At the brink of nuclear waste repository crises, viable alternatives for the long term radiotoxic wastes are seriously being considered worldwide. Minor actinides serve as one of these targeted wastes. Partitioning and transmutation in fission reactors is one possible incineration option and could potentially serve as a source of nuclear fuel required for sustainability of energy resources. The objective of this research was to evaluate the neutronic performance of the pebble-bed Very High Temperature Reactor (VHTR) configurations with various fuel loadings. The configuration adjustments and design sensitivity studies specifically targeted the achievability of spectral variations. The development of several realistic full-core 3D models and validation of all modeling techniques used was a major part of this research effort. In addition, investigating design sensitivities helped identify the parameters of primary interest. The full-core 3D models representing the prototype and large scale cores were created for use with SCALE 5.0 and SCALE 5.1 code systems. Initially the models required the external calculation of a Dancoff correction factor; however, the recent release of SCALE 5.1 encompassed inherent double heterogeneity modeling capabilities. The full core 3D models with multi-heterogeneity treatments are in agreement with available pebble-bed High Temperature Test Reactor data and were validated through benchmark studies. Analyses of configurations with various fuel loadings have indicated promising performance and safety characteristics. It was found that through small configuration adjustments, the pebble-bed design can be tweaked to produce desirable spectral shifts. The future operation of Generation IV nuclear energy systems would be greatly facilitated by the utilization of minor actinides as a fuel component. This would offer development of new fuel cycles, and support sustainability of a fuel source.

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 169-172).

Pebble-fueled high temperature gas-cooled reactor (HTGR) technology was first developed by the Federal Republic of Germany in the 1950s. More recently, the design has been embraced by the People's Republic of China and the Republic of South Africa. Unlike light water reactors that generate heat from fuel assemblies comprised of fuel rods, pebble-fueled HTGRs utilize thousands of 60-mm diameter fuel spheres (pebbles) comprised of thousands of TRISO particles. As this reactor type is deployed across the world, adequate methods for safeguarding the reactor must be developed. Current safeguards methods for the pebble-fueled HTGR focus on extensive, redundant containment and surveillance (C/S) measures or a combination of item-type and bulk-type material safeguards measures to deter and detect the diversion of fuel pebbles. The disadvantages to these approaches are the loss of continuity of knowledge (CoK) when C/S systems fail, or are compromised, and the introduction of material unaccounted for (MUF). Either vulnerability can be exploited by an adversary to divert fuel pebbles from the reactor system. It was determined that a solution to maintaining CoK is to develop a system to identify each fuel pebble that is inserted and removed from the reactor. Work was performed to develop and evaluate the use of inert microspheres placed in each fuel pebble, whose random placement could be used as a fingerprint to identify the fuel pebble. Ultrasound imaging of 1 mm zirconium oxide microspheres was identified as a possible imaging system and microsphere material for the new safeguards system concept. The system concept was evaluated, and it was found that a minimum of three microspheres are necessary to create enough random fingerprints for 10,000,000 pebbles. It was also found that, over the lifetime of the reactor, less than 0.01% of fuel pebbles can be expected to have randomly the same microsphere fingerprint. From an MCNP 5.1 model, it was determined that less than fifty microspheres in each pebble will have no impact on the reactivity or temperature coefficient of reactivity of the reactor system. Finally, using an ultrasound system it was found that ultrasound waves can penetrate thin layers of graphite to image the microsphere fingerprint.

No description available.

Thesis (M.S.)--Oregon State University, 2010. / Printout. Includes bibliographical references (leaves 53-56). Also available on the World Wide Web.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2008. / "May 2008." / Includes bibliographical references (leaves 112-113). / The Modular Pebble Bed Reactor is a small high temperature helium cooled reactor that is being considered for both electric power and hydrogen production. Pebble bed reactors are being developed in South Africa, China and the US. To make smaller 120 Mwe reactors economically competitive with larger 1500 Mwe traditional light water reactors changes in the way these plants are built are needed. Economies of production need to be sufficiently large to compete with economies of scale. MIT (Berte) has been working on developing a modular design and construction strategy for several years. This thesis builds on that work by demonstrating the technical feasibility of implementing the modularity approaches previously developed. The MIT approach uses "space frames" containing all the components, piping, valves and needed cables, instrumentation in a specified volume. These space frames are built in a factory to assure high quality in manufacture. They are then shipped by train or truck to the site and assembled "lego" style. It is expected that with the improved quality in the factory setting, and rapid assembly at the site that the total time and cost of construction of the plant will be greatly reduced (Kadak). To make this process work, it is vitally important to assure that when the space frames and internal components are manufactured, they are done to rigid tolerances to assure line up when assembled in the field. By using many advanced three dimensional measurement technologies, including the use of digital photography, lasers, and photogrammetry, companies are now capable of fabricating pieces to extremely precise specifications at a relatively affordable cost. This thesis evaluates the feasibility of manufacture of space frames and internal components to the required tolerances, the accuracy control needed and how the plant can be assembled with details of each space frame interfaces. / (cont.) A global reference system was determined and a basic plant map for space frame placement developed. Deviations from exact placement from this map due to tolerance allowances were factored in and methods and techniques for overcoming any variations was developed. In order to enable each frame and it's respective components to be accurately fabricated to ensure interfacing parts will mate, a local coordinate system was developed for each frame and used to describe the exact location of the required interfaces for each specific frame. Crucial concepts of accuracy control and "best fit" are outlined and incorporated. Based on independent verification of the processes and the design proposed, this modularity approach appears to be feasible. A comparative economic analysis was also performed to assess the potential cost savings of the modularity approach compared to traditional "stick build" approaches presently being used in nuclear construction. Manhour, learning curve and overall cost savings of over 30 % can be expected which suggests that if modularity approaches as those proposed are used, smaller reactors can compete with larger economies of scale plants. / by Jaime E. Hanlon-Hyssong. / S.M.

Pressure drops over a packed bed of pebble bed reactor type are investigated. Measurement of porosity and pressure drop over the bed were carried out in a cylindrical packed bed facility. Air and water were used for working fluids. There are several parameters of the pressure drop in packed beds. One of the most important factors is wall effect. The inhomogeneous porosity distribution in the bed and the additional wetted surface introduced by the wall cause the variation of pressure drop. The importance of the wall effects and porosity can be explained by using different bed-to-particle diameter ratios. Four different bed-to-particle ratios were used in these experiments (D/dp = 19, 9.5, 6.33 and 3.65). A comparison is made between the predictions by a number of empirical correlations including the Ergun equation (1952) and KTA (by the Nuclear Safety Commission of Germany) (1981) in the literature. Analysis of the data indicated the importance of the bed-to-particle size ratios on the pressure drop. The comparison between the present and the existing correlations showed that the pressure drop of large bed-to-particle diameter ratios (D/dp = 19, 9.5and 6.33) matched very well with the original KTA correlation. However the published correlations cannot be expected to predict accurate pressure drop for certain conditions, especially for pebble bed with D/dp (bed-to-particle diameter ratio) </= 5. An improved correlation was obtained for a small bed-to-particle diameter ratio by fitting the coefficients of that equation to experimental database.

High Temperature Gas-cooled Reactor (HTGR) is a Generation IV reactor under consideration by Department of Energy and in the nuclear industry. There are two categories of HTGRs, namely, Pebble Bed Modular Reactor (PBMR) and Prismatic reactor. Pebble Bed Modular Reactor is a HTGR with enriched uranium dioxide fuel inside graphite shells (moderator). The uranium fuel in PBMR is enclosed in spherical shells that are approximately the size of a tennis ball, referred to as \fuel spheres". The reactor core consists of approximately 360,000 fuel pebbles distributed randomly. From a reactor design perspective it is important to be able to understand the fluid flow properties inside the reactor. However, for the case of PBMR the sphere packing inside the core is random. Unknown flow characteristics defined the objective of this study, to understand the flow properties in spherically packed geometries and the effect of turbulence models in the numerical solution. In attempt to do so, a steady state computational study was done to obtain the pressure drop estimation in different packed bed geometries, and describe the fluid flow characteristics for such complex structures. Two out of the three Bravais lattices were analyzed, namely, simple cubic (symmetric) and body centered cubic (staggered). STARCCM commercial CFD software from CD- ADAPCO was used to simulate the flow. To account for turbulence effects several turbulence models such as standard k-epsilon, realizable k-epsilon, and Reynolds stress transport model were used. Various cases were analyzed with Modified Reynolds number ranging from 10,000 to 50,000. For the simple cubic geometry the realizable k-epsilon model was used and it produced results that were in good agreement with existing experimental data. All the turbulence models were used for the body centered cubic geometry. Each model produced different results what were quite different from the existing data. All the turbulence models were analyzed, errors and drawbacks with each model were discussed. Finally, a resolution was suggested in regards to use of turbulence model for problems like the ones studied in this particular work.

Thesis (M.Sc.(Chemistry))--University of Pretoria, 2007. / Includes bibliographical references (leaves 80-85).

The fission product releases from spherical fuel elements used in modern high temperature gas cooled reactors are one of the first source terms used in describing the safety of planned nuclear plants during normal and accident conditions. The verification and validation of the model and code used to predict the gaseous fission product behaviour and release from spherical fuel elements for the PBMR were documented in this dissertation. The PBMR is the latest design in high temperature gas cooled reactor technology utilizing spherical fuel elements based on the LEU TRISO-coated particle design. Fission products can be divided into relatively short-lived noble gas and halogens, and relatively long-lived metallic fission and activation products. Each group is described by its own models and sets of transport parameters. The noble gases and halogen fission product releases from the fuel elements are direct indications of fuel performance and are modelled by the Booth equation. The fission product release legacy code NOBLEG for noble gases and halogens was developed previously to calculate this diffusion model for high temperature reactors. The model and code are verified and validated for use in PBMR design and analyses under normal operating conditions. The history of irradiation experiments conducted on coated fuel particles and spherical fuel elements was investigated, and the most suitable irradiation tests with their post irradiation investigations were identified for the purpose of validation of the model and code. The model used to determine gaseous fission product behaviour and release from spherical fuel elements is described in detail. The application of this model in the code is verified mathematically with the Booth model, and by inspection of the source code. The thermohydraulic model used by NOBLEG to calculate fuel temperatures is verified with code to code comparisons with the core neutronics code VSOP. The irradiation tests HFR-K5 and -K6 were selected to validate the gaseous fission product code NOBLEG. An investigation was done into the development of NOBLEG to calculate gaseous fission product release under oxidizing conditions caused by water ingress events. New relationships were derived from water vapour injection tests done during the irradiation experiment HFR-K6, that allows NOBLEG to estimate the increase in gaseous fission product release under oxidizing conditions. A new model was proposed to explain peculiarities observed during the water injection tests. / Prof. P.P. Coetzee