An Integrated Approach for Monitoring Gas Accumulation in Safety Related Systems

Abstract

Gas intrusion into safety related systems, such as the Emergency Core Cooling System (ECCS), Decay Heat Removal (DHR) and Containment Spray (CS) system in nuclear power plants is undesirable and has led to pump binding and damaging water hammer events. Furthermore, total or momentary loss of hydraulic performance in safety related pumps has occurred, which has led to pump damage rendering the pumps unable to perform their safety functions or reduce the pump discharge pressure and flow capacity to the point that the system cannot perform its design function. Extreme cases of water hammer can result in physical damage to system piping, components and supports, and possible relief valve lifting events with consequential loss of inventory. The U.S. Nuclear Regulatory Commission (NRC) issued Generic Letter GL-2008-01, “Managing Gas Accumulation in Emergency Core Cooling, Decay Heat Removal, and Containment Spray Systems,” requiring U.S. utilities to demonstrate that suitable design, operation and testing measures are in place to maintain licensing commitments. GL-2008-01 outlines a number of actions that are detailed in nature, such as establishing pump void tolerance limits, establishing limits on pump suction void fractions, assuring adequate system venting capability, identifying all possible sources of gas intrusion, preventing vortex formation in tanks and determining acceptable limits of gas in system discharge piping. Regarding one of these issues, GL-2008-01 indicates that the amount of gas that can be ingested without significant impact on pump operability and reliability is not well established and is known to depend on pump design, gas dispersion and flow rate. Each U.S. nuclear power plant licensee is required to evaluate their ECCS, DHR and CS system design, operation and test procedures to assure that gas intrusion is minimized and monitored in order to maintain system operability and compliance with the requirements of the U.S. code of federal regulations 10 CFR 50 Appendix B. Typically, gases get into the safety related systems through a number of mechanisms, such as maintenance, gas desorption, vortex activities, cavitation, etc. This paper discusses the sources of gas into safety related systems and the challenges associated with management of gas voids in these systems. A number of technologies exist that can detect the gas that accumulates in the safety related piping. These technologies are discussed and an integrated approach for monitoring gas accumulation in safety related pipes is presented. Issues such as methods to get rid of gases and venting periodicity are discussed. Industry efforts to address the management of gases in these systems are also presented.