Coping with complexity

The author presents a short talk on intentional complexity, a form of complexity self-created by individuals such as occurs in experimental set ups     

Forty-One Years with Zirconium

The MEGAPIE (Megawatt Pilot Experiment) Project has been primarily initiated in response to interest in the Accelerator Driven System (ADS). The ADS is a nuclear reactor concept with a fuel cycle which can burn minor actinide waste products from conventional reactors, while also taking advantage of the safety characteristics of a sub-critical reaction process. By this means, the fission chain reaction is maintained by additional neutrons generated by protons in a liquid-metal spallation target. The goal of the international MEGAPIE project is to design and construct such a target and install and test it over a period of one year in the SINQ spallation neuron source facility at the Paul Scherrer Institut (PSI). To aid the design process, Computational Fluid Dynamics (CFD) is being used to optimise the thermal- hydraulic behaviour of the target. Results obtained so far indicate that it should be possible to remove 700 kW of heat deposited by the proton beam in the target, under steady-state conditions (40kg/s total Lead Bismuth Eutectic flow rate), without peak window temperature rising above about 385°C, when using a bypass flow of 2.5 kg/s and a slanted- end guide tube. A tentative peak window temperature of 400°C is currently considered allowable, on the basis of material strength after irradiation, and CFD simulation is currently being validated by suitable LBE experiments in similar geometry.     

Design and optimization of an equilibrium reload with MOX fuel with minor actinides

In this work the design and optimization of an equilibrium core for a boiling water reactor (BWR), loaded with fuel composed of plutonium and minor actinides (Np, Am and Cm), is presented. The plutonium and minor actinides are obtained from the recycling of the spent fuel of a BWR, and are mixed with depleted uranium obtained from the enrichment tails. The design and optimization of the equilibrium reload is achieved in two steps. In the first step, the fuel assembly is adjusted and the reload pattern is designed, in order to obtain the target cycle length. In order to improve the shutdown margin, two actions were taken: to increase the boron-10 content in the control rods, and to add a burnable absorber (gadolinia) in some fuel rods. In the second step, the reload pattern, obtained in the first step, is optimized to maximize the energy, under the thermal and reactivity margins constraints; a system based on Genetic Algorithms was used in the optimization process. Results show that 5% more energy was obtained with the optimized reload.     

Dynamic characteristics of rectangular twisted beam with fins surrounded with fluid

This study investigates the vibration characteristics of rectangular twisted beam and the safety assessment of the potential for fretting-wear damages caused by foreign object. To get the natural frequency, corresponding mode shape and participation factor, modal analyses are performed for a twisted beam with fins submerged in fluid of hexagonal cylinder. Special emphases are on the effects of fins, boundary conditions at both ends, fluid surrounding the beam and the number of turns on the modal characteristics, which are expressed in terms of the natural frequency and corresponding mode shape. Also, the wear rate caused by foreign object is calculated using the Archard formula and the remaining life of the beam is predicted.     

Diffusion with reversible trapping in a slab with an eroding boundary

The problem of transient diffusion with reversible trapping of atoms in a slab which has an eroding boundary and subjected to boundary conditions of the third kind has been solved. Numerical results are presented using the zeroth order solution. The results are found to be sufficiently accurate for all cases of practical interest except when the extraneous source distribution peaks very close to the moving boundary.It is also shown that a quasi-steady-state condition can be assumed to exist after the initial transients have passed; with such an assumption the analysis is significantly simplified. The accuracy of the quasi-steady-state analysis is examined by comparing numerical results obtained by the quasi-steady-state analysis against those obtained by the general analysis.     

Evaluation of the weldability of heavy-section forgings with special emphasis on areas with segregation

This paper describes comparative tests performed on a component, a test coupon and a test specimen (implant) to evaluate the weldability of the material 20 MnMoNi 5 5 particularly in areas exhibiting segregation. The results show that the methods used are suitable for evaluating susceptibility to cold cracking. The test welds on the coupon, however, yielded different results with respect to the minimum preheat temperature required. Possible reasons for this are discussed.     

ITER Safety Analyses with ISAS

Detailed analyses of accident sequences for the International Thermonuclear Experimental Reactor (ITER), from an initiating event to the environmental release of activity, have involved in the past the use of different types of computer codes in a sequential manner. Since these codes were developed at different time scales in different countries, there is no common computing structure to enable automatic data transfer from one code to the other, and no possibility exists to model or to quantify the effect of coupled physical phenomena. To solve this problem, the Integrated Safety Analysis System of codes (ISAS) is being developed, which allows users to integrate existing computer codes in a coherent manner. This approach is based on the utilization of a command language (GIBIANE) acting as a glue to integrate the various codes as modules of a common environment. The present version of ISAS allows comprehensive (coupled) calculations of a chain of codes such as ATHENA (thermal-hydraulic analysis of transients and accidents), INTRA (analysis of in-vessel chemical reactions, pressure built-up, and distribution of reaction products inside the vacuum vessel and adjacent rooms), and NAUA (transport of radiological species within buildings and to the environment). In the near future, the integration of S AFALY (simultaneous analysis of plasma dynamics and thermal behavior of in-vessel components) is also foreseen. The paper briefly describes the essential features of ISAS development and the associated software architecture. It gives first results of a typical ITER accident sequence, a loss of coolant accident (LOCA) in the divertor cooling loop inside the vacuum vessel, amply demonstrating ISAS capabilities.     

Airline experience with reliability-centered maintenance

Reliability-Centered Maintenance is a process for developing preventive maintenance programs. Its concepts evolved from the post WWII experience of the airline community. Its genesis was in a paper by F. Stanley Nowlan and Thomas D. Matteson of United Airlines for the American Institute of Aeronautics and Astronautics in 1967. Its first application was to the Boeing 747. It has subsequently been adopted by the FAA and the Department of Defense and applied to many new transport and military aircraft. Its objective is applicable and effective preventive maintenance and it has proven to be a highly effective replacement for the prior intuitive processes for selecting preventive maintenance tasks. It focuses on system functions, functional failures, then dominant failure modes and effects. It then uses a decision tree to classify failure criticality and identify applicable and effective tasks. The result is a program focused on maintaining inherent safety and reliability at minimum cost.     

A simple method with imaging plates for examination of soil contaminated with radioactive caesium

A simple and convenient system with imaging plates (IPs) was well constructed for the radioactivity examination of soil contaminated with radioactive caesium. A set of two IP strips was vacuum-sealed with thin polyethylene film to prevent the IP strips from direct contacting soil and from being moisturized. In the examination of radioactive soil, a stainless steel pipe including this IP strip monitor was put into the ground and then it was kept in a lead container after being pulled out of the ground for the reduction of the surrounding gamma-ray background. In addition, a sensitive reader was well made for the measurement of the photostimulated luminescence through the scanning of a laser beam on the IP strip. It was confirmed from experiments on actual radioactive-caesium-contaminated soil that the IP strip monitor with the reader effectively detected beta-rays from radioactive caesium and could be satisfactorily used for the examination of radioactive soil.     

Interaction of uranium-molybdenum fuel with an aluminum matrix with deep burnup

A thermodynamic analysis is used to calculate the phase and component composition of uraniummolybdenum fuel with burnup 200 GW·days/ton. The equilibrium composition of the gas phase, consisting mainly of gaseous cesium whose pressure reaches 30 kPa, is determined more accurately. The quantitative composition of the phase of solid solutions of tellurides, whose formation degrades the structure of a fuel granule, is presented. Thermal tests of the fuel composition (U–Mo)–Al were performed. The investigation was performed in the presence of simulators of the chemically active fission products of cesium and iodine at different temperature. The interaction zone of (U–Mo)–Al is investigated by means of metallography and scanning electron microscopy. The data obtained on the composition of the indicated zone made it possible to conjecture the character of interaction between the fuel material and the aluminum matrix.     

Coherent systems with multistate components

The basic rules of the Boolean algebra with restrictions on variables are briefly recalled. This special type of Boolean algebra allows one to handle fault trees of systems made of multistate (two or more than two states) components.Coherent systems are defined in the case of multistate components. This definition is consistent with that originally suggested by Barlow in the case of binary (two states) components. The basic properties of coherence are described and discussed. Coherent Boolean functions are also defined. It is shown that these functions are irredundant, that is they have only one base which is at the same time complete and irredundant. However, irredundant functions are not necessarily coherent.Finally a simplified algorithm for the calculation of the base of a coherent function is described. In the case that the function is not coherent, the algorithm can be used to reduce the size of the normal disjunctive form of the function. This in turn eases the application of the Nelson algorithm to calculate the complete base of the function. The simplified algorithm has been built in the computer program MUSTAFA-1. In a sample case the use of this algorithm caused a reduction of the CPU time by a factor of about 20.     

Temperature measurement with irradiated materials

The principle of operation of a crystalline temperature indicator for measuring the maximum temperature is described and examples of its application are given. The operation of the device is based on the expansion of the crystal lattice of diamond or silicon carbide under irradiation in a reactor and a decrease of this expansion depending on the duration and temperature of heating. The results obtained during flights of shuttle models Bor-4 and −5 are presented as examples of practical applications. In the tests, the temperature of the heat-shielding coatings intended for installation on the Buran shuttle was tested. Another example of an application is thermometry of the Siemens (Germany) GTX-800 power-generation turbine. In this case, the temperature of the working and guiding blades of different steps of the motor as well as the gas in direct proximity to the blades was measured. The temperature in the space behind a VVER-1000 vessel was measured with the sensor.     

Oxygen ion source with plasma cathode

An ion source with a plasma cathode has been developed for long lifetime use in ion implanters. In this ion source, a plasma cathode replaces the conventional metallic filament used in a Freeman-type ion source. This ion source consists of two compartments, namely a plasma generator and an ion source chambers interconnected by a tapered narrow duct. The pressure difference between the two parts, maintained by differential pumping, prevents the feed gas from flowing into the plasma generator. With any combination of an argon plasma cathode and a feed gas of either fluoride (AsF₅, PF₅) or oxygen, the lifetime was found to be more than 90 h with an extraction voltage of 40 kV and a correspoding ion current density of 20 mA/cm², and a considerable amount of As⁺, P⁺, O⁺, and O₂⁺ ions were observed in mass spectra.     

Neutron-capture therapy with ultra-cold neutrons

The status of neutron-capture therapy of malignant tumors and its problems – damage to healthy tissue as a result of neutron transport to the irradiation location and presence in the therapeutic beam of a background consisting of γ rays and fast neutrons – are presented. To solve these problems, the authors have proposed using ultracold neutrons with energy less than 10^–7 eV, whose unique capability is to undergo total reflection from the surface of a condensed substance at any angle of incidence. Numerous works have demonstrated that such neutrons can be transported along neutron guides. The cross section for inelastic scattering of neutrons by hydrogen-containing substances – water, ethyl alcohol, and biological tissue – has been measured in an IR-8 beam of ultracold and very cold neutrons. At temperature 200–300 K, the experimental data are in very good agreement with calculations, but as temperature decreases further a discrepancy appears, which could be due to the inaccuracy of the model spectra of the oscillations hydrogen-containing substances used in the calculations. The use of ultracold neutrons opens up new possibilities of neutron-capture therapy for treating malignant tumors localized in body cavities or organs.     

Computational experience with non-linear dynamic analysis of thin reinforced concrete structures with the initial-stress approach

Computational experience gained in the area of the dynamic non-linear analysis of thin reinforced concrete structures is described. Parasitic damping is shown to be related to the residual stress. Using only one modified three-dimensional isoparametric element across the thickness of a thin plate, capable of representing a bi-linear stress distribution, the ultimate moment can be accurately calculated. The difficulties in reaching convergence when the stress resultants are redistributed throughout a structure are discussed. Formulae for initial stress when multiple cracks occur are derived.     

Reactor statics with feedback

A diffusion theory model is presented for a slab reactor with coolant temperature feedback. A dimensionless feedback number is introduced and shown to be the determining physics parameter for (power dependent) critical sizes and spatial flux distributions. For the latter, the exact solution contains elliptic functions, but an approximation with three spatial modes is found to give satisfactory results. Two methods to evaluate the coefficients of the modal expansion are presented.     

Scatter reduction with energy-weighted acquisition

The theory of energy-weighted acquisition (EWA) in nuclear medicine imaging is outlined, and a system that implements EWA is described. EWA reduces the effects of scattered radiation by allowing events of all energies to contribute to image formation, processing each energy with its own short-range spatial filter. This approach implements short-range energy-dependent filtering with an image buffer called a weighted acquisition module, providing scatter reduction with controllable noise and resolution properties. The system's response to point sources and planar distributions of radioactivity embedded in radiation-scattering media have been measured. EWA is compared to conventional energy-window acquisition, showing that the EWA approach provides improved image contrast     

Accelerator mass spectrometry with time-of-flight measurement

A time-of-flight detection system for the measurement of microscopic concentrations of long-lived radioisotopes has been set up at the Munich MP tandem accelerator. The technique allows an unambiguous mass determination for heavy ions up to the actinides. As a first test of the system, the concentration of ¹²⁹I in iodine has been measured for several samples of mineral and biological origin. The smallest ¹²⁹I/¹²⁷I ratios measured were a few times 10⁻¹³. The high sensitivity of the method was achieved by using a 90° injector with a good mass resolution, a special technique of stabilizing the terminal voltage with an auxiliary beam, and a time-of-flight system with a mass resolution of 800.