Dae-Hyun Hwang

Assessment of a tube-based bundle CHF prediction method using a subchannel code

Abstract At the conceptual design stage for advanced water-cooled reactors (AWCRs), a general critical heat flux (CHF) prediction method with a wide applicable range and reasonable accuracy is essential to the thermal-hydraulic design and safety analysis. A basic idea for general CHF prediction is to utilize the tube-based CHF models covering wide applicable ranges with the help of supplementary terms for bundle effects. In this study, feasibility assessments have been performed with the bundle CHF data relevant to pressurized water reactors. A subchannel analysis is adopted in order to be able to consider the geometrical variations properly even in untested fuel bundle geometries. As a result, a CHF look-up table method (i.e., the use of a round tube CHF table with appropriate bundle effect factors) turns out to be a promising way to fulfill the needs in many aspects among some selected correlations and theoretical models. Though improvements of the supplementary factors, especially for the cold wall and the bundle heated length effects, are desirable to make better predictions, the tube-based bundle CHF prediction method clearly shows a potential as a general CHF predictor.

Development of a bundle correction method and its application to predicting CHF in rod bundles

Abstract A bundle correction method, based on the conservation laws of mass, energy, and momentum in an open subchannel, is proposed for the prediction of the critical heat flux (CHF) in rod bundles from round tube CHF correlations without detailed subchannel analysis. It takes into account the effects of the enthalpy and mass velocity distributions at subchannel level using the first derivatives of CHF with respect to the independent parameters. Three different CHF correlations for tubes (Groenevelds CHF table, Katto correlation, and Biasi correlation) have been examined with uniformly heated bundle CHF data collected from various sources. A limited number of CHF data from a non-uniformly heated rod bundle are also evaluated with the aid of Tongs F- factor . The proposed method shows satisfactory CHF predictions for rod bundles both uniform and non-uniform power distributions.

Development of a phenomenological model for the prediction of dryout locations under flooding-limited critical heat flux conditions

Abstract A physical model for the prediction of dryout locations in a boiling channel with a closed bottom end is developed on the basis of the liquid film dryout model and the two-phase mixture level theory. It is assumed that the first dryout occurs at the boundary between the two-phase mixture region and the countercurrent annular flow region in the present boiling system. The mass and energy conservation equations are applied to the liquid film assuming no entrainment from the film region. A drift flux formulation is used for the calculation of the void fraction in the two-phase mixture region. In view of the results compared with Kattos experimental data, the present model is found to predict the upper and lower bounds of the dryout locations measured in the boiling system with a closed bottom end. The model accuracy improves as the length-to-diameter ratio increases.

Accuracy and Uncertainty Analysis of PSBT Benchmark Exercises Using a Subchannel Code MATRA

In the framework of the OECD/NRC PSBT benchmark, the subchannel grade void distribution data and DNB data were assessed by a subchannel code, MATRA. The prediction accuracy and uncertainty of the zone-averaged void fraction at the central region of the 5 × 5 test bundle were evaluated for the steady-state and transient benchmark data. Optimum values of the turbulent mixing parameter were evaluated for the subchannel exit temperature distribution benchmark. The influence of the mixing vanes on the subchannel flow distribution was investigated through a CFD analysis. In addition, a regionwise turbulent mixing model was examined to account for the nonhomogeneous mixing characteristics caused by the vane effect. The steady-state DNB benchmark data with uniform and nonuniform axial power shapes were evaluated by employing various DNB prediction models: EPRI bundle CHF correlation, AECL-IPPE 1995 CHF lookup table, and representative mechanistic DNB models such as a sublayer dryout model and a bubble crowding model. The DNBR prediction uncertainties for various DNB models were evaluated from a Monte-Carlo simulation for a selected steady-state condition.

An improved physical model for flooding limited CHF under zero and very low flow conditions

Abstract This paper presents an improved physical model to obtain comprehensive understanding of the CHF characteristics and to predict the CHF values under zero and very low flow conditions. The improved model gives reasonable agreements with the experimental data for CHF locations and values. It provides details of physical information on the CHF phenomena, and analytically confirms that the flooding is a triggering mechanism of the countercurrent annular flow CHF under zero and very low flow conditions. The present model reveals that the heat flux effect such as entrainment due to nucleation should be considered for the analysis of the flooding limited CHF as the L/D ratio decreases.

Critical Heat Flux Tests for an Application of the Three-Pin Fuel Test Loop in HANARO

Critical heat flux (CHF) tests in a three-rod bundle were carried out, and a CHF database was established in order to obtain an operational license for the three-pin fuel test loop (FTL) in HANARO (High-flux Advanced Neutron Application Reactor). Two kinds of CHF tests are currently being performed for the CANDU and PWR-type fuel test loop. In this study, an experimental work on the CHF tests for the CANDU-type fuel test loop is explained, and a new CHF prediction methodology is proposed. In all, 108 experimental data points have been obtained, and the data are analyzed and compared with the available CHF correlations for a bundle or an annulus geometry. The 1986 AECL look-up table with a bundle correction factor and the Doerffers correlation for annuli are compared with the present CHF data. It is found that the 1986 AECL look-up table with a bundle correction factor results in a better prediction than the Doerffers correlation for annuli geometry. A three-pin correction factor is developed to account for the geometric effects of the three-rod bundle and to improve the prediction accuracy. It is concluded that the best estimate thermal hydraulic system code, MARS 3.0, which uses the same look-up table for a CHF prediction, can be used for a safety analysis of the CANDU-type three-pin FTL to obtain a license if it is corrected by the developed three-pin correction factor.