THE CONTROL OF COBALT CONTENT IN REACTOR GRADE STEELS
A reassessment of sources of long-lived gamma radiation in reactor grade steels indicates that besides cobalt other potentially hazardous residual elements are niobium, silver and, of less importance, europium. Although the decay of 60-Co dominates the gamma-actigity for several decades after reactor shutdown, the longer-lived 94-Nb and 108-Ag isotopes eventually become higher contributors to the radiation dose rate. Analyses of four heats of Type 304 stainless steel (reactor grade, with cobalt contents in the range 140-350ppm) revealed niobium and silver contents of up to 85 ppm and 0.8 ppm respectively; the europium content of the steels was insignificant at <4 x 10**-4 ppm. The main sources of cobalt, niobium and silver in steels are scrap materials and ferroalloys used in steelmaking. Since these elements are not easily oxidised they are retained in the steel. However, the residual levels of these potential gamma radiation sources can be decreased by using higher purity raw materials in steelmaking. The use of sponge iron and high purity nickel and chromium should enable residual levels of <50 ppm Co, about 5ppm Nb and <0.05ppm Ag to be achieved in Type 304 steel.
Bibliographic Reference: EUROPEAN APPLIED RESEARCH REPORTS - NUCLEAR SCIENCE AND TECHNOLOGY SECTION, VOL. 5 (1983), NO. 2 PP. 121-144 (EUR 8655 E).
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Record Number: 1989122015600 / Last updated on: 1987-01-01
Available languages: en