Space Shuttle Propulsion

                In my last couple of blogs, I have been writing about nuclear power. For this blog and the next few to follow, I will be talking about some things relating to space travel. The subject that I will be writing about in this post will be the problems with propulsion for space travel today. For simplicity, the space craft that I will be examining will be the Space Shuttle.

                 The first thing that is a major problem is the fuel. An amazing amount of fuel is required to get anything into space. The fuel for the space shuttle weighed a total of 3,869,475lb, or nearly 1935 tons of fuel! That is more than 23 times the weight of the empty shuttle itself!

There were three different types of fuel used in the Space Shuttle. One was a mixture of liquid hydrogen and liquid Oxygen. This was contained in the Space Shuttle external tank, weighing a total of 1,621,722lb. This burns almost invisibly, and is the fire that is coming out of the bottom of the shuttle itself. The second type of fuel was a mix of 69.6% ammonium perchlorate as an oxidizer, 16% aluminum as a fuel, .4% iron oxide to control burn speed, 12.04% PBAN (Polybutadiene acrylonitrile) to hold the fuel together and act as a secondary fuel, and 1.96% an epoxy curing agent. This was contained in the Space Shuttle Solid Rocket Boosters, weighing 1,100,000lb each for a total of 2,200,000lb. This is the fuel that causes the classic huge plumes of smoke and fire during a launch. The last type of fuel used in the Space Shuttle was two chemicals that, when mixed, combusted to produce thrust. These two chemicals were monomethylhydrazine as a fuel, and dinitrogen tetroxide as an oxidizer. These were contained in the Space Shuttle Orbital Maneuvering System, with a combined weight of 47,753lb. These thrusters are small and mostly used in space, so you cannot see them during the launch.

All three of these fuels are either toxic and carcinogenic or cryogenically cooled, making them very difficult, expensive, and dangerous to handle. They also are impractical for more mass or longer travel, as this would require more fuel. This increase in fuel would need even more fuel to lift it. This means that at a certain amount of weight or distance to travel, it would simply require too much fuel or be too massive to achieve liftoff. In my opinion this needs much improvement. In my next blog, I will be writing about some ideas for improved fuel efficiency and different types of engines and fuels.

Comparison of hybrid reactor

                In my last blog, I described my design for a nuclear reactor core that would, in my opinion, be superior to the current design in multiple ways. In this blog, I will be comparing my reactor design to the design of modern day fission reactors.

                The first point of comparison is safety. This reactor would have a much higher operating temperature than current reactors. This makes a meltdown sound much more dangerous, but in fact it would be less of a problem, because any explosion that would occur would simply be caused by too much heat or pressure, not an uncontrolled nuclear chain reaction. The likelihood of a meltdown is also drastically reduced because of the plethora of failsafe mechanisms and intrinsic safety of the reactions.

                The second point of comparison is the production of nuclear waste. This nuclear reactor would produce only short half-life radioisotopes that could be used in simpler power plants as fuel because they would produce large amounts of heat from radiation that could be used to generate electricity, or possibly even for other uses such as industrial heating. This reactor would also be able to consume long half-life nuclear waste as fuel, solving many of the storage and environmental concerns that are usually associated with nuclear waste.

                The last point of comparison is the availability, sustainability, and economic feasibility of fuel. This reactor would take two types of fuel, one with relatively large atoms and one with relatively small atoms. The small atom fuel, for the fusion reactor component, would be produced in the reactor from the cooling water. The neutrons used to produce this would come mostly from the splitting of the larger fuel. This larger fuel could be nuclear waste as mentioned above, or it could be thorium or natural uranium. This would mean that there would be no need for processing, and would theoretically last us millions to billions of years. This means that it would be considered a renewable resource.

                In comparison to current nuclear reactors, or worse, coal plants, in my opinion, this would win by a landslide. So what would this all mean for you, the average person? The main thing that it would mean is dirt cheap electricity. This would cause an increase in the use of electric vehicles. This increased use of electric vehicles coupled with the zero emissions from the power plant would remove the top two sources of greenhouse gasses. Overall, I think that the benefits of this would be massive and cause good effects across almost everything indirectly economically or environmentally.

A hybrid reactor design

                In my last few blog posts, I have described some ideas for nuclear power that I think are worth considering for use in the future. In this blog post, I will be writing about what I think would be the best design for a nuclear reactor for the future. My reactor design is a hybrid of the three reactors that I discussed in my previous posts.

                For the design of the nuclear reactor core, it would be a toroid, or donut shape, with three shells. The center would be a combination thermonuclear and inertial confinement fusion reactor. The first shell would be molten fluorine and fuel salts. The second shell would be a circulated coolant molten salt. The outermost third layer would be a layer of supercritical water coolant.

                The plasma in innermost fusion reactor would be spinning which will make the heavier elements, or wastes from the fusion reaction, be centrifuged out to the sides of the chamber for removal. While wastes are being removed, more fuel could be constantly added, eliminating the need to stop and refuel.

                The first shell of the reactor would be the same as a molten salt reactor, but would be able to use much more nuclear waste or depleted uranium as fuel, as the fusion reactor would produce high energy neutrons similar to those produced from a particle accelerator. A lot of the heat from the fusion reactor would be deposited in this layer, as the salt would be very dense. The salt would be continuously circulated and processed to remove wastes and add more fuel, eliminating the need to stop and refuel, which is a major problem with the molten salt reactors.

                The second shell would be the main cooling shell. It would be some other molten salt that would be used to transfer the heat to the power generation part of the reactor. The extremely high operating temperature allows very high thermodynamic efficiency.

                The outermost shell would be a layer of water that would be used for further heat transfer, but also would absorb many neutrons from the reactor, making deuterium and tritium for use in the fusion reaction at the center.

                This design would have most of the benefits of the constituent designs while minimizing the negatives. In my next blog post, I will compare this design to the design of the nuclear reactors that we use today.

Molten salt reactor

                In my last couple blogs, I have wrote about three different nuclear reactor designs. In this blog post, I will be writing about another. This next reactor design is the liquid fluoride thorium molten salt reactor. This means that the fuel or coolant (or both) is in the form of a molten salt, allowing it to run at higher operating temperatures for better thermodynamic efficiency. I will compare this design to current reactors in the three ways that I have compared the others in my previous blogs.

                The first concern for most people when they hear about a new nuclear reactor design is how safe it is. This reactor would have some easy safety features, one of which is a freeze plug. A freeze plug is a plug in a pipe that is cooled to freezing of the salt by an electric fan or pump that would stop functioning in the event of a power failure. The plug would also melt if the reactor got too hot from loss of coolant or control malfunction. When the plug melts, it would allow the entire contents of the fuel salt into emergency tanks that would stop the reaction and cool the fuel, stopping the meltdown. In this reactor, thorium-232 absorbs a neutron and turns into uranium-233 which is hit by another neutron, causing it to split and give off large amounts of heat and two or three neutrons that continue the process. One concern risen in regard to this cycle is the possibility of using such a reactor to produce uranium-233 for use in nuclear weapons. This is a real concern, as it would be easier to make a nuclear weapon out of this reactor than conventional reactors.

                The second main concern is the production of nuclear wastes. This is not as much of a problem with this type of reactor, but would still be a problem, as it still produces nuclear waste, just not as much. This type of reactor can be configured to be a “waste-burner”. This would be run with a combination of thorium and nuclear waste as the fuel. Thorium produces much less of the long-lived and problematic wastes than conventional reactors. This results in the reactor consuming more nuclear waste than it produces.

                The last major concern is the fuel for the reactor. As I have wrote in a previous blog post, Thorium, the main fuel for this reactor, is hundreds of times as abundant as useable uranium. This reactor can have other things added to the fuel to replace some of the thorium. Some of the thorium can be replaced with nuclear waste as stated above, and if we run out of nuclear waste, it can also use depleted uranium to replace some of the thorium. One fuel related problem is that it would need a considerable amount high grade fuel to start it every time it is shut down for maintenance. Overall, in my opinion, this choice is superior to the accelerator driven subcritical reactor, but only for a couple more years, as accelerator technology is improving rapidly to make more energy efficient accelerators.

Thermonuclear Fusion

                In my last blog post, I wrote about how an accelerator driven subcritical reactor would be superior to current fission reactors. In this blog post, I will be writing about an alternative to these two choices. This alternative is fusion. Simply put, fission is splitting large atoms, while fusion is combining small atoms. The type of fusion I will be talking about is thermonuclear fusion.

                The first point of comparison will be regarding safety. Nuclear fusion reactors are safer than current fission reactors. Thermonuclear fusion requires a large amount of heat to sustain itself. If the containment is breached such as in the event of a nuclear meltdown, the temperature would drop and it would stop producing heat. The fallout of such an event would be minimal, as it would not have the same energy as a nuclear explosion or the radioactivity, as it would become no longer radioactive when it cools.

                Another problem with conventional nuclear reactors that is overcome in fusion reactors is the production of nuclear waste. The waste from a fusion reaction is almost always stable helium. There are some radioactive isotopes created, but those have extremely short half-lives ranging from less than a second to 12.3 years depending on what fuel is used.

                As with conventional fission reactors, fuel would be a problem for fusion reactors. The fuel for fusion reactors can be different elements and isotopes and mixes of those with varying efficiency, required operating heat, radioactivity, and difficulty to produce. One of the most commonly discussed reactions is between deuterium and tritium. Deuterium is hydrogen with one neutron. It can be extracted from natural water such as the ocean, but current extraction techniques are costly, energy intensive, and inefficient, comparable to enriching uranium. Another way to get deuterium is to make it. This can be done by irradiating hydrogen (normally bonded with oxygen in the form of water) as neutron shielding of fission reactors, or even of other fusion reactors. Tritium is made by irradiating deuterium. Therefore, using water as neutron shielding will produce fuel.

This reactor design, if implemented, would not be dependent on mining of rare elements for fuel, but would be much more difficult and expensive to build. Overall, this type of reactor may not be practical with today’s relative abundance of fission fuels, its cost, and our current technologies for containing the reaction, but the massive amount of power able to be produced in this reaction make it something worth developing.

Accelerator driven subcritical reactor

In my last post, I wrote about some drawbacks or flaws in the current design of nuclear fission reactors. In this blog post, I will be writing about an accelerator driven subcritical reactor. I feel that this type of reactor is superior in multiple ways. I will compare this to the current fission reactor design in three main ways. The first of which is safety.

The accelerator driven subcritical reactor would be much safer than current nuclear fission reactors. In normal fission reactors, the reaction is self-sustaining, meaning that if the reactor lost power, it would continue to generate heat, making it possible to cause a nuclear meltdown. The accelerator driven subcritical reactor, however would, in the event of a loss of power, become inert by no longer producing heat. Fission reactors produce heat by splitting atoms. The conventional fission reactor uses a fuel that can sustain a chain reaction. One atom will split and give off more than one neutron. These neutrons will cause other atoms to split, giving off more neutrons, and the cycle continues. In an accelerator driven subcritical reactor, the fuel is not able to sustain a chain reaction. Instead, it is split by high energy neutrons from a particle accelerator. If the particle accelerator has no power, it will stop splitting atoms, and therefore stop producing heat.

An accelerator driven subcritical reactor would not produce the long half-life nuclear waste that is produced in a conventional fission reactor. As an addition to not producing these dangerous and long-lived isotopes, it would also be able to use these as fuel, solving most of the problems with nuclear waste. The only nuclear waste that would be produced by the accelerator driven subcritical reactor would be short half-life isotopes that would be highly radioactive, but for a shorter period of time. The high radioactivity could be used in a second reactor to generate power. This would remove the need for expensive long term storage, empty existing long term storage, and generate more power.

Lastly, in conventional fission reactors, the fuel is rare and expensive to process. This is not the case for accelerator driven subcritical reactors. Other than being able to use nuclear waste as fuel, which would actually gain money, as it costs money to store it otherwise, it can also use thorium. Thorium is estimated to be three to four times as abundant in the earth’s crust as uranium. This, along with the fact that less than .4-.6% of uranium can be used in reactors, makes thorium hundreds of times as abundant as uranium. Also, thorium only occurs as one natural isotope, so it does not need the expensive enrichment process required by uranium. It is also thought that thorium will give more energy per weight than enriched uranium. This, if implemented, would make power substantially cheaper, safer, and environmentally friendly.

Problems with modern nuclear reactors

For this blog, I will be stating my opinion on how nuclear power has room for improvement. There are three major drawbacks of nuclear power from fission reactors. Fission reactors are the only non-experimental power producing nuclear reactors that exist today. The first drawback is safety. Some people are worried about nuclear reactors exploding. This is not a realistic concern due to the fact that the fuel is not the same grade, or concentration of useable uranium, as that which is used in nuclear bombs. There are also many failsafes that have been developed or implemented since events like 3 mile island or Chernobyl. People are also concerned about nuclear reactors emitting radiation, even though you get more radiation exposure from living in a brick or stone house than you do from living next door to a nuclear power plant.

The second drawback is the production of nuclear waste. Most nuclear waste is in the form of spent fuel rods. These contain fission products, most of which are radioactive, and therefore are poisonous and hazardous to the environment. Nuclear waste is a mix of radioactive isotopes with varying half-lives. The half-life of something is the amount of time it takes for half of the atoms to decay into something else. Some constituents of the nuclear waste have a relatively short half live. This means that they are very radioactive, but become less radioactive faster. Other parts of the waste have very long half-lives these are a problem because, despite being less radioactive, will stay radioactive for millions of years.

The last major drawback is that the fuel for fission reactors is expensive to process and rare. Natural uranium is only 0.7% U²³⁵ (Uranium that can be used as fuel), meaning that it has to be enriched to be used for fuel in nuclear reactors. This enrichment process takes some of the U²³⁵ from natural uranium and concentrates it. The uranium with a lower concentration of U²³⁵ (under .3%) is called “depleted uranium” and is used for other things. The uranium with a higher concentration of U²³⁵ (usually from 3% to 5%) is called enriched uranium and can be used in nuclear reactors. This enrichment process takes large amounts of energy, time, and infrastructure. This along with all of the safety equipment needed makes nuclear power very expensive in comparison to what it could be with further development. In my next blogs, I will be writing about some ideas for development that could minimize or negate the problems above.

Opinions

For this blog post I am going to be writing about how difficult it is to write an opinion article on any form of science. My opinion is that writing an article about a theory, design, discovery, etc. only consisting of my opinion is nearly impossible. What I was planning on doing for this blog posting assignment was to read a scholarly article about the findings of a study and write about that. We were supposed to write about something that interested us, and that is what interests me. However, we are not supposed to post “recycled information”, or what someone has already said. This means I cannot write a simplification, summary, or post regarding the practical applications of the subject discussed in the article.

As for writing an opinion on the article, whenever I write an opinion on something, it becomes more of a compare and contrast article than anything. This would be nearly indistinguishable from just listing the pros and cons of implementing the idea discussed in the article. Also, for the majority of subjects, the pros and cons are already somewhere on the internet or in a book or article. This means that everything that I would have wrote has already been wrote.