Stochastic contraction-based observer and controller design algorithm with application to a flight vehicle

This paper focuses on a stochastic version of contraction theory to construct observer-controller structure for a flight dynamic system with noisy velocity measurement. A nonlinear stochastic observer is designed to estimate the pitch rate, the pitch angle, and the velocity of an aircraft example model using stochastic contraction theory. Estimated states are used to compute feedback control for solving a tracking problem. The structure and gain selection of the observer is carried out using Itô's stochastic differential equations and the contraction theory. The contraction property of integrated observer-controller structure is derived to ensure the exponential convergence of the trajectories of closed-loop nonlinear system. The upper bound of the state estimation error is explicitly derived and the efficacy of the proposed observer-controller structure has been shown through the numerical simulations.     

ASCE Journal of Management in Engineering: Review of the years 1985–2002

This study considers the content of the papers published by ASCE’s Journal of Management in Engineering, which has witnessed a growth in number of papers and breadth of participation, particularly of international origin, during the 1985–2002 period. The content is analyzed in two main dimensions: the type of subject and its composing topics, and the type of contributors to the journal. The analysis shows the evolution of the engineering management discipline as represented in the journal. Over the years the discipline has been enriched by an increasing number of contributions on corporate strategies and programs, organizational change, and cultures issues, as well as a growing interest in project management topics, such as quality planning and evaluation of alternative project delivery systems. The number of papers by practitioners, initially the largest contributing group to the journal, has declined significantly over the years.     

Point Scattered Function (PScF) for fast neutron radiography

Fast neutron radiography opened up a new range of possibilities to image extremely dense objects. The removal of the scattering effect is one of the most challenging problems in neutron imaging. Neutron scattering in fast neutron radiography did not receive much attention compared with X-ray and thermal neutron radiography. The purpose of this work is to investigate the behavior of the Point Scattered Function (PScF) as applied in fast neutron radiography.The PScF was calculated using MCNP as a spatial distribution of scattered neutrons over the detector surface for one emitting source element. Armament and explosives materials, namely, Rifle steel, brass, aluminum and trinitrotoluene (TNT) were simulated. Effect of various sample thickness and sample-to-detector distance were considered. Simulated sample geometries included a slab with varying thickness, a sphere with varying radii, and a cylinder with varying base radii. Different neutron sources, namely, Cf-252, DT as well as DD neutron sources were considered. Neutron beams with zero degree divergence angle; and beams with varying angles related to the normal to the source plane were simulated.Curve fitting of the obtained PScF, in the form of Gaussian function, were given to be used in future work using image restoration codes. Analytical representation of the height as well as the Full Width at Half Maximum (FWHM) of the obtained Gaussian functions eliminates the need to calculate the PScF for sample parameters that were not investigated in this study.     

Neutronic characterization of the MEGAPIE target

The MEGAPIE project aimed to design, build and operate a liquid metal spallation neutron target of about 1 MW beam power in the SINQ facility at the Paul Scherrer Institut (Villigen, Switzerland). This project is an important step in the roadmap towards the demonstration of the accelerator driven system (ADS) concept and high power liquid metal targets in general. Following the design phase, an experimental program was defined to provide a complete characterization of the facility by performing a “mapping” of the neutron flux at different points, from the center of the target to the beam lines. The neutronic performance of the target was studied using different experimental techniques with the goals of validating the Monte Carlo codes used in the design of the target; additionally, the performance was compared with the solid lead targets used before and after the MEGAPIE experiment.     

Sectored Compact Space Reactor (SCoRe) concepts with a supplementary lunar regolith reflector

Design concepts of the Sectored Compact Space Reactor for Small power (SCoRe-S) have been developed for the avoidance of single-point failures in reactor cooling and energy conversion and a wide range of thermal powers. These modular, fast neutron spectrum, lithium cooled reactors with 16.0 cm thick BeO radial reflector are designed for at least +$2.00 hot-clean excess reactivity, and with a sufficient reactivity shutdown margin. They employ ¹⁵⁷GdN additives in the UN fuel and a 0.10 mm thick coating of ¹⁵⁷Gd₂O₃ on the outer surface of the reactor vessel to ensure that the bare reactors, when submerged in wet sand and flooded with seawater following a launch abort accident, remain at least ?$1.00 subcritical. In addition to identifying the smallest SCoRe-S concept that satisfies the design reactivity requirements, the benefit of using a lunar regolith as a supplementary reflector to decrease the thickness of the BeO radial reflector and hence, the launch mass of the SCoRe-S concepts for a lunar outpost is investigated. Calculations performed using MCNP5 confirmed that SCoRe-S₇ with a 16 cm thick BeO reflector is the smallest to satisfy the stated reactivity requirements. Results also show that a lunar regolith reflector alone is inadequate for this reactor to achieve a critical state at the beginning of life. However, when the regolith is used in conjunction with a BeO reflector of a reduced thickness, this reactor not only becomes critical, but also satisfies the reactivity design requirements at a significantly reduced launch mass. Using a supplementary reflector of regolith decreases the thickness of the BeO reflector for the SCoRe-S₇ from 16 cm to 8.0 cm, and to 5.7 cm for the SCoRe-S₁₁ of the largest core. The resulting decreases in the launch mass of the SCoRe-S concepts are ～34% or 150–200 kg.     

Methods for determining operation life and reactivity depletion for space reactors with fast energy spectra

Space reactors with fast neutron energy spectrums are preferred for their compactness and high fission power density, but require a high fissile inventory. The operation life estimates of these reactors are important to mission planning. This paper examines a number of fuel depletion and neutronics code packages for determining the operation lives of two space reactors with hard fast neutron energy spectra. These are: the lithium-cooled, Sectored, Compact Reactor (SCoRe-S₁₁), and the submersion subcritical safe space (S^4) reactor, cooled with a He–Xe binary gas mixture (40 g/mol). This work investigated the code packages of Monteburns 2.0, MCNPX 2.6C and TRITON and validated their prediction with fuel depletion data for a PWR fuel bundle, with satisfactory results. The operation life predictions of the two space reactors using these code packages are compared with those calculated using a simplified method that couples MCNP5 to a burnup analysis model using the Simulink^® platform. This method considers only the 10 most probable low-Z and high-Z elements of the fission yield peaks plus ¹⁴⁹Sm, and neglects the depletion of fission products due to capture and radioactive decay. The simplified method requires significantly shorter running time and its predictions of the operation lives for the two space reactors are within 0.29–12.5% of those obtained using Monteburns 2.0 and MCNPX 2.6C code packages. This method, however, is not recommended for operation life predictions for space or commercial reactors with thermal neutron spectrums.     

Performance analyses of VHTR plants with direct and indirect closed Brayton cycles and different working fluids

This paper investigated the performance of Very High Temperature Reactor (VHTR) power plants with helium working fluid and direct and indirect Closed Brayton Cycles (CBCs), and with binary mixture working fluids of He–Xe and He–N₂ (molecular weight of 15 g/mole) and indirect CBCs. Also investigated are the effects of using low- and high-pressure compressors with intercooling, versus a single compressor, using bleed cooling the reactor pressure vessel in direct CBC helium plants, and varying the reactor exit temperature from 700 °C to 950 °C on the plant thermal efficiency, cycle pressure ratio and the size of and number of stages in the turbine and compressor. Analyses are performed for a shaft rotation speed of 3000 rpm, reactor thermal power of 600 MW and a temperature pinch of 50 °C in the intermediate heat exchanger (IHX) for the indirect CBCs.     

Reactivity control options of space nuclear reactors

This paper compares two ex-core control options of the gas-cooled Submersion Subcritical Safe Space (S^4) reactor with a fast neutrons energy spectrum: (a) rotating BeO drums with 120° thin segments of enriched B₄C in the BeO radial reflector; and (b) sliding segments in the BeO radial reflector. Investigated are the effects on the beginning-of-life (BOL) excess reactivity, reactivity depletion rate and operation life, and the spatial neutron flux distributions and fission power profiles in the core. Also investigated is the effect of reducing the thickness of the enriched B₄C segments in the control drums on the BOL excess reactivity, when one or two of the 6 drums are stuck in the shutdown position. Reducing the thickness of the B₄C segments from 0.5 mm to 0.238 mm, with one drums stuck in the shutdown position, increases BOL cold and hot-clean excess reactivity from +$1.71 and +$0.47 to +$2.38 and +$0.89, respectively. These reactivity values are almost identical to those of the reactor with one of the six reflector segments stuck open in the shutdown position. Results also showed that the control options made little difference in the reactor performance. The power peaking in the reactor core with sliding reflector segments is slightly lower and the spatial power profiles are relatively flatter. The operation life of the reactor with a sliding reflector segments control, when operating at a nominal thermal power of 471 kW, is only 22 full power days longer than with rotating drums control.     

Surface damage of poly(methylmethacrylate) under fretting loading

The initial fretting damage in a glass/PMMA contact was investigated by means of experiments and numerical (F.E.M.) simulations. Both micro-crack nucleation at the contact edges and particle detachment were identified on the PMMA's surface. Micro-crack initiation was related to the combination of high tensile stresses and positive hydrostatic pressures which are known to enhance crazing. During the early stages of the fretting tests, the distribution of the detached particles within the contact was correlated to the spatial distribution of the cumulative interfacial energy dissipated by friction. As the number of cycles was increased, it was observed that detached particles moved toward the middle of the contact. On the basis of FEM simulations, this particle displacement within the contact was attributed to the existence of differential micro-displacements during the fretting cycle.