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Understanding gas flows in capillaries has many applications in modeling the transport of gases in nano-structured, porous, or fractured media. A network of capillaries can often approximate these media, and also information on gas-surface interactions obtained from capillary experiments can be used for modeling flows in these media. Experimental data on flows of different mixtures of He-Ar, He-N2, and He-air in the slip regime and in stainless steel capillaries at high temperatures were obtained by using a two-bulb apparatus. An accurate expression for the thermal creep slip coefficients with the Lennard-Jones potential parameters and diffuse-specular reflection gas-surface interaction conditions were then used to obtain the accommodation coefficients for the different gases. The experimental data are best described with values of accommodation coefficients in the range of 0.1–0.3 for He and 0.5–1.0 for Ar, N2, and air. The use of values in this range is suggested for modeling gaseous flows in capillaries and nano-structured, porous, or fractured media if other direct measured values for a particular medium are unavailable. [ABSTRACT FROM AUTHOR]

The diffusion of fission products (FPs) in reactor materials affects the nuclear source term. The diffusion coefficient itself is measured through various techniques. In the release method, it is of interest to know the initial distribution of the FPs in nuclear graphite or other materials from an exterior measurement like mass surface flux or cumulative mass release. In this paper, a Fredholm integral of the first kind is considered, relating the initial distribution to the cumulative release fraction of a diffusant from a spherically symmetric body. The Tikhonov regularization, conjugate gradient least-squares (CGLS) method, and algebraic reconstruction technique (ART) with nonnegativity and conserved mass constraints were compared to fractional release data from a simulated linear profile using data for Cs diffusion in a 0.32-cm sphere NBG-18 at 1090 K. The Tikhonov regularization was shown to provide a better estimation of the initial linear distribution than the CGLS and ART methods. The performance of the Tikhonov regularization was further examined with simulated constant, quartic, and exponential initial distributions. The Tikhonov regularization provided a reasonable recovery of the exponential profile, while the estimation of the linear, constant, and quartic profiles suffers from several issues. [ABSTRACT FROM AUTHOR]

Measurements and data are reported for the total hemispherical emissivity of Grade 91 steel [ASTM International (ASTM) A387 Grade 91] for the temperature range of 400 K to 1048 K using ASTM standard C835-06. The surfaces studied included (1) an electric discharge machining (EDM) cut, (2) Grade 91 steel sandblasted with 320-grit-sized alumina beads, (3) EDM-cut Grade 91 steel oxidized in air, and (4) Grade 91 steel sandblasted with 320-grit-sized alumina beads and oxidized in air. The EDM-cut Grade 91 steel was oxidized at 873 K and 1023 K for 1, 3, and 5 h. The 320-grit sandblasted Grade 91 steel was oxidized at 1023 K for 5 h. Air oxidations were carried out in a three-zone furnace at fixed airflow. It appears that under some conditions, Grade 91 steel can reach the emissivities required for passive heat removal during an air ingress accident. [ABSTRACT FROM AUTHOR]

The effect of long-term oxidation on the total hemispherical emissivity of Type 316L stainless steel (SS 316L) is of interest in nuclear plant safety and is reported on here. ASTM standard C835-06 [American Society for Testing and Materials, 2006] was used for measuring the total hemispherical emissivity of this material for the following surface conditions: (1) "as-received" from the manufacturer (essentially unoxidized) and (2) oxidized in air at 573 K for up to 3000 h. The emissivity of the as-received samples varied within the range from 0.24 at 434 K to 0.34 at 1026 K. Oxidation in air at 573 K for 500 h increased the emissivity range of the oxidized sample from 0.28 at 429 K to 0.38 at 1096 K. There was no further significant change in emissivity observed following an increase in the oxidation time from 500 to 3000 h. It is suspected that the emissivity ceased to increase during the additional oxidation time because of chromium oxide that formed on the SS 316L surface inhibiting further oxidation. [ABSTRACT FROM AUTHOR]

Total hemispherical emissivity of Alloy 617 was measured for applications in very high temperature reactors with apparatus based on ASTM Standard C835-06. The emissivity data were obtained for the following surface conditions: (1) as-received (rolled sheets) from manufacture, (2) sandblasted with aluminum oxide beads, (3) oxidation in air at temperature of 1153 K, and (4) coated with graphite powder. For the as-received Alloy 617, emissivity increased from 0.26 to 0.34 over the temperatures 593 K to 1164 K. Sandblasting Alloy 617 with alumina beads increased the emissivity to 0.46 to 0.73 in the temperature range 600 to 1300 K (emissivity increased further when higher grit size beads were used). The oxidation of Alloy 617 gave a slight increase in emissivity from 900 to 1250 K with larger increases above 1100 K. Coating of graphite onto as-received and 60-grit sandblasted increased the emissivity by roughly 0.12 and 0.20, respectively, over the measured temperature range. [ABSTRACT FROM AUTHOR]