Accelerated Aging of Concrete Dry Cask Storage Systems for Nuclear Waste

Abstract

Dry cask storage systems (DCSS) are widely used worldwide for storage of spent nuclear fuel (SNF). Particularly, in the United States, other than the SNF pools, DCSS are the only means for storage of SNF. In the United States, the DCSS are licensed for an initial 20 years (with a possible extension of 40 years). The absence of a long-term (or perma-nent) storage facility has brought up concerns regarding the long-term performance of DCSS, which may now have to be used for extended durations reaching over 100 years. The DCSS with an exposed concrete overpack account for ap-proximately 61% of the DCSS inventory in the United States. The corrosion of the steel reinforcing bars (rebar) and the alkali-silica reactivity (ASR) of concrete have been identified as two of the main degradation mechanisms. In this paper, the accelerated aging of reinforced concrete (RC) overpacks of a vertical DCSS is evaluated experimentally at the struc-tural scale. Three 1/3-scale specimens were fabricated. The first specimen was built using a conventional self-consolidating concrete to serve as a control. The second and third specimens were prepared using special concrete mix-tures, designed to accelerate the corrosion of rebar and ASR. All three casks were observed for 2 years for aging-induced deterioration using various non-destructive approaches including visual inspection, half-cell potential, Schmidt hammer, and ultra-sonic pulse velocity (UPV) measurements. The RC overpacks have been observed to exhibit signifi-cant distress due to these aging mechanisms. The overall conclusion is that accelerating ASR and corrosion through use of reactive aggregates and/or addition of chemicals (NaOH and CaCl2 in this particular case) is a viable and practical approach for large-scale studies. Although accelerated aging of concrete structures have been extensively studied in the literature, this is one of the first studies on the long-term degradation in DCSS due to corrosion and ASR.