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The forced recirculation head from the recirculation pumps is very useful in controlling power, however, and allows achieving higher power levels that would not otherwise be possible. The thermal power level is easily varied by simply increasing or decreasing the forced recirculation flow through the recirculation pumps.
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.
These pumps are powered by wet-rotor motors with the housings connected to the bottom of the RPV and eliminating large diameter external recirculation pipes that are possible leakage paths. The 10 internal recirculation pumps are located at the bottom of the annulus downcomer region (i.e., between the core shroud and the inside surface of the RPV).
Babcock & Wilcox plants (e.g., Three Mile Island) have smaller steam generators that force water through the top of the OTSGs (once-through steam generators; counter-flow to the feedwater) and out the bottom to be recirculated by the reactor coolant pumps. The horizontal design has proven to be less susceptible to degradation than the vertical ...
These are called the pump curves. They are determined by studies, whose methodology is standardized. These curves are specified when water is pumped with a density of 1000 kg/m3 and kinematic viscosity of 1 mm2/s. When the circulating pump is used for liquids of different density and viscosity, the pump curves have to be recalculated.
The pump must be beneath the steam drum to take advantage of the pressure due to the height of the water. If the pump was not there; when it reaches the steam separator and water returns to the pump, the pressure may become low enough in the eye of the impeller to cause cavitation and subsequent damage. [6]
Fast breeder reactors do not use moderators, since they run on fast neutrons, but the coolant (often lead or sodium) may serve as a neutron absorber and reflector. For this reason they have a positive void coefficient. Magnox reactors, advanced gas-cooled reactors and pebble bed reactors are gas-cooled and so void coefficients are not an issue ...
The main circulating pumps have the capacity of 5,500–12,000 m 3 /h and are powered by 6 kV electric motors. The normal coolant flow is 8000 m 3 /h per pump; this is throttled down by control valves to 6,000–7,000 m 3 /h when the reactor power is below 500 MWt.