Tim -
Thanks for the .pdf, there is no substitute for just knowing - with definition.
Gerald,
The SFPs are an unrealized real problem. The heat of decay, as all nuke plant operators know, is the issue. You may scram - attempt to shut the reactor off - a reactor, but without the continued presence of coolant circulating through the core, the core will be in danger of melting, even though not in operation for a week or more. The culprit is the heat-of-decay of all the short half life elements decaying that were created as a normal process of reactor operation. This danger lasts for in excess of a week or more. That heat (heat-of-decay) is created spontaneously even though the reactor is technically non-critical, or turned off for the first week or so after all criticality is terminated by the full insertion of the all the control rods that stop all nuclear fission; thus terminating critical uranium/plutonium chain reactions. The half-live of the various isotopes may be found at -
http://www.iem-inc.com/toolhalf.html. These are useful numbers for nuclear fallout purposes also.
Unfortunately many years ago the movie the China Syndrome glorified nuclear core melt downs. In so doing the nature of the danger resident in the spent fuel pools (SFP) was completely ignored; the rational being that the "spent fuel rods" are spent - right? Wrong. It is the spent fuel that can be seen as possibly the greater danger, as it is the spent fuel that has been undergoing chain reaction irradiation for several years - usually 1/3 of a reactor's fuel elements are replaced every 3 years. The "spent fuel" is the hottest of the hot material. President Carter - that last wonder thinker to occupy the WH - made the decision that all spent fuel will be stored on site at each nuclear power generating facility, and the spent fuel will not be reprocessed to reclaim the residual uranium, plutonium, and other useful by-products of reactor operation.
The term "spent fuel" is in fact a misnomer. New nuclear power plant reactor fuel elements usually have a limited number of atomic elements within them: a 5% concentration of U235 (the fissionable element necessary to be split and thus generate heat for electrical power generation), 95% U238 (the non-fissionable element, some of which over time captures a neutron and spontaneously decays within a couple of days into plutonium, an element that will fission), zirconium tubes that house the uranium, and an inert gas surrounding the uranium inside the zirconium tubes so that the uranium won't react with anything. Spent fuel elements still have a 2.5% concentration of U235, but additionally have a plethora of newly created radio isotopes, not the least of which is Pu239 in concentrations of 1% - 2.5% (also a fissionable element). These newly created elements - the normal by-product of reactor operation - have half-lives (time intervals where they decay into some other element generating spontaneous energy - heat) of anywhere from a few moments to thousands of years - and there are lots of them, and they are a real witches brew.
The message is: the spent fuel rods are anything but spent - they are the hottest of the hot regarding radioactivity. It takes years for spent fuel rods to "cool" to the point where they may be safely stored without water to cool them. As a normal part of the spent fuel handling process the fuel elements - rods - are stored in pools that are constantly being replenished with an appropriate amount of water to keep the spent elements submerged and cooled - should anything interrupt that SFP water replenishment (a tsunami in the case of Fukashima) what is normally a relatively innocuous process of letting the spent fuel rods just sit in a pool of water for years, becomes a very large problem of keeping the very hot - both physically, and radioactively speaking - spent rods from melting and aggregating - coming together - in the bottom of the pool and continuing to generate enough heat to evaporate all the water from the pool.
Now we get to the really nasty part; uranium can and will generate enough heat, if not immersed in a coolant, so that it will physically melt, come together self-heat, and catch fire and physically burn, thus spreading oxides of, not just uranium, but of uranium and all the other nasty by-products of the nuclear fission process - an unintended, and unanticipated consequence of having the SFPs dry out. Again the same web site noted above giving the half lives of the various radio isotopes -
http://www.iem-inc.com/toolhalf.html
A quick note on half-lives. If you have one of some thing and a half-life of, oh say, two years, at the end of two years you will have half of the original item, and the other half will have "decayed" - changed - into some thing(s) else. That something else, if still radioactive, will have, depending upon what radio isotope it has become, it's own half life, and the new half life is unrelated to the starting element's half life. To illustrate U238 - non-fissionable - will capture a neutron and immediately emit a beta particle - an electron - and have one of the neutrons in its nucleus change into a proton and become neptunium 239. Neptunium 239, an element that has a half life of 2.3 days, where upon after 2.3 days half of the neptunium 239 will spontaneously have decayed into plutonium 239, an element with a half life of 24,000+ years. All radioactive decay in nuclear fallout, and nuclear reactor operation deals in half-lives with all radioactive materials.