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    An Empirical Evaluation of the Irradiation Sensitivity of Reactor Pressure Vessel Materials


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    There is a general consensus that copper content increases the neutron irradiation sensitivity of reactor vessel materials. Within the past few years, several attempts have been made to introduce factors other than copper to explain the greater variance in irradiation sensitivity of weld metal compared with plate, such as the model presented in 1976 by Biemiller and Byrne (ASTM STP 611). This model implicated the role of the weld metal's major alloying elements with the use of a theoretically derived relation. The objective of this investigation was to further develop an improved and continuous correlation between irradiation-induced nil-ductility transition temperature (NDTT) shift and the residual and alloying elements in a typical reactor pressure vessel weldment. This correlation can be used for predicting, in advance to neutron exposure, a material's irradiation sensitivity.

    Using the model developed by Biemiller and Byrne as the investigative basis, a comprehensive computerized data bank was used in performing a series of selected iterations to develop a best-fit chemistry relationship. The approach was to determine, by weighting the empirical chemistry relation, the synergistic role of residual and alloying elements with copper on weld irradiation sensitivity.

    In the new empirical model, the alloying elements found to increase irradiation sensitivity were nickel, silicon, carbon, and manganese, while molybdenum was found to decrease sensitivity. A decreasing role of the carbide formers in acting beneficially toward weld irradiation sensitivity was noted.

    Reactor vessel plate irradiation data were also evaluated using the empirical weld metal chemistry relationship. The results indicated that plate material exhibited a lower irradiation sensitivity than welds when their corresponding compositions were similar. As principal chemistry increased in content, the irradiation sensitivity of the plate and weld material became less distinguishable.

    The most significant conclusion of this investigation is that the influence of alloying elements on weld irradiation sensitivity can be controlled provided a proper chemistry balance, especially copper content, is maintained. This model supports the assumption that, for a given copper content, specific alloying elements in the weld account for the large variation in weld irradiation behavior. The chemistry relationship developed provides a significantly improved method for predicting weld irradiation behavior than was possible with the earlier model as well as with relations involving residual elements alone.


    neutron irradiation, pressure vessel weld metal, copper, nickel, nil-ductility transition temperature shift, irradiation sensitivity

    Author Information:

    Varsik, JD
    Development engineer and principal engineer, Nuclear Power SystemsCombustion Engineering, Inc., Windsor, Conn.

    Byrne, ST
    Development engineer and principal engineer, Nuclear Power SystemsCombustion Engineering, Inc., Windsor, Conn.

    Committee/Subcommittee: E10.07

    DOI: 10.1520/STP38169S