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A reactor vessel head for a pressurized water reactor. This structure is attached to the top of the reactor vessel body. It contains penetrations to allow the control rod driving mechanism to attach to the control rods in the fuel assembly. The coolant level measurement probe also enters the vessel through the reactor vessel head.
The pressurizer vessel itself may be maintained much hotter than the rest of the reactor plant to ensure pressure control, because in the liquid throughout the reactor plant, pressure applied at any point has an effect on the entire system, whereas the heat transfer is limited by ambient and other losses.
The time required for the molten metal of the core to breach the primary pressure boundary (in light water reactors this is the pressure vessel; in CANDU and RBMK reactors this is the array of pressurized fuel channels; in PHWR reactors like Atucha I, it will be a double barrier of channels and the pressure vessel) will depend on temperatures ...
The reactor pressure vessel is manufactured from ductile steel but, as the plant is operated, neutron flux from the reactor causes this steel to become less ductile. Eventually the ductility of the steel will reach limits determined by the applicable boiler and pressure vessel standards, and the pressure vessel must be repaired or replaced ...
In the most severe of such scenarios, if the fuel remains insufficiently cooled for prolonged periods of time, the integrity of the primary system (second barrier) can be threatened as well, for example if a substantial mass of molten fuel reaches the bottom of the reactor pressure vessel and melts through it.
2: reactor cover [10] or vessel head [11] 3: Reactor pressure vessel 4: inlet and outlet nozzles 5: reactor core barrel or core shroud 6: reactor core 7: fuel rods The arrangement of hexagonal fuel assemblies compared to a Westinghouse PWR design. Note that there are 163 assemblies on this hexagonal arrangement and 193 on the Westinghouse ...
The Reactor Protection System (RPS) is a system, computerized in later BWR models, that is designed to automatically, rapidly, and completely shut down and make safe the Nuclear Steam Supply System (NSSS – the reactor pressure vessel, pumps, and water/steam piping within the containment) if some event occurs that could result in the reactor entering an unsafe operating condition.
The High Pressure Coolant Injection (HPCI) System consists of a pump or pumps that have sufficient pressure to inject coolant into the reactor vessel while it is pressurized. It is designed to monitor the level of coolant in the reactor vessel and automatically inject coolant when the level drops below a threshold.