Main steam isolation valves (MSIVs) for operating commercial reactors are large, fast-acting valves responsible for isolating steam systems in the event of a design basis accident (DBA). Due to the fast closure times required for MSIVs to perform their safety function and the large size of the valve, the valve actuators are mechanically complex. At Plant Vogtle (Units 1 and 2), malfunctions associated with MSIVs have resulted in thirteen unplanned outages and/or reactor trips since both units began commercial operation in the late 1980s. Seven of these unplanned outages have occurred in the last 20 years. In addition to unplanned outages, maintenance associated with fast-acting MSIVs is expensive and often requires specialty vendors to perform the required routine maintenance evolutions.
The majority of the operating PWR fleet has one MSIV per steam line. However, Plant Vogtle in the Southern Nuclear fleet has two MSIVs per steam line. This leads to double the single point vulnerabilities (SPVs), double the maintenance costs, and double the probability of an unplanned outage resulting from an MSIV malfunction for Vogtle as compared to the current operating fleet.
Approximately 40 units in the U.S. fleet utilize fast-acting electrohydraulic isolation valves as a part of the main steam or feedwater systems. There is evidence that these valve types have had negative impacts on maintenance and plant capacity factors resulting from numerous unplanned outages and complex and expensive maintenance evolutions.
To improve these maintenance costs and overall plant reliability, Plant Vogtle has implemented an innovative valve actuator technology referred to as system media actuators (SMAs).