Development of a new apparatus for modeling deep excavation related problems in geotechnical centrifuge

Underground space is commonly exploited both to maximise the utility of costly land in urban development and to reduce the vertical load acting on the ground. Deep excavations are carried out to construct various types of underground infrastructure such as deep basements, subways and service tunnels. Although the soil response to excavation is known in principle, designers lack practical calculation methods for predicting both short- and long-term ground movements. As the understanding of how soil behaves around an excavation in both the short and long term is insufficient and usually empirical, the judgements used in design are also empirical and serious accidents are common. To gain a better understanding of the mechanisms involved in soil excavation, a new apparatus for the centrifuge model testing of deep excavations in soft clay has been developed. This apparatus simulates the field construction sequence of a multi-propped retaining wall during centrifuge flight. A comparison is given between the new technique and the previously used method of draining heavy fluid to simulate excavation in a centrifuge model. The new system has the benefit of giving the correct initial ground conditions before excavation and the proper earth pressure distribution on the retaining structures during excavation, whereas heavy fluid only gives an earth pressure coefficient of unity and is unable to capture any changes in the earth pressure coefficient of soil inside the zone of excavation, for example owing to wall movements. Settlements of the ground surface, changes in pore water pressure, variations in earth pressure, prop forces and bending moments in the retaining wall are all monitored during excavation. Furthermore, digital images taken of a cross-section during the test are analysed using particle image velocimetry to illustrate ground deformation and soil–structure interaction mechanisms. The significance of these observations is discussed.

Open cycle thorium–uranium-fuelled nuclear energy systems

The potential for countries that currently have a nominal nuclear energy infrastructure to adopt thorium–uranium-fuelled nuclear energy systems, using a once-through ‘open’ nuclear fuel cycle, has been suggested by the International Atomic Energy Agency. This review paper highlights generation II, III and III+ nuclear energy technologies that could potentially adopt an open thorium–uranium fuel cycle and qualitatively highlights the main differences between the open thorium–uranium and open uranium fuel cycles.

Reliability analysis of deep excavation based on a semi-empirical approach

The design and construction of deep excavations in urban environment is often governed by serviceability limit state related to the risk of damage to adjacent buildings. In current practice, the assessment of excavation-induced building damage has focused on a deterministic approach. This paper presents a component/system reliability analysis framework to assess the probability that specified threshold design criteria for multiple serviceability limit states are exceeded. A recently developed Bayesian probabilistic framework is used to update the predictions of ground movements in the later stages of excavation based on the recorded deformation measurements. An example is presented to show how the serviceability performance for excavation problems can be assessed based on the component/system reliability analysis. © 2011 ASCE.

Numerical modeling of deep soil mixing excavation support

The finite element method (FEM) is growing in popularity over the pressure diagram/hand calculation method for analysis of excavation systems in general and deep soil mixing excavations in particular. In this paper, a finite element analysis is used to study the behavior of a deep mixed excavation. Through the use of Plaxis (a FEM software program), the construction sequence is simulated by following the various construction phases allowing for deflections due to strut or anchor installation to be predicted. The numerical model used in this study simulates the soil cement columns as a continuous wall matching the bending stiffness of the actual wall. Input parameters based on laboratory tests and modeling assumptions are discussed. An example of the approach is illustrated using the Islais Creek Transport/Storage Project in San Francisco, California. Copyright ASCE 2006.

Deep mixing for excavation support: Design issues

Deep mixing is effectively used in excavations both in conjunction with and in substitution of traditional techniques, where it results in more economical and convenient solutions for the stability of the system and the prevention of seepage. Deep mixed walls constructed as part of a soldier pile and tie-backs system act also as temporary support, prevent seepage like a sheet pile wall, but require a lower amount of steel. The deep mixed treatment can also contribute to the stability of the wall system against deep-seated failures. Although deep mixing is currently used for excavation control in numerous projects, no standard procedure has been developed and the different applications have not been evaluated. As this technique emerges as a more economical and effective alternative to traditional excavation shoring, there is a need for guidelines describing proven procedures for evaluation of design, analysis and construction. This paper presents comparisons in the design of excavation support using deep mixing and other traditional techniques. Issues important for design, analysis, and construction of deep mixed excavation walls are also discussed.

Conceptual design of Americium nuclear battery for space power applications

This work presents simplified 242mAm-fueled nuclear battery concept design featuring direct fission products energy conversion and passive heat rejection. Optimization of the battery operating characteristics and dimensions was performed. The calculations of power conversion efficiency under thermal and nuclear design constraints showed that 5.6 W e/kg power density can be achieved, which corresponds to conversion efficiency of about 4%. A system with about 190 cm outer radius translates into 17.8 MT mass per 100 kW e. Total power scales linearly with the outer surface area of the battery through which the residual heat is rejected. Tradeoffs between the battery lifetime, mass, dimensions, power rating, and conversion efficiency are presented and discussed. The battery can be used in a wide variety of interplanetary missions with power requirements in the kW to MW range. Copyright © 2007 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

The smallest thermal nuclear reactor

There is a growing need for very small nuclear reactors for space applications and as portable high-intensity neutron sources. This technical note investigates the question of what is the smallest possible thermal reactor. It was found that the smallest reactor is a spherically shaped solution of 242mAm(NO3)3 in water. The weight of such a reactor is 4.95 kg with 0.7 kg of 242mAm nuclear fuel. The radius of the reactor in this case is 9.6 cm.

Americium nuclear battery for space power applications

The work presents simplified242mAm fueled nuclear battery concept design featuring direct fission products energy conversion and passive heat rejection. The performed calculations of power conversion efficiency under thermal and nuclear design constraints showed that 14 W/kg power density can be achieved, which corresponds to conversion efficiency of about 6%. Total power of the battery scales linearly with its surface area. 144 kW of electric power can be produced by a nuclear battery with an external radius of about 174 cm and total mass of less than 10300 kg. The mass of242m Am fuel for such a system is 3200 gram.

Ultra-thin 242mAm fuel elements in nuclear reactors

There is a growing interest in using 242mAm as a nuclear fuel. The advantages of 242mAm as a nuclear fuel derive from the fact that 242mAm has the highest thermal fission cross section. The thermal capture cross section is relatively low and the number of neutrons per thermal fission is high. These nuclear properties make it possible to obtain nuclear criticality with ultra-thin fuel elements. The possibility of having ultra-thin fuel elements enables the use of these fission products directly, without the necessity of converting their energy to heat, as is done in conventional reactors. There are three options of using such highly energetic and highly ionized fission products. 1. Using the fission products themselves for ionic propulsion. 2. Using the fission products in an MHD generator, in order to obtain electricity directly. 3. Using the fission products to heat a gas up to a high temperature for propulsion purposes. In this work, we are not dealing with a specific reactor design, but only calculating the minimal fuel elements' thickness and the energy of the fission products emerging from these fuel elements. It was found that it is possible to design a nuclear reactor with a fuel element of less than 1 μm of 242mAm. In such a fuel element, 90% of the fission products' energy can escape.

Sensitivity analysis and optimization of the nuclear fuel cycle

A sensitivity study has been conducted to assess the robustness of the conclusions presented in the MIT Fuel Cycle Study. The Once Through Cycle (OTC) is considered as the base-line case, while advanced technologies with fuel recycling characterize the alternative fuel cycles. The options include limited recycling in LWRs and full recycling in fast reactors and in high conversion LWRs. Fast reactor technologies studied include both oxide and metal fueled reactors. The analysis allowed optimization of the fast reactor conversion ratio with respect to desired fuel cycle performance characteristics. The following parameters were found to significantly affect the performance of recycling technologies and their penetration over time: Capacity Factors of the fuel cycle facilities, Spent Fuel Cooling Time, Thermal Reprocessing Introduction Date, and incore and Out-of-core TRU Inventory Requirements for recycling technology. An optimization scheme of the nuclear fuel cycle is proposed. Optimization criteria and metrics of interest for different stakeholders in the fuel cycle (economics, waste management, environmental impact, etc.) are utilized for two different optimization techniques (linear and stochastic). Preliminary results covering single and multi-variable and single and multi-objective optimization demonstrate the viability of the optimization scheme.

Preliminary evaluation of a nuclear scenario involving innovative gas cooled reactors

In order to guarantee a sustainable supply of future energy demand without compromising the environment, some actions for a substantial reduction of CO 2 emissions are nowadays deeply analysed. One of them is the improvement of the nuclear energy use. In this framework, innovative gas-cooled reactors (both thermal and fast) seem to be very attractive from the electricity production point of view and for the potential industrial use along the high temperature processes (e.g., H 2 production by steam reforming or I-S process). This work focuses on a preliminary (and conservative) evaluation of possible advantages that a symbiotic cycle (EPR-PBMR-GCFR) could entail, with special regard to the reduction of the HLW inventory and the optimization of the exploitation of the fuel resources. The comparison between the symbiotic cycle chosen and the reference one (once-through scenario, i.e., EPR-SNF directly disposed) shows a reduction of the time needed to reach a fixed reference level from ∼170000 years to ∼1550 years (comparable with typical human times and for this reason more acceptable by the public opinion). In addition, this cycle enables to have a more efficient use of resources involved: the total electric energy produced becomes equal to ∼630 TWh/year (instead of only ∼530 TWh/year using only EPR) without consuming additional raw materials. © 2009 Barbara Vezzoni et al.