Let's return for a short graduate course from Hormesis U. about "splitting the atom."
We've already seen that U238 is an isotope of uranium with a half-life of 4.5 billion years. [I realize I said I was going to refer to isotopes in the form of 238U or uranium 238. But U235 and U238 are such commonly used abbreviations to denote these isotopes that I will be using them in this chapter.]
With a lump of this element and the proper instruments, you would find there is another isotope, U235, which amounts to only 0.7% of the total mass. Yet it is this tiny fraction that makes uranium the tremendous source of safe and reliable energy - not to mention the fearful master - that it has become.
U235, like its more plentiful sibling, is an alpha emitter - but has a considerably shorter half-life... a mere 3,800,000 years, meaning that it was considerably more plentiful a billion or so years ago. [U235 is sometimes referred to as "actinium" or "uranoactinium."] It, along with plutonium 239 and U233, are the only isotopes that are fissionable - a phenomenon described below.
Under normal conditions, we can expect to see a U235 atom occasionally decay into an isotope of thorium and a helium nucleus (an alpha particle) similar to all radioactive isotopes experiencing alpha decay. ["Occasionally" takes on a new meaning in the atomic world. Our roughly penny-sized gram of U235 would experience approximately 80,000 nuclear disintegrations per second.]
But let us suppose that a stray neutron smacks into the nucleus of an unsuspecting U235 atom. If the energy of the neutron is within a certain range, our U235 target atom fissions, that is, breaks into pieces. [This was first observed by an unbelieving Lise Meitner in December 1938. She had observed barium, with an atomic number of 56, arising when she bombarded "actinium" with neutrons.]
It usually splits into two roughly equal parts, and most important, ejects about two neutrons. Obviously no atom could eject or emit about two neutrons, but, on average, that is what a fissioning U235 atom sends out of its nucleus.
Imagine, then, one of these neutrons hitting another U235 atom, which emits two neutrons with at least one of these splitting another atom... and so on, and so on. As you have no doubt already figured out, this is what is known as a chain reaction. When the ratio of fissioned atoms in successive generations is equal to one - that is, when one splitting atom causes exactly one more to split - the reaction is said to go critical. What happens to the other neutrons? They either escape from the volume of uranium, or they are absorbed - either unintentionally by structural material, or purposely by control rods made of boron, aluminum, cadmium, or several other neutron-absorbing materials - in order to keep the reaction under control (that is, to keep it from going super-critical). Does a super-critical reaction cause a bomb-like explosion? Not at all; if it did, bomb development by the Manhattan project would have been relatively simple rather than requiring the best theoretical physics minds on two continents. But super-criticality is no picnic. It causes rapid rises in fission reactions, leading to very high temperatures that cause structural damage, torrents of neutrons, and "steam explosions." Bad, yes, but still light years away from the mushroom-shaped cloud.
Let's look at a few different types of reactors, with an eye for those that might allow decentralization of electric power generation.
Showing posts with label fissionable. Show all posts
Showing posts with label fissionable. Show all posts
Sunday, March 27, 2016
Friday, January 8, 2016
Just How Dangerous is Radiation?
In March 1954, sailors onboard the Lucky Dragon were exposed to fallout from a hydrogen bomb test conducted on Bikini atoll. While his two compatriots suffered from radiation sickness, one sailor died the following September. I was a teenager at this time, and yet I can remember a huge amount of news regarding the incident. I suspect it shaped my fear of radiation.
In July 2000, a joint U.S.-Russian Federation report gave as sixty the total number of "criticality" accidents that had occurred in the United States, Russia, France, the United Kingdom, Canada, Argentina and Japan. These accidents occur when too much "fissionable" material comes together for whatever reason and produces for a few moments the same conditions as would be found inside a nuclear reactor. It doesn't cause a "nuclear explosion" but a flash of blue light and a large spike of heat energy. Mr. Harry Daghlian has the unenviable distinction of being the first criticality accident victim in August 1945, during the Manhattan Project. Since then, there have been twenty-one similar deaths, with seven having occurred in the United States.
The most recent were in 1999 when an accident at a Japanese enrichment facility killed two workers. A third worker survived after experiencing severe radiation sickness. I saved the newspaper with that story (prior to the deaths) with the headline "Japanese contain radioactive gas leak" emblazoned across five of six columns at the top of the front page. On the same page there was a notice "Deadly Quake Strikes Mexico/Page 7A." Oh well, I guess earthquake deaths just aren't as fashionable.
Yes, sadly some individuals have died from exposure to radiation. But it is surprising how few, and since there are so few we can account for all or nearly all of them. (This obviously doesn't count the unfortunates who were victims at Hiroshima and Nagasaki, almost all of whom died from blast and heat, but would very likely have died of radiation sickness had they survived the primary causes.)
It is unclear what killed the thirty-one firemen and rescue workers at the Chernobyl disaster. The graphite reactor was in flames (not possible in U.S. power reactors) and was convecting extremely radioactive materials from the core. The probably cause of the firemen's death was heat, since death by radiation generally takes several days to do its work on internal organs. But they, as in the case of the Japanese bomb victims, would very likely have died from radiation.
[I cannot allow Chernobyl to be considered in the same light as other nuclear power facilities. It was built by a Communist government with no concern for the safety of its citizens - as evidenced by the lack of a containment structure to prevent what did happen from happening - and constructed of graphite, rather than water, as a moderator in order that it could be used to produce bomb-grade plutonium. In my opinion, the firemen were murdered by a lack of responsibility on the part of the Soviet government.]
In July 2000, a joint U.S.-Russian Federation report gave as sixty the total number of "criticality" accidents that had occurred in the United States, Russia, France, the United Kingdom, Canada, Argentina and Japan. These accidents occur when too much "fissionable" material comes together for whatever reason and produces for a few moments the same conditions as would be found inside a nuclear reactor. It doesn't cause a "nuclear explosion" but a flash of blue light and a large spike of heat energy. Mr. Harry Daghlian has the unenviable distinction of being the first criticality accident victim in August 1945, during the Manhattan Project. Since then, there have been twenty-one similar deaths, with seven having occurred in the United States.
The most recent were in 1999 when an accident at a Japanese enrichment facility killed two workers. A third worker survived after experiencing severe radiation sickness. I saved the newspaper with that story (prior to the deaths) with the headline "Japanese contain radioactive gas leak" emblazoned across five of six columns at the top of the front page. On the same page there was a notice "Deadly Quake Strikes Mexico/Page 7A." Oh well, I guess earthquake deaths just aren't as fashionable.
Yes, sadly some individuals have died from exposure to radiation. But it is surprising how few, and since there are so few we can account for all or nearly all of them. (This obviously doesn't count the unfortunates who were victims at Hiroshima and Nagasaki, almost all of whom died from blast and heat, but would very likely have died of radiation sickness had they survived the primary causes.)
It is unclear what killed the thirty-one firemen and rescue workers at the Chernobyl disaster. The graphite reactor was in flames (not possible in U.S. power reactors) and was convecting extremely radioactive materials from the core. The probably cause of the firemen's death was heat, since death by radiation generally takes several days to do its work on internal organs. But they, as in the case of the Japanese bomb victims, would very likely have died from radiation.
[I cannot allow Chernobyl to be considered in the same light as other nuclear power facilities. It was built by a Communist government with no concern for the safety of its citizens - as evidenced by the lack of a containment structure to prevent what did happen from happening - and constructed of graphite, rather than water, as a moderator in order that it could be used to produce bomb-grade plutonium. In my opinion, the firemen were murdered by a lack of responsibility on the part of the Soviet government.]
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