Factors affecting performance of engineered barriers

For the Yucca Mountain Viability Assessment (VA), a reference design was tentatively selected` In September 1997, and a series of model abstractions are being prepared for the performance assessment (PA) of that design. To determine the sensitivity of peak dose rate at the accessible environment to engineered components, several design options were subjected to the PA models available late in FY97

Major repository design issues

The Yucca Mountain Project is focused on producing a four-part viability assessment in late FY98. Its four components (design, performance assessment, cost estimate, and licensing development plan) must be consistent. As a tool to clarify design issues and compare design and performance assessment options, a series of repository sketches were developed for the sequential time phases of a repository. The boundaries of the time phases correspond to evolution In the engineered barrier system (EBS)

Neutron activation of four ferritic steels

Four ferritic steels (Table 1) were compared using two simplified fusion reactor geometries. In the MFE geometry, a 14.1-MeV point source was centered inside a 5 m radius vessel. The 2 cm thick first wall was surrounded by a reflector consisting of 78 cm Li, 18 cm graphite, 2 cm B/sub 4/C, and a 10 cm thick outer wall. In the ICF geometry, the neutron source was in a compressed plasma (rho R = 3 g/cm/sup 2/), and the region between 0.5 and 2.0 m was filled with Li at 50% of the natural density

Sequence and Expression of Amphioxus Alkali Myosin Light Chain (AmphiMLC-alk) Throughout Development: Implications for Vertebrate Myogenesis

AbstractThe lower chordate amphioxus, widely considered the closest living invertebrate relative of the vertebrates, is a key organism for understanding the relationship between gene duplications and evolution of the complex vertebrate body plan. In tetrapod vertebrates, the alkali myosin light chain genes (MLC-alk), which code for proteins associated with the globular head of the myosin heavy chain, constitute a large family with stage-, tissue-, and fiber-type-specific expression of different isoforms thought to have arisen by duplication of a single ancestral gene. In protostome invertebrates, e.g., arthropods, molluscs, and nematodes, only one MLC-alk gene has been found, but the number of such genes in deuterostome invertebrates and lower vertebrates is unknown. The present report, describing the sequence and expression throughout development of the amphioxus gene for alkali myosin light chain (AmphiMLC-alk), thus fills a major gap in understanding the relation between gene duplication and increasing diversity of muscle-cell types. A full-length clone (1 kb) of AmphiMLC-alk was isolated from a larval amphioxus cDNA library. It coded for a 149-amino-acid protein most closely related to the vertebrate embryonic form of MLC-alk. Southern blot analysis revealed only one copy of AmphiMLC-alk and suggested that it is the only MLC-alk gene in amphioxus. Northern blot analysis indicated that this gene produces only one transcript, which is expressed at all stages of development and in adults. In situ hybridizations showed expression initially in the myotomes of somites 2-5 of neurula embryos and soon thereafter in the myotomes of somite 1 and of newly forming somites progressively added posteriorly. Myotomal expression continues throughout larval development and into the adult stage as the myotomal cells differentiate into striated, mononucleate muscle cells—unlike vertebrate striated muscle cells, those of amphioxus never become multinucleate. In late larvae and adults myotomal expression of AmphiMLC-alk is localized along the medial edge of the myotome and at the ends of the cells. This is the first demonstration of intracellular localization of MLC transcripts in muscle cells of any animal. Expression of AmphiMLC-alk was also detected in smooth muscles as well as in striated muscles not derived from the myotome. These expression data are consistent with the Southern blot analysis in suggesting that there is only one MLC-alk gene in amphioxus. Thus, duplication of an ancestral vertebrate MLC-alk gene probably occurred after the vertebrate and amphioxus lineages split. We conclude that development of a segmented axial musculature preceded the evolution of multiple MLC-alk isoforms, which evidently arose about the time of multinucleation. Since myogenesis in amphioxus is similar to but far simpler than myogenesis in vertebrates at both the structural and gene levels, an understanding of myogenesis in amphioxus can give insights into both the evolutionary history and the detailed mechanisms of vertebrate myogenesis

Pulse Star inertial confinement fusion reactor

Pulse Star is a pool-type ICF reactor that emphasizes low cost and high safety levels. The reactor consists of a vacuum chamber (belljar) submerged in a compact liquid metal (Li/sub 17/Pb/sub 83/ or lithium) pool which also contains the heat exchangers and liquid metal pumps. The shielding efficiency of the liquid metal pool is high enough to allow hands-on maintenance of (removed) pumps and heat exchangers. Liquid metal is allowed to spray through the 5.5 m radius belljar at a controlled rate, but is prohibited from the target region by a 4 m radius mesh first wall. The wetted first wall absorbs the fusion x-rays and debris while the spray region absorbs the fusion neutrons. The mesh allows vaporized liquid metal to blow through to the spray region where it can quickly cool and condense. Preliminary calculations show that a 2 m thick first wall could handle the mechanical (support, buckling, and x-ray-induced hoop) loads. Wetting and gas flow issues are in an initial investigation stage

Origin of Low-Energy Excitations in Charge-Ordered Manganites

The low-energy excitations in the charge-ordered phase of polycrystalline La0.25Ca0.75MnO3 are explored by frequency-domain terahertz spectroscopy. In the frequency range from 4 cm^-1 to 700 cm^-1 (energies 0.4 meV to 90 meV) and at temperatures down to 5 K, we do not detect any feature that can be associated with the collective response of the spatially modulated charge continuum. In the antiferromagnetically ordered phase, broad absorption bands appear in the conductivity and permittivity spectra around 30 cm^-1 and 100 cm^-1 which are assigned to former acoustic phonons optically activated due to a fourfold superstructure in the crystal lattice. Our results indicate that characteristic energies of collective excitations of the charge-ordered phase in La0:25Ca0:75MnO3, if any, lie below 1 meV. At our lowest frequencies of only few wavenumbers a strong relaxation is observed above 100 K connected to the formation of the charge-ordered state.Comment: 5 pages, 3 figure