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.
Pressure is Key
So, for a Boiling Water Reactor, pressure becomes key to the rate at which fission occurs. As water boils, steam bubbles develop. Steam bubbles result in a decrease in moderator density, meaning fewer neutron-water collisions occur in the reactor, causing power to decrease. This is the inherent safety feature designed into a Light Water Reactor. The hotter the reactor gets, the fewer fissions occur, the more power goes down. In this manner, an American reactor tends to regulate power by itself. If the reactor were to overheat, the fission reaction would slow, bringing the temperature back down. This is why American reactors are referred to as Under Moderated.
This was the big difference in design that prevented the Three Mile Island event from being catastrophic like the Chernobyl disaster. At Three Mile Island, the water level in the reactor dropped so low that the coolant function was lost and the fuel melted due to decay heat, destroying the reactor core. However, with the water level down, the moderator function was lost. This caused the reactor to cease, preventing a catastrophe.
Graphite Moderator at Chernobyl
Chernobyl, a Russian RBMK reactor, was a graphite-moderated plant. Water acts as a coolant in the RBMK but the graphite acts as the moderator. However, as voids form in an RBMK, power increases because the graphite still moderates the neutrons and now more neutrons reach the moderator because there is no water to absorb them. Thus, as the temperature rises in an RBMK, power rises, causing temperature to rise further, and so on.
It is because of this that the RBMK reactor is considered Over Moderated. During the Chernobyl disaster, the operators intended to shut down the reactor when the temperature began to rise too rapidly. But the control rods had graphite in the tip, as they were inserted to shut down the reactor they displaced water, adding moderation to the neutrons. This caused power to rise. In an instant, temperature rose, increasing power in a feedback loop that caused a steam bubble to flash at the bottom of the reactor. When that happened, power instantly spiked to 1,000%, causing a steam explosion that hurled the 2,000 ton reactor-head through the roof of the plant, thus expelling the entire core into the atmosphere.
If Chernobyl had been an American Reactor, this would have been impossible. There was much more happening at Chernobyl than described above, and if you have not read Mathew’s previous article I encourage you to do so. For the clarity of this post however, understand that moderation was just one difference between the TMI Event and the Chernobyl disaster.
Image Credit
Neutron Moderator image courtesy of Wikimedia Commons published under the CC license.
One Comment
One interesting consequence of the under-regulation (as I understand it) is that the boiling-water reactors especially can be controlled by changing the flow-rate of the water through the core, since faster water will have fewer steam bubbles in it and so will increase the fission rate.
I often stress the differences between Chernobyl and Three-Mile Island, of which there are many in both origins and consequences, but one similarity is that the operators overrode automatic systems, and made things worse. I have to observe though that in the Three-Mile Island meltdown the reactor was correctly stopped using control rods before the water boiled off from decay heat.
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[...] reactors use either water or graphite to moderate the reactor core. Chernobyl was a graphite moderated reactor that used water only for cooling. United States (and now Russian) reactors are all required [...]