Embrittlement correlation methods Japan

The Japanese embrittlement correlation equations (JEAC4201-1991 ) were presented in the open literature in 1994. Similar to US Regulatory Guide 1.99, Revision 2, the mean predictive equations differ between base metal and welds. Japanese surveillance data as well as test reactor data and some US surveillance data were considered in order to make the data distribution homogeneous in the parameter space. The JEAC4201-1991 formulas are  

The most noticeable points are that there is a synergistic effect of Cu and Ni, a beneficial effect of Si in the case of welds, and a decreasing fluence exponent similar to US Regulatory Guide 1.99, Revision 2. 

In the development of a new correlation method, microstructural characterization of the surveillance materials of some PWR plants was also performed in order to understand the embrittlement mechanism of RPV steels with different Cu contents. At the same time, another test reactor irradiation project, the PLIM project, was also conducted by Japan Nuclear Energy Safety (JNES), where extensive microstructural characterization of base metals and weld metals with a wide range of chemical compositions in terms of Cu and Ni was performed using APT, transmission electron microscopy and positron annihilation to obtain new insights with the embrittlement mechanism at high fluences.The mechanism of embrittlement identified or confirmed in these projects was summarized as follows  

Soneda et al. tried to translate the above conclusions into mathematical equations. They developed a set of time-evolution equations to estimate the microstructural evolution as follows  

Since the details of these equations are explained elsewhere, only key ideas are briefly described here. One of these is to classify the solute atom clusters into irradiation-induced clusters and irradiation-enhanced clusters. Irradiation-induced clusters correspond to solute atom clusters with or without Cu atoms, whose formation mechanism is assumed to be the segregation of solute atoms based on point defect cluster or matrix damage (heterogeneous nucleation). On the other hand, the irradiation-enhanced clusters correspond to so-called CRPs (Cu-rich precipitates) or CELs (Cu-enriched clusters), and the formation mechanism is the clustering of Cu atoms above the solubility limit enhanced by the excess vacancies introduced by irradiation. This model also assumes that the formation of solute atom clusters and matrix damage is not independent to each other, which is a very different model from the conventional two-feature models as described in the previous sections. Another key idea is the introduction of a concept of a thermal vacancy contribution in the diffusivity model. This idea is essentially identical to that shown in Rg. 11.11. This is a direct modeling of the results of atomic-level computer simulations.  

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N. Soneda, K. Nakashima, K. Nishida and K. Dohi, High fluence surveillance data and recalibration of RPV embrittlement correlation method in Japan, Proceedings ASME 2013 Pressure Vessels and Piping Division Conference, American Society of Mechanical Engineers, New York, 14-18, 2013, PVP2013-98076. 

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