Very Long-Term Dry Storage Systems for Spent Nuclear Fuel: Effect of Canister Weld Corrosion on System Integrity
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Very Long-Term Dry Storage Systems for Spent Nuclear Fuel: Effect of Canister Weld Corrosion on System Integrity Carlos A. W. Di Bella,1 David J. Duquette,2 and Douglas B. Rigby1 1 U.S. Nuclear Waste Technical Review Board, 2300 Clarendon Blvd. Suite 1300, Arlington, VA 22201 U.S.A. 2 Department of Materials Science & Engineering, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, NY 12180 U.S.A. ABSTRACT Termination of the Yucca Mountain repository program implies that U.S. spent nuclear fuel (SNF) and high-level radioactive waste may be stored for very long periods, i.e., more than 100 years. Many of the storage systems for commercial SNF are large, cylindrical, welded, stainless-steel canisters containing 24-80 SNF assemblies in an inert gas (helium) environment. Each canister is contained in a massive reinforced concrete structure for shielding, safety, and security reasons. Airflow in the annular space between the canister and the concrete structure removes SNF decay heat by natural convection. Undetected defects in the canister-lid welds and stress corrosion cracking (SCC) in the canister-lid weld area may combine to form pathways for slow escape of the inert fill gas from the canister. SCC may not occur until many decades after SNF is loaded into the canister because SNF decay heat initially would keep the canister surface temperatures above the temperature where liquid water, a prerequisite for SCC, could condense. The period required for inert gas to escape from the canister once pathways have formed and for air subsequently to enter the canister may also be measured in decades, depending on the size and shape of the pathways. In this paper, the authors explore the limits of weld-flaw detectability; when necessary conditions for SCC could occur in the canister-lid weld area; the period for loss of inert fill gas if through-weld cracks occur; and the possible consequences of fill gas loss. The authors also develop a list of research and study needs to address the possibility of SCC resulting in the loss of fill gas. INTRODUCTION The US nuclear power industry adds approximately 2,000 metric tons of spent nuclear fuel (SNF) per year to the inventory of approximately 65,000 metric tons of commercial SNF that already exists. With few exceptions, the SNF is stored in water-filled pools adjacent to reactors from which the SNF was discharged or in dry storage systems on concrete pads at the reactor sites. Currently, approximately 50,000 metric tons of commercial SNF are in pools and 15,000 metric tons in dry storage. SNF is always discharged from reactors into pools first, but when pools reach their ultimate storage capacity, nuclear utilities build new dry storage capacity to accommodate the older SNF rather than new pool storage capacity. Thus, the ratio of SNF in dry storage to SNF in pool storage is increasing, and by 2030 the amount
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of SNF in dry storage is predicted to be greater than the amount of SNF in pool storage [1]. The March 2011 events at the Fukushima Dai-ichi nuclear complex in Japan
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