PSI - Issue 19

Lloyd Hackel et al. / Procedia Structural Integrity 19 (2019) 346–361 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

347

2

Keywords: Type your keywords here, separated by semicolons ;

1. Introduction

The Multi-Purpose Canister (MPC) was conceptualized by the U.S. Department of Energy as a single versatile package equally suitable for on-site storage, transport, and permanent disposal in a future repository. These dry canisters are a temporary approach for storing high-level radioactive waste such as spent nuclear fuel that has been cooled for a required time in a liquid pool. The past and current industry design life of MPCs ranges between 60 and 80 years. The Nuclear Regulatory Commission (NRC) provided initial 20 years license for the facilities using qualified MPCs. Many facilities are applying for license extensions which range from 20 to 40 years [1,2]. The design and license years were based on the plan that the US Government was to start receiving used fuel from the industry in 1998 based on contracts executed between the Government and the nuclear utilities under the Nuclear Waste Policy Act of 1982. As of the issue date of this paper, a repository is not available and potential availability ranges from 50 to 100 years. In the United States, Yucca Mountain was expected to open in 2017 as a permanent dry storage site. At present that has not occurred so fuel is being stored at the power plant sites in pools and dry storage. Some dry storage sites, such as those in coastal areas, face more challenging environmental exposure. Interim storage sites, such as Eddy-Lea in New Mexico or Waste Control Specialists in West Texas, may become available in the future. MPCs are typically made of 304 and 316L stainless steel. They are roll-formed and welded to cylindrical shape and after loading with spent fuel, sealed with a welded or bolted lid. The cylinders are typically filled with an inert gas such as helium and are placed vertically or horizontally in concrete vaults to provide additional radiation shielding and sufficient air flow for passive cooling. The dry canister system is being used in over 20 countries worldwide. Chloride induced stress corrosion cracking (CISCC) is the initiation and growth of cracks in a chlorine-dominated corrosive environment. The cracking typically develops following the formation and growth of corrosive pits. Austenitic stainless steels are susceptible to CISCC and because of their wide use, especially in the nuclear and oil industries, very extensive research has been done on this problem [3,4,5]. The chemical environment that causes CISCC for a given alloy is often one which is otherwise only mildly corrosive to the metal. Deliquescence of salt deposits, for example, may not occur at temperatures below 80oC so although crack growth rate may be more prone to accelerate at higher temperatures, initiation may be more problematic at lower temperatures [3,6]. To occur this cracking is generally considered to require the presence of three concurrent factors: 1) material susceptible to CISCC, 2) a corrosive environment and 3) tensile stress above a threshold, for example, tensile stress developed in component welds. With respect to canister materials, 304 and 316L stainless steels are in common use; although resistant to corrosion they are known to be susceptible to CISCC. With respect to a corrosive environment, spent fuel storage is predominantly being done at reactor sites which are typically located near a body of water for reactor cooling. Many of the sites are located along ocean coastal areas with the consequent high humidity and salt atmosphere. During long term exposure, surface pitting occurs in the corrosive environment thereby creating weakened sites for crack initiation. The CISCC develops and propagates perpendicular to the vector direction of tensile residual stress. Welding for example is a source of such tensile stress developing along the weld line direction as well as transverse to the weld seam. For welded canisters, the tensile stress developed during the weld cool-down exposes grain boundaries of the sensitive material to the corrosive environment. Parrot and Pits report that fracture mechanics tests have shown that CISCC propagation can begin at low stress intensities in the range of 2 MPa.m0.5 to 10 MPa m0.5 and that crack propagation is strongly dependent on temperature but is relatively unaffected by stress intensity [3]. Chen and Kelly have developed predictions of the maximum size of a hemispherical pit in type 304 and 316 stainless steels after exposure to atmospheric conditions [7]. The results of the calculations agree well with several sets of data for near seacoast exposures on three continents for exposure times out to 26 years. Further evaluations have placed maximum pit size at around 0.2 mm [8]. For structures containing tensile residual stresses, the critical depth of localized corrosion to initiate would be in the range of <1mm [9]. Generally the environment, such as exposure to ocean salt air, is not an 2. Stress Corrosion Cracking