Analysis of Stainless Steel Grades for MOX Piping and Components

Abstract

In 1999, the National Nuclear Security Administration signed a contract with Shaw AREVA MOX Services, LLC to design, build, and operate a Mixed Oxide (MOX) Fuel Fabrication Facility. This facility will be a major component in the United States� program to dispose of surplus weapon-grade plutonium. The facility will be capable of turning 3.5 metric tons of weapon-grade plutonium into MOX fuel assemblies annually. Before surplus weapon-grade plutonium can be used in mixed oxide fuel, the weapons-grade plutonium oxide must be purified. The purification process is a chemical process called aqueous polishing, which is based on the same process the French nuclear industry has used safely and successfully used for over 30 years. The aqueous polishing uses various chemicals to remove impurities such as gallium, americium and uranium. The plutonium oxide is first dissolved in nitric oxide. Impurities are then chemically removed with a phosphate solution. Finally, the purified plutonium is converted back to plutonium oxide, packaged and stored in durable cans for future production of MOX fuel pellets. Clearly, safety and reliability are key factors in the construction and operation of such a facility. Long life, mechanical performance, and corrosion resistance are all required for materials used in a MOX aqueous polishing system and materials selection is an important step in assuring both safety and reliability. Typical materials for these applications include 304 and 316 stainless steels, which are well suited for such applications. In the United States, ASME and ASTM standards are used to assure uniformity in composition and properties between various heats of steel. However, other nations may follow different standards and procedures. As noted above, the aqueous polishing process is based on French technology. As a result, there is motivation to use the same grade of 304 and 316 steel as French facilities to ensure the same performance level. However, this MOX grade steel differs in composition from the ASME grades readily available in the United States. Further, while domestic vendors may be able to provide MOX grade steel, this may not be economical. The objective of this paper is to examine differences in MOX and ASME grades of 304 and 316 stainless steels. Once differences in composition have been identified, the impact of these elemental variables on long-term stability, mechanical performance, weldability and corrosion were explored. The availability and cost-impact of using MOX grade versus ASME grade steel were assessed. Finally, the advantages and disadvantages of using a mixture of 304L and 316L versus all 316L are presented.