Just for comparison’s sake, this Wiki article states that the total world power consumption is on the order of 10[sup]13[/sup] watts, while the total solar flux received by the earth is on the order of 10[sup]18[/sup] watts. So even if all the energy produced by the world’s power plants went into heat,[sup]1[/sup] it’dd still only be a 0.01% effect.
[sup]1[/sup] Come to think of it, this probably isn’t a bad approximation - even the “useful” energy produced by a power plant eventually ends up as heat anyways.
I don’t think it is the heat we are adding to the atmosphere by our activities that causes a problem so the relative amount of man-made vs. solar heat is irrelevant. It is the addition of greenhouse gasses that trap the heat of the sun in the atmosphere that is the problem. The sunlight strikes the earth heating the surface. The incoming light has much of its energy at wavelengths in the visible range that pass right through the greenhouse gasses. When this visible light heats the surface it radiates in the infra red. Without the greenhouse gasses these wavelengths would pass into space and keep the earth at an even temperature. However with the greenhouse gasses these infra red wavelengths are absorbed and the atmosphere heats up. Some of this heat is radiated into space but some of it is radiated back to the surface and is thus trapped on earth. This raises the temperature of the earth and is what is called global warming.
First of all, it’s a mistake of thinking of the fission reaction of a [sup]235[/sup]U atom as the the atom "evaporating’, or indeed, anything disappearing. Technically speaking, the [sup]235[/sup]U atom absorbs a thermal (slow-moving) neutron, converting it to [sup]236[/sup]U, which owing to its inherent instability, fissions spontaneously into a couple of smaller products plus a series of neutrons and ionizing (gamma) radiation. The energy doesn’t come from a direct conversion of matter per se, but rather a release of the nuclear binding energies required to hold the large nucleus together. Because the process of neutron capture is statistical–that is, a nucleus has a “capture cross-section” and for a given flux there is a certain ratio of capture to escape–and because the flux drops over time as individual atoms decay, the total sample demonstrates a half-life, or exponential reduction in decay events and resultant radiation emission. Note that [sup]238[/sup]U is relatively stable (in comparison), and so not all isotopes of uranium are fissile (capable of sustaining a chain reaction or emitting significant amounts of radiation).
Second, David Simmons has an excellent, if politically indefensible point when he says:
In fact, much of the hazard of nuclear waste disposal comes from moving it around and concentrating it in supposedly-geological stable and hermetically-sealed underground storage sitees. In this older [thread=328522]thread[/thread] (the sensible talk stops after page 3) we talked about storage and processing of nuclear waste. Essentially, there are two types of nuclear waste; high level “concentrated” wastes such as residual radioactives in used fuel rods and the like; these have to be isolated or, if possible, processed to mitigate the manifest hazard this poses to human and animal life, although it isn’t clear that dumping it in an underground storage bunker in Nevada is really the best way to go about it.
The other form is low level waste; essentially, secondard products that are activated by exposure to ionizing radiation. Currently, these are disposed of by a variety of means, depending on the form and local/state/federal regulations, but the hazard posed by these wastes is not significant if exposure is moderate. While we have been conditioned to fear any exposure to radiation, the truth is that we are routinely exposed to radioactive products, including but not limited to fossil fuel exhaust, tobacco smoke, vegetable matter high in potassium and niacin, dairy products, activated isotopes from cosmic radiation, et cetera. Although all of this exposure does damage at the cellular and genetic level, the vast majority of that damage is reperable *as long as exposure is below a tolerable threshhold. Even high level waste–like redisual fallout from nuclear testing–poses only moderate risk, as evidenced by the results of atmospheric testing in the 1950’s. Dilution and distribution may well be the best policy with handling many forms of nuclear waste. But to suggest so is political suicide. [post=6829003]Here[/post]'s an old post on the different types of radiation and their effects. Of course, noted science ficiton author Larry Niven has a novel approach to the handling of nuclear waste. Consider it evolution in action.
To the OPs question, the amount of actual heat pumped into the atmosphere by nuclear powerplants is insignificant. The great concern, regarding alleged human culpability for global warming, are waste gases that either erode the Earth’s protective stratospheric ozone layer or that act to trap radiated heat from exiting the Earth (CO[sub]2[/sub], methane, et cetera). How much of this contributes to the natural fluctuation of surface and atmospheric variation in temperature is unclear even to atmospheric scientists. Obviously, there is some benefit in reducing dependence upon fossil fuels even without consideration to the potential for climate change, and at this point, nuclear fission represents the best option for sustainable non-atmospheric polluting energy generation in the near future. I still hold out hope for some way to make muon-catalyzed fusion an energy-positive process, but other options (solar, wind, geothermal, thermal fusion, et cetera) are either too limited in scale or to restricted by technological limits to be of near-term use.
This is misleading in that it doesn’t apply to a nuclear reactor. The thermal neutron flux in a reactor is the result of a very complex interaction, and the loss of fuel due to burnup is one of the factors that is controlled for. There isn’t a reactor core “half-life”, fission is not a decay event, and the radiation emission doesn’t drop exponentially.
The Moon never gets down to -270 C. That’s the temperature you’d get for something in the center of one of the vast intergalactic voids, billions of lightyears from the nearest star. The average temperature of the Moon is around 0 C, as would be the Earth’s, if we had no atmosphere. The bigger problem would be variability of temperature: It’d sometimes be much hotter, and sometimes much cooler.
uh, finding areas that will allow you to scatter it widely is likely to be a problem. Talk about a not in my backyard problem! Seriously, that is why atmospheric testing was banned. Even though it wasn’t much, the fallout ended up scattered widely and was very unpopular. I am afraid storing the waste in a secure location is the best idea around. Personally, I would build a big mound out in the desert and post signs. As long as people can read, no one would mess with it. After they can’t read anymore, it would be the least of their health problems. Either way, it isn’t going to hurt anyone.
Go back and reread the post. I was referring to radioactive waste, not fuel elements in a reactor (in which, as you allude, the rate of reaction is moderated by additional mechanisms to obtain a marginally critical state at the desired energy output). Fission is, in fact, a decay event in which the nucleus decays into (typcially) two smaller nuclei, some neutrons, and some gammas that account for the binding energies; you can make the distinction between spontaneous fission, which is “pure” decay, and induced fission, which is a nuclear interaction (neutron absorption) followed by a decay event, but in either case decay occurs. An unmoderated “pile” of fissile material will demonstrate an exponential half-life based upon the amount and distribution of radioactive isotopes and the geometrical configuration.
The problem in your statement here is the assumption that, in burying the wastes in the ground, they won’t leak out and become a hazard. Part of the umbrage about the Yucca Flats site (as detailed in the link I previously posted) are concerns that the material could leak into the water table and prove to be a concentrated source of high grade pollution. “Out of sight, out of mind” is bad policy with regard to toxic and radioactive wastes, as illustrated by the Love Canal fiasco. One also has to consider the logistical and public safety hazards of transporting wastes to a centralized location. It is possibly better to retain the high level wastes on site, in properly designed and inspected storage, than to bury it deep within a national repository where it may not be regularly inspected. Better yet, processing and reactor-breeding technology could extend the life of the existing fuel material, thus resulting in less overall waste and greater utilization of finite fissile ore.
Although atmospheric testing has gotten a (deservedly) bad rep, the fact is that there is only a tenuous and marginal that it increased chronic illness among exposed populations and virtually no short-term illness; not even to people immediately exposed to fallout, as with the cast of The Conqueror. Exposure to moderate amounts of low level waste is almost certainly less harmful than exposure to other, non-fission forms of radioactive waste, including coal-fired plant exhaust and cigarette smoking. This isn’t to marginalize the potential dangers of a large scale breach of radioactive waste or a catastrophic core fire/meltdown of a nuclear pile, but to put the degree of hazard in perspective you have to appreciate the damage done by other pollutants. In that context, release or distribution of dilluted wastes are, while not desireable, not the great evil that they are alleged to be.
The majority of the nuclear reactors in the United States are light water reactors, utilizing thermal (read “slow”) neutrons to cause neutron capture in fissile U-235 and Pu-239, though predominately U-235. The enrichment of commercial reactor fule is usually about 3-5%, meaning the U-235 is 3-5% of the bulk uranium mass. In Naval reactors (subs, aircraft carriers, cruisers) the enrichment can be much, much higher ( I believe this may be classified).
The fuel is loaded into fuel rods, usually aluminum alloy, that are normally tubular in shape. The fuel rods maintain the integrity of the fule to prevent fuel and fission products from escaping inot the reactor coolant, which is used to transfer heat of fission, when critical, and heat of decay, when shut down, from the reactor core. The core itself can contain hundreds of fuel rods. The condition of the reactor (sub critical - fewer neutrons released by fuel than escaped from core, critical - escaped neutrons equal neutrons released from core, supercritical - more released than escaped) is determined by many factors, but the primary control is maintained by position (height) of the control rods. The control rods are made of an element that has a high neutron absorption cross section, but remains very stable upon absorbing multiple neutrons.
When a fission occurs, as stated previously, and corectly so, two neutrons and gammas are released, as well as two fission products with protons that add up to 92 and neutrons that add up to 234 (235+1-2). The energy released in the reaction is determined by the mass defect (difference) bewteen the original U-235 atom plus the neutron and the resultant particles produced from the fission. While the amount of mass defect is very small, the commonly known equation E=MC^2 applies, with m being mass, c being the speed of light (^2 = squared), and E being energy. It takes a very small conversion of mass to create a whole lot of energy.
The heat energy released is transferred through the fuel rod out into the bulk reactor coolant that is being circulated through the reactor core. The coolant travels through a loop that includes a steam generator. It is in the steam generator that the heat is transferred out of the reactor coolant into another coolant loop that turns that water into steam. It is important to know that these two coolant loops do not directly interface. The reactor coolant loop is a closed loop. The secondary coolant loop is clean water.
The steam produced in the steam generators is used to turn large turbines that are coupled to generators. The exhaust from the turbines is condensed and sent back to the steam generators. This cooling is performed by another coolant loop that itself is cooled in cooling towers. Again, there is no intimate contact between these coolants.
Anyways, just some info to help out a little bit.
Oh, and the energy released in fission is huge compared to anything seen in natural decay.
On an interesting note, though, PU-238 in its metallic state, glows bright red due to the heat released during its decay. This is highly useful in powering deep space probes such as the Galileo and Cassini probes.
Some of the later technologies that are being used to stabilize waste for long-term are glassification (high-level waste) and grouting (low-level waste). To actually get the cut-off line for what is high or low level waste, you would have to ask a politician. Both processes make the waste an integral part of the matrix and thus make it extremely stable. The rampant idea in the public of rolling millions of 55 gallon drums filled to the brim with nuclear waste is a vision perpetrated by opponents to long-term storage to fuel fear. The robustness with which we design, build and test our shipping and storage containers sometimes seems obsurd, but it is done to protect our public, even when their perception of the way things are handled is far from the reality of today.
Stanger is correct. The decay occurring during fission, though induced, is the result of te nucleus of the atom becoming unstable when it absorbs a neutron. Essentially, and this is not cut and dry, the additional mass in the nucleus overcomes the ability of the strong nuclear force to hold the nucleus together, and boom, it splits into two, lowering its energy state. All decay is a lowering of the isotopes energy state. The eventual outcome of most actinide non-fission decay is lead, even if it takes billions of years to get there. Gold has eluded us once again!
Vitrification (or glassification, if you like) is a good way to handle moderate amounts of solid fuel residue. There are some conderns about sublimation, but by and large it has been strongly validated as a technical solution to high level solid waste. Of course, it also makes the material essentially inaccessible for later processing; in the current American climate this isn’t an issue, but future needs may dictate adopting reprocessing technology, and it would be, IMHO, foolish to limit the amount of material available for that.
This doesn’t apply for liquid waste, such as inner loop coolant, however. Whether you claim this as low-level or high-level waste depends on what it is composed of, how contaminated it gets in process, and the politics surrounding the issue. Then there’s low level waste and activated materials that have to be disposed of but are inconvenient to store long term.
The desire to put waste underground–rather than in secure, managable on-site bunkers–seems to be more of a NIMBY issue than a technical one. Vitirifed waste should be sufficiently stable to be stored above ground. Liquid waste is a potential hazard for leakage and contaimination of the water table regardless of where you store it. Low level wastes, if properly handled/dispersed, don’t represent a significant hazard, but owing to political sentiment and ignorance, people are terrified of disposing of it in any way that releases it back into the environment.
