Development of Engineered Structural Barriers for Nuclear Waste Packages
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DEVELOPMENT OF ENGINEERED STRUCTURAL BARRIERS FOR NUCLEAR WASTE PACKAGES*
R. E. WESTERMAN, S. G. PITMAN, R. P. ELMORE, AND J. L. NELSON Pacific Northwest Laboratory,** Richland, Washington, USA
ABSTRACT The development of structural barriers for nuclear waste packages involves selection of candidate materials, their screening by mechanical and corrosion testing, rigorous accelerated testing, and evaluation and comparison with other package elements. This document presents results from work conducted on titanium and cast steels.
INTRODUCTION A program directed toward the development of licensable structural engineered barrier elementst for high-level nuclear waste packages has been in progress at Pacific Northwest Laboratory (PNL) since January 1979. The objective of the program, which is currently sponsored by the Office of Nuclear Waste Isolation (ONWI), is to characterize and recommend materials for structural barrier elements that are capable of contributing substantially to maintaining radioisotope release rates at near-zero values for time periods of -1000 years. Past work in the Engineered Barrier Development Program at PNL has involved corrosion screening of a variety of candidate alloys and ceramic materials; thermal- and radiation-degradation of polymers; and some preliminary investigations of the susceptibility of titanium (Ti)-grade 2 and Ti-grade 12 to environmentally induced fracture [l]. These early studies revealed no unusual deterioration of the mechanical integrity of the titanium alloy due to Hanford basaltic ground water or air environments at temperatures to 250'C. The program currently emphasizes the accelerated testing of Ti-grade 2 and -grade 12 for environmentally induced fracture in simulated Hanford basaltic ground water and the corrosion screening of ductile cast iron and two steels in the same environment. MATERIALS Five materials are currently emphasized in this study: Ti-grade 2 and -grade 12, ductile cast iron (ASTM A536 grade 60-40-18), and two cast steels (ASTM A217 grades WC6 and WC9). Ti-grade 2 is a commercial purity grade; Ti-grade 12 has similar composition limits except that approximately 0.8% Ni and 0.3% Mo have been added. These additions improve resistance to pitting and crevice corrosion at elevated temperatures, particularly in high-chloride environments [2,3]. Ductile iron was chosen as a candidate material because of its low cost, high toughness and ductility, and promising performance in preliminary corrosion tests. The two
*PNL-SA-9543, work supported by the U.S. Department of Energy (DOE) under contract DE-ACO6-76RLO 1830. **Operated for DOE by Battelle Memorial Institute, Columbus, Ohio. tThe canister, overpack, and hole sleeve are all potential structural barrier elements.
364 cast steels studied (nominally 2-1/2 Cr, 1 Mo and 1-1/4 Cr, 1/2 Mo) were chosen for possible improvements in corrosion resistance relative to ductile cast iron. EXPERIMENTAL METHODS Only brief descriptions of experimental methods are given here. More thorough descriptions are
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