Radiation damage in ferritic/martensitic steels for fusion reactors: a simulation point of view

Abstract

Low activation ferritic/martensitic steels are good candidates for the future fusion reactors, for, relative to austenitic steels, their lower damage accumulation and moderate swelling under irradiation by the 14 MeV neutrons produced by the fusion reaction. Irradiation of these steels, e.g. EUROFER97 which contains 8,9 % wt. Cr, 1,1 % W, 0,47 % Mn, 0,2 % V, 0,14 %Ta, 0,11 % C and Fe for the balance, is known to produce hardening, loss of ductility, shift in ductile to brittle transition temperature (DBTT) and reduction of fracture toughness and creep resistance starting at the lowest doses. The mechanisms underlying the degradation of mechanical properties are not well understood, despite numerous experimental and theoretical studies on the evolution of the microstructure under irradiation. This impedes our ability to predict and extrapolate materials' behavior at higher doses for use in the future fusion reactors. In this paper an overview on some simulations studies of irradiation induced damage accumulation and their impact on the mechanical properties is presented. In general these simulations describe so-called model alloys, as available theoretical tools cannot at this point describe real materials with their actual chemical composition. F/M steels are typically modeled by pure Fe, for it constitutes their basic structure. Helium (He), produced by transmutation by the fusion neutrons of 14 MeV, is known to impact mechanical properties, but its effect at the microstructural level is still unclear. Molecular dynamics simulations of the primary state of damage in Fe(He) and of the mobility of an edge dislocation in iron (Fe) are performed to study the effect of He, either as a gas in solid solution or in cavities.

The EU programme for modelling radiation effects in fusion reactor materials: An overview of recent advances and future goals

“…This was investigated in [74], where it was found that helium clusters (bubbles) do not give rise to substantial hardening, whereas the self-interstitial dislocation loops emitted by helium bubbles do represent strong obstacles for the propagation of dislocations. Only the helium bubbles characterized by high helium per vacancy ratio and hence approaching the threshold for the spontaneous emission of SIA loops are able to significantly impede the propagation of dislocations [64]. Simulations of cascades carried out assuming randomly distributed helium atoms did not show a significant effect of helium on the Frenkel pair production [75].…”

“…Simulations performed at low temperatures show that a moving dislocation line experiences strong resistance when encountering an SIA cluster, while the high temperature simulations show the partial dissolution of an SIA cluster (a nano-dislocation loop) by a moving dislocation. The significant part played by the screw component of a dislocation line was illustrated by a recent MD study of interaction between an edge dislocation and a cavity [64] where it was found that the obstacle strength was largely determined by the length of the screw segment formed just before the release of the dislocation from the obstacle.…”

Stability of helium bubbles in alpha-iron: A molecular dynamics study

“…However the atomistic properties of He in metal are difficult to identify experimentally. Thus atomistic simulations such as molecular dynamics provide useful tools to study the formation and the stability of these clusters and their impact on moving dislocations, vector of plasticity [2].…”

The formation energy and bonding characteristics of small helium–vacancy clusters on the low-index surface of α-Fe by first principles calculations

“…These helium bubbles give rise to hardening, embrittlement, and swelling of the materials. The interaction of He with other defects such as vacancies and SIA also needs to be investigated to complete our understanding of the effects of He on the mechanical properties of structural materials used in the nuclear reactor [1][2][3]. The effect of He on the mechanical properties of low activation ferrite-based steels has been widely studied both theoretically and experimentally.…”

“…The effect of He on the mechanical properties of low activation ferrite-based steels has been widely studied both theoretically and experimentally. It remains an important and unsolved issue towards the understanding of irradiation damage on the reactor structural materials [1][2][3].…”