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Atom Probe Tomography has been performed on as-irradiated and post-irradiation annealed surveillance weld samples from Ringhals Unit 3. The weld contains low Cu (0.07at.%) and high Ni (1.5at.%). A ...

We have undertaken a combined experimental and modelling study of plasticity development in samples containing notches and cracks in a reactor pressure vessel steel that exhibits the Luders phenomenon in uniaxial testing. We have used digital image correlation to study the development of plasticity in plane sided, waisted and notched samples of a mild steel that exhibits Luders behaviour. We have developed a constitutive model for use within elastic–plastic finite element analyses of structural geometries made from materials which exhibit Luders strain behaviour, and validated it against experimental data. The model is capable of describing plasticity development in smooth sided, waisted and notched specimens. With the assistance of the model we have determined that Luders behaviour affects the detailed development of plasticity not only in smooth sided and waisted specimens, which is readily observed experimentally, but also in sharp notched specimens. In sharp notched specimens the plastic zone is more constrained and stress intensification higher in the presence of Luders behaviour.

A model has been produced to describe irradiation damage in high nickel submerged arc welds (1.6% Ni, 1.5% Mn, 0.5% Mo). It is fitted to a database of hardness change results obtained for 12 such welds with copper contents from about 0.02 to about 0.6% irradiated in materials test reactors. Irradiation dose rates ranged from about 6×10−9 dpa/s down to about 1×10−10 dpa/s. In addition to the hardness change data, Charpy shift data were also available. For some specimens irradiation induced changes in microstructure have been characterised using small angle neutron scattering (SANS) and energy compensated position sensitive atom probe (ECoPoSAP) techniques. The model was empirically fitted to the data, but available physical understanding and the evidence from the microstructural studies were used to guide selection of the equations fitted. Physical understanding and microstructural evidence were also used to help choose between alternative models with similar statistical goodness of fit. The model has two components. First, ‘matrix’ damage, which is insensitive to copper content and irradiation dose rate, and which appears to increase with phosphorus and sulphur. Second, ‘precipitation damage’, which increases with copper and manganese content, and reduces with increasing silicon. Precipitation damage also increases with reducing irradiation dose. The ECoPoSAP data show that the precipitates are composed of primarily of Ni, Mn, Cu and Si, with a substantial amount of iron entrained within them. In the low copper welds the same instrument shows significant compositional fluctuations in these elements, but no visible clusters. The Charpy data have been used to develop an empirical correlation between transition shift and hardness change. This is linear up to about 250 °C shift.

The effect of irradiation on microstructure of four irradiated reactor pressure vessel steels (a low copper A533B-1 plate, a low copper A508-3 forging, a high copper Linde 80 flux weld and a high copper Linde 1092 flux weld) was determined by using complementary microstructural techniques such as optical position-sensitive atom probe (OPoSAP), field emission gun scanning transmission electron microscopy (FEGSTEM) and small angle neutron scattering (SANS). In the low copper steels, irradiation resulted in small shifts in transition temperature and small changes in hardness increments. The microstructural analyzes showed that this response was dominated by matrix damage. In contrast, both copper-enriched clusters and matrix damage formed in the high copper welds. This information was then used as input to the Russell–Brown model to predict the change in hardness resulting from copper-enriched clusters. The calculated hardness increments were found to be consistent with the experimental data.

The principles are reviewed which underlie trend curve development for irradiated ferritic Reactor Pressure Vessel steels where the dominant embrittlement mechanism is cluster hardening. The extent to which the form of the equations employed reflect the underlying physical mechanism is assessed.

Displacement cascades produced by energetic lattice recoils are the primary damage state in neutron-irradiated metals, among which austenitic stainless steels are an important class of technological alloys. The vacancy constituent of this damage that survives in the form of collapsed vacancy loops is a major component of this damage state, and has been investigated in the present work as a function of alloy composition and the temperature. This has been done by irradiating single crystal foils of a range of high-purity model ternary Fe - xNi -15% Cr alloys ( x = 15–70%) with heavy ions, and then analysing the resulting damage by transmission electron microscopy. A full analysis has been achieved by measuring the areal density of loops and their size distribution for each irradiation condition, and hence obtaining the important parameters defect yield and collapse efficiency. By appropriate choice of ion energy, ion mass, ion dose and specimen temperature, we have been able to vary the factors such as cascade size, cascade energy density, cascade defect density and cascade overlap, and thus study their influence on cascade collapse. The results have been assessed in terms of current models of cascade processes in pure metals and alloys.

The irradiation-induced changes occurring at the nanometer level in the microstructure of the reactor pressure vessel have been shown to be responsible for a degradation in mechanical properties that occur in large structures with dimensions in the tens of metres. These changes can place severe restrictions on the reactor both at the startup and under continual operation; and in the long term could compromise the safe operation of the plant. The large financial and safety related matters have given the necessary driving force to study the factors controlling the microstructural evolution, resulting in a multi disciplinary approach to the problem. This paper aims to review the current understanding of the subject, giving were possible examples of the approach and techniques used to obtain this. We also highlight areas of current research activity and indicate the type of work still required to provide information on aspects that currently lack a full understanding.

In this paper we review the approach employed in the UK to characterise the embrittlement of thermal reactor pressure vessels (RPVs). The experimental techniques developed to characterise the neutron energy spectrum and fluence are described; these include the use of multifoil activation packs and a direct damage monitor. The techniques for the retrospective assessment of the exposure of service irradiated components and the methods for the prediction of end-of-life doses in RPVs are discussed. Displacements per atom (dpa) is the main exposure and correlation parameter employed and the success of using it to measure damage dose is evaluated. A variety of techniques is used to characterise the changes in mechanical properties in both materials test reactors and power reactors. These include fracture toughness, Charpy impact and tensile testing. Mechanical properties inter-relationships are particularly helpful when evaluating the condition of irradiated materials; the Charpy-tensile, Charpy-hardness and Charpy-fracture toughness correlations are reviewed. An important aspect of research into materials behaviour in the UK has been the development of mechanistic models of the embrittlement phenomena. These models have been reasonably successful in the interpretation of embrittlement in copper-containing materials, particularly in rationalising data obtained at different dose rates. The approaches are summarised. Microstructural techniques based on transmission electron microscopy, small angle neutron scattering and atom probe/field ion microscopy have been developed to characterise the microstructure and microchemistry of unirradiated and irradiated copper-containing materials. The contributions these techniques have made to the understanding and prediction of embrittlement processes are described.

The cascade processes are discussed which potentially give rise to a dependence of microstructural evolution on the recoil spectra set up by a flux of fast particles. It is emphasised that the thermal spike phase of cascade development is central to these processes. Recent results from Molecular Dynamics simulations are presented to highlight the potential importance of interstitial clustering in cascades and the impact of cascades on pre-existing features of the microstructure. A qualitative discussion is presented of the likely importance on microstructural evolution at low and high doses of the different cascade processes.

Irradiation damage structures induced in α-iron by low-dose fisson-neutron irradiation at reactor ambient temperature have been investigated by transmission electron microscopy. Materials of three purities were irradiated to doses between 1022 and 1024 neutrons (with E > 1 MeV) m−2 in the PLUTO experimental reactor at Harwell. In a high-purity iron no visible damage was observed below a dose of 8 × 1022 neutrons m−2. Above this dose the damage was restricted to the decoration of in-grown dislocations and low-angle boundaries by dislocation loops. No visible damage was found in the less pure materials after irradiation to doses

The influence of small amounts of Cu on the neutron irradiation induced embrittlement of reactor pressure vessel steels is of considerable practical importance. Previous work has shown that the embrittlement is associated with the formation of copper rich precipitates but uncertainties remain regarding their composition and form. The present paper reports preliminary results from a study of such precipitates in solution treated and aged Fe-Cu alloys with additions of Ni and P, using a combination of atom probe analysis in a Field Ion Microscope (FIM) and Small Angle Neutron Scattering (SANS).

In this paper the progress made by the author and co-workers in transmission electron microscope studies of the irradiation damage structure induced in pure metals (Cu, Mo and α-Fe) and dilute Cu-Ge alloys by low-dose fission neutron irradiation will be reviewed. Neutron doses (>1 MeV) were generally 10 21 − 10 22 n/m 2 and irradiation temperatures were between 80° and 600° C. A common feature of the results in all the systems studied was that frequently the damage showed considerable spatial variation with the development of highly damaged regions around in-grown dislocations. The form of the damage between the dislocations depended sensitively on the metal, irradiation temperature and neutron dose. For example, in copper at elevated temperatures the damage was predominantly vacancy in nature, with a complex interdependence between stacking fault tetrahedra and voids. An extensive parallel study has been undertaken of cascade collapse in different metals as a function of material purity, irradiation temperature, and cascade parameters. The considerable insight these results have given into the neutron damage structures are discussed.

We have irradiated foils of single-crystal pure copper in the Harwell AEI EM-7 microscope at several temperatures from 140 to 320°C, and have measured interstitial dislocation loop densities and growth rates. We have interpreted the experimental results using the rate theory of microstructural evolution during radiation damage. We have achieved a good agreement between loop growth measurements and rate theory calculations only when using a very recently developed dislocation sink strength. In this case we predict a temperature-dependent dislocation bias for interstitials and a vacancy migration energy of about 0.75 eV, which is consistent with recent independent experimental data.

The role of the refractory metal alloys in relation to their possible application for future thermonuclear fusion reactor systems is considered in this review. Assessment of the availability and properties of the refractory metals indicates that alloys of vanadium, niobium and molybdenum are most likely to play a major role in reactor systems. The operating environment of a fusion reactor and the materials constraints that are imposed by radiation damage effects and plasma-wall interactions are considered briefly. The metallurgical aspects of the refractory metals and their alloys are then reviewed with a subsequent assessment of the existing database on the effects of neutron and ion irradiation on physical and mechanical properties. Consideration is also given to the effect of specific environments on alloy properties and to the industrial base and availability of refractory metal alloys. The review concludes with a brief summary in which it is proposed that alloys based on vanadium offer the best potential for long-term fusion reactor applications for reasons of low induced radioactivity and good structural stability under irradiation. It is further concluded that the current database for vanadium alloys is inadequate in respect of fusion reactor design, safety and environmental requirements and that extensive research will be necessary if the potential of these alloys is to be realized.

An electron microscope study has been made of damage in Type 316 stainless steel irradiated to low doses at room temperature with 40, 80, 120 and 200 keV Cr + ions. The visible clusters are dissociated Frank loops and stacking fault tetrahedra and it is deduced that they are vacancy in nature having formed by collapse of displacement cascades. Quantitative analysis of the structures have shown that the defect yield and cascade efficiency are approximately independent of incident ion energy and are much lower than in self-ion irradiated pure copper. This and the fact that reducing the level of carbon in solution in the stainless steel increases the cascade efficiency is attributed to the role played by the alloy elements and impurities in defocussing focussed collision sequences and thereby enhancing interstitial-vacancy recombination in cascades. Lastly, the present results are considered in terms of their implication to the formation and growth of voids during irradiation of Type 316 stainless steel at elevated temperatures.

The collapse of displacement cascades in pure gold at either 4.2 K or 285 K has been investigated by irradiating (011) single-crystal foils with Bi + ions having energy in the range 2.5 to 20 keV. Dose effects over the range 5 × 10 11 to 10 15 ions cm −2 have also been studied. Parameters such as the yield of collapsed cascades and the efficiency with which vacancies are retained during collapse have been determined by examination of vacancy loops in a transmission-electron microscope. The yield in gold is small, particularly at low temperature, and decreases with increasing dose due to cascade overlap effects. In cascades which do collapse, however, the efficiency is high and most vacancies survive in the resulting dislocation loops.

Zones of reduced long-range order created at displacement cascade sites in well-ordered Cu3Au may be directly imaged in the transmission electron microscope so that quantitative information can be obtained on individual cascade events. This technique has been used to characterise the cascade damage created by three fast particles (3.5 MeV protons, a source of moderated fission neutrons and a source of fusion neutrons with energies peaking at 14.8 MeV) with the aim of comparing the experimental observations with the relevant collision models. In each case, disordered zone number densities, sizes and shapes were determined, and were found to be characteristic of each irradiation, with the sizes of disordered zones and the proportion of zones of complex shape increasing on going from 3.5 MeV protons to fission neutrons to fusion neutrons. The quantitative results are largely consistent with the different calculated primary recoil spectra, although in the fusion neutron case some discrepancies are found which cannot readily be explained by limitations in the experimental technique. More specifically, more and larger disordered zones are found than expected from the calculated recoil spectrum. Subcascade formation was observed only in the neutron irradiations, with the distributions of sizes and shapes of individual sub-cascades being very similar in the two cases (in marked contrast to those obtained from sizing total cascade events). Finally, the production of point-defect clusters at cascade sites was studied. The efficiency of cascade collapse increased on going from 3.5 MeV protons to fission neutrons to fusion neutrons.