DAMAGE-FUNCTION ANALYSIS OF NEUTRON-ENERGY AND SPECTRUM EFFECTS UPON THE RADIATION EMBRITTLEMENT OF STEELS.

Abstract

Critical evaluations of irradiation effects data for steels exposed in different reactor environments depend upon fluence measurements that reflect the neutron population and the corresponding influence of damage mechanisms inherent to those neutron energy spectra. For the present research investigation, theoretical models describing neutron damage in reactors were adjusted with data on mechanical property changes of A302-B steel irradiated at <450°F (232°C). As described previously, this procedure yielded a damage function that more properly accounts for the energy dependence of damage. The present investigation was centered on a new comprehensive experiment which yielded wide variations in spectra and in corresponding measurements of neutron embrittlement. The experiment and the resultant data have validated the damage function. An important new part of the damage function analysis technique provides the percent contributions of neutrons of all energy levels to the embrittlement process. Values of the damage function, averaged for a typical reactor physics spectral calculation group structure, are presented with suitable descriptions of their applications to a wide variety of spectra. One major conclusion reached in this study is that detailed as well as accurate neutron dosimetry measurements of fast and thermal fluxes, corrected to reactor operating temperatures, are necessary if good correlations among irradiation-effects data are to be obtained. This requirement applies to both experimental irradiations and irradiations at reactor component surveillance locations. The study has shown further that an independently derived damage function for irradiation of structural steels such as A302-B at <450°F (232°C) is realistic and can be applied to new experimental data conforming to those conditions. The contributions of thermal and low-energy neutrons to the embrittlement process in low-alloy steel are shown to be of major importance to the interpretation of radiation-effects data. (auth)