As shown in FIG. 1, a conventional nuclear reactor, such as a Boiling Water Reactor (BWR), may include a reactor pressure vessel (RPV) 12 with a generally cylindrical shape. RPV 12 may be closed at a lower end by a bottom head 28 and at a top end by a removable upper head 29. A cylindrically-shaped core shroud 34 may surround reactor core 36, which includes several nuclear fuel elements that generate power through fission. Shroud 34 may be supported at one end by a shroud support 38 and may include a removable shroud head 39 and separator tube assembly at the other end. Fuel bundles may be aligned by a core plate 48 located at the base of core 36. One or more control blades 20 may extend upwards into core 36, so as to control the fission chain reaction within fuel elements of core 36. Additionally, one or more instrumentation tubes 50 may extend into reactor core 36 from outside RPV 12, such as through bottom head 28, permitting instrumentation, such as neutron monitors and thermocouples, to be inserted into and enclosed within the core 36 from an external position.
A fluid coolant, such as water, is circulated up through core 36 and core plate 48 and is at least partially converted to steam by the heat generated by fission in the fuel elements. The steam is separated and dried in separator tube assembly and steam dryer structures 15 and exits RPV 12 through a main steam nozzle 3 near top of RPV 12. The coolant circulated through and boiled in RPV 12 is typically pure and deionized, except for some additives that enhance coolant chemistry. During the course of operation, fission products may be inadvertently leaked into the coolant through failure of fuel elements in core 36, or hydrogen may be produced in fuel elements through radiolysis and other reactions. These and other events may cause non-condensable gasses to migrate up through coolant in RPV 12 and gather in upper head 29, where they decrease RPV volume and present explosion or other fluid-mechanical risks and problems.
Conventionally, a head vent 80 is provided in upper head 29 to prevent non-condensable gas accumulation in upper head 29 and also to provide a vent for air escape during RPV 12 filling in plant testing and outages. Head vent 80 includes a sealable passage through upper head 29 and external piping that routes gasses to a steam line 3 or sump area (not shown) outside of RPV 12.