Kemal O. Pasamehmetoglu

The U.S. accelerator transmutation of waste program

Abstract A national project to develop a future capability to separate actinides and long-lived fission products from spent fuel, to transmute them, and to dispose off the remaining waste in optimal waste forms has begun in the United States. This project is based on the Accelerator-driven Transmutation of Waste (ATW) program developed during the 1990s at Los Alamos National Laboratory, and has its technological roots in several technologies that have been developed by the multi-mission laboratories of the U.S. Department of Energy (DOE). In the Fiscal Year 1999 Energy and Water Appropriation Act, the U.S. Congress directed the DOE to study ATW and by the end of FY99 to prepare a “roadmap” for developing this technology. DOE convened a steering committee, assembled four technical working groups consisting of members from many national laboratories, and consulted with several individual international and national experts. The finished product, “A Roadmap for Developing ATW Technology – A Report to Congress,” recommends a five-year,

Saturated pool nucleate boiling mechanisms at high heat fluxes

281 M, science-based, technical-risk-reduction program. This paper provides an overview of the U.S. Roadmap for developing ATW technology, the organization of the national ATW Project, the critical issues in subsystems and technological options, deployment scenarios, institutional challenges, and academic and international collaboration.

The effect of Helmholtz instability on the macrolayer thickness in vapor mushroom region of nucleate pool boiling

Abstract A new model has been developed where the coupled transient two-dimensional conduction equation is solved for the heater and the liquid macrolayer, while allowing for the time-wise thinning of the macrolayer. The major conclusions are: (1) dominant evaporation occurs at the liquid-vapor-solid contact point (triple point) and is required to match boiling curve behavior quantitatively; (2) evaporation at the stem interface and bubble-macrolayer interface is negligible (except near critical heat flux); and (3) the results are sensitive to closure relationships, especially to the active site-density correlation.

Modeling of heat and mass transfer in accelerator targets during postulated accidents

Abstract In the previously postulated relationship between the macrolayer thickness in saturated pool boiling and the Helmholtz instability wavelength is further investigated in the ligth of experimental data that recently appeared in the open literature. The study shows that the Helmholtz wavelength in the vapor stems is strongly dependent on parameters affected by surface chemistry. These parameters same order of magnitude as the ones reported in the literature, the Helmholtz instability model was able to successfully predict the macrolayer thickness data. This result suggests that the Helmholtz instability model must not be ruled out, unless further experimental research proves otherwise.

A numerical investigation of the effect of heating methods on saturated nucleate pool boiling

The modeling of thermal-chemical behavior of targets used in accelerator applications is an important part of safety analysis. Tungsten is considered as a target material to produce tritium in a linear proton accelerator. The prediction of the chemical reactivity of tungsten in a steam flow at high temperatures is the most important part of a safety analysis of target design. The oxidation and volatilization of tungsten in steam at high temperatures is a complex phenomenon that involves various mechanisms (depending on the temperature), steam pressure, and steam velocity. A simple diffusion model that considers chemical equilibrium at the reaction interface and effective diffusion thickness, including the boundary and oxide layers, is proposed for predicting the volatilization rate. The proposed simple model predicts the available data reasonably well. The proposed model is implemented into a computer program that is developed to predict the radiological releases during postulated loss-of-coolant accidents (LOCAs). The computer program models heat production, heat transfer, and oxidation reactions in the multiple radiation enclosures representing the accelerator target elements. It treats each element of the radiation enclosures as a lumped control volume, or heat structure. Each heat structure may generate or lose heat by conduction, convection, or radiation and is subject to mass loss as a result of oxidation, melting, and volatilization. Postulated beyond-design-basis LOCAs are simulated with this computer program for the accelerator-production-of-tritium target. Sample calculations demonstrate oxidation/volatilization model capabilities and sensitivity to the assumptions selected.

A simple forced convection film boiling model

Abstract Using a numerical model, the effect of heating methods on saturated nucleate pool boiling is investigated parametrically for smooth and rough nickel and copper heater plates. The boiling curve moved right with decreasing thickness for the smooth and rough nickel and copper heaters in the constant-heat-flux heating method. This trend was reversed in the constant-temperature heating method; the boiling curved shifted left with decreasing heater thickness. However, the later trend was not affected by the heater material and thickness and the surface roughness (mean cavity radius). The boiling curves were identical for the constant internal generation rate and the constant-heat-flux heating method. The use of ac instead of dc resistive heating caused the boiling curve generally to move left. This behavior was not linear with the heat flux, heater material, or surface conditions. No hysterisis was found when the heat flux was increased and then decreased gradually to original values.

Numerical modeling of a nucleate boiling surface

Abstract A simple analysis of the steady laminar forced convection film boiling flow over a horizontal flat plate is presented. A novel analogy drawn between the mobile liquid-vapor interface in forced convection film boiling and a moving surface in a flowing single-phase fluid enables a simple analysis of this complex flow. As a result, easy to apply closed-form expressions to predict theoretical wall heat transfer and skin friction are obtained, circumventing the need for previous complex analyses.

On the relationship between the macrolayer thickness and the vapor-stem diameter in the high-heat-flux, pool nucleate boiling region

ABSTRACT A computer program developed to analyze nucleate boiling over a heated surface is described. The model solves the three-dimensional transient conduction equation within the heater. The conduction solution is coupled with closure relationships to mimic the bubble dynamics and the associated heat transfer coefficients. Sample problems are run using a copper surface subject to partial nucleate boiling in saturated water at atmospheric pressure. The results are shown to be in good qualitative agreement with the pertinent experimental observations.

Modeling of radiation heat transport in complex ladder-like structures placed in rectangular enclosures

Abstract The authors consider the high-heat-flux, pool nucleate boiling region of a horizontal surface and propose a theoretical basis for estimating the constant of proportionality between the mean macrolayer thickness and the mean vapor-stem diameter at critical heat flux. This constant is found to be of the same order of magnitude (−10% difference) as that suggested by Gaertner [5]. While Gaertners estimate is based on photographic evidence, the current approach uses a theoretical basis together with experimental data.

Heat transfer from a high temperature safety rod placed in a perforated guide tube and surrounded by an array of cold tubes

Abstract Complex ladder-like structures recently have been considered as the target design for accelerator applications. The decay heat, during a postulated beyond design-basis loss-of-coolant accident in the target where all normal and emergency cooling fails, is removed mainly by radiation heat transfer. Modeling of the radiation transport in complex ladder-like structures has several challenges and limitations when the standard net-radiation model is used. This paper proposes a simplified lumped, or ‘hot-rung’ model, that considers the worst elements and utilizes the standard net-radiation method. The net-radiation model would under-predict structure temperatures if surfaces were subject to non-uniform radiosity. The proposed model was assessed to suggest corrections to account for the non-uniform radiosity. The non-uniform radiosity effect causes the proposed hot-rung model to under-predict the center-rung temperatures by ≈4–74°C when all parametrics, including temperatures up to 1500°C, were considered. These temperatures are small. The proposed model predicted that an important effect of decreasing the emissivity was smoothing of non-isothermal effects. The radiosity effects are more pronounced when there are strong temperature gradients. Uniform rung temperatures tend to decrease the radiosity effects. We concluded that a relatively simple model that is conservative with respect to radiosity effects could be developed.