV, defect production varies with the changing nature of the cascade configuration, resulting in more defects per unit damage energy. The simulated annealing of a low fluence of interacting cascades revealed an interstitial shielding effect on depleted zones during Stage I recovery.     

Study of a neutron irradiated reactor pressure vessel steel by X-ray absorption spectroscopy

Reactor pressure vessel (RPV) reference steel samples submitted to neutron irradiations followed by thermal annealing were investigated by X-ray absorption fine structure (XAFS) spectroscopy. Several studies revealed that Cu and Ni impurities can form nanoclusters. In the unirradiated sample and in the only-irradiated sample no significant clustering is detected. In all irradiated and subsequently annealed samples increases of Cu and Ni atom densities are recorded around the absorber. Furthermore, the density of Cu and Ni atoms determined in the first and second shells around the absorber is found to be affected by the irradiation and annealing treatment. The comparison of the XAFS data at Cu and Ni K-edges shows that these elements reside in arrangements similar to bcc Fe. However, the local irradiation damage yields vacancy fractions which were determined from the analysis of XAFS data with a precision of ∼5%.