Stop valve for installation in a pipeline of a nuclear facility to stop a fluid flow through the pipeline in the event of an operational failure, the stop valve comprising a valve housing including a flow channel passing through the valve housing, a closure member arranged at least partially in the flow channel and reversibly transferrable between an open position and a closed position such as to open or close the flow channel through the valve housing, a non-electrically driven actuator mechanism operatively coupled to the closure member for transferring the closure member at least from the open position to the closed position, wherein the actuator mechanism is configured to be activated by a fluid flow through the flow channel reaching or exceeding a switching temperature and/or switching flow rate during operation, and at least one position indicator (30, 34) to indicate whether the closure member is in the open position or in the closed position, characterized in that the position indicator (30, 34) comprises at least one indicator member (31, 32) movably arranged in or at the valve housing between a first position and a second position, wherein the indicator member (31, 32) is magnetically coupled to the closure member directly by a direct magnetic interaction between at least a portion of the closure member and at least a portion of the indicator member or indirectly by a coupling element fixedly coupled to at least one of the closure member or the indicator member which magnetically interacts with the respective other member or another coupling element that is fixedly coupled to the respective other member such that the indicator member (31, 32) is magnetically transferred into the first position when the closure member is transferred into the open position, and into the second position when the closure member is transferred into the closed position. a valve housing including a flow channel passing through the valve housing, a closure member arranged at least partially in the flow channel and reversibly transferrable between an open position and a closed position such as to open or close the flow channel through the valve housing, a non-electrically driven actuator mechanism operatively coupled to the closure member for transferring the closure member at least from the open position to the closed position, wherein the actuator mechanism is configured to be activated by a fluid flow through the flow channel reaching or exceeding a switching temperature and/or switching flow rate during operation, and at least one position indicator (30, 34) to indicate whether the closure member is in the open position or in the closed position, characterized in that the position indicator (30, 34) comprises at least one indicator member (31, 32) movably arranged in or at the valve housing between a first position and a second position, wherein the indicator member (31, 32) is magnetically coupled to the closure member directly by a direct magnetic interaction between at least a portion of the closure member and at least a portion of the indicator member or indirectly by a coupling element fixedly coupled to at least one of the closure member or the indicator member which magnetically interacts with the respective other member or another coupling element that is fixedly coupled to the respective other member such that the indicator member (31, 32) is magnetically transferred into the first position when the closure member is transferred into the open position, and into the second position when the closure member is transferred into the closed position.