Impurity effects on reduced-activation ferritic steels developed for fusion applications

Reduced-activation steels are being developed for fusion applications by re stricting alloying elements that produce long-lived radioactive isotopes wh en irradiated in the fusion neutron environment. Another source of long-liv ed isotopes is the impurities in the steel. To examine this, three heats of reduced-activation martensitic steel were analyzed by inductively coupled plasma mass spectrometry for low-level impurities that compromise the reduc ed-activation characteristics: a 5-ton heat of modified F82H (F82H-Mod) for which an effort was made during production to reduce detrimental impuritie s, a 1-ton heat of JLF-1, and an 18-kg heat of ORNL 9Cr-2WVTa. Specimens fr om commercial heats of modified 9Cr-1Mo and Sandvik HT9 were also analyzed. The objective was to determine the difference in the impurity levels in th e F82H-Mod and steels for which less effort was used to ensure purity. Silv er, molybdenum, and niobium were found to be the tramp impurities of most i mportance. The F82H-Mod had the lowest levels, but in some cases the levels were not much different from the other heats. The impurity levels in the F 82H-Mod produced with present technology did not achieve the low-activation limits for either shallow land burial or recycling. The results indicate t he progress that has been made and what still must be done before the reduc ed-activation criteria can be achieved. (C) 2000 Elsevier Science B.V. All rights reserved.