Akira Tokuhiro

Benchmark Simulation of Turbulent Flow through a Staggered Tube Bundle to Support CFD as a Reactor Design Tool. Part I : SRANS CFD Simulation

Time-invariant and time-variant numerical simulations of flow through a staggered tube bundle array, idealizing the lower plenum (LP) subsystem configuration of a very high temperature reactor (VHTR), were performed. In Part I, the CFD prediction of fully periodic isothermal tube-bundle flow using steady Reynolds-averaged Navier-Stokes (SRANS) equations with common turbulence models was investigated at a Reynolds number (Re) of 1.8 × 104, based on the tube diameter and inlet velocity. Three first-order turbulence models, standard k-ε turbulence, renormalized group (RNG) k-ε, and shear stress transport (SST) k-ω models, and a second-order turbulence model, Reynolds stress model (RSM), were considered. A comparison of CFD simulations and experiment results was made at five locations along (x, y) coordinates. The SRANS simulation showed that no universal model predicted the turbulent Reynolds stresses, and generally, the results were marginal to poor. This is because these models cannot accurately model the periodic, spatiotemporal nature of the complex wake flow structure.

Design of Liquid Metal Phase Change Heat Exchanger for Next-Generation Nuclear Plant Process Heat Application

The Next Generation Nuclear Plant will most likely produce electricity and its reactor heat will be further utilized for the production of hydrogen, oil recovery from tar sands and oil shales, and other process heat applications, that will further the nations pursuit of energy independence. An intermediate heat exchanger is required to transfer heat from the Next-Generation Nuclear Plant to the hydrogen plant (or other processes) in the most efficient way possible. Phase change heat exchangers are quite attractive in this regard, as they can transfer process heat more efficiently than for the single phase due to the advantage of high-enthalpy transport that includes the sensible heat of liquid, the latent heat of vaporization, and possible vapor superheat. This paper explores the overall heat transfer characteristics and pressure drop of the phase change heat exchanger with helium as the primary and sodium as the secondary heat exchanger coolant. For a two-phase boiling regime, the convective heat transfer coefficient is based on the concept of an additive, interacting mechanism of micro- and macroconvective heat transfer. In this analysis an improved design is proposed for given conditions, so as to obtain a lower overall pressure drop and a moderate/high overall heat transfer coefficient. The analysis presented in this paper will be useful as a guide for future experimental work for Next Generation Nuclear Plant process heat transfer.

Theoretical Design of a Thermosyphon for Efficient Process Heat Removal from Next Generation Nuclear Plant (NGNP) for Production of Hydrogen

The work reported here is the preliminary analysis of two-phase Thermosyphon heat transfer performance with various alkali metals. Thermosyphon is a device for transporting heat from one point to another with quite extraordinary properties. Heat transport occurs via evaporation and condensation, and the heat transport fluid is re-circulated by gravitational force. With this mode of heat transfer, the thermosyphon has the capability to transport heat at high rates over appreciable distances, virtually isothermally and without any requirement for external pumping devices. For process heat, intermediate heat exchangers (IHX) are required to transfer heat from the NGNP to the hydrogen plant in the most efficient way possible. The production of power at higher efficiency using Brayton Cycle, and hydrogen production requires both heat at higher temperatures (up to 1000oC) and high effectiveness compact heat exchangers to transfer heat to either the power or process cycle. The purpose for selecting a compact heat exchanger is to maximize the heat transfer surface area per volume of heat exchanger; this has the benefit of reducing heat exchanger size and heat losses. The IHX design requirements are governed by the allowable temperature drop between the outlet of the NGNP (900oC, based on the current capabilities of NGNP), and the temperatures in the hydrogen production plant. Spiral Heat Exchangers (SHE’s) have superior heat transfer characteristics, and are less susceptible to fouling. Further, heat losses to surroundings are minimized because of its compact configuration. SHEs have never been examined for phase-change heat transfer applications. The research presented provides useful information for thermosyphon design and Spiral Heat Exchanger.

An Economic Model of Nuclear Reprocessing Using Vensim

Even under the call for solutions to climate change and alternative energy sources to meet increasing energy demands, the imminent “nuclear renaissance” is debated by those who want to know the final destination of spent nuclear fuel. One of the alternatives to direct storage of spent fuel in a geological repository includes partial to full fuel reprocessing such that fission products such as actinides can be removed, as well as the recycling of plutonium and uranium into mixed oxide fuel (MOX). With the anticipated construction of ‘new build’ nuclear power plants (NPPs), as well as the continued operation of the existing fleet, we anticipate that the inventory of spent fuel destined for storage in Yucca Mountain (or similar) will continue to grow. Thus the U.S. DOE is promoting a sensible consideration of reprocessing, burning MOX in existing and near-terms LWRs and continuing R&D on sodium-cooled fast reactors (SFRs) for their eventual commercial introduction. However, countries that have chosen to reprocess are facing high costs and lingering political opposition, while others who have chosen not to reprocess equally face opposition to licensing and operating an adequate federal repository. This research continues ongoing research by the authors on existing and planned realization of NPPs and the associated fuel cycle. That is, we have to date developed models of the construction and decommissioning of NPPs in the U.S., developed an associated model that includes construction of reprocessing facilities, and finally, accounts for the mass flow within the partially closed fuel cycle. From early on, we included the gradual introduction of MOX-burning LWRs and SFRs into the existing and anticipated LWR fleet over the next 100 years. All models were created using Vensim, a software tool that facilitates development, analysis and compartmentalization of dynamic processes with feedback models. Our model has been benchmarked against the MIT and U. Chicago reports on the future of nuclear energy. The current work presents cost estimates and uncertainties assigned to the mass flow model to evaluate the cost of NPP-based electricity generation, with and without a fuel reprocessing. Preliminary results demonstrate that the high cost of reprocessing can be offset by the larger expense of having to construct ‘multiple’ Yucca Mountain-type repositories, under current NPP growth forecasts and insistence of the once-through fuel cycle. Details and results on various, sensible scenarios will be presented.Copyright

Optimization method to branch-and-bound large SBO state spaces under dynamic probabilistic risk assessment via use of LENDIT scales and S2R2 sets

Traditional probabilistic risk assessment (PRA) methods have been developed to evaluate risk associated with complex systems; however, PRA methods lack the capability to evaluate complex dynamic systems. In these systems, time and energy scales associated with transient events may vary as a function of transition times and energies to arrive at a different physical state. Dynamic PRA (DPRA) methods provide a more rigorous analysis of complex dynamic systems. Unfortunately DPRA methods introduce issues associated with combinatorial explosion of states. In order to address this combinatorial complexity, a branch-and-bound optimization technique is applied to the DPRA formalism to control the combinatorial state explosion. In addition, a new characteristic scaling metric (LENDIT – length, energy, number, distribution, information and time) is proposed as linear constraints that are used to guide the branch-and-bound algorithm to limit the number of possible states to be analyzed. The LENDIT characterization is divided into four groups or sets – ‘state, system, resource and response’ (S2R2) – describing reactor operations (normal and off-normal). In this paper we introduce the branch-and-bound DPRA approach and the application of LENDIT scales and S2R2 sets to a station blackout (SBO) transient.

The Potential Application of Metal Binding Hydro- and Silica-Gels in Low-Level Radioactive Waste Processing

Within the management of radioactive waste, we sought to consider a new approach radioactive hazardous waste processing in aqueous or similar (low-level waste; LLW) forms LLW and in fact, ‘contaminants of concern’ is often stored as diluted aqueous solutions of radioactive (or non) elements and contained in storage containers. One of the general problems associated with mixed liquid waste is the lack of an efficient, effective, and inexpensive means of processing (separating) its constituents. Two of the objectives in processing solid, radioactive laden liquid LLW are as follows: 1) to separate/extract the radioisotopes from the rest of the mixed constituents, and 2) to produce stable solidified forms encapsulating radioactive elements. Recent R&D in the physical chemistry of gel materials, have identified promising approach to simultaneously achieve the above objectives. That is, by utilizing and manipulating the physicochemical properties of various silica- and polymer-based gels at the nanoscale, we have demonstrated a process by which to specific chemical species are encapsulated.Copyright

An Initial Study on Modeling the United States Thermal Fuel Cycle Mass Flow Using Vensim

According to current forecasts, nuclear power plant construction and nuclear-generated electricity production is projected to increase in the next half-century. This is likely due to the fact that nuclear energy is an ‘environmental alternative’ to fossil fuel plants that emit greenhouse gases (GHG). Nuclear power also has a much higher energy density output than other alternative energy sources such as solar, wind, and biomass energies. There is also growing consensus that processing of low- and high-level waste, LLW and HLW respectively, is a political issue rather than a technical challenge. Prudent implementation of a closed fuel cycle not only curbs build-up of GHGs, but can equally mitigate the need to store nuclear used fuel. The Global Nuclear Energy Partnership (GNEP) is promoting gradual integration of fuel reprocessing, and deployment of fast reactors (FRs) into the global fleet for long-term uranium resource usage. The use of mixed oxide (MOX) fuel burning Light Water Reactors (LWR) has also been suggested by fuel cycle researchers. This study concentrated on modeling the construction and decommissioning rates of six major facilities comprising the nuclear fuel cycle, as follows: (1) current LWRs decommissioned at 60-years service life, (2) new LWRs burning MOX fuel, (3) new (Gen’ III+) LWRs to replace units and/or be added to the fleet, (4) new FRs to be added to the fleet, (5) new reprocessing and MOX fuel fabrication facilities and (6) new LWR fuel fabrication facilities. Our initial work [1] focused on modeling the construction and decommissioning rates of reactors to be deployed. This is being followed with a ‘mass flow model’, starting from uranium ore and following it to spent forms. The visual dynamic modeling program Vensim was used to create a system of equations and variables to track the mass flows from enrichment, fabrication, burn-up, and the back-end of the fuel cycle. Sensible construction and deployment rates were benchmarked against recent reports and then plausible scenarios considered parametrically. The timeline starts in 2007 and extends in a preliminary model to 2057; a further mass flow model scenario continues until 2107. The scenarios considered provide estimates of the uranium ore requirements, quantities of LLW and HLW production, and waste storage volume needs. The results of this study suggest the number of reprocessing facilities necessary to stabilize and/or reduce recently reported levels of spent fuel inventory. Preliminary results indicate that the entire national spent fuel inventory produced over the next ∼50 years can be reprocessed by a reprocessing plant construction rate of less than 0.07 plants/year (small capacity) or less than 0.05 plants /year (large capacity). Any larger construction rate could reduce the spent fuel inventory destined for storage. These and additional results will be presented.Copyright