The Chernobyl Accident: Reactor Basics
In order to understand what went wrong at Chernobyl we need to think about nuclear chain reactions. A heavy nuclide (a species of atom) is split using a neutron. The nucleus yields at least two free neutrons upon splitting. The freed neutrons proceed to split additional nuclei causing the chain reaction.
There are a couple of issues. 1)The neutrons that are produced do not typically have the optimal energy to be “caught” by additional fissionable nuclei. 2)Unless the chain reaction is controlled in some way……well you got a mess on your hands. The first issue is addressed using a moderator: a substance that will interact with the neutrons, causing them to lose enough energy that they will interact with successive nuclei. Water and graphite are two popular moderator choices. The second issue is addressed by using control rods: rods made of material that will absorb neutrons (boron compounds for example). These rods can be inserted into the reactor or removed to cause the chain reaction to decelerate of accelerate. This nifty cartoon of the effects of a moderator and control rods is part of the web site of the Paks Nuclear Power Plant in Hungary. Note how the cartoon illustrates the neutrons slowing down as they pass through the moderator (the blue matrix).
We’ll consider this in greater detail later. But we’ll close today noting that some moderators (water) could be lost as a result of plumbing mishaps, leaks, or boiling. Other moderators (graphite) are solid phases that will remain in place in the reactor system (barring some catastrophic event of course).
Posted: April 27th, 2008 under Chernobyl, Nuclear Power.