Container Materials for High-Level Nuclear Waste at the Proposed Yucca Mountain Site

Abstract

Candidate container materials for high-level nuclear waste packages to be emplaced at the proposed Yucca Mountain repository site are being considered for their long-term resistance to corrosion, oxidation, embrittlement and other kinds of degradation. Selection criteria have been established, and a method has been developed for recommending a material for advanced container design activities. An extensive compilation of the degradation phenomena for six candidate materials is complete, and further studies have begun on the degradation modes affecting additional candidate materials. Phenomenological models for predicting container degradation rates are being advanced for environmental conditions applicable to Yucca Mountain. An experimental program is underway to evaluate the susceptibility of container materials to localized corrosion, stress corrosion cracking, and enhancement of corrosion and oxidation attack by gamma radiation. Initial evaluations of container fabrication and welding processes have identified some processes that appear to alleviate some long-term corrosion susceptibility concerns.

Yucca Mountain project canister material corrosion studies as applied to the electrometallurgical treatment metallic waste form

“…Local galvanic cells in specific regions of the container materials are of concern [56]. These cells can be initiated due to compositional and microstructural differences made by various fabrication processes used to make different parts of the container.…”

“…Production of these substances increases the proton concentration and decreases the local pH. Some well-known micro-organisms oxidize and reduce the various oxidation states of sulfur and iron, and make the environment hostile to metals and alloys [56].…”

“…Certain micro-organisms can survive and propagate in environments previously thought to be too harsh for these organisms [56]. The micro-organisms can remain dormant for long periods of time under unfavorable conditions and become active when conditions are favorable.…”

“…When considering air and &/water systems together, the most important radiolytic products are the oxidation products of nitrogen (NO, N20, N02, N2O5, HNO2, and HNO3), the reduction products of nitrogen (ammonia), and ozone [16]. Many of the chemical species formed by radiolysis are quite oxidizing to container materials (ozone, oxygen, oxides of nitrogen, nitric acid, hydrogen peroxide, and a number of free radicals) [56], and these products have the potential to concentrate in droplets that condense on canister materials [631. Specifically, irradiation of air with water can form nitric acid which will attack the inner components of the waste package, and radiation will form nitric acid within moisture layers that could affect corrosion rates [12].…”