Project Subjects
Irradiation Embrittlement and Structural Integrity of Reactor Pressure Vessel
Background and Objective
The reactor pressure vessels (RPVs) of light water reactors (LWRs) are subjected to neutron irradiationthat causes changes in the mechanical properties of RPV steels. This phenomenon, known as neutron irradiation
embrittlement, is one of the most important issues that need to be accurately predicted for the long term and safe
operation of aged nuclear power plants. CRIEPI has been promoting a research project on understanding themechanism of embrittlement at the microstructural level, and has developed a new method to predict the amountof embrittlement, which has been adopted in Japan Electric Association Code, JEAC4201-2007. The improvementof embrittlement prediction capability at high ﬂuence regime as well as the development of advanced methods forthe evaluation of the structural integrity of RPVs are necessary for the expected long term operation of LWRs.
This project aims at understanding embrittlement mechanism at high ﬂuence regime for the validation andimprovement of current embrittlement prediction method, and at the development of new technology for reliablestructural integrity evaluation.

Main results
1. Effect of neutron ﬂux on embrittlement
Test reactor irradiation at high neutron ﬂux conditions is the only way to achieve high neutron ﬂuencelevels, in short irradiation time, that correspond to extended end-of-life conditions of RPV steels. CRIEPI has
studied, under a joint research project with the University of California, Santa Barbara, the effect of neutron ﬂux
on microstructural changes at the atomistic level by means of Atom Probe Tomography (APT) technique. It was
found that the size and number density of solute atom clusters formed during irradiation (see Fig. 1) are smaller
and higher at higher ﬂux conditions, respectively. Furthermore, the changes in such microstructures after postirradiation annealing are larger at higher ﬂux conditions as shown in Fig. 2. These results suggest that microstructures formed during irradiation are affected by neutron ﬂux [Q09021].
2. Computer simulation of microstructural evolution
Atomistic-scale computer simulation is a very promising tool to study the mechanism of solute atom cluster formation as observed by APT. In this study, ﬁrst-principles molecular dynamics calculations were performed
to calculate the interaction energies between a solute atom and a vacancy, and then the numerical simulationof cluster formation through the diffusion of solute atoms by vacancy mechanism. Cluster nucleation around avacancy and cluster growth by agglomeration of solute atoms were identiﬁed as shown in Fig. 3 [Q09002].
3. High precision hardness measurement by nano-indentation
Hardness is known as a good measure of the amount of embrittlement although the scatter in measured
hardness has often been a problem for precise evaluation of embrittlement. We investigated here the applicabilityof nano-indentation technique with a few grams weight and more than 1,000 measurements. The results show thatthe nano-hardness distribution in a 2mm x 2mm region has a one-to-one correspondence with the microstructure
as shown in Fig. 4, and a comparison of the histogram of hardness allows us to detect a small change in hardness
caused by neutron irradiation [Q09014].
Other reports
6[Q09029], [Q09026], [Q09015]

Nuclear TechnologyFig. 1 Atom map obtained by 3-dimensional Atom Probe measurement
54 million atoms are contained in the measured volume. Only Cu and Si atoms are displayed as orange and gray dots,respectively. Formation of solute atom cluster containing Cu atoms at very high number density is evident.

Fig. 2 Microstructural changes after annealing
Changes due to annealing are small at low ﬂux, while they
are large at higher ﬂux. This result suggests that the characteristics of microstructures are affected by ﬂux.

Fig. 3 Formation of solute atom clusters by diffusion of vacancies (Cu, Ni, Si and vacancy
are shown in green, blue, yellow and red
dots, respectively)
Clusters are formed around vacancies.

Fig. 4 Hardness distribution in 2mm x 2mm
region measured by nano-intentation
The harder regions that consist of yellow and red data
points contain many carbides indicating good correlation
between hardness and microstructure.

Fig. 5 Histograms of the hardness of neutron
irradiated reactor pressure vessel steel
Comparison of histogram allows us to measure smallchanges in hardness with sufﬁcient accuracy.

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