Quote:
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Originally Posted by wraithe
...nuclear power plants have a major shutdown problem? How do you pump cooling into something that is getting higher in pressure as it gets hotter, without releasing that pressure...
Sorry its an age old questions that is yet to be answered and has proven that we dont have the answer
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The new pebble bed reactor design has some nice answers, like actually producing
less energy as they get hotter...
(cut n' pasted from the
wikipedia entry)
"When a pebble-bed reactor gets hotter, the more rapid motion of the atoms in the fuel decreases the probability of neutron capture by 235U atoms by an effect known as Doppler broadening. When the uranium is heated, its nuclei move more rapidly in random directions, and therefore see and generate a wider range of relative neutron speeds. 238U, which forms the bulk of the uranium in the reactor, is much more likely to absorb fast-moving neutrons.[3] This reduces the number of neutrons available to cause 235U fission, reducing the power output by the reactor. This natural negative feedback places an inherent upper limit on the temperature of the fuel, without any operator intervention.
The reactor is cooled by an inert, fireproof gas, so it cannot have a steam explosion as a light-water reactor can.
The coolant has no phase transitions—it starts as a gas and remains a gas.
The moderator is solid carbon. It does not act as a coolant, move, or have phase transitions (i.e. between liquid and gas) as the light water in conventional reactors does.
A pebble-bed reactor thus can have all of its supporting machinery fail, and the reactor will not crack, melt, explode or spew hazardous wastes. It simply goes up to a designed "idle" temperature, and stays there. In that state, the reactor vessel radiates heat, but the vessel and fuel spheres remain intact and undamaged. The machinery can be repaired or the fuel can be removed.
These safety features are not just theoretical. This exact test was performed (and filmed) with the German AVR reactor. All the control rods were removed, and the coolant flow was halted. Afterward, the fuel balls were sampled and examined for damage. There was none."