You will remember from chapter 21 that we have two neutrons, on average, emitted whenever a U235 atom undergoes fission - or "splitting." One of these is necessary to fission another atom to keep the chain reaction going. But what happens to all of those second neutrons? Some of them, as mentioned, are absorbed by the structure of the reactor or by the control rods, which slide in and out of the reactor to keep the reaction at - or just very slightly above - the critical point. But others smash into, and are captured by, the plentiful U238 atoms that make up from 95% to 96.5% of the fuel rod contents. When this happens, a truly miraculous thing takes place: This practically worthless material is transformed into one of the most concentrated sources of energy on Earth - or in the universe for that matter - plutonium 239, and element so evil that it was named for the god of the underworld. (Not really, but that's what some would have you believe.) [Plutonium was named in honor of the discovery of the planet Pluto, just as neptunium and uranium were named for Neptune and Uranus.]
This happens in every one of the world's 500 power reactors, plus thousands of research reactors, every day they are in operation. In fact, a sizable fraction (up to about 30%) of electrical energy generated by a power plant comes from this plutonium, which arises as a natural consequence of the uranium fission reaction - without any effort on our part - and supplements the scarce U235 fuel.
Some reactors, however, are designed to intentionally make plutonium. If it is to be used in bombs, it is normally made in a reactor with another modulator - such as the carbon-modulated reactor at Chernobyl. A reactor designed specifically to make only fuel-grade plutonium is called a breeder reactor, since new fuel is "bred" from an almost worthless byproduct of the refining cycle. [Breeder technology seems to be on hold for a couple of reasons: (1) in the prevailing anti-nuclear climate, few entrepreneurs or speculators are willing to make investments in nuclear power for fear of laws that can make their investment instantly worthless; and (2) at the present time there is a glut of plutonium available from the dismantlement of nuclear weapons.]
Are we speculating here on new technology like "fusion" power? Hardly.
The first reactor ever to produce electric power from nuclear energy was a "liquid metal fast breeder reactor" known as the EBR-I. (By the way, liquid metal means that the coolant was not our old friend water, but liquid sodium; fast means that it used "fast neutrons," not the slowed-down, moderated variety.) Designed by physicist Walter Zinn in 1944, his brainchild went critical at 11 am, December 20, 1951 - producing the first steam in history produced by man-made nuclear heat. Like the Manhattan reactor in Chicago and the SLOWPOKE reactor in Canada, EBR-I was not designed to produce electrical power but to prove the concept of fuel breeding (which it did along with its successor, EBR-II). [Declared a national landmark in 1966, the EBR-I is open to the public from mid June to mid September. Located eighteen miles southeast of Arco, Idaho, on Highway 26, visitors must be at least sixteen years old (too much neutron violence?) and U.S. citizens (fear of spies who might steal this technology?).]
The EBR-II had "on the spot reprocessing," which reprocessed 35,000 fuel elements between 1965 and 1969. But the facility was not without problems: the fence around it kept out the coyotes, causing the rabbit population to outbreed the reactor.
Does the ERB-II sound a little familiar? It should since it has another name we used in chapter 21 - the Integral Fast Reactor (IFR).
While many U.S. politicians have never heard of breeder technology, Europeans have. Sadly, "Green" activists there have been successful in shutting them down or keeping them from ever starting up.
Showing posts with label U235. Show all posts
Showing posts with label U235. Show all posts
Sunday, April 3, 2016
Wednesday, March 30, 2016
Power Reactors
In the United States, power reactors are entirely of the PWR (pressurized water reactor) or the BWR (boiling water reactor) types. In both cases, water is used as the coolant and the moderator, which provides a very interesting advantage that probably no one has bothered to mention to you: If the coolant is lost, the chain reaction stops. Depending on the length of time the fuel has been producing power, the fuel rods may or may not be thermally and radioactively "hot" from the daughters of the fissioning process. Even in the worst case, the heat generated is no more than 1% or 2% of that during normal operation. This is why the "disaster" at Three Mile Island didn't really happen - except in the minds of the uninformed.
While the Japanese installed the first Advanced Boiling Water Reactor (ABWR) in 1996, none of the new, modular designs have seen the light of day in this country. Not only have we been blinded by the non-threat of low-level radiation, but the cost of building a nuclear plant has escalated by a factor of seventeen, after considering inflation - mostly from construction delays caused by environmentalist lawsuits. (The above-mentioned Japanese ABSR plant took fifty-two months to build - compared with more than eleven years for the most recent plant in the United States.) I would say the new designs are even safer than the old - but how do you get safer than no deaths, no injuries, and no negative effects to the public from several thousand reactor years of operation with thousands of gigawatt-hours of life-enhancing electrical energy having been generated? [Some of the media scream "disaster" when ten gallons of water with 1/80 the radioactivity of salad oil leak out in the process of heating and otherwise providing life-giving energy to an entire city. Why doesn't it make front-page news when some one falls off the roof to his death trying to clean the solar collector - which provides a few puny kilowatts of solar energy for warming the hot water... when the sun is shining?]
Nonetheless, neither the PWR or BWR has much promise for miniaturization and "local" use as - by nature - they operate with high-power densities, which have the potential to cause a messy and expensive loss-of-coolant accident. They also require pumps, back-up pumps, and relatively elaborate controls.
All of these U.S. power reactors use enriched uranium as a fuel, as do reactors in France (where 80% of the electrical power comes from nuclear energy), Japan, England, and most other countries. The enrichment process starts with natural uranium, which is dissolved in acid to produce uranium hexafluoride gas. This ultra-corrosive gas is then pumped thousands of times through membranes where the lighter U235 passes through just a little bit easier than the U238. For power reactors, the U235 is enriched from 0.7% to about 3.5%, which takes not only lots of time but considerable energy. ["Bomb grade" U235 must be enriched to 90% - an extremely difficult process. Thank goodness, or any crackpot might be able to do it.)]
While the Japanese installed the first Advanced Boiling Water Reactor (ABWR) in 1996, none of the new, modular designs have seen the light of day in this country. Not only have we been blinded by the non-threat of low-level radiation, but the cost of building a nuclear plant has escalated by a factor of seventeen, after considering inflation - mostly from construction delays caused by environmentalist lawsuits. (The above-mentioned Japanese ABSR plant took fifty-two months to build - compared with more than eleven years for the most recent plant in the United States.) I would say the new designs are even safer than the old - but how do you get safer than no deaths, no injuries, and no negative effects to the public from several thousand reactor years of operation with thousands of gigawatt-hours of life-enhancing electrical energy having been generated? [Some of the media scream "disaster" when ten gallons of water with 1/80 the radioactivity of salad oil leak out in the process of heating and otherwise providing life-giving energy to an entire city. Why doesn't it make front-page news when some one falls off the roof to his death trying to clean the solar collector - which provides a few puny kilowatts of solar energy for warming the hot water... when the sun is shining?]
Nonetheless, neither the PWR or BWR has much promise for miniaturization and "local" use as - by nature - they operate with high-power densities, which have the potential to cause a messy and expensive loss-of-coolant accident. They also require pumps, back-up pumps, and relatively elaborate controls.
All of these U.S. power reactors use enriched uranium as a fuel, as do reactors in France (where 80% of the electrical power comes from nuclear energy), Japan, England, and most other countries. The enrichment process starts with natural uranium, which is dissolved in acid to produce uranium hexafluoride gas. This ultra-corrosive gas is then pumped thousands of times through membranes where the lighter U235 passes through just a little bit easier than the U238. For power reactors, the U235 is enriched from 0.7% to about 3.5%, which takes not only lots of time but considerable energy. ["Bomb grade" U235 must be enriched to 90% - an extremely difficult process. Thank goodness, or any crackpot might be able to do it.)]
Sunday, March 27, 2016
Enter the Atom
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.
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.
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