Cetin Unal

Possible mechanisms of macrolayer formation

Abstract This paper critically compares the mechanisms proposed for the formation of the liquid-rich macrolayer on heater surfaces during nucleate boiling. These mechanisms include Helmholtz instability analysis applied to vapor stems above active nucleation sites, liquid trapped by lateral coalescence of discrete bubbles that initally form during the mushroom bubbles waiting period, and the limitation of liquid resupply after mushroom departure as a result of vapor flow from active sites.

Saturated pool nucleate boiling mechanisms at high heat fluxes

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.

A phenomenological model of the thermal hydraulics of convective boiling during the quenching of hot rod bundles Part I: Thermal hydraulic model

Abstract In this paper, a phenomenological model of the thermal hydraulics of convective boiling in the post-critical-heat-flux (post-CHF) regime is developed and discussed. The model was implemented in the TRAC-PF1/MOD2 computer code (an advanced best-estimate computer program written for the analysis of pressurized water reactor systems). The model was built around the determination of flow regimes downstream of the quench front. The regimes were determined from the flow-regime map suggested by Ishii and his coworkers. Heat transfer in the transition boiling region was formulated as a position-dependent model. The propagation of the CHF point was strongly dependent on the length of the transition boiling region. Wall-to-fluid film boiling heat transfer was considered to consist of two components: first, a wall-to-vapor convective heat-transfer portion and, second, a wall-to-liquid heat transfer representing near-wall effects. Each contribution was considered separately in each of the inverted annular flow (IAF) regimes. The interfacial heat transfer was also formulated as flow-regime dependent. The interfacial drag coefficient model upstream of the CHF point was considered to be similar to flow through a roughened pipe. A free-stream contribution was calculated using Ishiis bubbly flow model for either fully developed subcooled or saturated nucleate boiling. For the drag in the smooth IAF region, a simple smooth-tube correlation for the interfacial friction factor was used. The drag coefficient for the rough-wavy IAF was formulated in the same way as for the smooth IAF model except that the roughness parameter was assumed to be proportional to liquid droplet diameter entrained from the wavy interface. The drag coefficient in the highly dispersed flow regime considered the combined effects of the liquid droplets within the channel and a liquid film on wet unheated walls. The heat-transfer and interfacial drag models used were based on the flow-regime map noted above with length averaging of the flow-regime length if more than one regime existed in a give hydraulic cell.

Calibration under uncertainty for finite element models of masonry monuments

Historical unreinforced masonry buildings often include features such as load bearing unreinforced masonry vaults and their supporting framework of piers, fill, buttresses, and walls. The masonry vaults of such buildings are among the most vulnerable structural components and certainly among the most challenging to analyze. The versatility of finite element (FE) analyses in incorporating various constitutive laws, as well as practically all geometric configurations, has resulted in the widespread use of the FE method for the analysis of complex unreinforced masonry structures over the last three decades. However, an FE model is only as accurate as its input parameters, and there are two fundamental challenges while defining FE model input parameters: (1) material properties and (2) support conditions. The difficulties in defining these two aspects of the FE model arise from the lack of knowledge in the common engineering understanding of masonry behavior. As a result, engineers are unable to define these FE model input parameters with certainty, and, inevitably, uncertainties are introduced to the FE model.

A Resource Allocation Framework for Experiment-Based Validation of Numerical Models

In experiment-based validation, uncertainties and systematic biases in model predictions are reduced by either increasing the amount of experimental evidence available for model calibration—thereby mitigating prediction uncertainty—or increasing the rigor in the definition of physics and/or engineering principles—thereby mitigating prediction bias. Hence, decision makers must regularly choose between either allocating resources for experimentation or further code development. The authors propose a decision-making framework to assist in resource allocation strictly from the perspective of predictive maturity and demonstrate the application of this framework on a nontrivial problem of predicting the plastic deformation of polycrystals.

Fuel-Cladding Chemical Interaction in U-Pu-Zr Metallic Fuels: A Critical Review

Abstract Fuel-cladding chemical interaction (FCCI) is a phenomenon that occurs at the fuel-cladding interface during the irradiation of U-Zr and U-Pu-Zr metallic nuclear fuel and stainless steel cladding. The inter-diffusion zone that develops places both the fuel and cladding at risk through the reduction in cladding strength and the formation of a (U,Pu)/Fe eutectic in the fuel. Due to the impact FCCI has on limiting fuel pin burnup, there is a need for better understanding of the governing FCCI mechanisms in order to make accurate predictions using fuel-performance codes. By performing a critical review of previous work, the physics of FCCI can be separated into individual phenomena so that targeted models can be developed for each. Through examination of experiments conducted both in- and out-of-reactor, the behavior of lanthanides provides a natural separation of models by tracking their behavior through (1) production and transport in the fuel to the clad, (2) interaction with macroscopic changes in fuel topography including cracking and swelling, and finally (3) inter-diffusion at the fuel-cladding interface. Informed by past experience, phenomenological models can be built for each separate effect and subsequently combined in an integral fuel-performance simulation. Prototypical simulation approaches at each level have been included, as well as suggestions for several experiments to help bolster the understanding of irradiated fuel. A robust and predictive FCCI model will provide fuel-performance codes with the ability to predict clad failure and/or fuel eutectic melting. Armed with this information, advanced concepts such as palladium doped fuel, ODS steels, or mitigating reactor designs may be able to reduce FCCI enough to extend fuel burnup beyond its current limits, potentially boosting safety margins and reducing cost through higher fuel utilization.

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

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

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.

Ab initio molecular dynamics study of the properties of cerium in liquid sodium at 1000 K temperature

For liquid-sodium-cooled fast nuclear reactor systems, it is crucial to understand the behavior of lanthanides and other potential fission products in liquid sodium or other liquid metal solutions such as liquid cesium-sodium. In this study, we focus on lanthanide behavior in liquid sodium. Using ab initio molecular dynamics, we found that the solubility of cerium in liquid sodium at 1000 K was less than 0.78 at. %, and the diffusion coefficient of cerium in liquid sodium was calculated to be 5.57 × 10−9 m2/s. Furthermore, it was found that cerium in small amounts may significantly alter the heat capacity of the liquid sodium system. Our results are consistent with the experimental results for similar materials under similar conditions.

A phenomenological model of the thermal-hydraulics of convective boiling during the quenching of hot rod bundles Part II: Assessment of the model with steady-state and transient post-CHF data

Abstract After completion of the thermal-hydraulic model developed in a companion paper, we performed developmental assessment calculations of the model using steady-state and transient post-critical heat flux (CHF) data. This paper discusses the results of those calculations. The overall interfacial drag model predicted reasonable drag coefficients for both the nucleate boiling and the inverted annular flow (IAF) regimes. The predicted pressure drops agreed reasonably well with the measured data of two transient experiments, CCTF Run 14 and a Lehigh reflood test. The thermal-hydraulic model for post-CHF convective heat transfer predicted the rewetting velocities reasonably well for both experiments. The predicted average slope of the wall temperature traces for these tests showed reasonable agreement with the measured data, indicating that the transient-calculated precursory cooling rates agreed with measured data. The hot-patch model, in conjunction with the other thermal-hydraulic models, was capable of modeling the Winfrith post CHF hot-patch experiments. The hot-patch model kept the wall temperatures at the specified levels in the hot-patch regions and did not allow any quench-front propagation from either the bottom or the top of the test section. The interfacial heat-transfer model tended to slightly underpredict the vapor temperatures. The maximum difference between calculated and measured vapor temperatures was 20%, with a 10% difference for the remainder of the runs considered. The wall-to-fluid heat transfer was predicted reasonably well, and the predicted wall temperatures were in reasonable agreement with measured data with a maximum relative error of less than 13%.