The evaluation of corrosion processes for engineered barrier systems

The U.S. Nuclear Regulatory Commission (NRC) established a corrosion program to evaluate the predicted performance of the U.S. Department of Energy’s proposed engineered barrier system for the potential high-level waste geologic repository at Yucca Mountain, Nevada. The risk-informed, performance-based regulation established by the NRC for the potential licensing of the proposed repository is introduced in this article with a discussion of resolution methods for various technical issues. These resolution methods include probability or consequence screening, deterministic confirmatory research investigations, and the use of the Total-System Performance Assessment Code to determine the risk significance of specific materials degradation processes on the total system performance. An example of the deterministic analysis method is the evaluation of the susceptibility of mill-annealed and gas-tungsten-arc welded alloy 22 to lead-assisted stress corrosion cracking in deaerated PbCl₂ and Pb(NO₃)₂ solutions at 95°C. Also in this article is an evaluation of the risk significance of high-temperature deliquescence-induced localized corrosion of the alloy 22 waste package outer barrier on the total system performance. Author’s Note: This paper was prepared to document work performed by the Center for Nuclear Waste Regulatory Analyses (CNWRA) for the Nuclear Regulatory Commission (NRC) under Contract No. NRC-02-02-012. The activities reported here were performed on behalf of the NRC Office of Nuclear Material Safety and Safeguards, Division of High-Level Waste Repository Safety. This paper is an independent product of the CNWRA and does not necessarily reflect the view or regulatory position of the NRC. The NRC staff views expressed herein are preliminary and do not constitute a final judgment or determination of the matters addressed or of the acceptability of a license application for a geologic repository at Yucca Mountain. For more information, contact A. Csontos, U.S. Nuclear Regulatory Commission, Office of Nuclear Material Safety & Safeguards, Washington, D.C. 20555-0001; (301) 415-6352; fax (301) 415-5397; e-mail aac@nrc.gov.     

Evaluation of the conduction shape factor with a CFD code for a liquid–metal heat transfer in heated triangular rod bundles

A heat transfer due to conduction through a coolant itself is not negligible in a liquid–metal cooled reactor (LMR). This portion of a heat transfer is frequently described with a conduction shape factor during the thermal-hydraulic design of an LMR. The conduction shape factor, which is highly dependent on a pitch-to-diameter (P/D) ratio, is defined as the ratio of the local conduction heat flux at a gap between two subchannels to the reference heat flux calculated by the averaged subchannel temperatures. The shape factors in heated triangular rod arrays for three different pitch-to-diameter ratios are generated through CFX calculations in the present study. The flow paths of 1.0–2.0 m in length are meshed into 180,000–360,000 volumes depending on the flow velocities. The SSG Reynolds stress model is used as a turbulent model in the calculations. The evaluated data fell between the heated-rod data and the plane-source data obtained by theoretical investigations. The conduction shape factors were found to be independent of the heating pattern of the rod arrays. Based on the evaluated data, a correlation for a liquid sodium coolant is suggested, which will improve the accuracy of the subchannel analysis codes for the thermal-hydraulic design of an LMR. When it is compared with the existing correlations, the suggested correlation is expected to enhance the reliability of the conduction shape factor because the data is evaluated by a more realistic numerical experiment.     

Investigation of ammonium polyphosphate dilution with ground eggshells and lignin through the study of natural fibre composite flammability

The interactions of Calcium carbonate (e.g., eggshell powder) and Lignin with ammonium polyphosphate (APP) when used as fire retardants were investigated. Three mixing ratios - 1:3, 1:1, and 3:1, were used with natural fibre reinforced composites containing a hemp mat and an epoxy matrix manufactured using a light resin transfer moulding (L-RTM) process. The thermal decomposition of the retardant mixtures and composites was investigated using thermogravimetric analysis (TGA). The findings showed that even though the decomposition reactions of APP with eggshell powder and lignin mixtures interacted and overlapped, the same interactions could not be seen in the composites. In the composite form while the residue was affected by the retardant, the decomposition reactions were driven primarily by the hemp and epoxy. Flammability of the composites was studied by testing to 20, 35, 50, and 75 kW/m² with a cone calorimeter, and determining the critical heat flux. While the samples with eggshell powder had higher ignition times, the critical heat flux for ignition was 13 kW/m² for all sample groups except for a ratio of 1:3 APP to eggshell powder, which was 14 kW/m². The lowest burning rates (mass loss and heat release) occurred in composites containing only APP, however, the addition of eggshell powder or lignin at even a ratio of 3:1 APP to either provided a notable reduction.     

Case study of attempts to enact self service tobacco display ordinances: a tale of three communities

OBJECTIVE: To examine self service tobacco displays (SSTDs) and youth retail tobacco access by comparing longitudinal illegal tobacco sales rates in three communities in Santa Barbara County, California, that considered or implemented ordinances banning SSTDs. A confirmatory survey was also conducted to substantiate the longitudinal data. DESIGN: A longitudinal case study design was utilised. Five undercover tobacco buys were conducted between 1994 and 1997 (n = 332). In addition, one confirmatory survey was conducted in a geographically separated area, which had no ordinances banning SSTDs (n = 57). RESULTS: Decreases in youth buy rates were reported in all three communities. Most notably, the first city to enact a SSTD ban, Carpinteria, achieved a 0% sales rate, which was maintained throughout the study period. In contrast, Santa Barbara and Goleta experienced considerable drops in their illegal sales rates, but neither community obtained results as dramatic as those found in Carpinteria. The confirmatory survey showed that 32.1% of stores with SSTDs sold cigarettes to minors; this compares to a sales rate of 3.4% in stores without SSTDs (chi(2) (1) = 8.11, p = 0.004). CONCLUSIONS: Efforts to enact self service bans are likely to meet with retail and tobacco industry opposition, as was the case in this study's three communities. The process of community debate, resultant publicity surrounding the issue, and enactment of SSTD ordinances may serve to not only increase merchant awareness of youth tobacco laws and their penalties but also may contribute to reduced youth cigarette sales rates. Implications and limitations of the findings are discussed.     

Characterization of gold nanoparticle binding to microtubule filaments

Microtubule (MT) protein filaments were used as templates for fabricating Au nanowires as a bottom-up approach for fabricating building blocks for future integrated circuits. Photochemical reduction methods were employed to form Au nanoparticles which bind and uniformly cover the MT filaments. Synthesis of the MT-templated Au nanowires was characterized using UV/vis spectroscopy and transmission electron microscopy (TEM). In addition, binding between the MT filaments and Au nanoparticles was investigated using surface enhanced Raman spectroscopy (SERS) and X-ray photoelectron spectroscopy (XPS) to establish the nature of the binding sites. A variety of functional groups were identified by SERS to interact with the Au including imidazole, sulfur, aromatic rings, amine, and carboxylate. The imidazole ring in the histidine is the most prominent functional group for Au binding. The results from these studies provide better understanding of the binding between Au and the biotemplate and give insight concerning methods to improve Au coverage for MT-templated Au nanowires.     

Grain-boundary chemistry and intergranular corrosion in alloy 825

Alloy 825, a former candidate material for radioactive high-level waste containers, was investigated to assess its thermal stability and the time-temperature conditions for sensitization. Alloy specimens with a carbon content of 0.01 wt pct in the mill-annealed (MA) and solution-annealed (SA) conditions were studied after thermal exposure to temperatures ranging from 600 °C to 800 °C for periods of up to 1000 hours. Sensitization was evaluated by using corrosion tests that were correlated to grainboundary chemistry analyses. Sensitized microstructures were found to contain M₂₃C₆-type carbides and a chromium-depleted region in the vicinity of the grain boundaries. Thermal aging at 700 °C for 100 hours resulted in the highest sensitization. While heat treatment at 640 °C showed a progressive development of sensitization with time, healing was found to occur after aging at 800 °C for 100 hours. The degree of sensitization, quantified by an equivalent chromium-depleted-zone size, correlates well with the corrosion rate in the nitric acid test. Thermodynamic models were used to calculate the interfacial chromium concentration, chromium depletion profile, and the depleted-zone width. Comparisons between experimental measurements and model calculations indicate that reliable prediction depends on the selection of key model parameters.     

Measurement of mechanical properties of electronic materials at temperatures down to 4.2 K

Operating temperatures of spacecraft components in the ‘subzero’ range are encountered during solar eclipse periods or when voyaging on deep-space missions. Moreover some spacecraft instruments or parts of them, e.g. sensors, have to be cooled to obtain an improved performance, e.g. in spacecraft missions like the infrared space observatory (ISO) and CryoSat. Materials utilized in the assembly of electronic circuits can be subjected to mechanical loading at cryogenic temperatures. [Semerad E, Scholze P, Schmidt M, Wendrinsky W. Effect of new cleaning liquids on electronic materials and parts. ESA metallurgy report no. 3275; January 2002, [1]].Within the present work the mechanical properties of electronic materials at cryogenic temperatures down to liquid helium temperature were analysed. Specifically the tensile properties of solders (63Sn37Pb, 62Sn36Pb2Ag, 60Sn40Pb, 96Sn4Ag, 50In50Pb, 70Pb30In, 96.8Pb1.5Ag1.7Sn, 96.5Sn3Ag0.5Cu), PC boards (MLB polyimide glass fibre, MLB epoxy glass fibre, MLB Thermount), conformal coatings (Arathane 5750, Sylgard 184, Scotchcast 280, Solithane 113, CV-1144-0, Mapsil 213, Conathane EN4/EN11) as well as OFE Cu were characterised at room temperature, at liquid nitrogen and at liquid helium temperature by tensile tests.The fracture surface of tested samples was examined by means of optical microscope and if necessary with scanning electron microscope.     

Surface roughness effects onmicroparticle adhesion

A new self-consistent model is developed to treat the static contact of a microparticle with a flat barrier in the presence of molecular adhesion and surface roughness. Separation between their mean datum planes is modeled considering the elastic deformation of the microparticle and surface. The contact pressure is computed from the Lennard-Jones law following the Derjaguin approximation. The elastic deflection of the mean datum plane is calculated from the effective pressure by the half-space elastic theory. Roughness is modeled by introducing a Gaussian distribution to the gap between the surfaces. An effective pressure is defined as the statistical average of the contact pressure over the roughness heights. A solution satisfying all of the above conditions gives a self-consistent method of modeling adhesion between the microparticle and the flat barrier. Using collocation methods the equations are discretized as a large system of nonlinear algebraic equations. A continuation method is used to find the multiple numerical solutions for the mean separation and the effective contact pressure. Finally, adhesive contacts of both smooth and rough surfaces are simulated in a comparative manner to elucidate the features of surface roughness in the presence of molecular adhesion. The standard deviation of the Gaussian distribution is used as a parameter to assess the effects of roughness on the pull-off force. It is shown that increasing surface roughness significantly reduces the pull-off force and decreases the tendency for the microsphere to snap-on and snap-off.