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111.
The natural nuclear reactors were geological arrangements of uranium and water where, like the 2 Ga-old uranium deposits discovered in Oklo (Gabon, Africa), uranium chain fission processes took place. Ten years after its discovery the phenomenon of Oklo is still neglected in Precambrian evolutionary studies. We consider some probable reasons for this and show that natural reactors might have been important, specific, localized sources of ionizing radiation during both the criticality and shut-down periods. Some of the long-lived fission products which migrated from the reactor core could also have been effective radiation energy sources after fixation in the environment or upon uptake by the earliest forms of living matter. The results presented here concern the examination of conditions for nuclear criticality on the Precambrian Earth, the dose-rates of ionizing radiation available and the estimate of the number of natural nuclear reactors that could have been active in the past. 相似文献
112.
113.
This study is concerned with the influence of the glacial-isostatic adjustment caused by the last Pleistocene deglaciation
on the present-day sea level. The viscoelastic deformation caused by the time-variable ice and ocean loads is simulated by
computing the resulting perturbations for a spherical, self-gravitating, incompressible, Maxwell-viscoelastic earth model.
The associated variation of the earth rotation is described in terms of the Liouville equation, which is solved by means of
the MacCullagh formulae. This allows the determination of the vertical displacement and geoid height and, thus, the solution
of the sea-level equation. We test several viscosity and ice models and evaluate them by comparison of the computed response
with the Holocene relative sea-level record. Using the optimum combination of viscosity and ice models, we then estimate the
influence of the last Pleistocene deglaciation on the tide-gauge measurements. A comparison between the observational and
residual linear trends for the tide-gauge measurements shows a significant reduction of the variance and geographical variability
for the latter, in particular for the formerly ice-covered regions of North America and Scandinavia. The favoured value determined
for the global mean sea-level rise is (1.46±0.2) mm a−1. 相似文献
114.
The conditions governing side bar origination in canals and river channels are considered based on the analysis of the data of experiments conducted in the State Institute of Hydrology. The scheme of bed load transport for the side bar-type channel process is presented. The results of calculations using a formula proposed earlier to describe the velocity of side bar displacement are compared with the experimental and field data. 相似文献
115.
116.
Z. Mrz 《国际地质力学数值与分析法杂志》1980,4(1):45-55
A comparative discussion of constitutive rate equations generated by using the hypoelasticity and plasticity theories is presented. If is shown that these two approaches are equivalent for the loading process but differ essentially in defining the unloading (or reverse loading) paths. The multisurface plasticity formulation proves to be more convenient in incorporating memory rules of particular loading events, whereas loading-unloading criteria used in hypoelasticity may result in an unrealistic prediction of material response for some loading paths, and in particular, for cyclic loading. The violation of the continuity condition between loading–unloading domains in proposed constitutive relations3,4 limits their applicability to some particular classes of loading paths. 相似文献
117.
On the surface of a degerate star, a slow nuclear combustion process with a wake of escaping matter can exist. The evolution of this process has been studied by solving the nonadiabatic, time-dependent hydrodynamic equations. It has been found that this flow is almost stationary and is stable against small spherical symmetric perturbations. 相似文献
118.
119.
Gamma-ray bursts: post-burst evolution of fireballs 总被引:3,自引:0,他引:3
Y. F. Huang Z. G. Dai D. M. Wei & T. Lu 《Monthly notices of the Royal Astronomical Society》1998,298(2):459-463
The post-burst evolution of fireballs that produce γ-ray bursts (GRBs) is studied, assuming the expansion of fireballs to be adiabatic and relativistic. Numerical results as well as an approximate analytic solution for the evolution are presented. Owing to the adoption of a new relation between t R and γ, our results differ markedly from previous studies. Synchrotron radiation from the shocked interstellar medium is carefully calculated, using a conventional set of equations. The observed X-ray flux of GRB afterglows can be reproduced easily. Although the optical afterglows seem much more complicated, our results can still present a rather satisfactory agreement with observations. We also find that the expansion will no longer be highly relativistic about 4 d after the main GRB. We thus suggest that the marginally relativistic phase of the expansion should be investigated so as to check the afterglows observed a week or more later. 相似文献
120.
Summary The morphology of the Wadati-Benioff zone in the Kermadec region, based on the distribution of 1100 earthquake foci, verified the existence of an intermediate aseismic gap and its relation to active andesitic volcanism, and the non-uniformity of subduction due to the hampering effect of the main structural features of the subducting Pacific plate. Two cycles of the recently active subduction in the Tonga-Kermadec island arc were found. 相似文献