George Yadigaroglu

Trends and needs in experimentation and numerical simulation for LWR safety

Abstract The very complex phenomena that need to be considered in safety analyses require use of sophisticated analytical tools. Basically, one-dimensional (1D) system codes have been used for a long time and have reached a degree of maturity. There are, however, limits to their capabilities and further developments are underway; these are outlined. The development of new generations of tools and methods can profit from the availability of increasingly powerful computers and advances in multiphase flow, information technology and numerical techniques. Three-dimensional (3D) situations need also to be addressed more frequently now. Certain developments in these directions that are already taking place in various EURATOM research programs and elsewhere are briefly reviewed; case studies of applications are discussed and lessons drawn. Future safety analyses for nuclear power plants may include use of Computational Fluid Dynamics (CFD) for parts of the primary system and the containment. First applications in this direction have already been made. Although 3D, single-phase CFD computations are commonplace, the size of the systems considered make these quite challenging. The real challenges lie, however, in two-phase flow CFD applications that are still at their very infancy. Coupling of neutronic and thermal-hydraulic codes is also necessary for certain problems.

Direct Numerical Simulation of Turbulent Heat Transfer Across a Mobile, Sheared Gas-Liquid Interface

The impact of interfacial dynamics or turbulent heat transfer at a deformable, sheared gas-liquid interface is studied using Direct Numerical Simulation (DNS). The flow system comprises a gas and a liquid phase flowing in opposite directions. The governing equations for the two fluids are alternated solved in separate domains and then coupled at the interface by imposing continuity of velocity and stress. The deformations of the interface fall in the range of capillary waves of waveslope ak=0.01 (wave amplitude a times wavenumber k), and very small phase speed-to-friction velocity ratio, c/u*. The influence of low-to-moderate molecular Prandtl numbers (Pr) on the transport in the immediate vicinity of the interface is examined for the gas phase, and results are compared to existing will-bounded flow data. The shear-based Reynolds number Re* is 171 and Prandtl numbers of 1, 5, and 10 were studied. The effects induced by changes in Pr in both wall-bounded flow and over a gas-liquid interface were analyzed by comparing the relevant statistical flow properties, including the budgets for the temperature variance and the turbulent heat fluxes

Numerical investigation of the entrainment and mixing processes in neutral and stably-stratified mixing layers

The direct numerical simulation (DNS) of a temporally-growing mixing layer has been carried out, for a variety of initial conditions at various Richardson and Prandtl numbers, by means of a pseudo-spectral technique; the main objective being to elucidate how the entrainment and mixing processes in mixing-layer turbulence are altered under the combined influence of stable stratification and thermal conductivity. Stratification is seen to significantly modify the way by which entrainment and mixing occur by introducing highly-localized, convective instabilities, which in turn cause a substantially different three-dimensionalization of the flow compared to the unstratified situation. Fluid which was able to cross the braid region mainly undisturbed (unmixed) in the unstratified case, pumped by the action of rib pairs and giving rise to well-formed mushroom structures, is not available with stratified flow. This is because of the large number of ribs which efficiently mix the fluid crossing the braid region. ...

Numerical investigation of the formation of three-dimensional structures in stably-stratified mixing layers

A temporally-growing mixing layer has been directly simulated with a pseudospectral technique, for initial bulk Richardson numbers from 0.0 to 0.2 and for Prandtl numbers from 0.00535 to 2.2. Several different initial conditions for the velocity fluctuations were imposed. For the two-dimensional (2-D) case only purely-deterministic conditions were used, whereas purely-deterministic, combined deterministic-random, or purely-random conditions were imposed in the three-dimensional (3-D) cases. The numerical procedure allowed fields with very different characteristic lengths to be resolved, with spectral accuracy maintained. The evolution of the velocity, active (temperature), and passive scalar fields were followed independently by adaptively redistributing collocation points in the regions of high shear and rapid scalar variations. The vertical boundary conditions were imposed at infinity to eliminate any boundary-layer effects and an exponential mapping was used to translate infinite physical space into fi...

Turbulence and heat exchange in condensing vapor-liquid flow

Turbulence and heat exchange during condensation of a vapor stream countercurrently flowing to a subcooled liquid stream in a slightly inclined channel has been investigated by direct numerical simulation (DNS). Condensation rates and imposed pressure gradients have been varied, and capillary-gravity waves have been allowed to develop at the (deformable) vapor-liquid interface. These simulations extend our previous DNS of turbulence and scalar exchange in stratified gas-liquid flows without condensation. The previous studies indicated that for conditions in which the gas-liquid interface remained continuous, i.e., did not “break,” scalar exchange rates on both the gas and liquid sides were largely determined by sweeps which brought high momentum fluid from the bulk flow to the interface. As sweep frequencies were found to scale with interfacial friction velocities, scalar exchange coefficients could be parametrized with a surface renewal theory. The issue addressed in the current work is how these finding...

Reflooding With Steady and Oscillatory Injection: Part I—Flow Regimes, Void Fraction, and Heat Transfer

Simultaneous void fraction and wall temperature measurements were made during bottom-reflooding of a vertical Inconel tube under both constant and oscillatory injection rates. To support interpretation of these data, flow regime visualization experiments were also conducted by reflooding a heated quartz tube. With constant, high reflooding rates, inverted annular, transition, and dispersed flow regimes exist above the quench front, with typical chordal-average void fractions in the ranges of 10–30 percent, 30–70 percent, and 70–90 percent, respectively. Each regime exhibits different heat transfer rates. With lower injection rates or higher heating rates, annular droplet and dispersed flow regimes appear with void fractions above 80 percent. For reflooding with oscillatory inlet flow and high injection rates, large oscillations are seen in void fraction and wall temperature, indicating periodic changes in the flow regime near the quench front: The regime alternated between inverted annular (during an upstroke) and annular droplet flow (during a downstroke). These flow regimes were observed in the flow visualization experiments to be qualitatively similar to those for the constant injection cases. Heat transfer rates are substantially affected by the flow regime and increase (or decrease) as the void fraction falls (or rises). Compared to the constant-injection tests, increased rates of entrainment were observed during the forced-oscillation tests.

On Farmer’s Line, Probability Density Functions, and Overall Risk

Limit lines used to define quantitative probabilistic safety goals can be categorized according to whether they are based on discrete pairs of event sequences and associated probabilities, on probability density functions (pdfs), or on complementary cumulative density functions (CCDFs). In particular, the concept of the well-known Farmers line and its subsequent reinterpretations is clarified. It is shown that Farmers lines are pdfs and, therefore, the overall risk (defined as the expected value of the pdf) that they represent can be easily calculated. It is also shown that the area under Farmers line is proportional to probability, while the areas under CCDFs are generally proportional to expected value.

Planned experimental studies on natural-circulation and stability performance of boiling water reactors in four experimental facilities and first results (NACUSP)

Within the 5th Euratom framework programme the NACUSP project focuses on natural-circulation and stability characteristics of Boiling Water Reactors (BWRs). This paper gives an overview of the research to be performed. Moreover, it shows the first results obtained by one of the four experimental facilities involved. Stability boundaries are given for the low-power low-pressure operating range, measured in the CIRCUS facility. The experiments are meant to serve as a future validation database for thermohydraulic system codes to be applied for the design and operation of BWRs

Equilibrium quality and mass flux distributions in an adiabatic three-subchannel test section

An experiment was designed to measure the fully developed quality and mass flux distributions in an adiabatic three-subchannel test section. The three subchannels had the geometrical characteristics of the corner, side, and interior subchannels of a boiling water reactor (BWR-5) rod bundle. Data collected with Refrigerant-114 at pressures ranging from 7 to 14 bars, simulating operation with water in the range 55 to 103 bars are reported. The average mass flux and quality in the test section were in the ranges 1,300 to 1,750 kg/m{sup 2} {center_dot} s and {minus}0.03 to 0.25, respectively. The data are analyzed and presented in various forms.

Nuclear Safety and Thermal Hydraulics: Personal Thoughts and Some Recent Progress

Abstract An attempt is made in the first part of this paper to review the history of reactor safety and draw conclusions about trends that could be avoided and directions that could lead to robust reactor designs that would not be susceptible to severe accidents. In the second part, progress in reactor thermal hydraulics is observed by reviewing the list of conference sessions. Finally, a report is made on some recent work on two computational problems: the prediction of departure from nucleate boiling and the potential spatial coupling of computational multifluid dynamics methods to achieve multiscale, high-resolution simulations.