Heater selection criteria for engineered safety features atmosphere filtration systems

US Nuclear power plants incorporate engineered safety features atmosphere filtration system (ESFAFS) in their design as a means of limiting radiation exposure to individuals in the event of a design basis radiological accident. These systems typically utilize heaters to limit the relative humidity of the filtered air to 70 percent or less in order to obtain the desired level of efficiency from the charcoal adsorbers. ANSI/ASME N509 requires that these heaters be sized to reduce the maximum expected relative humidity of the airstream to approximately 70 percent at the system design flowrate. In some cases the criteria used to select these heaters may not consider the worst case conditions that could occur both during and following a design basis accident. Degraded electrical system voltage, airflow variations, and accident condition heat loads are significant factors in the heater selection process that are commonly overlooked. The Vogtle Electric Generating Plant ESFAFS heaters were originally selected without considering these worst case conditions. As a result, several systems were found to have insufficient heater capacity to maintain the previously assumed humidity levels and filtration efficiency. To compensate for the undersized heaters, the Technical Specifications and accident analysis were revised to reflect the potential impact of increased relative humidity on the charcoal adsorber filtration performance. This paper describes the ESFAFS at the Vogtle Electric Generating Plant. Attention is given to individual factors that are often overlooked in EFSAFS heater selection. An overview of the system analysis and the corrective actions prompted by the analysis are presented. 11 refs., 2 tabs