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 permanent) 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 approximately 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 structural scale. Three 1/3-scale specimens were fabricated. The first specimen was built using a conventional selfconsolidating concrete to serve as a control. The second and third specimens were prepared using special concrete mixtures, designed to accelerate the corrosion of rebar and ASR. All three casks were observed for 2 years for aginginduced 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 significant 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 CaCl 2 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.

A Computational Study of the Shear Behavior of Reinforced Concrete Beams Affected from Alkali–Silica Reactivity Damage

“…A comprehensive experimental program was performed by the authors of this study on the effect of ASR on the mechanical properties of concrete using accelerated aging [ 19 ]. A 0.8% sodium hydroxide by weight of cement was added to concrete and degradation was evaluated over time.…”

Multi-hazard performance of reinforced concrete dry casks subjected to chloride attack and tip-over impact

“…The aging process was monitored via destructive and non-destructive methods over a two year period. The details on the non-destructive testing of the casks are provided in a previous publication (Attar et al, 2016). Three tip-over tests were performed after two years followed by damage evaluation of the specimens.…”

“…Under normal conditions, it will take several years for chloride ions to diffuse into the concrete and (or carbonation front to) reach the reinforcing steel, initiate corrosion, and cause a noticeable amount of degradation. There are methods for accelerating the corrosion process such as adding chloride directly into the concrete mixture at the time of mixing (Attar et al, 2016, Lee et al, 2000, passing electrical current through the reinforcement (Galvano static method) (Abosrra et al, 2011, Cheng et al, 2005, and using an artificial environmental exposure that is more conducive to corrosion (i.e., high temperature and wet-dry cycles) (Yuan et al, 2007, Şahmaran andLi, 2009). However, most of these techniques are very difficult to apply in large-scale investigations.…”

“…The rebar was extracted from the concrete at select locations and the diameter of the rebar was measured, using calipers with a precision of 0.0001 mm, at twenty different locations on two rebar and the smallest diameter was about 9.07 mm, which compared to the original diameter of 9.52 mm, indicates a 9% decrease in the cross-sectional area. More details regarding the non-destructive testing and long-term monitoring of the casks is available in Attar et al (2016) and Attar (2016). To correlate the observed deterioration under accelerated conditions to real field conditions a study was performed by Attar (2016) using the same experiments described in this paper.…”

ASR Degradation and Expansion of Plain and Reinforced Concrete

“…Concrete surface cracking is the first externally observable sign of ASR and it leads to the degradation of concrete mechanical and durability properties. Since © ASCE ASR is a long-term process, researchers have adopted accelerated ASR techniques such as raising the alkali content of the concrete or curing the hardened concrete specimens in a high alkaline solution (Fan and Hanson 1998, Smaoui et al 2005, Attar et al 2016. Different and sometimes contradictory results have been reported in literature regarding the effect of ASR on the concrete properties since various parameters are involved in this chemo-thermo-mechanical and time dependent process.…”