This letter report presents a probabilistic performance assessment model to evaluate the probability of canister failure (through-wall penetration) by SCC. The model first assesses whether environmental conditions for SCC – the presence of an aqueous film – are present at canister weld locations (where tensile stresses are likely to occur) on the canister surface. Geometry-specific storage system thermal models and weather data sets representative of U.S. spent nuclear fuel (SNF) storage sites are implemented to evaluate location-specific canister surface temperature and relative humidity (RH). As the canister cools and aqueous conditions become possible, the occurrence of corrosion is evaluated. Corrosion is modeled as a two-step process: first, pitting is initiated, and the extent and depth of pitting is a function of the chloride surface load and the environmental conditions (temperature and RH). Second, as corrosion penetration increases, the pit eventually transitions to a SCC crack, with crack initiation becoming more likely with increasing pit depth. Once pits convert to cracks, a crack growth model is implemented. The SCC growth model includes rate dependencies on both temperature and crack tip stress intensity factor, and crack growth only occurs in time steps when aqueous conditions are predicted. The model suggests that SCC is likely to occur over potential SNF interim storage intervals; however, this result is based on many modeling assumptions. Sensitivity analyses provide information on the model assumptions and parameter values that have the greatest impact on predicted storage canister performance, and provide guidance for further research to reduce uncertainties.
Deep Borehole Disposal (DBD) of high-level radioactive wastes has been considered an option for geological isolation for many years (Hess et al. 1957). Recent advances in drilling technology have decreased costs and increased reliability for large-diameter (i.e., ≥50 cm [19.7”]) boreholes to depths of several kilometers (Beswick 2008; Beswick et al. 2014). These advances have therefore also increased the feasibility of the DBD concept (Brady et al. 2009; Cornwall 2015), and the current field test design will demonstrate the DBD concept and these advances. The US Department of Energy (DOE) Strategy for the Management and Disposal of Used Nuclear Fuel and High-Level Radioactive Waste (DOE 2013) specifically recommended developing a research and development plan for DBD. DOE sought input or expression of interest from States, local communities, individuals, private groups, academia, or any other stakeholders willing to host a Deep Borehole Field Test (DBFT). The DBFT includes drilling two boreholes nominally 200m [656’] apart to approximately 5 km [16,400’] total depth, in a region where crystalline basement is expected to begin at less than 2 km depth [6,560’]. The characterization borehole (CB) is the smaller-diameter borehole (i.e., 21.6 cm [8.5”] diameter at total depth), and will be drilled first. The geologic, hydrogeologic, geochemical, geomechanical and thermal testing will take place in the CB. The field test borehole (FTB) is the larger-diameter borehole (i.e., 43.2 cm [17”] diameter at total depth). Surface handling and borehole emplacement of test package will be demonstrated using the FTB to evaluate engineering feasibility and safety of disposal operations (SNL 2016).
We describe here a method for modifying the bulk composition (pH, salinity, hardness) of fracturing fluids and overflushes to modify wettability and increase oil recovery from tight formations. Oil wetting of tight formations is usually controlled by adhesion to illite, kerogen, or both; adhesion to carbonate minerals may also play a role when clays are minor. Oil-illite adhesion is sensitive to salinity, dissolved divalent cation content, and pH. We measure adhesion between middle Bakken formation oil and core to verify a surface complexation model of reservoir wettability. The agreement between the model and experiments suggests that wettability trends in tight formations can be quantitatively predicted and that the bulk compositions of fracturing fluid and overflush compositions might be individually tailored to increase oil recovery.
The majority of existing dry storage systems used for spent nuclear fuel (SNF) consist of a welded 304 stainless steel container placed within a passively-ventilated concrete or steel overpack. More recently fielded systems are constructed with dual certified 304/304L and in some cases, 316 or 316L. In service, atmospheric salts, a portion of which will be chloride bearing, will be deposited on the surface of these containers. Initially, the stainless steel canister surface temperatures will be high (exceeding the boiling point of water in many cases) due to decay heat from the SNF. As the SNF cools over time, the container surface will also cool, and deposited salts will deliquesce to form potentially corrosive chloride-rich brines. Because austenitic stainless steels are prone to chloride-induced stress corrosion cracking (CISCC), the concern has been raised that SCC may significantly impact long-term canister performance. While the susceptibility of austenitic stainless steels to CISCC in the general sense is well known, the behavior of SCC cracks (i.e., initiation and propagation behavior) under the aforementioned atmospheric conditions is poorly understood. A literature survey has been performed to identify SCC crack growth rate (CGR) studies conducted utilizing conditions that may be relevant to existing SNF interim storage canisters, the results of which are presented in this document. The data presented here have been restricted to those representing atmospheric corrosion of stainless steels due to deliquescence of marine salts, or marine salt components, on the metal surface. A suite of experimental studies representing both long-term field tests and accelerated laboratory tests has been identified. Potentially relevant data are summarized in Figures 1-1 (304 SS) and Figure 1-2 (316 SS). In the Figures, when a particular reference utilized a series of samples, the range is shown as a bar, and the average value shown with a symbol. A summary of the test methods, sample geometry, and environmental conditions for each study is given in Table 1-1. While the surveyed studies all explore SCC of austenitic stainless steels under atmospheric conditions, the methods through which each researcher approached the problem do differ, as illustrated in Table 1-1. The surveyed studies utilized a variety of metal treatments including as-fabricated, solution annealed, welded, and sensitized material. Furthermore, different surface treatments (polished vs ground) were also used. In addition, most of these studies were accomplished using techniques that are not generally accepted for high-fidelity crack growth rate measurements, and in cases where more traditional approaches were taken, these methodologies may not be applicable to the atmospheric conditions of interest here. The wide variety of methods and materials results in the observed large scatter in measured CGRs. Each of the data sets in Figures 1-1 and 1-2 is described in more detail in the following sections. A short summary of crack growth rates based on operational experience is also presented.
This report fulfills the M3 milestone M3FT-15SN0802042, “Evaluate the Frequencies for Canister Inspections for SCC” under Work Package FT-15SN080204, “ST Field Demonstration Support – SNL”. It reviews the current state of knowledge on the potential for stress corrosion cracking (SCC) of dry storage canisters and evaluates the implications of this state of knowledge on the establishment of an SCC inspection frequency. Models for the prediction of SCC by the Japanese Central Research Institute of Electric Power Industry (CRIEPI), the United States (U.S.) Electric Power Research Institute (EPRI), and Sandia National Laboratories (SNL) are summarized, and their limitations discussed.