ASTM E 900-87, Standard Guide for Predicting Neutron Radiation Damage to Reactor Vessel Materials, provides methods for predicting the effects of fast neutron irradiation on the toughness of reactor pressure vessel materials. The prediction methodology is similar but separate for the steel base metal and weld metal. The methodology relies on a correlation between the chemical content of the steel and the fast neutron fluence. The prediction methodology is used to assess the margin of safety against fracture of the reactor vessel during normal operation and postulated transients.
The purpose of this paper is to describe the results of an effort to improve the accuracy of the predictions of transition temperature shift. Numerous issues have been identified which could influence the calculational approach. Those issues include: • the quality of the input data • the dependence on irradiation temperature • differences in irradiation response between unique subsets (e.g., between welds deposited by different fabrication process and consumables) • saturation of neutron damage at “end-of-life” neutron fluences • saturation of neutron damage above copper solid solubility limits (i.e., modification of the “chemistry factor” above 0.25 to 0.30 weight percent copper) • the significance of minor variations in copper or nickel content within the base metal or weld to the relative sensitivity to neutron damage
This evaluation was performed using a “scrubbed” data base of reactor vessel surveillance data coupled with best-estimate irradiation temperatures; the latter were based on the reactor vessel cold leg temperatures rather than the “go” / “no go” thermal monitor data. A term for dependence on irradiation temperature was derived and added to the correlation for irradiation induced shift of the transition temperature. The assumption used was that differences in irradiation temperature were a significant contributor to the scatter, or standard deviation. Once that scatter was reduced, the other more subtle factors identified above could be quantified. Factors considered in this evaluation were weld fabrication process and saturation at copper solubility limits.
The ultimate objective is to contribute to the development of an improved understanding of the empirical relationships which, when coupled with theoretical models of irradiation damage mechanisms, will result in more accurate correlations for inclusion in ASTM E 900. The data base improvements, addition of a temperature term, consideration of saturation effects (with neutron fluence or copper content) and separation by material subsets are intended to assure that the best estimate predictions and their uncertainty are as realistic as possible.