Jack Gamble explains very clearly how moderation is required to enable nuclear fission, why American reactors are inherently unable to runaway like Chernoble. It really isn’t that hard to understand, so do read the whole thing.
(…) Everything in moderation. It would seem the universal slogan for healthy living applies to subatomic particles as well. Moderation is the term used in nuclear science for the slowing of neutrons to a speed conducive to nuclear fission.
When an atom sheds a neutron by fission, it’s considered a fast neutron. A fast neutron has a kinetic energy of about 1 MeV (Million Electron Volts). To put that in perspective, a single molecule of air at room temperature has about 0.0000000125 MeV of kinetic energy. A kinetic energy of 1 MeV is about the same as a neutron moving at 14,000 km/s or 31.3 million miles per hour.
Slow Down Those Neutrons
At this speed, it is very unlikely that an Uranium 235 atom can ‘grab’ it to produce fission. Therefore, it is necessary to moderate (slow down) the neutron. The only way to slow down a neutron is to rob it of kinetic energy. The best way to do that is to have it bump into something. This is where a moderator comes into play. In order to achieve fission consistently, neutrons need to slow down to an energy of about .000000025 MeV. At this point, the neutron is considered a thermal neutron and is ready for fission. To get to this energy, a neutron will have had multiple collisions with a moderator.
American commercial nuclear power plants—Boiling Water Reactors (BWR) and Pressurized Water Reactors (PWR)—use water as both a moderator and a coolant. The coolant function removes the heat energy from the fuel and transfers it to either the turbine for a BWR or the Steam Generator in a PWR. The moderator function causes the neutrons to collide with water molecules, slowing down the neutrons so that fission can take place. The image above shows the neutrons (red) at high speed colliding with water molecules (blue) and slowing down. It’s not a perfect metaphor, but you get the idea.
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Please continue reading the Nuclear Fissionary tutorial.
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