Operator Action–Induced Two-Phase Flow Condition Resulting in Performance Degradation of Interfacing Passive System

Abstract

This study investigates the degradation of the heat transfer performance of a closed-circuit intermediate natural circulation heat transport loop used as a passive safety system in a nuclear power plant (NPP). The degradation arises from the strong thermal-hydraulic (TH) coupling of the loop operating characteristics, saturation temperature and pressure, and natural circulation flow rate, which determine the heat rejection rate to the TH boundary conditions imposed on the hot side of the loop by the transitory state of the primary reactor coolant system (RCS) of the NPP. Several operator actions related to a feed-and-bleed emergency operating procedure (F&B) are postulated, and system TH code simulations are performed to demonstrate how the F&B can induce two-phase flow conditions in the RCS. Natural circulation two-phase flow regimes in the RCS hot leg can significantly reduce the heat transfer to the circulating working fluid of the interfacing heat transport loop over long periods, sometimes lasting over 24 h, of passive system mission time. A transient performance indicator for the passive system mission is introduced for use in the passive reliability assessment and quantitative comparison of transient simulations. The need to consider human factors in the design and operation of NPP passive safety systems is stressed.

Keywords:

• Passive system degradation
• natural circulation
• reverse flow
• operator actions

Acknowledgments

This work has been carried out under the nuclear long-term research and development program sponsored by the Korea Ministry of Science and Information and Communication Technologies and Ulsan National Institute of Science & Technology Internal Project 1.180088.01. All simulations were performed at the Korea Atomic Energy Research Institute.

Notes

^a Operators routinely use nonsafety-grade systems for accident mitigation at operating NPPs designed with safety-grade active systems. For example, loss-of-coolant accident events at CANDU heavy water reactors have been successfully mitigated without the actuation of the emergency coolant injection system (ECIS).⁷ Following the injection of light water into the CANDU heat transport system by the ECIS, very expensive upgrading of the diluted heavy water must be performed to return the plant to service. Thus, operators have the incentive to use heavy water charging and makeup systems and to manually perform a controlled cooldown and depressurization.

^b An exception is the use of peak clad temperature to evaluate the performance of the accumulator or safety injection tank passive systems used at most operating PWRs. Accumulators are specifically designed to limit the peak clad temperature during the refill and reflood stages of a large-break loss-of-coolant accident (LBLOCA).