Fine structure of the optic cusion in the asteroid Nepanthia belcheri

The optic cushion of Nepanthia belcheri (Perrier) is a prominent pigmented sense organ situated on the oral surface below the terminal tentacle. The distal region contains up to 170 optic cups, whilst proximally are numerous pyriform glandular cells traversed by supporting fibres. The outer margin of the optic cup is formed by alternating pigmented and photoreceptor cells. The pigmented cells contain numerous densely staining granules of scarlet pigment. The distal ends of the photoreceptors are elaborated into many long microvilli regularly arranged about a modified cilium. There is a clear circumciliary space delimiting the cilium from the microvilli.     

Physicochemical mechanisms controlling the periodic release of flammable gases from hanford tanks

Hanford tanks contain more than 60 million gallons of high-level wastes produced by decades of extracting plutonium from irradiated uranium fuel. The wastes were concentrated to a thick slurry consistency by evaporation prior to storage to minimize space. The resulting concentrated waste properties introduced unanticipated, detrimental conditions affecting workers' and the public's health and safety and involving the release of potentially flammable gases. The released gases consist primarily of hydrogen, nitrous oxide, and ammonia. Dilution and sluicing were initially proposed to mitigate the flammable gas safety conditions. As a result of evaluations, the mechanisms and conditions that are thought to control the accumulation and spontaneous release of flammable gases were identified and confirmed. The technical rationale was established for developing operational approaches to mitigate the periodic generation of flammable gases in existing tanks and to avoid any reoccurrence of this serious safety problem during future waste management activities. The chemistry of the two highest risk tanks was examined to test the potential for reversing the conditions causing gas buildup and the consequences of sluicing without appropriate chemical conditioning. The identified mechanisms apply equally to the remaining flammable gas tanks at Hanford as well as to other waste tanks in the DOE complex, particularly those at Savannah River. Passive means of mitigating the flammable gas condition require less than 1:1 dilution, and sluicing wastes from tank 106-C can be accomplished without creating a flammable gas condition. Carbonate equilibria reactions and their effect on aluminum speciation are largely overlooked and provided the key for explaining the episodic release of flammable gases from tank wastes. The reaction of atmospheric carbon dioxide with a sodium hydroxide-rich waste solution produces carbonate precipitates. More importantly, this reaction lowers the pH of the waste and precipitates aluminum hydroxide as a gel. The wastes contain substantial amounts of complexing agents such as ethylene diamine tetraacidicacid (EDTA), hydroxy ethylene diamine triacidic acid (HEDTA), and their degradation products. These complexing agents stabilize the aluminum hydroxide gel together with chromium, manganese and iron hydroxides, and oxybydroxides under the resulting pH conditions. These complex species may coprecipitate and accumulate as a metastable layer in the middle and lower levels of th tank. The complexed aluminum hydroxide acts as a binding agent trapping other particulates in a microcrystalline mat. Microcrystalline particles such as sodium nitrite provide the structural strength for the mat. Once the gas accumulation below the gel layer achieves a critical buoyancy sufficient to rupture the microcrystalline mat, a gas release event occurs. The cycle of gas buildup and release continues each time the buoyancy of the trapped gas exceeds the hydrostatic pressure and the gels' plasticity modulus. Stokes Law predicts a particle settling rate in the tank of less than 50 days, well within the bistorical periodicity of GREs. Laboratory tests, forming the basis of a recent patent application, verify that large quantities of complexed aluminum hydroxide gel are produced by passing carbon dioxide through simulated waste solutions (Hobl, 1993) equivalent to those found in tank 101-SY. It was confirmed that a simple adjustment of pH witll redissolve the gel, thereby reducing viscosity and safely facilitating continuous flammable gas release. Additional experiments were undertaken to provide a basis for understanding the role of complexed aluminum hydroxides in the CO₂/NaOH/Al(OH)₃ (complexing agents)/NaAlO₂ system. This article examines a plausible mechanism for the periodic release of flammable gas and considerations for: (1) remediating existing flammable gas tanks through a combination of chemical treatment and mixer pumps; (2) diluting, combining, retrieving, and storing wastes; (3) preventing clogging of transfer lines; (4) sludge and soil washing; and (5) cribs, ponds, basins, and ground-water cleanup. This study provides a singificant breakthrough for tank waste management by explaining key mechanisms controlling episodic release of flammable gases. The breakthrough provides the bases for removing the tanks classified as flammable gas from the wathclist and has broad operational applications with a potential for billions of dollars in cost savings.     

Petroleum exploitation and contract terms in developing countries after the oil price collapse

In resource taxation the critical fiscal requirement is a scheme which performs the twin functions of ensuring that projects which are viable on a pre-tax basis remain so post-tax, and that the host government obtains a reasonable share of the economic rent. Royalty, income tax, production-sharing and the resource rent tax are examined in the light of their efficiency at meeting these two criteria. The resource rent tax is probably the most appropriate fiscal instrument because this tax permits a threshold rate of return to be earned before the tax becomes payable. One drawback is that during the early years of a project the host government may receive little or no revenue. An advance resource rent tax can be used to eliminate this shortcoming. A further problem is the disincentive to exploration which can be produced.     

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