The trip of the plant then caused the other two reactor coolant pumps to trip ( as they had been receiving power directly from the main generator ) and natural circulation had to be used during the shutdown.
42.
Some early BWRs and the proposed ESBWR ( Economic Simplified BWR made by General Electric Hitachi ) designs use only natural circulation with control rod positioning to control power from zero to 100 % because they do not have reactor recirculation systems.
43.
Instead, the designers of the " simplified boiling water reactor " used thermal analysis to design the reactor core such that natural circulation ( cold water falls, hot water rises ) would bring water to the center of the core to be boiled.
44.
These include : Core heat removal through natural circulation; direct injection of emergency core coolant system ( ECCS ) water in fuel; and the availability of a large inventory of borated water in overhead gravity-driven water pool ( GDWP ) to facilitate sustenance of core decay heat removal.
45.
Physically Natural circulation evaporators use a short tube bundle within the batch pan or by having an external shell and tube heat exchanger outside of the main vessel ( as shown in the diagram ) External heating through heat exchangers is normally used as it has the advantage that it is not dependent on the calandria size or shape.
