Microstructure and phase identification in type 304 stainless steel-zirconium alloys
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INTRODUCTION
STAINLESS steel-zirconium alloys are of interest for the containment and disposal of radioactive metal isotopes isolated from spent nuclear fuel.m These isotopes are isolated by using electrorefining techniques that were developed to process a wide variety of spent nuclear fuels, t2,3,4] The major waste streams generated by this refining operation are (1) a "mineral waste" in the form of elemental chlorides, which will be incorporated into the crystal structure of a zeolite matrix, and (2) a "metal waste," which includes stainless steel from cladding hulls and miscellaneous assembly hardware, zirconium from alloy fuels, and noble metal fission products such as Pd, Ru, and Ag that are not removed during the processing operation. This metal waste will be consolidated into a monolithic waste form of sufficient corrosion resistance and mechanical strength and canned in a high-level waste container for disposal in a geologic repository. The fuel cladding hulls comprise over 80 wt pct of the metal waste stream. Since ZIRCALOYS* and stainless *ZIRCALOYS is a trademark of Westinghouse Electric Company, Pittsburgh, PA.
steels are prevalent cladding materials, and since zirconium metal is present in certain nuclear fuels (for example, the driver fuel in the Experimental Breeder Reactor II is U-10 wt pct Zr~5[), the metal waste form will be predominantly a mixture of stainless steel and zirconium. Stainless steels contain significant amounts of chromium and nickel, in addition to iron, so the metal waste form may be approximated by the Cr-Fe-Ni-Zr quaternary. However, very little reliable information exists on either the Cr-Fe-Ni-Zr quaternary or Cr-Fe-Zr ternary alloys. Recently, Raghavan
DANIEL P. ABRAHAM, Metallurgical Engineer, and SEAN M. McDEAVITT, Nuclear and Materials Engineer, Chemical Technology Division, and JANGYUL PARK, Staff Scientist, Energy Technology Division, are with Argonne National Laboratory, Argonne, IL 60439. Manuscript submitted August 10, 1995. METALLURGICALAND MATERIALSTRANSACTIONSA
remedied this situation partially, by presenting a solidification scheme for Cr-Fe-Zr alloys.t6j He also showed a schematic of the Cr-Fe-Zr liquidus and tentative isothermal sections at various temperatures for these ternary alloys. To a first approximation, the behavior of stainless steelzirconium alloys may be predicted from the Fe-Zr phase diagram. However, many uncertainties exist in the Fe-Zr binary, as evidenced by the differing phase diagrams presented by various authors, t7 1o1 For example, there is controversy over the existence of the ZrFe 3 phase, which was first reported by Svechnikov et al., who noted that it forms at 1480 ~ and has an fcc structure with a = 1.169 nm."" Aubertin et al., however, did not observe this compound when the elements were annealed at 950 ~ [121They proposed that either ZrFe3 existed only above 950 ~ or it did not exist as a stable compound and resulted only from specimen contamination. Both Arias and Abriata[7] and KubachewskitSJ indicate the possible formation
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