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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.
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 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 March 2002, plant staff discovered that the borated water that serves as the reactor coolant had leaked from cracked control rod drive mechanisms directly above the reactor and eaten through more than six inches [23] (150 mm) of the carbon steel reactor pressure vessel head over an area roughly the size of a football (see photo).
At about 5:20 a.m., after almost 80 minutes with a growing steam bubble in the reactor pressure vessel head, the primary loop's four main reactor coolant pumps began to cavitate as a steam bubble/water mixture, rather than water, passed through them. The pumps were shut down, and it was believed that natural circulation would continue the water ...
The force impinged on the lid of the reactor vessel, causing water and steam to spray from the top of the vessel. This extreme form of water hammer propelled top head shielding, remnants of fuel plates, five loose shield plugs, a nozzle flange, and the entire reactor vessel upwards. A later investigation concluded that the 26,000-pound (12,000 ...
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.