However, this last sentence is why no one has ever seriously looked into this before. If millions of American homes could be totally heated without needing heating oil or natural gas, very large and powerful Corporations would not be pleased, as their profits would be less. Ditto regarding people having a free (local, on or actually under the property) source of electricity.
Around 40 years ago, a technology was developed called an "isotopic power generator". Such devices used various isotopes such as Strontium-90 in an enclosed chamber. The concept was to have all the radiation get absorbed in the container, which then became very hot, and then various methods of generating electricity were applied to get productive use of that radiation. NO "chain reaction" was ever involved, so the concept was very different than in nuclear power reactors, which require such chain reactions to function.
A variation of that approach seems compatible with the spent fuel material from nuclear reactors. A relatively conventional water well hole would be bored, 100 feet deep, and the device would be lowered down there. The hundred feet of soil and groundwater and some shielding on the device itself would eliminate any radiation hazard. Being 100 feet below ground, the small amounts of spent nuclear fuel in any homeowner's "well" would be extremely "secure", too. Heat could be captured, and then heat-exchanged (twice) to provide heating for the home and the domestic hot water as well as supplying electricity. It seems prudent for us to adapt that 40-year-old technology to solve both the spent-nuclear-fuel-disposal problem and the overwhelming energy needs of our country.
In any case, the amount of spent nuclear fuel currently being stored is pretty huge. If it were all collected together, it would cover an entire football field nearly 20 feet deep.
Let's start to figure. The total spent nuclear fuel removed from the 103 operating American nuclear reactors in a year is about 2,000 metric tons, which is the same as 2 * 109 grams. Since each pellet weighs 7 grams, that means that about 280 million spent nuclear fuel pellets are removed each year. A lot!
When a pellet is first put into a reactor, it starts out with an available energy of about 22 million Btus. (If you know that an average American house requires about 40 million Btus for an entire winter's heating needs, you are already starting to get way ahead of me!) Remember, every year, there are about 280 million spent pellets removed from reactors! And there are around 70 million family residences in the US.
Let's check these numbers. If each pellet starts out with 2.2 * 107 Btus, and 2.8 * 108 new pellets are added each year, that means that they are adding a total of 6.2 * 1015 Btus of new energy to the US reactors each year. The published total nuclear electric power created in 2004 was 7.89 * 1011 kWh of electricity. One kWh = 3412 Btu, so this means that 2.692 * 1015 Btus of electricity was produced. So the thermal efficiency of the nuclear power plants in 2004 was 2.692/6.2 or around 43%. That is a little higher than it actually is, but close enough to indicate that the numbers are all basically right.
At some point a nuclear fuel pellet is determined to have a reduced remaining energy that it is considered not being worth keeping in the reactor. It seems very difficult to tell exactly when that is, as the decision is apparently generally made by a visual inspection of the pellets! But it is reasonable to assume that it is when the nuclear fuel is around 1/2 or 2/3 used up. The reason they are removed then is because the amount of steam generated (to power steam turbines which turn electric alternators) has dropped and the equipment all requires high and consistent production of steam.
A more technical explanation is that the pellets then have too few decaying 235U atoms (each of which creates TWO neutrons in the process, a VERY important fact!) where there are enough neutrons created to act like pool cue balls to hit and break apart (fission) other 235U atoms. It was discovered in the 1940s that if you have at least 90% 235U present (with the other ten percent being the natural 238U) there are plenty of targets for the neutrons to hit and the fission can occur REALLY fast (in a nuclear weapon). It was also discovered then that if there is roughly 4% 235U present, there are ABOUT enough target 235U atoms so that a "controlled chain reaction" can be created (for creating electricity in a nuclear power plant). Once the percentage of 235U has dropped below around 1.5% or 2%, the fuel pellets barely create enough neutrons to be "economically practical" in a large scale nuclear power plant. However, there is still a good amount of fission that can and will still occur in that spent nuclear fuel pellet. Just not enough to PROFITABLY make electricity in power plants! So those pellets are "thrown away" and new pellets replace them, ones that create lots of neutrons! (Over the years of nuclear power generation, the fuel pellet percentage of 235U has been raised from the old 4% or so [which worked fine] to around 5% today. There is no actual need for that to have been done, and the added processing to raise the percentage up to 5% is fairly expensive, but power plant operators now find that they can use those pellets FOR A LONGER TIME BEFORE HAVING TO REPLACE THEM! It was entirely an economic decision! An unfortunate side effect of the higher 235U percentage is that a reactor is therefore slightly more susceptible to bad things happening. If safety devices fail, as in Three Mile Island or Chernobyl, that higher percentage of 235U can cause really bad things to happen pretty fast. Not nuclear-weapons-fast, but still a LOT faster than when they used to use 4% fuel. It was decided that power plants and Operators were capable of ensuring that the higher potency fuel would never become uncontrolled. We shall see.
So it seems reasonable to assume that spent nuclear pellets have 1/2 to 1/3 of their original energy content. Given the numbers above, that suggests that a spent 7 gram nuclear fuel pellet probably has between 7 million and 11 million Btus of energy left in it. And that energy is all left to gradually waste itself away, stored in some field somewhere! There is also normal radiation from all the 238U in the pellet.
The point of all this is that ALL spent nuclear fuel constantly produces and gives off heat. So there is a rather large amount of discarded spent fuel that is just sitting in storage, constantly and continuously giving off heat. The thought is to see if there might be some way of capturing that (low-grade) heat.
So I am looking at a SAFE concept, one that has NO chance of aiding any terrorists! (Keep in mind that right now we (the US) has fields and fields of drums and other containers which are chock full of this spent fuel, in HUGE quantities! Also, with the pellet 100 feet down, there is NO radiation from it up where we are, since the ground absorbs any radiation coming upward.
There are a number of processes that could be used to capture that nuclear radiation. Most involve using the Rankine Cycle. The Indirect Rankine Cycle, using water as both heat capture and heat transfer media seems worth considering. This is a standard and proven process, used in many operating reactors, although they usually use liquid sodium as the first circuit heat exchange fluid, to get higher power output. In this usage, we are not faced with trying to get any tremendous output. Both cycles of water are sealed, the lower one gradually becoming radioactive, but the upper one generally not. The upper circuit is the one that creates the steam to drive a steam turbine or other mechanism (also inside the well casing, which turns an alternator to produce electricity or transfers heat to a third circuit that is standard hydronic house heating.
Note that in a VERTICAL well, a very simple and natural "thermosiphon" effect could be used to move that water around. As water got hot inside the fuel chamber, its density becomes less and it would rise up one passageway, to carry the useful heat upward up to the next thermosiphon loop of water. No pump is necessary, so there is less that could ever break down, and the water will flow at all times. (Both TMI and Chernobyl failed when water circulation stopped, the chamber overheated, water boiled and exploded, etc. That could not happen with this, partly because of the thermosiphon and partly because of the very small amount of low-grade nuclear fuel present.)
There are also gas-cooled technologies, which generally use the extremely cheap carbon dioxide as the heat transfer medium. CO2 has the advantage of virtually not ever becoming radioactive. Many, many other technologies are known, and some are probably ideally suited to this "low-power" application. In general, all the research has gone into achieving higher power densities, so the field of low-power applications seems wide open.
This premise would have some Government representative stop by every few months and drop another pellet (or a few of them) down, down, down, into this well casing to add to the power generating capability. The bottom chamber would be made so that maybe 200 pellets would fit (roughly the size of a closed fist).
There are actually two completely different ways of considering this. Water and/or graphite could be used as a moderator to slow down neutrons to accomplish a very minimal "chain reaction". This is where the (two) neutrons (mentioned above) are slowed down enough to be "thermal neutrons" which are rather good at causing some other Uranium atom to split (fission) when they hit them. That is the process that all nuclear power plants (and weapons) use. However, with a very small mass of Uranium, the reaction cannot sustain itself enough to be dangerous. The thermal neutrons would simply increase the natural rate of radioactivity of the Uranium, to produce more power, faster.
An alternative is to not even consider fission reactions, and simply capture the heat that is always given off (by natural radioactivity) by the Uranium. This is far less effective, so more pellets would be needed, but the technology is extremely simple. The chamber around the nuclear pellets would be made of material that did not allow energy to escape, so that most or all of the energy produced would wind up heating water, boiling it, and causing steam as described above. This approach would be quite similar to the isotopic power generators invented about forty years ago.
This approach is technically not like nuclear reactors, because absolutely natural nuclear reactions occur. And again, the technology to do this is over 40 years old!
So there are generally two large new nuclei, which are driven off with about 168 MeV of kinetic energy (energy of motion). That energy gets converted to heat as the nuclear chunks (and neutrons) slow down in the moderating (water). There are also those two neutrons emitted, which start out with a total of 5 MeV of kinetic energy. Again, in conventional fission reactors, they are slowed down such that they each could cause new fissions in nearby atoms of 235Uranium
There is also about 10 MeV of energy given off in gamma-rays (radiation), which again can be captured when the radiation hits matter and causes kinetic energy and then heat energy. Another 5 MeV is carried away by beta-rays (actually electrons), which also can be captured easily as heat. Another 11 MeV is carried away as neutrinos. The total energy released when a single atom of 235U> fissions is therefore 199 MeV. If suitable materials surround it, virtually all of this can be captured. Whatever is not captured simply becomes a slight warming of the soil and rocks around the area. No danger, and potentially nearly 100% thermal efficiency.
This total amount of energy is equal to 1.2 * 10-11 watt-second. This means that 1.12 * 1017 atoms of 235U must fission to produce 1 kWh of energy.
One fuel pellet has a mass of 7 gm as mentioned above. That means there are (approximately) (.007/235) * 6.02 * 1026 atoms of Uranium in that pellet, or 1.8 * 1022 atoms. Only four percent of these are the fissionable 235U so there are 7.2 * 1020 atoms of 235U in a newly made pellet.
This means that a new pellet starts out with 6,400 kWh of energy in it, or 6.4 MWh. Since a watt-hour is 3.412 Btu, this is 21.9 MBtu, in good agreement with the number given above.
However, spent fuel pellets have a number of additional sources of radioactivity (some of which are potentially dangerous, being the assorted fission product nuclei. Nearly all of those daughter nuclei have too many neutrons to be stable, so they all tend to soon have some sort of secondary radioactive decay where additional radiation is given off.) The total ambient radiation is therefore far higher than for natural Uranium. I have not been able to locate any data on exactly how much higher. The assortment of different elements/isotopes in the spent nuclear fuel pellets is very complex.
But the point of this section is that natural radioactivity would work for this purpose, but only if thousands of spent fuel pellets were dropped down into this system. That would still not remotely be enough to inspire any terrorist to try to dig up a hundred-foot-deep device, but it may be considered impractical. (There are other radio-nuclides that are commonly used in this way, but they each have far shorter half-lives, but their usage would not result in the usage of scrap spent nuclear material. Polonium210, for example, has a half-life of about 139 days, and produces a typical power density of 1210 watts per cubic centimeter. Unfortunately, there is not much of that isotope and it is rather dangerous to handle!
This implies that the only possibility of this premise being of practical use is by using at least some of the remaining fission capability of the spent fuel pellet, by suitable moderation (to slow the emitted neutrons down, which water does pretty well, and heavy water does even better) and a chamber that might reflect some neutrons back toward the Uranium atoms.
One other note: When a 235U fissions, the neutrons are emitted at extremely high speed (they carry a lot of kinetic energy away). They are moving so fast that other nearby 235U nuclei cannot easily capture them (unless they happen to be hit dead on, which is relatively rare). So the neutrons need to be slowed down, or "moderated" so that they can more easily be captured. Research in the 1940s determined that if the neutrons could be slowed to around 5,000 mph, when they are considered "thermal neutrons" then other nearby 235U nuclei can very easily capture them and the effect of a chain reaction can be encouraged. It turns out that if you surround 235U with around double that much water, the neutrons are slowed enough to be easily captured. This is why the references to water being used as a moderator in this discussion.
A relatively obvious configuration seems worth experimenting with. If a grid-structure (made of a material that would not degrade due to radiation) was made so that spent nuclear fuel pellets were kept spaced so that an appropriate amount of water was always between pellets, a significant number of thermal neutrons would be present to inspire other 235U nuclei to capture them, which would greatly increase the performance of this very simple system.
I have seen government numbers which indicate that the "reasonably assured" natural Uranium resources, within practical mining capabilities, in the US was around 581,000 metric tonnes in 1975, AND IS ESSENTIALLY NOW AROUND ZERO, and in Canada was around 204,000 metric tonnes (but additional Canadian deposits were discovered. Only Sweden, South Africa and Australia have reserves on that scale, with no other country higher than 1/10 of these amounts.
No one seems to publicized that the US currently has NO uranium mines! They were all closed before 1992 because nearly all the accessible ores had already been mined! Even in 2002, the US IMPORTED around 92% of the uranium used in our power reactors, with the other 8% primarily coming from re-processed nuclear warheads.
Natural Uranium only contains 0.711% 235U, and that has to be enriched up to at least 4% for use as nuclear fuel pellet material. What this means is that nearly 5/6 of the mined natural Uranium must later be discarded as "depleted uranium" in order to collect the 235U in the enriched portion. This is NOT "spent nuclear fuel" because it never even became fuel, it was all simply discarded! There are two significant points here:
Really large amounts of Depleted Uranium wind up being discarded in that process. You may have heard of "Depleted Uranium artillery shells"? Did you ever wonder why they use it there? Did you think it was simply a way of using it up? Well, maybe, but that is not the main reason. It turns out that Uranium has a Density that is about as high as anything known, more than two and a half times the density of steel. When a high speed shell has such high density, traveling at a similar speed, it therefore has 2.5 times the momentum of a steel shell. When such a shell hits the side of a tank made of steel, the extremely dense uranium tends to pierce right through. Not much can stop a very high speed piece of flying uranium!
The second point is more of what we are interested in here. The enriching process can only get (0.711 / 4.0) (around 18%) of enriched uranium that can be used for power plants. This means that the world's known reserves are really equivalent to only about 1/6, with the other 5/6 being immediately discarded.
Did you notice above that we already have 40,000 metric tonnes of spent fuel pellets? There are certainly also hundreds of thousands of metric tonnes of depleted Uranium. Keep in mind that we are only getting around 20% of our electricity from nuclear. In another 20 years (of ONLY getting 20% from the existing nuclear reactors) we will have used up 80,000 metric tonnes of fuel and equivalent amounts of depleted uranium will exist. This is a problem! It implies that even only providing 20% of our supply, our American supplies of Uranium from mines has already essentially been all used up, and even if we maintain close friendships with Australia and Canada, even their nuclear resources will be used up around 2030, roughly the same time that oil and natural gas (worldwide) will be running low.
Now, you have undoubtedly seen where Washington Politicians have started pushing nuclear again, now that they are starting to realize the immense crisis regarding oil and natural gas. There are only 100 currently operating American nuclear power plants. Imagine that our politicians decide (because of lobbyists' pressure, to authorize another hundred to be built. In the short run, that SOUNDS good, reducing foreign oil dependence and all. But if we use up the world's nuclear supplies twice as fast in providing 40% of our electricity, we will cause the entire world to be out of nuclear reserves even faster, maybe by 2025 or so.
This presentation is not meant to be "political" but merely informational and accurate. But if people or politicians become so averse to oil and natural gas that they run to nuclear as a "saving grace", they will certainly be seriously disappointed.
The Earth's Rotation as a Source for Energy
Waste Nuclear Power For Making Electricity And Heat?
The Physics of Efficiency In Electric Power Plants
Individual Ways of Reducing Your Energy Usage
Methods of Storing Energy for Later
How Much Energy Comes From the Sun? And Why is there Global Warming?
How does the Sun create so much energy?
Inventions Which Might Help Deal With Coming Energy Catastrophes
An Invention to Efficiently Make Electricity from Solar
Enormous Heating of the Atmosphere by the Alaska Pipeline
Air Conditioning without Huge Electric Bills and without Freon
A Method of Storing Summer Heat to (Nearly) Entirely Heat a House all Winter
An Extremely Highly-Efficient (and Fast, 200.0 mph) Transportation System for People and Products
The Sophisticated Woodstove I Invented in 1973
The Physics of Wood as a Heating Fuel
Why is the North Pole Heating Faster than the rest of the Earth?
A Possible way to greatly reduce Aerodynamic Drag of Airplanes
( http://mb-soft.com/public/index.html )
C Johnson, Physicist, Physics Degree from Univ of Chicago