Chang-Hyun Chung

Enhanced Natural Convection in a Metal Layer Cooled by Boiling Water

Abstract An experimental study is performed to investigate the natural convection heat transfer characteristics and the solidification of the molten metal pool concurrently with forced convective boiling of the overlying coolant to simulate a severe accident in a nuclear power plant. The relationship between the Nusselt number (Nu) and the Rayleigh number (Ra) in the molten metal pool region is determined and compared with the correlations in the literature and experimental data with subcooled water. Given the same Ra condition, the present experimental results for Nu of the liquid metal pool with coolant boiling are found to be higher than those predicted by the existing correlations or measured from the experiment with subcooled boiling. To quantify the observed effect of the external cooling on the natural convection heat transfer rate from the molten pool, it is proposed to include an additional dimensionless group characterizing the temperature gradients in the molten pool and in the external coolant region. Starting from the Globe and Dropkin correlation, engineering correlations are developed for the enhancement of heat transfer in the molten metal pool when cooled by an overlying coolant. The new correlations for predicting natural convection heat transfer are applicable to low-Prandtl-number (Pr) materials that are heated from below and solidified by the external coolant above. Results from this study may be used to modify the current model in severe accident analysis codes.

Xenon Stability Analysis Using the Generalized Nyquist Criterion

Xenon spatial oscillation may give rise to operational difficulties in a nuclear power plant. In this study, in order to investigate xenon instability for a pressurized water reactor, the frequency-domain technique is adopted by using the generalized Nyquist criterion, which is more general and suitable for the multi-input/multi-output system. Also linearized modal fluxes are obtained by modal expansion. This model has been implemented to test the axial xenon stability of the KNU 7 unit with changes in plant operating parameters.

Quantification Frameworks and Their Application for Evaluating the Software Quality Factor Using Quality Characteristic Value

Many problems, related with safety, frequently occur because Digital Instrument & Control Systems are widely used and expanding their ranges to many applications in Nuclear Power Plants. It, however, does not hold a general position to estimate an appropriate software quality. Thus, the Quality Characteristic Value, a software quality factor through each software life cycle, is suggested in this paper. The Quality Characteristic Value is obtained as following procedure: 1) Scoring Quality Characteristic Factors (especially correctness, traceability, completeness, and understandability) onto Software Verification and Validation results, 2) Deriving the diamond-shaped graphs by setting values of Factors at each axis and lining every points, and lastly 3) Measuring the area of the graph for Quality Characteristic Value. In this paper, this methodology is applied to Plant Control System. In addition, the series of quantification frameworks exhibit some good characteristics in the view of software quality factor. More than any thing else, it is believed that introduced framework may be applicable to regulatory guide, software approval procedures, due to its soundness and simple characteristics.Copyright

Development of Reliability Function and Failure Rate Considering Information of Censored Data

Generally, there are two methods to estimate reliability of system or component failure in engineering, or survival function of patient death in medicine: 1) parametric and 2) non-parametric methods. The parametric method has been used widely since there is few data on severe accidents in nuclear power plants. However, if we have a lot of data on system failure or patient death, the non-parametric method could be used to estimate reliability or survival function. In this paper we focus on the latter case in which one problem arises in treating the data. The problem is how to reflects efficiently the censoring information into reliability or survival function. In this study, two works have been done: 1) develop the model to reflect censoring information and 2) show how efficient the model is using the Monte-Carlo Simulation. In the WASH-1400 (NUREG-75/014), Failure to Operate of Motor Operated Valves from Nuclear Experience is suggested 1×10−3 /d. The new developed method can estimate closer to the failure rate 1×10−3 /d than the other existing methods. Also the failure rate using the new method satisfies the conservative limit.Copyright