Organization, driving themes:
The inherent stability of American nuclear reactors.
Two negative feedback mechanisms
- Nuclear reactors use the energy released when a neutron splits
uranium-235 into smaller atoms and a few neutrons: nuclear fission.
- The fission process is controlled by diluting the density of uranium-235
with non-fissioning uranium-238 that absorbs some of the neutrons.
- If the reactor temperature were to increase, non-fissioning
uranium-238 would absorb more neutrons and decrease the fission
process, thus lowering the temperature: a process called
fuel-temperature negative feedback.
- Water lowers the energy of neutrons into an energy range where
fissioning uranium-235 is enhanced; the neutrons are said to be
- If the reactor temperature were to increase, water would expands
and be less efficient at moderating neutrons, thus decreasing the rate
of fission: a process called moderator feedback.
Nuclear accidents: Three Mile Island compared to Chernobyl
- Even a shutdown reactor generates heat from the radioactive decay
of the fission products, a heat that, if not removed, could melt the
- The potential danger of a reactor-core meltdown is avoided in two
ways: the moderator water is continuously circulated to remove the
heat; a steel-reinforced concrete building could contain even a molten
- At Three Mile island, even with repeated operator mistakes, the
reactor shuts itself off stopping the fission process; even tho the
core melted, all the radioactivity was contained.
- The non-US Chernobyl incident resulted from a lack of inherent
stability -- the use of carbon moderator that provided no feedback --
and no containment building to prevent radioactivity escaping.