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A loss-of-pressure-control accident (LOPA) is a mode of failure for a nuclear reactor that involves the pressure of the confined coolant falling below specification. [1] Most commercial types of nuclear reactor use a pressure vessel to maintain pressure in the reactor plant.
The reactor vessel used in the first US commercial nuclear power plant, the Shippingport Atomic Power Station. Photo from 1956. A reactor pressure vessel (RPV) in a nuclear power plant is the pressure vessel containing the nuclear reactor coolant, core shroud, and the reactor core.
In less than 16 hours, the reactor core melted [42] and dropped to the bottom of the pressure vessel, burning a hole through the vessel. By that time, water was pumped into the reactor in an effort to prevent the worst-case scenario – overheating fuel melting its way through the containment and discharging large amounts of radionuclides in ...
The night before a rain shower did hit the building, and water might have reached the - 38C - reactor container lid, or might have reached hot fuel left behind in the reactor vessel. At that moment, the ambient temperature was 20.3 C, and the humidity was 91.2 percent.
Therefore, it was needed to locate all molten fuel in and outside the reactor-vessel. [70] [73] On 3 November 2011 TEPCO said that the tiny amounts of xenon-135 detected in the reactor's containment vessel atmosphere came from spontaneous nuclear fission with curium-242 and curium-244, substances that were present in the nuclear fuel.
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 ...
They believe that most of the core debris has breached the pressure vessel and lays at the bottom of the containment vessel for reactors 1 and 3, although some debris seems to be lodged in the control rod drive mechanism for reactor 1. However, they believe that most debris may remain at the bottom of the pressure vessel for reactor 2. [316]
Hydrogen was released from the reactor pressure vessels, leading to explosions inside the reactor buildings in units 1, 3 and 4 that damaged structures and equipment and injured personnel. Radionuclides were released from the plant to the atmosphere and were deposited on land and on the ocean.