Erasmus : “Wow”. That’s a mouthful, but you’re probably right. Power generation is no laughing matter. We need to do a few things better if we are stake a claim to a place in the universe. (Ah well, maybe just a place in the galaxy).
Current technology for power generation includes:
- Nuclear Fission
- Burning Fossil fuels such as gas or goal or oil or liquid fuels
- Collecting solar energy and translating this into electrical energy or pressure or heat energy
- Geothermal energy extraction
- Wave or wind energy collection
- Hydroelectric power generation
Unfortunately fusion power has yet to be invented.
(Well, in effective terms for power generation at this time).
Antimatter power generation also has a long way before it becomes mainstream.
Windmill Alternate Power.. Solar Voltaic Power
Kinkajou : I think even if we could use antimatter for Power generation and if we could get antimatter at one dollar a kilo, there will be substantial public non-acceptance of this technology. Antimatter is dirty (as gamma radiation is produced) and dangerous (unstable).
Fission Reactors: Properties
Kinkajou : The only option here for self-propelled isolated worlds such as small spaceships is Fission power. So let’s look at what we have now.
Erasmus : Fission reactors are:
- Not scalable to a small size: You would not build a nuclear reactor to generate 100 kW of electrical power. It’s not that this is impossible. It’s just that it makes much more commercial sense to build it bigger. You would not build a nuclear battery for a watch.
- Big and heavy requiring substantial shielding
- Radioactive with a large number of long half-life radioactive by-products
Some of our views of fission reactors relate to our method of fuelling. Currently we use uranium 235 as a component of fuel rods for nuclear fission reactors for commercial power. However, there has been some usage of thorium as a fuel.
Thorium is potentially safer, less able to be sidelined into weapons grade uranium production, potentially cheaper as thorium is more plentiful in the Earth’s crust than is uranium and has less long-term toxic by-products produced as a result of the fission process.
Thorium Power Station China
Thorium-based nuclear power is predominantly fuelled by the fission of uranium 233 produced from the element thorium. Thorium has a greater abundance on the earth, has advantageous physical properties and nuclear fuel properties leading to reduced nuclear waste production, stability and safety. However the technology to use standard fuels such as uranium 235 isotopes is easier to implement. Hence the barrier to the usage of thorium.
Erasmus : A nuclear reactor is fuelled by specific fissionable isotopes which practically are:
- Uranium 235 generated by breeder reactors from naturally mined uranium 238. When the percentage of uranium 235 is especially enhanced, it becomes suitable for weapons grade usage.
- Plutonium-239, transmuted from uranium-238 obtained from natural mined uranium. Plutonium is also used for weapons.
- Uranium-233, transmuted from thorium-232, derived from natural mined thorium. That is this article's subject.
Uranium Atomic Structure
Erasmus : Naturally occurring uranium is composed of three major isotopes:
- Uranium 238 99.3% natural abundance
- Uranium 235 0.7% natural abundance
- Uranium 234 .005% natural abundance
All the isotopes a radioactive with very long half-lives.
The decay series of uranium-235 has 15 members that end in Lead-207.
Uranium-233 is made from thorium-232 by neutron bombardment.
The isotope uranium-235 is important for both nuclear reactors and nuclear weapons because it is the only isotope existing in nature to any appreciable extent that is fissile, that is, can be broken apart by thermal neutrons.
The isotope uranium 238 is use exists in abundance in nature. It can absorb neutrons producing an intermediate isotope that subsequently decays into isotope plutonium 239, which is also fissionable.
Radioactive Hazard Sign
Erasmus :Thorium nuclear fuel is:
- Three times more abundant than uranium in the Earth’s crust. The mind material thorium 232 is all directly usable as reactor core fuel. Uranium 238 by contrast needs to be enriched to uranium 235 to be used as a fissionable fuel in reactor fuel rods.
- Thorium reactor's plutonium production rate would be less than 2 per cent of that of a standard reactor, and the plutonium's isotopic content would make it unsuitable for a nuclear detonation
Several uranium 232/233 bombs have been designed and tested. However high radiation output from inherent uranium 232 made the fissionable material difficult to handle and uranium 233 was much more unstable, increasing the risk of possible pre-detonation. Also separating uranium 232 from uranium 233 is more difficult than separating uranium 235 from uranium 238.
Nuclear Fission Explanation
Erasmus : The nuclear waste generated by thorium fission is approximately 1% of that generated by using uranium as a nuclear fuel. Also the radioactivity of the by-products of the thorium fission reaction drop to safe levels after just a few hundred years compared to many thousands of years for current uranium nuclear waste.
Estimates have given that one time of thorium can produce much energy as 200 tons of uranium or 30 million tons of coal.
Thorium as a fuel is more amenable to safety features implemented in the reactor design than is uranium. If the temperature in a reactor exceeds a specific level, liquid fluoride thorium reactors have been designed to melt a drain plug at the bottom of the reactor, draining the thorium fuel into an underground tank for safe storage.
Thorium is easy to extract by mining techniques than is uranium. Thorium is derived generally from the ore monazite, which in nature has been found to have generally high concentrations of thorium than the percentage of uranium normally found in its respective ores. Radon gas more common uranium mines and found less often in thorium mines.
Nuclear Fission Example
Kinkajou : Sounds like all go.
Other problems with using thorium nuclear power ?
Breeding uranium 233 from thorium 232 is a slow process and requires extensive reprocessing. Reprocessing is difficult.
Thorium is currently a technology with which we have limited experience. Theoretical considerations suggest advantages. However, we need experience to make informed decisions. Most current research on thorium is based on the liquid fluoride thorium reactor and pilot plants are being considered in a number of countries.
There is a higher cost of fuel fabrication and reprocessing than those that use traditional solid fuel rods.
Thorium, thorium can break down into uranium 232 which is very dangerous due to a high gamma ray emission rate.
Kinkajou : So tell us about the countries in the world that are thinking about thorium.
India's government is the only country in the world with a detailed, funded, government-approved plan to focus on thorium-based nuclear power. It is currently developing up to 62, mostly thorium reactors, which it expects to be operational by 2025. Figures released by the government reveal that it expects to increase the nuclear share of the country’s power production from 2% (current) to 25% by 2025.
Part of the reason India is favouring Thorium as a nuclear fuel is because it is believed to have about 2/3 of the world’s supply of the main Thorium containing mineral, monazite, which are found in heavy mineral sands deposits on the south and east coasts of India.
Thorium is mostly found with the rare earth phosphate mineral, monazite, which contains up to about 12% thorium phosphate, but 6-7% on average. World monazite resources are estimated to be about 12 million tons,
Types of thorium-based reactors
According to the World Nuclear Association there are seven types of reactors that can be designed to use thorium as a nuclear fuel. The first five of these have all entered into operational service at some point. The last two are still conceptual, although currently in development by many countries:
- Heavy water reactors (PHWRs)
- High-temperature gas-cooled reactors (HTRs)
- Boiling (light) water reactors (BWRs)
- Pressurized (light) water reactors (PWRs)
- Fast neutron reactors (FNRs)
- Molten salt reactors (MSRs, LFTRs)
- Accelerator driven reactors (ADS)
Additionally, in the 1958 Atoms for Peace publication entitled Fluid Fuelled Reactors, Aqueous Homogeneous Reactors (AHRs) were proposed as a fluid fuelled design that could accept naturally occurring uranium and thorium suspended in a heavy water solution.
Kinkajou : Again, we humans are not really good at micro engineering.
Erasmus : I remember reading a book series called Asimov’s Foundation series. One of the criticisms of the Old Empire was their inability to micro scale their commercial productions. They could build an atomic reactor that would power a world, but not one that could power a hand held device.
At Terminus, the scientists of the First Foundation confronted with the need to conserve valuable resources in this resource poor world at the edge of the galaxy, were forced to develop efficient devices for the generation of micro scale power.
Kinkajou : I think you’re right Erasmus. That’s; exactly where the human race needs to be heading now. Let’s just hope we don’t start doing this because the population of the earth is heading for 21 Billion and our resources are becoming similarly scarce.
Erasmus : Back to the characteristics of Nuclear Fission reactors. They are
Dirty: They throw out nuclear contaminants that are hazardous to life and have an extremely long half life
Have a low power/mass ratio: they are not very energy dense. Currently, we are not building nuclear fission reactors for spaceships. The shielding required means they are very heavy and for a given weight they generate too little output power. I suppose one solution is to run the reactors much hotter: in short a lot closer to the threshold of meltdown or explosion.
Power generation in a fission reactor is much higher the further up the exponential reaction curve you go, but perhaps not very safe. An alternate proposal is to develop some new way to extract energy from the fission reaction: i.e. not using water as a steam generator or coolant.
Erasmus : The solution is perhaps some new method of generating power / energy which is more capable of dealing with spaceship sized amounts of output energy, (vs. city size energy outputs). Fusion power generators and matter antimatter power generators both perhaps are more likely to allow the scaling of plants to smaller sizes and more energy/mass efficient power plants.
Erasmus : Let’s look at these technologies.
Fusion Power Generators:
Fusion power generators, create energy from the controlled fusion of atoms. The easiest reaction, and most immediately achievable nuclear reaction using current technologies, according to the Lawson criteria, (A measure of the conditions required to achieve a fusion reaction)) is:
21 D + 31 T → 42 He + 10 n
In a fusion reactor, these light small atomic nuclei are fused together to form a single heavier nucleus. To achieve this fusion of atomic nuclei, immense pressure and temperature conditions are generally required. The binding energy of the new atomic nucleus is lower than that of the initial nuclei. This energy is released as energy in the fusion process and can be used for power generation.
Erasmus : Fusion reactions compare favourably to fission reactions at a number of levels.
Running a fission reaction in the exponential range of the power generating curve, will generate a lot of energy. But malfunctions in this power generation range will be deadly. By contrast, fusion reactors malfunctioning will simply stop working.
If the fusion containment vessel is breached for any reason, the reactions cease : immediately. Fission reactors, ,in contrast, continues to generate heat through decay nuclear fuel for several hours or even days after reactor shut-down, meaning that melting of fuel rods is possible even after the reactor has been stopped due to continued accumulation of heat.
The total amount of fusion fuel in the fusion reactor is small, typically a few grams. If the fuel supply is closed, the reaction stops within seconds. In comparison, a fission reactor is typically loaded with enough fuel for one or several years, and no additional fuel is necessary to keep the reaction going.
Waste products of fusion reactions are generally light atomic nuclei and have shorter half-lives. The waste products are less toxic. Fusion reactors often use liquid lithium as fuel and as a neutron capture agent to convert stray neutrons into tritium which can then be extracted and returned to the reaction chamber.
Although lithium is flammable it poses only a local chemical risk of short duration. Calculations suggest that the total amount of tritium and other radioactive gases in a typical fusion power plant would be so small, about 1 kg, that they would have diluted to legally acceptable limits by the time they blew as far as the perimeter fence of a typical fusion reactor plant.
Reactor Nuclear Waste Effluent: Although tritium (an isotope of hydrogen) is volatile and can enter and bond to biological systems,
the health risk posed by this chemical is much lower than that of most radioactive contaminants,
- due to tritium's short half-life (12 years),
- very low decay energy (~14.95 keV),
- And due to the fact that it readily is washed out of the body as water,
with a biological half-life of 7 to 14 days).
The half-life of the radioisotopes produced by fission is much greater than those produced by nuclear fusion. Most of the radioactive material in a fusion reactor would be the reactor core itself, which would be dangerous for about 50 years, and low-level waste another 100.
Although this waste will be considerably more radioactive during those 50 years than fission waste, the very short half-life makes the process very attractive, as the waste management is fairly straightforward. By 300 years the material would have the same radioactivity as coal ash. Fusion reactor waste by products in contrast last for thousands of years.
In a fission reactor, many materials are required for their specific neutron cross section. The choice of materials used building a fusion reactor is less constrained than in a fission design. This allows a fusion reactor to be designed using materials that are selected specifically to be "low activation", materials that do not easily become radioactive.
- Vanadium in a reactor core, would likely become much less radioactive than stainless steel.
- Carbon fibre materials are also not prone to nuclear activation into highly reactive intermediary chemical radio- isotopes
Erasmus : Although there has been considerable research and some breakthroughs, the technology of fusion power generation has proven to be difficult to implement. Research has focused on sustaining the fusion reaction long enough and safely enough to make sustained power generation practical.
The difficulties with early versions of these technologies relate to the instability of the containment fields which invariably leaked plasma at far higher rates than theory would predict. As researchers attempted to add more containment fields or more powerful containment fields, the cost and complexity of designs increased, making commercial exploitation of this technology increasingly unlikely.
Despite optimism dating back to the 1950s about the wide-scale harnessing of fusion power, there are still significant barriers standing. It is a substantial step between current scientific and engineering capabilities and the practical development of fusion as an energy source.
Research, has also continually thrown up new difficulties. It remains unclear whether an economically viable fusion plant can be built with current technology, although research work still continues.
Fusion Process Diagram
The main competing fusion technologies currently are:
- Magnetic toroidal shaped reactors: known as Tokomak reactors and
- Inertial Laser triggered reactors creating and manipulating nuclear plasma, and finally
Cold Nuclear Fusion Sun
Cold Fusion Sun and Nuclear Fusion
Kinkajou : Cold fusion sounds like something really interesting and really really safe after all the problems you’ve been telling me about.
A number of experiments claiming nuclear cold fusion have proven to be false. No by-products of nuclear reactions were detected. (In short, something happened, but probably not nuclear cold fusion). Theoretical considerations looking at how the reactions were supposed to work, made the concepts proposed seem very unlikely. Consequently, the science of cold fusion gained a reputation as charlatan science.
However some researchers have suggested some novel approaches, which fall in the cold fusion reaction scope. Some researchers have suggested using unusual nuclear particles to catalyse a fusion reaction. One suggestion proposes that muons (having a mass of 207 times that of an electron)
, could be used to catalyse a fusion reaction. Muons allow atoms to get much closer together and thus reduce the kinetic energy required to initiate fusion. Muons, deuterons and tritons form “heavy” hydrogen molecules. Due to the increased mass, the “Columbic” barrier decreases.
Fusion could occur at room temperature. The problem of course is that to make this method commercially viable requires a stream of muons. These are neither cheap not easy to produce in the required quantities.
Perhaps another option is the use of fuel materials cooled to almost absolute zero. Recent research into Bose-Einstein condensate (BEC) has found that by slowing down a body of atoms, to within a fraction of a degree Kelvin (near absolute zero), they coalesce into a “superatom”. This material may have different interactivity properties compared to normal matter and may be amenable to new approaches.
Erasmus : Another research team claimed to have devised a way of producing fusion using a machine that "fits on a lab bench". They used a chemical called lithium tantalate to generate enough voltage to smash deuterium atoms together. However, the process called pyroelectric fusion does not generate net power, so is not of use in power generation.
Finally a fusion technique called sonoluminescence claims to be able to induce fusion reactions. Some researchers call this technique bubble fusion. It is not widely accepted as a viable fusion technology in the scientific mainstream.
Kinkajou : How successful have we been so far in initiating nuclear fusion?
Erasmus : As of July 2010, the largest fusion experiment using magnetic confinement has been the Joint European Torus (JET). In 1997, JET produced a peak of 16.1 megawatts (21,600 hp) of fusion power (65% of input power), with fusion power of over 10 MW (13,000 hp) sustained for over 0.5 sec. To be commercially viable, a lot more output power needs to be generated than input power. Due to wasteage and inefficiencies, the output needs to be much higher than has yet been achieved.
By the end of the 1980s a lot of research had been completed and many improvements in Tokomak design in areas such as non-circular plasmas, internal diverters and limiters, superconducting magnets, and operation in the so-called "H-mode" island of increased stability, had been made
The Tokamak reactor dominates much modern thought on the design and building of fusion power reactors. The newest Tokomak designs are typified by the ITER. This reactor began construction at Cadarache, south France in 2007, with first plasma expected in late 2019.
Another successor already entering design phase called DEMO is already in the design phase with construction tentatively planned for 2024 - 2033. DEMO is intended as a prototype fusion power generator. The goal is to produce up to 2 GWatt of electrical power on a continual basis.
Erasmus : In addition to the problems arising from the design and construction of the fusion reactors, a number of practical problems exist. Engineering researchers need to find suitable "low activity" materials for reactor construction, propose practical methods for Tritium 31H extraction, and build reactor designs that allow their reactor core to be removed when its materials becomes embrittled due to neutron flux damage.
Kinkajou : Tell us how this technology works.
Erasmus : Overall principles of an Inertial Fusion Energy (IFE) reactor
the operation of an inertial fusion reactor can be considered to be very similar to the operation of a simple combustion engine.
- intake : The fusion fuel is fed into the reaction chamber, often as microdrops or microcapsules;
- compression of the fuel droplet is initiated with a fast burn type of laser;
- explosion of the plasma heated to a high temperature and pressure by the laser device, leading to the occurrence of nuclear fusion, resulting in the release of energy.
- Exhaust of the reaction residue, which will be collected and, recycled afterwards to extract all the reusable elements, mainly tritium.
To allow such an operation, an inertial fusion reactor is made of several components:
Fuel Injection system: this actually delivers both the fuel capsule and in some designs a containment vessel or containment device to the reaction chamber:
Ignition or spark creation system; depending on the technique, it can be:
- Lasers; or
- An ion beam accelerator;
- A z-pinch device;
The reaction chamber or confinement chamber. This is designed to keep the plasma dense enough and hot enough to undergo fusion, that usually incorporates
- An external wall made of metal;
- An internal blanket intended to perform a number of complex tasks such as
- Protect the external wall from the fusion shockwave and radiation,
- Collecting the emitted energy,
- And allowing the production of more tritium fuel for the reactor;
To induce the symmetrical explosion needed to create the fusion pressures, the pellet must be compressed to about 30 times solid density with high energy beams. If the beams are focused directly on the pellet, it is called direct drive. An alternative approach is one in which the heat is focussed on a shell and the shell radiates x-rays, which then implode the pellet. This is called indirect drive.
The beams are commonly laser beams, but heavy and light ion beams and electron beams have all been researched in inertial fusion systems.
Solar Fusion Containment
Erasmus : Inertial (laser) containment: Fast ignition
Laser-based design has other advantages over the magnetic design of the Tokomak reactors. The reactor core is mostly exposed which makes the problem of removing energy from the system somewhat simpler than in an enclosed system like the Tokomak reactor.
So access to the core of the device should be easier for maintenance or repair. The lack of strong magnetic fields also allows use of a wider variety of low-activation materials, which would reduce both the frequency of neutron activations and the rate of irradiation to the core.
Some concepts have proposed combinations between the magnetic field and the laser ignition designs, often called magneto-inertial fusion. Magnetic fields are currently cheaper and easier to generate than laser fields, but of course innovation in construction methods can change commercial realities quite quickly.
At very high temperatures such as those attained in a nuclear fusion reactor, the fuel is turned into plasma. Since plasma has some properties of an ionised gas and therefore has electrical conductivity properties, electromagnetic fields can be used to contain and direct the plasma. Systems used include:
- Magnetic mirror effects
- Toroidal or curved electromagnetic field line effects
- Electrostatic confinement effects holding ions in a reaction chamber
Solar Hydrogen Fusion Source
Kinkajou : Tell us about the Materials they used to construct fusion reactors.
Erasmus : Fusion reactors require materials that can withstand the hostile environment in the interior of a nuclear fusion reactor. Each atom in a blanket close to the reactor core can expect to be hit one hundred times by neutrons before being replaced. Hydrogen and helium are by-products of high energy neutron collision as atoms become converted. These will cause swelling, blistering or will make the reactor core construction materials brittle as they age in this environment. We still have a long time to go before we understand enough about the properties of various atoms in a neutron flux, such as exists in the centre of a nuclear fusion reactor.
Other parts of the reactor need to be able to withstand substantial heat loads even with the shielding used inside these reactors. The primary issue is the interaction with the plasma. The extreme heat inherent in reactor function makes the building material vaporize and then condense.
The structure is eroded and materials are deposited elsewhere. Tritium if it builds up in the condensate could load the reactor with up to many kilograms of this radioactive material. In the case of an accident, this material would then become a hazard in its own right. The sputtering of reheated condensate could destabilise the reactor as hot gases go where they are not supposed to go.
The consensus is that carbon as graphite while a good material for fusion reactor experiments is a poor material for long term operation as it reacts with and retains tritium from the nuclear fusion reaction. Tungsten is more stable and less reactive with tritium than is graphite carbon. Tungsten impurities in plasma are more hazardous than are carbon impurities. Tungsten however, can also create some issues with melting, electrical eddy currents and some radiological issues.
Erasmus : One can choose either
a low-Z material, typified by graphite, although for some purposes beryllium might be chosen, or a high-Z material, usually tungsten with molybdenum as a second choice.
Use of liquid metals (lithium, gallium, tin) has also been proposed, e.g., by injection of 1–5 mm thick streams flowing at 10 m/s on solid substrates.
Erasmus : A final consideration is the design for extracting electrical energy from the nuclear fusion reactor. Proposals include:
- Conventional steam turbines using the heat deposited by neutrons,
- Direct conversion of the energy of the charged particles into electricity. These are of little value with a Deuterium-Tritium fuel cycle, where 80% of the power is in the neutrons, but are indispensable with a-neutronic fusion, where less than 1% of the power is associated with energy bound up in the production of neutrons
- Charged particles flowing along electric field lines are guided to convert the random energy of the fusion products into directed motion. The particles are then collected on electrodes with very large voltages, creating current.
The Future: Fusion Power
Kinkajou : What about the future?
Erasmus : I think we stuck on the technology that exists today. We have to think to the future. The ability to generate gravity fields will likely revolutionise industrial processes of fission and fusion. Fusion particularly is likely to become substantially more practicable for commercial or industrial use generation of energy.
The problem with the use of electromagnetic fields to hold reaction materials at the focal point of a fusion reaction is that non-charged particles or incorrectly charged particles (for example from decay isotopes), will move in ways that the reactor design finds difficult to cope with, potentially leading to escape reactor fuel and reactor meltdown.
Using gravity field will allow charged and uncharged particles of all types to be controlled and concentrated within the fusion combustion zone of the reactor. This in short I think a new technology may well change how we do nuclear power generation and what we do it with.
It will be interesting to note whether fusion reactors can be designed to a micro level. This may make them appropriate for use in space ships or in military weapon systems. These applications cost is not a primary consideration.
Kinkajou : So what have you learned Goo?
Goo : I don’t think antimatter has much place in the future except as a military application or emergency grade propulsion system.
Fusion power with new technologies such as gravity-based containment mechanisms may well approach mainstream applicability for commercial and industrial uses. How the military uses this power source remains to be seen. And how spaceships are adapted to this power source also remains to be seen.
The biggest problem with escaping from the Earth is the need to generate energy quickly. While fusion reactors may well be able to generate energy at a rate that allows escape from the Earth’s gravity well, if you can generate gravity for the fusion reactor, it may be better to use the gravity directly as a drive system.(Yield of the processes is the issue).
Is interesting to note that fission power is not a mature technology. The consideration of thorium as a fuel opens up substantial options for acceptability the nuclear fission technologies in the world. Not quite clean and not green but hell of a lot safer and better than uranium 235 fission.
There are however many competing and quite ordinary technologies available. Solar power with an improvement in the yield harvested from photoreceptors becomes a very different animal to the solar power in existence today. Wind power and geothermal energy may well be practical contenders for some power generating systems of the future.
Coal Mining for Energy