This articles is from http://corbettharper.com/blog/2011/03/of-atoms-and-men
I am reposting it here for our info. This helps me so much better understand the horrendous disaster in Japan, esp after the quake and tsunami.
Thanks, Corbett, for sharing your knowledge with us.
Of atoms and menSince last week’s earthquake and tsunami, I’ve been very disturbed by reading these reports coming out of Japan about the reactor problems at the Fukushima power plant. Even though I’m not usually interested in most scientific or mathematical fields, I did spend three years studying nuclear chemistry, learning how nuclear reactors and atomic bombs operate and what physical principles they rely on. It is, after all, fascinating to understand how a lump of metal can somehow put out vast quantities of energy while changing its own nature in the process. Although I haven’t done much with that knowledge the past few years, I’ve remembered enough of what I learned to feel that the reports are not consistent and that there is something crucial going on at the plant that the Tokyo Electric Power Company is not being honest about.
For those of you not up on atomic power theory, here’s a quick overview. All atoms contain a nucleus (NOT “nuculus”, thank you very much, George Bush!) composed of protons and neutrons. The number of protons in a nucleus determines what chemical element the atom belongs to. The number of neutrons in the nucleus, however, is not necessarily consistent within an element and determines what isotope of that element the atom belongs to. While some isotopes of an element have similar properties, others are massively unstable, and their atoms will spontaneously degrade into lower-numbered elements, releasing energy in the process. Some stable isotopes can be converted to unstable isotopes to intentionally release energy, usually by bombarding the stable isotopes with free neutrons. This splitting of unstable elements is termed fission, and is the basic principle on which all current forms of atomic power rely.
Nuclear power reactions generally make use of uranium-238 in this process. U-238 is extremely stable, but U-239 is not and can thus be used to generate power. Atomic weapons and a few reactors (including the Fukushima 3 unit) are more likely to use plutonium-239, which has a higher energy output but is not found in nature. Power reactors usually use this energy to boil water and drive steam turbines. The reactor basically replaces a coal or oil fired boiler.
Inside the reactor, the rate of fission is maintained at a constant level because each fission event, in addition to producing heat energy, also produces more free neutrons. These transform other atoms of U-238 into U-239, which then fission in their turn to produce more energy and neutrons, a process called a chain reaction. Left to itself, this process would keep accelerating. It is limited by the control rods of the reactor, which are constructed of elements capable of absorbing large numbers of neutrons without becoming unstable. Power levels inside the reactor are regulated by the positioning of the control rods. When the reactor is shut down, the control rods are fully inserted into the reactor core to absorb all neutrons and halt the chain reaction.
How is this relevant to the events at Fukushima? It works like this. After the quake and tsunami, TPEC stated that all of the reactors at the plant had been shut down, and that it was continuing to cool the reactors to remove residual heat, but that the tsunami had flooded the backup generators and halted the cooling process.
Subsequently they began to report rising core temperatures in one and eventually three reactor units in spite of all the efforts made to cool them, leading gradually to fears that the cores would undergo meltdowns.
This is what has been bothering me. A reactor is shut down by fully inserting the control rods. The control rods absorb the free neutrons and make it impossible for fission events to take place inside the core at any noticeable rate. Without the fission events, the reactor cannot generate energy. So why are these reactors somehow getting hotter?
The second law of thermodynamics dictates that stored energy has to diminsh over time, not increase. Whether the reactor core was partially exposed due to evaporation or not, the flow of water through it would still be carrying heat away from the core. If the water was not moving, it would still be absorbing heat from the core and distributing it to other parts of the structure. If the core were completely exposed to air, the air would be serving the same role, albeit much more inefficiently. If the core were enclosed in a complete vacuum, energy would still be radiating away from it. Under no set of circumstances would the reactor core, simply sitting there with a certain amount of residual heat, be getting hotter. It would be cooling, possibly at an imperceptible rate, but still cooling.
The only conclusion I can come to is that TEPCO has lied about this situation from the beginning, and that the reactors were not properly shut down following the tsunami, due either to human error or damage to the control systems. They are still operational to some degree and generating heat energy, and the technicians have failed to halt the process, with the result that core temperatures are climbing.
Confirming this analysis is the report that TEPCO attempted to halt rising temperatures in the Fukushima 1 reactor by using seawater and boric acid. Boric acid, like all forms of boron, is a good neutron absorber and is heavily used in atomic industries for shielding and safety measures. But why put a neutron absorber into an idle reactor which is not generating any neutrons? The logical answer would be that the reactor is in fact generating neutrons as part of an ongoing fission reaction. Thus, the announcement that boric acid was being used in the attempt to prevent a meltdown is effectively an admission that the reactor is still active.
At this point, then, the Japanese PR teams are living on borrowed time. There will be no criticality accident―a reactor unintentionally reaching critical mass and beginning a self-sustaining chain reaction―because that has already occurred. There will also be no partial meltdowns. Unless some dramatic reversal happens, all three reactors will undergo meltdowns, which are already taking place, and the incident will not be closed until the meltdowns change the fuel configuration within the reactors and thereby halt the reactions.
How will this process work? Nuclear reactors are heavily dependent for stable operation on the manner in which fuel is loaded into them. Each fuel element within a reactor core contains many thousands of carefully sized and positioned fuel pellets. This is done so as to maximize the exposed surface area of the fuel and thereby the neutron capture rate. A meltdown occurs when the ongoing reactions raise the temperature of a portion of the fuel above its melting point, and the fuel literally runs out of the core to pool on the bottom of the containment vessel. This process greatly reduces its exposed surface area, cutting down on its ability to capture neutrons and keeping the fission reactions from sustaining themselves. Since a meltdown will always shut itself off given time, it is not as disastrous as popularly supposed as long as the containment vessel is not breached. However, it will always result in the complete loss of the reactor.
What the exact sequence of events leading up to the failure of the reactor control systems at the Fukushima plant was, I can’t begin to say. My best guess is that the earthquake and tsunami combined somehow misaligned or otherwise damaged the control systems so that the reactors could not be completely shut down. This incident may serve as a warning against one design danger of boiling water reactors, where the control rods have to be inserted by power from below the containment vessel; in a pressurized water reactor, the rods are inserted from above and can fall into the reactor freely due to gravity, making for a more foolproof system.
I realize this is an unsual post, but the contradictory statements and unexplained actions surrounding this accident are too unusual not to comment on. Maybe I have missed a piece here in examining this situation. But from the very beginning, it seemed strange. At first I thought TEPCO was simply refusing to shut the reactors down after the cooling loops had been damaged in order to keep power supplies going. That was the only way I could account for the news of rising core temperatures. When they announced that the reactors had in fact been shut down, it was baffling. According to the laws of physics, an increasing risk of a meltdown in an idle reactor is not possible. Yet that is exactly what the reports are asking us to believe has happened.
One final thought. If the reactors had been kept running when the earthquake struck, instead of being automatically shut down as a safety precaution, they would have been able to keep powering their own coolant systems and operating normally. Instead, because the operators were overcautious and didn’t want to risk the breach of a containment vessel surrounding an active reactor, they chose to rely on backup systems for which they did not have supplies. Now they face a higher risk of radiation leakage along with the certainty that half their plant will be destroyed. The meltdowns at Fukushima are one more example of how human overcaution can lead to even greater calamities than might have occurred had men faced a threatening situation boldly and aggressively.