Monday, June 8, 2009

Ni-Cr-Mo Alloys as Corrosion Barrier for the Rad-Waste Containers

Currently the Ni-Cr-Mo alloys are the leading candidates for the latest design of rad-waste containers for the Yucca Mountain Project. Alloy 22 is the leading candidate whereas alloy 59, an advanced Ni-Cr-Mo alloy, is under consideration as an equal or better alternative. The Nuclear Waste Policy Act of 1982 established an objective of Nuclear Waste disposal in a deep geological repository. This act was later amended in 1987, and established Nevada as the only site to be characterized. In 1994 a technical decision was made for a multipurpose container consisting of an outer barrier of carbon steel, alloy 400 or Cu-Ni 70/30 and an inner barrier of alloy 825. This concept was later modified to require a more corrosion resistant alloy for the inner barrier i.e., an alloy of the Ni-Cr-Mo family, alloy 22 (UNS N06022), titanium or a titanium alloy. Since then many papers (1-6) have been written comparing the corrosion resistant characteristics of various alloys such as alloy 825, 625, C-276 and alloy 22. The design waste package underwent several iterations with one of the latest design called "Enhanced Design Alternative" (EDA) which will consist of 20 mm thick alloy 22 as the outer container barrier. This will be shrunk fit to a 50mm thick inner barrier fabricated of type 316 nuclear grade or standard 316L SS. This waste package was then to be enclosed by a self-supported 20 mm thick Ti-grade 7 mailbox shaped drip shield. This design may be further modified as more comprehensive corrosion characteristics of uniform corrosion, localized corrosion, stress corrosion cracking, thermal stability, microbiological corrosion, galvanic corrosion, intergranular corrosion for both the base metal and more importantly, the weld joints in these waste containers under realistic repository environments are obtained. This paper presents data on a new but well established corrosion resistant alloy 59 (UNS N06059) of the Ni-Cr-Mo family. The alloy appears to have better corrosion resistance, both uniform and localized, better thermal stability and better weldability than alloy 22 as measured in standard ASTM laboratory tests and Modified Varestraint tests for measuring susceptibility to hot cracking. Data from some of these laboratory tests on alloy 59 and 22 along with the various interactions with Lawrence Livermore National Laboratories and the TRW Environmental Safety Systems, Management and Operating Contractor for the waste package design, are discussed.

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