F.A. Garner

Beam-contamination-induced compositional alteration and its neutron-atypical consequences in ion simulation of neutron-induced void swelling

ABSTRACT Although accelerator-based ion irradiation has been widely accepted to simulate neutron damage, neutron-atypical features need to be carefully investigated. In this study, we have shown that Coulomb force drag by ion beams can introduce significant amounts of carbon, nitrogen, and oxygen into target materials even under ultra-high vacuum conditions. The resulting compositional and microstructural changes dramatically suppress void swelling. By applying a beam-filtering technique, introduction of vacuum contaminants is greatly minimized and the true swelling resistance of the alloys is revealed and matches neutron behavior closely. These findings are a significant step toward developing standardized procedures for emulating neutron damage. GRAPHICAL ABSTRACT IMPACT STATEMENT An innovative ‘beam shaking’ technique is proposed to reduce beam-induced impurity contamination in accelerator-based ion irradiation. Without this technique, void swelling of ion-irradiated alloys can be greatly underestimated.

Investigation of Pitting Corrosion in Sensitized Modified High-Nitrogen 316LN Steel After Neutron Irradiation

The influence has been studied of thermo-mechanical treatment, sensitization conditions, and neutron irradiation on the pitting corrosion resistance of austenitic 316LN stainless steel variants in 10% FeCl3·6H2O at 22 °C. Variants of this steel were modified with additions of nitrogen, manganese, copper, and tungsten, as well as testing cast, cold-rolled, grain boundary engineered (GBE), and as-received variants. It was found that the 316LN steel variant with additions of 0.2% N and 2% Mn had the best pitting corrosion resistance of all studied conditions. When irradiated in a light water reactor (LWR) to a maximum fluence of 3 × 1017 n/cm2 (E > 1.1 meV, Tirr < 50 °C), neutron irradiation surprisingly increased the resistance of GBE steels to pitting corrosion. An anisotropy of corrosion resistance of GBE and cold rolled steels was observed.

Emulating Neutron-Induced Void Swelling in Stainless Steels Using Ion Irradiation

Self-ion irradiation is currently being used to explore the relative void swelling resistance of various candidate advanced alloys for a wide variety of nuclear systems, including light water reactor (LWR) reactors. The credibility of using this surrogate irradiation technique to evaluate potential in-reactor behavior requires that certain facets of neutron-induced behavior be reproduced in the ion simulation. Of particular importance is the ability of ion irradiation to produce the anticipated post-transient swelling rates for fcc and bcc iron-base alloys characteristic of reactor irradiation at ~1.0%/dpa and ~0.2%/dpa, respectively. Using a model duplex Fe-9Cr-C alloy irradiated at 450 ℃ with 8 MeV Fe+ ions it is shown in this study that a post-transient rate of ~0.2%/dpa is observed in the ferrite phase after an incubation period of ~60 dpa. It is also shown that the ferrite phase attains this rate first, while the tempered martensite phase exhibits a longer transient delay prior to the onset of high-rate swelling.

Carbon Contamination, Its Consequences and Its Mitigation in Ion-Simulation of Neutron-Induced Swelling of Structural Metals

Neutron-induced swelling in austenitic and ferritic steels is sensitive to the carbon level in the steel, as well as its distribution in matrix or precipitates. It has recently become known that ion-irradiation to high dpa levels leads to a progressive ion-beam-induced increase in carbon concentration and precipitation within the ion range, with concurrent reductions in void swelling. This neutron-atypical phenomenon imperils the credibility of ion simulation for light water reactor applications. A series of experiments involving pure iron and a structural alloy HT9, were conducted to identify the source and distribution of injected carbon. It was found that negatively-charged carbon atoms are entrained in the self-ion beam by a Coulomb drag effect, and thereby delivered at low drift energy to the irradiated surface, followed by ion-beam-mixing and diffusion. A technique for filtering out contaminants, especially carbon, oxygen and nitrogen, was developed and resulted in higher, more neutron-relevant swelling levels than achieved without filtering.