Microsc. Microanal. 23, 366–375, 2017 doi:10.1017/S1431927616012678
© MICROSCOPY SOCIETY OF AMERICA 2017
Analysis of Radiation Damage in LightWater Reactors: Comparison of Cluster Analysis Methods for the Analysis of Atom Probe Data
Jonathan M. Hyde,1,2 Gérald DaCosta,3 Constantinos Hatzoglou,3 Hannah Weekes,1 Bertrand Radiguet,3 Paul D. Styman,1,2,* Francois Vurpillot,3 Cristelle Pareige,3 Auriane Etienne,3 Giovanni Bonny,4 Nicolas Castin,4 Lorenzo Malerba,4 and Philippe Pareige3
1National Nuclear Laboratory, Culham Science Centre, Building D5, Abingdon, Oxfordshire OX14 3DB, UK 2Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK 3UNIROUEN, INSA Rouen, CNRS, Groupe de Physique des Matériaux, Normandie Université, 76000 Rouen, France 4Studiecentrum voor Kernenergie—Centre d’Etudes de l’Energie Nucléaire (SCK—CEN), Institute of Nuclear Materials Science, Expert Group of Structural Materials, Boeretang 200, B-2400 Mol, Belgium
Abstract: Irradiation of reactor pressure vessel (RPV) steels causes the formation of nanoscale microstructural features (termed radiation damage), which affect the mechanical properties of the vessel. A key tool for characterizing these nanoscale features is atom probe tomography (APT), due to its high spatial resolution and the ability to identify different chemical species in three dimensions. Microstructural observations using APT can underpin development of a mechanistic understanding of defect formation. However, with atom probe analyses there are currently multiple methods for analyzing the data. This can result in inconsistencies between results obtained from different researchers and unnecessary scatter when combining data from multiple sources. This makes interpretation of results more complex and calibration of radiation damage models challenging. In this work simulations of a range of different microstructures are used to directly compare different cluster analysis algorithms and identify their strengths and weaknesses.
Key words: atom probe field ion microscopy, statistical analysis, solute clustering INTRODUCTION
Most of the operating nuclear reactors in the world are pressurized water reactors (PWRs). In a PWR, the reactor pressure vessel (RPV) is the second barrier between the fuel and the outside world. They are made of low alloyed bainitic steels (NiMoCr, NiMoCrV, A-533B). During service, neu- trons produced in the reactor core generate displacement damage, resulting in a supersaturation of vacancies and self- interstitial atoms (SIAs). These supersaturated point defects (PDs) can agglomerate to form extended defects, but also enhance and modify solute diffusion causing solute redis- tribution in the material. This irradiation ageing is respon- sible for hardening and nonhardening embrittlement of RPV steels and the degradation can be life limiting for nuclear reactors. As irradiation damage occurs at the nm-scale, atom probe tomography (APT) (Miller et al., 1996; Miller, 2000; Gault et al., 2012) is a suitable tool to characterize irradiation-induced nanofeatures in terms of their nature, chemical composition, size, shape, and number density. Small clusters or precipitates containing Cu, Mn, Ni, Si,
and P are often observed using APT (Pareige et al., 1997; Miller et al., 2000, 2007; Carter et al., 2001; Radiguet et al., 2009; Huang et al., 2014). The chemical composition of these clusters, and their evolution with neutron fluence, depends
*Corresponding author.
paul.styman@
materials.ox.ac.uk Received July 4, 2016; accepted December 9, 2016
on many variables including bulk composition and detailed irradiation conditions (e.g., temperature and flux). However, the reported chemical compositions also depend on artifacts inherent to APT, the methodologies used to characterize the APT data and the specific scientists involved in the work (e.g., through the choice of analysis parameters). The result has been a plethora of inconsistent nomenclatures. For instance, the terms copper rich precipitates, copper enriched clusters, manganese nickel silicon precipitates, manganese nickel precipitates, nickel silicon precipitates have been used interchangeably by different researchers (Auger et al., 1995; Odette, 1995; Miller & Russell, 2007; Pareige et al., 1997; Carter et al., 2001; Takeuchi et al., 2010; Wells et al., 2014). A consistent and robust approach to data analyses (and
also nomenclature) is necessary to enable direct comparison of APT data obtained in different laboratories, whereas enabling modelers to analyze the outcome of atomistic simulations consistently with experiments, allowing direct and fair com- parison. There are three issues to address. First, it is necessary to assess the intrinsic limitations of APT techniques, so that the relationship between microstructure observed using APT and the actual microstructure is understood. Second it is necessary to ensure that the cluster analysis algorithms provide an accu- rate and robust description of the irradiation-induced micro- structural features. Third, it is necessary to agree on protocols to be used for the analysis of, especially, atomistic model results, so as to enable a fair comparison with APT results.
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