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Nuclear reactors generate heat internally; to convert this heat into useful power, a Coolant system is used. If this coolant is lost, the nuclear reactor may continue to generate the same heat while its temperature rises to the point of damaging the reactor. Particularly dangerous is the possibility that the high temperatures may prevent the control systems from slowing the reaction; if this happens, the temperature will continue to rise until something drastic happens.
  • If water is present, it may boil, bursting out of its pipes. (For this reason, Nuclear Power Plant s are equipped with pressure-operated Relief Valve s.)

  • If Graphite and air are present, the graphite may catch Fire , spreading Radioactive Contamination . (This situation exists only in AGR s, RBMK s, Magnox and weapons-production reactors, which use graphite as a Neutron Moderator .)

  • The fuel and reactor internals may melt; if the melted configuration remains critical, the molten mass will continue to generate heat, possibly melting its way down through the bottom of the reactor. Such an event is called a (and below) - however, current understanding and experience of nuclear fission reactions suggests that the molten mass would become too disrupted to carry on heat generation before descending very far; for example, in the Chernobyl Accident the reactor core melted and core material was found in the basement, too widely dispersed to carry on a chain reaction (but still dangerously radioactive).


A reactor may passively (that is, in the absence of any control systems) increase or decrease its power output in the event of a LOCA or of voids appearing in its coolant system (by water boiling, for example). This is measured by the s are designed to have steam voids inside the reactor vessel.

Modern reactors are designed to prevent and withstand loss of coolant using various techniques. Some, such as the Pebble Bed Reactor , passively shut down the chain reaction when coolant is lost; others have extensive Safety Systems to shut down the chain reaction.


THE THREE FINAL DEFENSES


A great deal of work goes into the prevention of a serious core event. If such an event was to occur, three different physical processes are expected to increase the time between the start of the accident and the time when a large release of radioactivity could occur. These three factors would provide additional time to the plant operators in order to mitigate the result of the event:

1. The time required for the water to boil away (coolant, moderator). Assuming that at the moment that the accident occurs the reactor will be SCRAM ed (immediate and full insertion of all control rods), so reducing the thermal power input and further delaying the boiling.

2. The time required for the fuel to melt. After the water has boiled, then the time required for the fuel to reach its melting point will be dictated by the heat input due to decay of fission products, the heat capacity of the fuel and the melting point of the fuel.

3. The time required for the molten fuel to melt its way through the pressure vessel. The time required for the molten metal of the core to burn through the bottom of the vessel will depend on temperatures, vessel materials. Whether or not the fuel remains critical in the conditions inside the damaged core or beyond will play a significant role.


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