摘要:

AbstractDuring the past few years, there has been an increasing effort to understand the behavior of nuclear power plants (NPPs) in normal and abnormal situations. To achieve this goal, large computer codes that allow the description of two-phase flows with sources and sinks of mass and heat have been built in many countries.The analysis of NPP situations often requires a large computation time, so efficient calculational methods are needed to minimize the cost of these studies. Thermal-hydraulic models and calculational methods used in the codes are reviewed. Computational methods and solution procedures involved in some European operating and safety codes are described. Developments concerning code optimization and adaptation of numerical methods to the new supercomputer architectures are discussed.

ISSN:0029-5639    

DOI:10.13182/NSE88-A23569

出版商:Taylor&Francis    

年代:1988

数据来源: Taylor

摘要:

AbstractDeveloped and applied to the WCOBRA/TRAC code for speed up is an implicit numerical method with an iterative solution scheme of semi-implicit format. For a sample problem of subcooled liquid, computation speed was increased by a factor of 2 to 3 or more. For problems involving phase transition and heat transfer, other techniques of iteration control, time-step control, adjustment of correlations, and numerical stabilizer were introduced. Consequently, computation speed was increased by a factor of 2.

ISSN:0029-5639    

DOI:10.13182/NSE88-A23570

出版商:Taylor&Francis    

年代:1988

数据来源: Taylor

摘要:

AbstractBy applying a finite volume approach to a finite element mesh, the ASTEC computer code allows three-dimensional incompressible fluid flow and heat transfer in complex geometries to be simulated realistically, without making excessive demands on computing resources. The methods used in the code are described, and examples of the application of the code are presented.

ISSN:0029-5639    

DOI:10.13182/NSE88-A23571

出版商:Taylor&Francis    

年代:1988

数据来源: Taylor

摘要:

AbstractMotivated by the enhancement of heat transfer under oscillating flow conditions in single-phase heated channels and by stability problems in two-phase systems such as those in boiling water reactors, density-wave oscillations have been analyzed by numerically solving the nonlinear, variable delay, functional, ordinary integrodifferential equations that result from integrating the nonlinear partial differential equations for the single- and two-phase heated channel regions along characteristics and along channel length for axially uniform heat fluxes. The cases of constant pressure dropΔPexacross the channel (steady-state feed pump operation), exponentially decayingΔPex(feed pump coastdown), and periodicΔPex(feed pump oscillations) were studied. In the constantΔPexcase, the system undergoes a supercritical Hopf bifurcation from a stable fixed point to a stable limit cycle as the parameters are moved into the linearly unstable region. In the exponentially decayingΔPexcase, depending on the initial and final pressures, the system travels along a hysteresis curve, jumps at the first turning point to another stable branch, and eventually evolves to a stable limit cycle. In the periodicΔPexcase when the system is in the linearly unstable region, it usually evolves asymptotically to one of several different attracting sets, depending on the frequency ofΔPex: stable period-N limit cycles, stable invariant tori, and a chaotic (or strange) attractor. The nature of the strange attractor was analyzed quantitatively by calculating its correlation dimension—an estimate of its fractal dimension—and the dimension of the phase space in which it can be embedded. These calculations indicate that the strange attractor is indeed a fractal object of fractional dimension 2.048±0.003 and embedding dimension 6. The results of these numerical studies suggest that the heated channel model can operate safely in the linearly unstable region in a dynamically stable mode without excessively large excursions when driven at many frequencies; however, at many other frequencies it cannot. The trajectories that do remain in bounded regions of phase space can be, depending on the forcing frequency, periodic with a short or very long period, very near periodic, or completely aperiodic or chaotic. Hence, it is possible to enhance heat transfer while maintaining safety in two-phase flow systems by operating them in an oscillatory mode.

ISSN:0029-5639    

DOI:10.13182/NSE88-A23572

出版商:Taylor&Francis    

年代:1988

数据来源: Taylor

摘要:

AbstractThe recent trend toward increased parallelism in supercomputer architectures, together with the need to use existing supercomputer resources more efficiently, has motivated this study of parallel algorithms designed specifically for implementation on parallel machines. Users of parallel machines like the ICL Distributed Array Processor (DAP) are all too familiar with the problem of having to reformulate their algorithms to incorporate the inherent parallelism of the DAP architecture. The individual processing elements of the DAP are comparatively slow, and the full potential of such a machine is only realized if the algorithm used is highly parallel. The very fast processing capabilities of pipeline machines like the Cray-1 or the Cyber 205 tend to obscure this point somewhat, and in fact most users of these machines are content with using existing codes, vectorized where possible. Such a strategy, however, is not appropriate for the parallel processors like the ICL DAP, AMT DAP, the Goodyear MPP, GEC GRID, etc.The application of parallel algorithms to the solution of fluid dynamics problems is considered. Most of the work concerns the solution of the two- and three-dimensional incompressible Navier-Stokes equations, steady and unsteady. The results are relevant for the above-mentioned parallel machines, but the methods can be adapted for use on vector machines.Concerning parallel algorithms, the easiest and possibly those with the widest potential application are relaxation techniques based on red-black (R-B) ordering. The use of R-B successive overrelaxation, R-B LSOR, etc., on two- and three-dimensional cavity flows, unsteady channel, and boundary layer flows at high Reynolds numbers is considered. While these methods are extremely problem dependent, in the favorable cases they can be as competitive as more sophisticated serial“vectorized”algorithms. As far as parallel algorithms are concerned, the most important criteria are what is the cost per iteration, and how fast do they converge. For application on serial machines, the latter factor seems to dominate.

ISSN:0029-5639    

DOI:10.13182/NSE88-A23573

出版商:Taylor&Francis    

年代:1988

数据来源: Taylor

摘要:

AbstractTwo new“low-”and“high-order”time-dependent nodal-integral methods were developed and applied to both incompressible fluid flow and natural convection. These new methods have a high level of accuracy on a coarse mesh, high efficiency, and an ability to reproduce results using various time-step sizes independent of a Courant condition. These new methods are applied to various benchmark problems, such as double-glazing, to verify their accuracy in space and time. Other applications to bifurcation searches and stability of flow fields verify their accuracy and their ability to duplicate natural phenomena without exhibiting problems with spurious solutions, turning points, and bifurcation points. The new methods are also used to verify the existence of critical values of the aspect ratio. The means by which alternative stable solutions can be obtained from a no-flow initial condition for a critical aspect ratio are also examined.

ISSN:0029-5639    

DOI:10.13182/NSE88-A23574

出版商:Taylor&Francis    

年代:1988

数据来源: Taylor

摘要:

AbstractAn expert system is developed for design synthesis and reliability analysis. Design synthesis uses a rule-based production system called KOPS, and reliability analysis is composed of the computer codes CAT, FTAP, and IMPORTANCE. These two system parts are linked in the LISP interpreter.This expert system is applied to the design of an auxiliary feedwater system that generates various design alternatives and performs a successful reliability analysis.

ISSN:0029-5639    

DOI:10.13182/NSE88-A23575

出版商:Taylor&Francis    

年代:1988

数据来源: Taylor

摘要:

AbstractTwo expert systems for on-line analysis of nuclear reactor transients are reported. During a hypothetical crisis in a nuclear facility, a team of the Institute for Protection and Nuclear Safety must assess the risk to the local population. Expert systems are intended to assist in this analysis. The first deals with the availability of the safety systems of the plant (e.g., emergency core cooling system), depending on the functional state of the support systems. A second expert system will be built to study the physical transient of the reactor (mass and energy balance, pressure, flows). To do this, as in the development of other expert systems, a physical analyzer is required. This is the aim of SEXTANT, which combines several knowledge bases concerning measurements, models, and qualitative behavior of the plant with a conjecture-refutation mechanism and a set of simplified models of the current physical state. A prototype is being assessed with integral test facility transients. Both expert systems require powerful shells for their development. SPIRAL is such a tool for the development of expert systems for the computer-aided management of complex processes.

ISSN:0029-5639    

DOI:10.13182/NSE88-A23576

出版商:Taylor&Francis    

年代:1988

数据来源: Taylor

摘要:

AbstractThe fuel shuffling problem is posed by the need to reposition partially burned assemblies to achieve minimum X-Y pin power peaks in reload cycles of pressurized water reactors. This problem is a classic artificial intelligence (AI) problem and is highly suitable for AI expert system solution assistance, in contrast to the conventional solution, which ultimately depends solely on trial and error. Such a fuel shuffling assistant would significantly reduce engineering and computer execution time for conventional loading patterns and, much more importantly, even more significantly for lowleakage loading patterns. A successful hardware/software demonstrator has been introduced, paving the way for development of a broadly applicable expert system program. Such a program, upon incorporating the recently developed technique of reverse depletion, would provide a directed path for solving the low-leakage problem.

ISSN:0029-5639    

DOI:10.13182/NSE88-A23577

出版商:Taylor&Francis    

年代:1988

数据来源: Taylor

摘要:

AbstractFor optimal operation of a power plant, it is now necessary to install an expert system in the control room, particularly to aid the operator in predicting, following, and explaining operating events. The three-dimensional MICROLUX code, foreseen for such a system, was tested on an operating event occurring in the Doel-3 reactor.After an ~6-h scram, the return to full power (PN) was limited at 15% PNbecause of an unacceptable axial offset deviation, which was believed to have been caused by xenon buildup during the shutdown. The reactor was then required to be operated at reduced power for 18 h before again reaching nominal power. From the study, however, it seems that there was no danger in returning directly to full power in spite of the ex-core indications.The three-dimensional calculations and the ex-core results show that there is a need to investigate the ex-core/in-core relationship when the reactor is operated outside the calibration field. These preliminary results indicate that the axial offset concept should be analyzed on the basis of a large number of transient operating conditions with the help of three-dimensional methods, which give a better description of the core behavior during transients. The use of such methods could contribute to a safer and more economical operation of the reactor.

ISSN:0029-5639    

DOI:10.13182/NSE88-A23578

出版商:Taylor&Francis    

年代:1988

数据来源: Taylor