Woong Sik Kim

Numerical Simulation of Subcooled Flow Boiling Heat Transfer in Helical Tubes

This paper addresses the numerical simulation of two-phase flow heat transfer in the helically coiled tubes of an integral type pressurized water reactor steam generator under normal operation using a computational fluid dynamics code. The shell-side flow field where a single-phase fluid flows in the downward direction is also calculated in conjunction with the tube-side two-phase flow characteristics. For the calculation of tube-side two-phase flow, the inhomogeneous two-fluid model is used. Both the Rensselaer Polytechnic Institute wall boiling model and the bulk boiling model are implemented for the numerical simulations of boiling-induced two-phase flow in a vertical straight pipe and channel, and the computed results are compared with the available measured data. The conjugate heat transfer analysis method is employed to calculate the conduction in the tube wall with finite thickness and the convections in the internal and external fluids simultaneously so as to match the fluid-wall-fluid interface conditions properly. Both the internal and external turbulent flows are simulated using the standard k-e model. From the results of the present numerical simulation, it is shown that the bulk boiling model can be applied to the simulation of two-phase flow in the helically coiled steam generator tubes. In addition, the present simulation method is considered to be physically plausible in the light of discussions on the computed results.

Fretting-wear Characteristics of Steam Generator Helical Tubes

This study investigates the safety assessment of the potential for fretting-wear damages caused by foreign object in operating nuclear power plants. To get the natural frequency, corresponding mode shape and participation factor, modal analyses are performed for the helical type tubes with various conditions. The wear rate of helical type tube caused by foreign object is calculated using the Archard formula and the remaining life of the tube is predicted, and discussed in this study is the effect of the vibration of the tube on the remaining life of the tube. In addition, addressed is the effect of the external pressure on the vibration and fretting-wear characteristics of the tube.

A Direction to Adopting Risk-Informed and Performance-Based Regulation (RIPBR) in Korea

The USNRC has pursued the incorporation of new regulatory approach of risk-informed and performance-based regulation (RIPBR) into nuclear safety regulation, as an alternative to improve existing nuclear safety regulation of nuclear power plants, which is deterministic and prescriptive. It focuses on the use of risk insight from probabilistic safety assessment (PSA). Recently, it becomes necessary to find a way to improve regulatory efficiency and effectiveness in order to cover the increasing regulatory needs in Korea. Also, the utility has optimized design and operation of the plant using PSA insight and equipment performance information. According to the increase of the necessity for regulatory improvement using risk and performance information, KINS (Korea Institute of Nuclear Safety) is developing, as a part of mid- and long-term project of Nuclear R&D program, how to adopt the RIPBR in Korean nuclear regulation. This paper presents the interim result of the study that comprises the assessment of feasibility to adopt the RIPBR, and basic directions and principles for implementing RIPBR model. It is concluded that adopting RIPBR is essential for risk management of the plant, public acceptance to nuclear safety, improvement of existing regulation, minimization of unnecessary regulatory burden, and effective use of regulatory resources, etc. Three basic directions and several principles that are necessary to implement RIPBR model are identified from the study. The application of the directions and principles to the assessment of RIPBR model to be established in the near future is expected to result in making the adoption of new regulatory system more objective and consistent.Copyright