No abstract available

No abstract available

[en] This research calculates the crack growth rate (CGR) of the cooling pipe in the nuclear power plant. The crack is caused by the tension left in welding as well as by the coolant chemicals. There is, by far, no satisfactory deterministic model that can predict the crack growth rate. Although Macdonald's CEFM model is more close to a deterministic one, it requires accurate calculation of the crack-tip strain rate before the crack growth rate can be obtained by using Faraday's equation. The empirical equation used by the CEFM model for estimating the crack-tip strain rate is a drawback in making it a totally deterministic model. This research is set to make up this critical defect by applying non-linear finite element method to calculate the crack-tip strain rate. According to the metallic property, there is a plastic zone forming around the crack-tip. This research found when applying finite element method, the number of the elements (namely the mesh density) in the vicinity of the crack-tip has significant effects on the crack-tip strain obtained. Therefore, this research pioneered by using different mesh density in the vicinity of the crack-tip for the calculation of the crack-tip strain, the convergence of which is used for selecting the appropriate mesh density. The selected mesh density is then used to determine the dimension of the plastic zone, in which the crack-tip strain rate can be calculated. When the FEM results are compared with the experimental data, the nonlinear finite element method combined with CEFM shows a very satisfactory prediction capability

[en] ¹⁵N NMR studies of the coliphage λ cro repressor are presented. The protein has been uniformly labeled with ¹⁵N, and individual amino acids have been incorporated. Although the four C-terminal residues (63-66) were not located in the original crystallographic studies of the protein it has been proposed that the C-terminus is involved in DNA binding. These experiments give direct verification of that proposal. [¹⁵N] Amide resonances are assigned for residues 56, 62, 63, and 66 in the C-terminus by enzymatic digestion and by ¹³C-¹⁵N double-labeling experiments. ¹⁵N{¹H} nuclear Overhauser effects show that the C-terminus is mobile on a nanosecond time scale. Exchange experiments using distortionless enhancement via polarization transfer, which is sensitive to proton exchange on the 1/J/sub NH/(10 ms) time scale, indicate that the amide protons in the C-terminus are freely accessible to solvent. It is thus a flexible arm in solution. The binding of both specific operator and nonspecific DNA is shown to reduce both the mobility and the degree of solvent exposure of this arm. Two-dimensional ¹⁵N-¹H correlation experiments using ¹⁵N-labeled cro reveal inconsistencies with previously reported ¹H NMR assignments for the lysine amides. This result suggests that those assignments require reexamination, illustrating the utility of ¹⁵N labeling for obtaining ¹H resonance assignments of biomolecules. Furthermore, isomerization of the peptide bond of Pro-59, which has been previously suggested and which would significantly affect the properties of the C-terminal arm, is shown to not occur

No abstract available

[en] Microstructure in the reaction interface between molten Al and dense mullite have been studied by transmission electron microscopy to provide insight into mechanisms for forming ceramic-metal composites by reactive metal penetration. The reactions, which have the overall stoichiometry, 3Al number sign iz01 + (8+ x)A1 + 13 AlzO + xA1 + 6Si, were carried out at temperatures of 900, 1100, and 1200oC for 5 minutes and 60 minutes, and 1400oC for 15 minutes. Observed phases generally were those given in the above reaction, although their proportions and interracial rnicrostructures differed strongly with reaction temperature. After reaction at 900oC, a thin Al layer separated unreacted mullite from the cx-AlzO and Al reaction products. No Si phase was found near the reaction front. After 5 minutes at 1100''C, the nxtction front contained Si, ct-A120, and an aluminum oxide phase with a high concentration of Si. After 60 minutes at 11O(YC many of the cx-A120g particles were needle-shaped with a preferred orientation. After reaction at 1200oC, the reaction front contained a high density of Si particles that formed a continuous layer over many of the mullite grains. The sample reacted at 140VC for 15 minutes had a dense ct-A120J reaction layer less than 2m thick. Some isolated Si particles were present between the a-AlzO layer and the unreacted mullite. Using previously measured reaction kinetics data, the observed temperature dependence of the interracial microstructure have been modeled as three sequential steps, each one of which is rate-limiting in a different temperature range

[en] E. coli Lac repressor N-terminal DNA binding domain (headpiece) has previously been isolated by cleavage with clostripain, chymotrypsin, and trypsin to yield N-terminal fragments of 51, 56, and 59 amino acids respectively. This method is both inefficient and limited by the specificity of available proteases. To circumvent these difficulties, the authors are cloning the DNA sequence encoding various lengths of the lac1 gene into a plasmid containing the P/sub L/ promoter from bacteriophage γ as well as an improved ribosome binding site. In the authors initial efforts they cloned a 56 amino acid headpiece (pAK3-4); they describe here the isolation and characterization of this headpiece. It has been shown that ¹⁵N NMR can be extremely useful in investigating the structure and dynamics of the bacteriophage γ cro repressor protein with its binding site O/sub R/3. Uniformly labelled lac headpiece (56 aa) has been prepared by growth of E. coli transformed with pAK3-4 in a minimal medium containing ¹⁵N ammonium sulfates as the sole nitrogen source. NMR experiments were used to measure ¹⁵N-{¹H} NOEs which are sensitive to mobility on a nanosecond timescale. They are thus able to observe changes in dynamics of individual amino acid side chains and backbone nitrogen of the peptide upon binding operator DNA

[en] The Jeffreys-Born integral in the WKB approximation is generalized to include the Coulomb potential. The Weyl-transform is used to obtain phase functions for potentials frequently used in fitting modified Coulomb scattering data. (orig.)

[en] The Reactor Coolant Sytem (RCS) of Combustion Engineering System 80 Nuclear Power Plant for Washington Public Power Supply System Unit 3 is modeled for a real-time operator's training simulator using the SEL 32/77 computer. The RCS consists of a reactor vessel; two parallel heat transfer loops, each containing a U-tube steam generator and two reactor coolant pumps; and a pressurizer connecting to one of the reactor vessel outlet pipe. The pressurizer pressure and water level is calculated by considering the RCS, including the pressurizer, as a two-phase tank and using the overall mass conservation on the entire system. The off-line testing of the validity of the RCS model includes the solid and empty conditions in the pressurizer due to the charging and letdown flows, the slow depressurization of the steam due to a stuck-open relief valve to draw a steam bubble in the reactor pressure vessel, the generation of noncondensible gas due to cladding failure, and the reversed coolant flows or the natural recirculation flows due to the combinatory trips of reactor coolant pumps. The model transients of these tests are presented and compared, whenever possible, with either the design report or the safety analysis of the plant

[en] Partial remelting behaviors of Mg-11Al-0.5Zn alloy and TiB₂/Mg-11Al-0.5Zn composite are investigated by the isothermal holding at the temperatures of 510 and 545 deg.C. The result shows that, although, the TiB₂ addition has no influence on the remelting characteristics of eutectic and liquidus temperatures of the composite, it has a great effect on the remaining liquid located at the grain boundary. With a processing temperature of 545 deg.C, the remaining liquid is dramatically decreased in the composite, compared with that in the unreinforced matrix alloy. Furthermore, the solid α-Mg grain size in the composite does not significantly reduce compared with that in the matrix alloy; however, the microhardness of the α-Mg grain is increased in the partial remelting microstructure in the composite. Although most of the TiB₂ particulates are present in the remaining liquid phases distributed at the grain boundaries, some liquid islands are entrapped inside α-Mg grains. As a consequence, the TiB₂ particulates can eventually reside within the grains after complete solidification. However, it is difficult that TiB₂ particulate is directly incorporated (engulfed) into the Mg-11Al-0.5Zn matrix alloy during the regular solidification process. Furthermore, the DTA, SEM and XRD results reveal that no reaction occurs between the TiB₂ particulate and matrix alloy during the partial remelting process, which indicates that TiB₂ particulate is thermodynamically stable at the Mg-11Al-0.5Zn matrix alloy