ABSTRACT

The localized corrosion susceptibility of Alloy 22, a candidate container material for the disposal of high-level nuclear waste in the proposed repository at Yucca Mountain, Nevada, was assessed in chloride containing solutions at temperatures ranging from 60 to 150 _C. Tests were conducted using the as-received material in the mill-annealed condition. In addition, the effects of fabrication processes were examined using both as-welded and thermally aged specimens. A comparison of the crevice corrosion repassivation potential and corrosion potential measured in separate tests suggests that fabrication processes and environmental conditions such as temperature and chemical composition of water contacting the waste packages (WPs) may influence the localized corrosion susceptibility of the Alloy 22 containers.

INTRODUCTION

The U.S. Department of Energy (DOE) is preparing a license application for Yucca Mountain, Nevada as the site for the permanent disposal of the nation?s high-level nuclear waste (HLW) including spent nuclear fuel (SNF) from boiling water reactors (BWR) and pressurized water reactors (PWR), DOE-owned SNF, and high-level waste (HLW) glass. The DOE has focused on providing containment of the HLW within an engineered barrier system (EBS) limiting the dose to the reasonably maximally exposed individual for the 10,000-year regulatory period.1 The U.S Code Of Federal Regulations (10 CFR Part 63) describes the regulatory requirements for the disposal of HLW in the proposed repository at Yucca Mountain, Nevada.

The proposed repository design and the EBS have several attributes that the DOE has suggested would contribute to the isolation of the HLW.1 The proposed repository horizon at Yucca Mountain will be located in the unsaturated zone that has a low water infiltration rate. Selection of the repository horizon in the unsaturated zone is based on a strategy of keeping the WP dry for an extended period after repository closure. The prolonged dry period and unsaturated zone location would also retard the migration of radionuclides in the event of WP failure. The current WP design is intended to provide containment of the radionuclides for thousands of years and consists of an Alloy 22 (56Ni-22Cr-13.5Mo-3W-4Fe) outer container surrounding a Type 316 nuclear grade (NG) stainless steel (SS) inner container to provide structural integrity.2,3 Descriptions of the fabrication sequence and nondestructive evaluation methods to be used to inspect the disposal container (DC) are provided in DOE documents.4,5

In order to reduce residual stresses in Alloy 22 final closure welds, laser peening will be used for the inner Alloy 22 lid weld. For the outer closure lid, local induction annealing of the extended outer shell is proposed as a method to eliminate residual tensile stresses in the Alloy 22 outer closure weld. The induction annealing process will heat the end of the Alloy 22 disposal container with the completed closure weld to a temperature of 1,150 _C. Forced air or water will be used to rapidly reduce the temperature of the closure weld region.4 Elevated temperature exposures during welding and annealing processes can result in the precipitation of topologically close packed (TCP) phases.6,7,8 Formation of TCP phases has been shown to increase susceptibility of Alloy 22 to localized corrosion9 and reduce both its ductility and impact strength particularly in weld material and weld heat affected zones.10,11

The objective of this investigation was to determine the localized corrosion susceptibility of Alloy 22 as a function of temperature, and solution chemistry. The effects of fabrication processes on the localized corrosion susceptibility of Alloy 22 were exa

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