E-Journal of Advanced Maintenance Vol.9-2 (2017) 33-37
Japan Society of Maintenology

Training for a wider application of atom probe tomography in reactor
ageing management
Dongyue CHEN 1,*, Kenta MURAKAMI2, Zhengcao LI3, *, Naoto SEKIMURA1
1

The University of Tokyo, Dept. of Nuclear Engineering and Management, JapanThe University of Tokyo, Nuclear Professional School, School of Engineering, Japan
3
Tsinghua University, School of Materials Science and Engineering, China
2ABSTRACTWith the higher safety requirements and the rising public concern of reactor safety, ageing management ofnuclear power plants are facing new challenges in recent years. Multiscale modeling is the tool for ageing
prediction during and beyond reactor design lifetime. It is important to update the latest predictions to existing
licensed reactors continuously, however, experimental data at microscopic scale is lacking for the validation andcalibration of modeling results. Atom probe tomography (APT) is a new and powerful analyzing tool withnear-atomic resolution. It is good at studying issues like second-phase precipitation and grain boundary cracking,and its 3D atom map is highly suitable for the comparison with modeling. APT is promising for future reactor
ageing management and maintainology, but only a small fraction of researchers can utilize the new techniquecurrently. In this work, training for potential APT users is proposed and designed, so that more researchers
could benefit from APT technique in the future. The advantages and the basic mechanisms of APT areintroduced. The possible errors that are most important to APT users and modeling researchers are explained. A
practice section is also designed, which enables the participants to experience the visualization of atom maps
and the tuning of precipitate definition. The concept, structure and experience of this training could be a goodstarting point for the design and organization of better APT training in the future.
KEYWORDS

ARTICLE INFORMATION

Ageing management, atom probe tomography, training

Article history:
Received 7 November 2016
Accepted 27 April 2017

1.

Introduction

The ageing management of nuclear power plants (NPPs) is the fundamental basis of reactor safe
long-term operation. In effective ageing management, the degradation of structures, systems andcomponents (SSCs) should be considered throughout the design, construction, commissioning,
operation and decommissioning phases of NPPs.
Although ageing management has been the focus of reactor safety for several decades, it is facing
new challenges in recent years with the higher safety requirements and the rising public concern. TheFukushima Daiichi accident in 2011 also rang the alarm that our previous knowledge and practices onreactor ageing management may not be proper and sufficient. The ageing management challenges
during and beyond reactor design lifetime are most important. First, for the safe operation during
design lifetime, technical obsolescence is one big issue. Technical obsolescence means the SSCs inoperating reactors and their related knowledge, standards and rules are becoming out of date incomparison with current knowledge and technology [1]. To mitigate obsolescence, a back-fitting
system is necessary for the continuous research on ageing prediction and the continuous update oflatest ageing knowledge to existing licensed reactors. Secondly, the prediction beyond reactor design
lifetime is another challenge. By the statistics of IAEA PRIS program, more than 250 NPPs in theworld have been operated for 30~47 years by now (Nov 18th, 2016) [2]. Therefore, they are facing thechoice of life extension or decommissioning. The decision making of license renewal should be based*Corresponding author, E-mail: dychen@safety.n.t.u-tokyo.ac.jp, zcli@tsinghua.edu.cn

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

33

D. Y. CHEN, et al./ Training for a wider applicationof atom probe tomography in reactor ageing managementon the ageing prediction beyond design lifetime. However, the prediction curve beyond lifetime could
not be simply extrapolated, and it requires a lot of research works and databases.
As a conclusion, the effective ageing management during and beyond reactor design lifetime
heavily relies on the accurate and up-to-date prediction of SSC behavior. The prediction uncertainty
should be continuously reduced with latest scientific and technical knowledge. Currently, multiscalemodeling is the major tool to make the prediction. Therefore, continuous efforts should be made to
improve the models.
However, the available experimental data for the validation of multiscale modeling is limited,
especially at microscopic scales. Fig. 1 shows a schematic of multiscale modeling tools commonly
applied in ageing prediction. At larger scales, the accumulated reactor operating data and mechanicalproperty databases can be used as the references for finite elements and dislocation dynamics
modeling. On the contrary, when the spatial scale or the time scale is small, it may be difficult toobtain enough experimental data to compare with modeling results. Especially, at the scale fromseveral nanometers to several hundreds of nanometers, different types of electron microscopy (EM)are the major tools for the direct observation of irradiation-induced defects, while EM has limited
sensitivity in distinguishing elements and second phases [3, 4]. Therefore, the experimental data about
irradiation-induced segregation and depletion is lacking. Without such data, it would be difficult tomodel and predict second-phase precipitation and grain boundary cracking phenomena in reactor
components.

Fig. 1. A schematic of multiscale modeling for ageing prediction

2.

Atom Probe Tomography in Ageing Management

Atom probe tomography (APT) is a new technique developed in recent years. Unlike EM, APT is
not sensitive to lattice defects, but it has high element sensitivity both for light and heavy elements.
APT can give the quantitative data about atom type and atom position of each atom that it detects.
After APT finishes the analysis of one specimen, it can reconstruct a 3D atom map of the specimen atnear-atomic spatial resolution with all the atoms it detected. Obviously, such kind of 3D atom map issuitable for the direct comparison with molecular dynamics (MD) or Monte Carlo (MC) modeling
results, for example in the study of precipitation [5, 6] and grain boundary depletion [7].
APT has been introduced into the field of reactor ageing management for only around 20 years,
but it has already made its contributions to the improvement of reactor maintainology. One example isthat APT has helped to clarify the irradiation hardening mechanism in low-Cu reactor pressure vessel(RPV) steels. Distinct irradiation hardening had been observed in low-Cu RPV steels at high fluences[8] and had confused the researchers. It was not until the 2000s that the formation of fine Mn-Ni-Si
precipitates in low-Cu RPV steels were confirmed by APT [9, 10], which explained the source ofhardening. Further APT study found that the Mn-Ni-Si precipitates are important to RPVembrittlement especially beyond 1019n/cm2 fluence. Therefore, APT has helped to find a new ageing
mechanism for the long-term operation of pressurized water reactors (PWRs). Recently, someresearchers have started trying the direct comparison of precipitate composition between APT andatomic kinetic MC modeling in order to calibrate modeling results [11].
APT is such a powerful and promising tool in future ageing management practices, however, it iscurrently deployed in very limited number of research institutes. Only few researchers and engineers
have the chances to benefit from this new technique. Besides, the 3D atom maps of APT contain alarge amount of information, while the APT results published in journals and reports could only reveal

34

D. Y. CHEN, et al./ Training for a wider applicationof atom probe tomography in reactor ageing managementa small part of it. If more people can get familiar with the new technique, it is certain that APT will
make more contributions to the improvement of reactor maintainology. Therefore, education andtraining for potential APT users, such as university students and young researchers, is proposed anddesigned in this work. The training is not designed to talk about everything about APT. The
fundamental knowledge that is most important to potential APT users is discussed here, which
outlines the basic requirements for a qualified APT user who can use APT technique and can utilize
APT data published by other researchers. It is hoped that the concept, structure and experience of thistraining could be a good starting point for the design and organization of better APT training in thefuture. The first training has been successfully performed on Dec 15th, 2016 in Japan, and is planned
to be held again late in this year with further improvement.

3.

Training Design

Since the training is not aiming at cultivating APT experts, the training time should be relatively
short, ranging from several hours to several days, so that more potential trainees could be able toparticipate. Generally speaking, the training is designed to show what APT can do, how it does, andwhat errors it may have. It is divided into two main sections, lecture and practice. Lecture section isthe traditional way of teaching in classrooms, delivering the basic knowledge of APT; while practicesection is going to give the participants a chance to visualize and post-analyze the 3D atom map bythemselves with the tools we provided. Fig. 2 lists the contents designed for the training, and they are
discussed in detail below.

Fig. 2. Design of APT training contents

3.1

Advantages of APT

The advantages of APT have been described above in section 2. The advantages will beintroduced to the participants so that they could think of APT technique when they are in need of APT
in the future. In order to leave a vivid impression to the participants, some examples of how APT has
solved the problems in reactor maintainology will be given.
3.2Basic APT knowledge

The detection mechanism of APT will be introduced in this section. The local electrode atom
probe (LEAP) will be taken as an example for mechanism explanation, as it is the latest type of APTcurrently. The participants need to know how atom species is distinguished by atom charge-to-mass

35

D. Y. CHEN, et al./ Training for a wider applicationof atom probe tomography in reactor ageing management

ratio, and how charge-to-mass ratio is determined by time-of-flight of the atom. The detailed 3D map
reconstruction process may be complicated for the beginners, but since it is the key step to define thex, y, z positions of each atom, some basic knowledge will also be introduced. Participants are notsupposed to be able to perform APT experiments by themselves; however, with such knowledge inmind they should better understand the meaning of APT data. The knowledge introduced here would
also make it easier for participants to understand the possible errors in APT in the next section.
The methods to prepare APT needle specimens will also be mentioned, as the specimen
preparation step is usually the bottleneck of APT experiments. The pros and cons of using focused ion
beam (FIB) method and electro-chemical polishing method will be briefly discussed.
3.3

Limitations of APT

APT is a power tool, whereas it is also a new tool still under rapid development. The current APT
technique has its limitations, which are important both for the people doing APT experiments and forthe people using APT data for modeling. In this section, the possible errors of APT are discussed.
Although the 3D atom map obtained by APT has near-atomic spatial resolution, the accuracy is
not always as good as it may look like. The spatial accuracy can be influenced by the inevitable
thermal vibration of the atoms, the sample tip shape which cannot be ideally spherical, the different
evaporation rate between different phases, and so on. Secondly, the element definition could be
another source of error. Elements are distinguished by charge-to-mass ratio, however, the peaks could
overlap and the background noise could be quite large. Finally yet importantly, since APT is often
applied in precipitation study, the definition of precipitates can greatly alter the output of precipitate
density, radius and composition, etc [12]. Training participants can experience the effects ofprecipitate definition in the next section by analyzing the precipitates by themselves.
3.4

Experience precipitate analysis

Since APT is often utilized for precipitation study and the selection of precipitate definition could
so much distinctively change the output, errors brought up by precipitate definition are the major
concerns of potential APT users. In this training, participants have the chance to experience the effects
of precipitate definition themselves. The following set of tools is prepared for this practice section.
 A freeware for 3D visualization of the atom maps. The open-source, multi-platform software
ParaView can be a good choice for atom map visualization.
 A free code for precipitate searching in the atom maps. A code based on the maximum separation
algorithm is developed by us for precipitate searching with tunable parameters, such as maximum
separation distance dmax, minimum number of atoms in precipitate Nmin, etc.
 A simulated atom map. The simulated atom map is used first instead of the actual atom map
because the simulated atom map can offer a simple and straightforward situation for easy
understanding. Only one or two precipitates are included in the simulated atom map for simplicity.
Different simulated atom maps are created by us for the understanding of dmax, Nmin, and themeaning of Guinier radius.
 An actual atom map created by APT. The participants can see what APT technique can really offerwith an actual atom map. They can visualize it, and can tune the precipitate definitions toexperience how they affect the outcomes.

4. Conclusion
To face the new challenges in ageing management, continuous updates on the current practices ofmaintainology is necessary. Multiscale modeling is the tool for ageing prediction during and beyond
reactor design lifetime, however, experimental data at microscopic scale is lacking for the continuous
improvement of modeling.
APT is a new and powerful analyzing tool with near-atomic resolution, and is sensitive inelement distinguishing. It is suitable for the comparison with modeling, and thus is promising for36

D. Y. CHEN, et al./ Training for a wider applicationof atom probe tomography in reactor ageing management

future reactor ageing management and maintainology. In this work, training for potential APT users isproposed and designed, so that more researchers could benefit from this novel technique in the future.
The essential knowledge necessary for a qualified APT user has been outlined. The advantages andthe basic mechanisms of APT were introduced. The possible errors that are most important to APT
users and modeling researchers were explained. A practice section is also designed, which enables theparticipants to experience the visualization of atom maps and the tuning of precipitate definition. Ourattempts in designing and organizing the training could provide references for future APT training,which is a necessary step to widely spread the novel APT technique.

Acknowledgement
The authors would like to thank Dr. Naoki Soneda, Dr. Kenji Dohi and Mr. Kenji Nishida in Central
Research Institute of Electric Power Industry for their helpful advices on designing the training
program.

References[1] IAEA: "Ageing Management for Nuclear Power Plants", Vienna, ISBN 978-92-0-112408-1 (2009).
[2] IAEA: "Operational Reactors by Age", http://www.iaea.org/PRIS/ (Accessed by Nov 18th, 2016)[3] T. Hamaoka, A. Nomoto, K. Nishida, K. Dohi, N. Soneda: "Accurate determination of the number density ofG-phase precipitates in thermally aged duplex stainless steel", Philos Mag, Vol.92, No.22, pp.2716-2732(2012).
[4] D. Y. Chen, K. Murakami, K. Dohi, K. Nishida, N. Soneda, Z. C. Li, L. Liu, N. Sekimura: "Depth
distribution of Frank loop defects formed in ion-irradiated stainless steel and its dependence on Siaddition", Nucl Instrum Meth B, Vol.365, pp.503-508 (2015).
[5] C. S. Becquart, C. Domain: "Introducing chemistry in atomistic kinetic Monte Carlo simulations of Fe alloys
under irradiation", Phys Status Solidi B, Vol.247, No.1, pp.9-22 (2010).
[6] L. Liu, K. Nishida, K. Dohi, A. Nomoto, N. Soneda, K. Murakami, Z. C. Li, D. Y. Chen, N. Sekimura:"Effects of solute elements on hardening and microstructural evolution in neutron-irradiated andthermally-aged reactor pressure vessel model alloys", J Nucl Sci Technol, Vol.53, No.10, pp.1546-1553(2016).
[7] T. Toyama, Y. Nozawa, W. Van Renterghem, Y. Matsukawa, M. Hatakeyama, Y. Nagai, A. Al Mazouzi, S.
Van Dyck: "Grain boundary segregation in neutron-irradiated 304 stainless steel studied by atom probe
tomography", J Nucl Mater, Vol.425, No.1-3, pp.71-75 (2012).
[8] G. R. Odette, G. E. Lucas: "Recent progress in understanding reactor pressure vessel steel embrittlement",
Radiat Eff Defect S, Vol.144, No.1-4, pp.189-231 (1998).
[9] M. K. Miller, A. A. Chernobaeva, Y. I. Shtrombakh, K. F. Russell, R. K. Nanstad, D. Y. Erak, O. O. Zabusov:"Evolution of the nanostructure of VVER-1000 RPV materials under neutron irradiation and post
irradiation annealing", J Nucl Mater, Vol.385, No.3, pp.615-622 (2009).
[10] K. Fukuya: "Current understanding of radiation-induced degradation in light water reactor structuralmaterials", J Nucl Sci Technol, Vol.50, No.3, pp.213-254 (2013).
[11] R. Ngayam-Happy, C. S. Becquart, C. Domain: "First principle-based AKMC modelling of the formationand medium-term evolution of point defect and solute-rich clusters in a neutron irradiated complex
Fe-CuMnNiSiP alloy representative of reactor pressure vessel steels", J Nucl Mater, Vol.440, No.1-3,pp.143-152 (2013).
[12] P. D. Styman, J. M. Hyde, K. Wilford, G. D. W. Smith: "Quantitative methods for the APT analysis ofthermally aged RPV steels", Ultramicroscopy, Vol.132, pp.258-264 (2013).

37