Nuclear reactors and hydrogen fuel cells
"The Generation of Electricity" is one of the most prominent expression with which could describe our society with. The quantity of electricity consumed nowadays is immense. Traditional methods of producing electrical power would have long failed to satisfy all of humanity's needs, technical evolution would have ceased eventually and several of the current items used every day would not have been invented, and the number of today's technological miracles would be significantly less.
Several things can be deduced from the above mentioned phrase, namely that this generation greatly relies upon the constant flow of electricity. We would find ourselves in great dilemma if this flow of electrical power would cease.
We can ask ourselves the question: With the growing need for electricity, how can we still satisfy our "apetite"? It may sound awkward, but the answer lies in the nuclear strikes on Hiroshima and Nagasaki. Robert Oppenheimer, Werner Heisenberg, Wolfgang Pauli, Edward Teller, Enrico Fermi, Pascual Jordan and Paul Dirac were all genious physicists who were members of the Manhattan Project and created the A-Bomb. Their research on fission and fusion reactors made the construction of the nuclear reactor and consequently of the nuclear power plant possible.
Thermal and fast reactors
There are many variations for the design and utilized components of a nuclear reactor in which the fission process is used to release heat, but all the reactors have certain features in common. There are two main types of reactors, fast and thermal reactors.
Fast reactors use fast neutrons to sustain the fission chain reaction. The neutrons released during this process have very high kinetic energies and thus travel with very high speeds. They are characterized by an absence of moderating material. Initiating the chain reaction requires enriched uranium or enrichment with plutonium 239. Fast reactors have the potential to produce less transuranic waste because all actinides are fissionable with fast neutrons, but they are more difficult to build and more expensive to operate. All in all, fast reactors are less commonly used than thermal reactors. Fast reactors were especially used in earlier power stations of an earlier design as well as some Russian ships. Construction of prototypes still continues, Generation IV reactors for instance (see them later on).
Thermal reactors use slow neutrons, (slow neutrons generally are referred to as thermal neutrons) and the majority of reactors are of this type. Neutron moderator materials that slow neutrons are general characteristics of these reactors. Slowing them down to reach the kinetic energies of the surrounding particles is necessary for the conduct of the reaction. Thermal neutrons are capableof fissioning uranium-235 at much higher efficiency. Thermal reactors need to have in order to function fuel, containing systems, shielding, and other necessary monitoring and safety devices for human operators to view.
The reactor consists of a core in which the fission chain is sustained and in which the energy generated by it is released as heat. The core contains nuclear fuel, which are usually long, thin cylindrical rods which are called fuel elements. In most cases fuel consists of a fissile nuclei which is U-235 together with a large amount of U-238. In the U.S.A. the fuel usually contains about 3% of U-235, fast reactors, on the other hand require at least 15% fissile material. The fissile species in a fuel can be in the form of metal, but an oxide or carbide is often used because it can withstand much higher temperatures without melting.
Moderators on Thermal Reactors
In the thermal reactor the core contains a material known as a moderator which decreases the speed of fast neutrons that are liberated in the fission process. In most cases this is light water and some added coolant because the hydrogen nuclei are effective in slowing neutrons down. In gas-cooled reactors graphite is the moderator. Fast reactors try to avoid moderators as far as possible, but some added oxides still contain them.
The core is surrounded by a neutron reflector which depends on the type of the reactor. The purpose of this reflector is to decrease the loss of neutrons which have escaped from the core by returning many of these. It contributes to the decreasing of the critical mass and an improved distribution of fission, this way improving the heat generation rate.
In thermal reactors, the reflector is usually of the same material as the moderator. For example a water moderated reactor would have a water coat as a reflector. In fast reactors, where the reflector is called a blanket, there is usually a chemical similar to the fuel (e.g. uranium-oxide), but the uranium is not enriched in fissile material.
In order to prevent the meltdown of a reactor, a suitable coolant must circulate around the reactor which may either be a liquid or gas. Water moderated reactors use water as a coolant, and graphite moderated ones gas which can be carbon dioxide, nitrogen or helium. Helium is the most favored coolant due to its low tendency to absorb neutrons and become radioactive. Some exceptions exist, for example the N-Reactor near Washington which is moderated by graphite and uses water as coolant. Liquid coolants include different oils, liquid fusible alloy, different liquid metals and molten salts.
An ideal coolant has high thermal capacity, low viscosity, is cheap, and is chemically inert thus posing no danger to the cooling system. Some applications also require the coolant to be an electrical insulator. The coolant can either keep its phase, stay liquid or gaseous, or can undergo a phase change, with the latent heat adding to the cooling efficiency. The latter, when used to achieve low temperatures, is more commonly known as refrigerant.
Shielding and Containment
Reactors can be dangerous toys as such, so they must be contained. For this purpose has biological shielding been developed. It attenuates neutrons and gamma rays escaping from the reactor so that they cannot threaten life in vicinity. In addition to this biological shield a thermal shield is often present.
The thermal shield is within the reactor vessel and is fairly close to the core. A few inches of iron or steel with the role of absorption is most likely to constitute the shield and it protects the biological shield from overheating and damage.
The biological shield is generally a layer of concrete several feet thick which surrounds the vessel. In order to be prepared for the damage of the reactor and to prevent radioactive material from spreading the entire nuclear steam supply system has to be enclosed in a vessel of its own. This vessel is made of reinforced concrete with a steel skeleton. It is designed for a worst-case scenario so it can withstand even the most serious accidents which could occur to a reactor.
There are several more features of reactors, they can be classified numerous ways and have even more other components, features which we did not want to mention, since an entire encyclopedia could be written on this toPic. So at this point you may ask yourself: "How is future involved in these 30 year-old facts?". And it is a just question indeed. The answer lies in experimental reactors, which await development.
Experimental and advanced reactors and reactors in concept stages
You may have noticed that all we have talked about to this point are fission-based reactors. Well, let's have some further discussion on them! Current technologies are the:
- PWR (Pressurized Water Reactor)
- BWR (Boiling Water Reactor)
- PHWR (Pressurized Heavy Water Reactor, also known as CANDU)
- RBMK (Reaktor Bolshoy Moschnosty Kanalniy, alias High Power Channel Reactor)
- GCR (Gas Cooled Reactor)
- LMFBR (Liquid Metal Fast Breeder Reactor).
Several advanced reactor designs are in various stages of development, some have their roots in the PWR, BWR and PHWR reactors, but quite a few are radical designs. The former are the:
- ABWR (Advanced Boiling Water Reactor of which two are already operational )
- the planned ESBWR (Economic Simplified Boiling Water Reactor which has a Gravity Driven Cooling System)
- AP1000 (a British design of which 6 have been already ordered in 2007)
The latter ones are the:
- IFR (Integral Fast Reactor, which could produce only a fracture of the waste of current reactors)
- the Pebble Bed Reactor, which is a HTGCR (High Temperature Gas Cooled Reactor). This design uses ceramic fuels and inert helium as the cooler. It is a much safer version because of the above mentioned and that it has up to seven layers (a normal reactor has 3)
Another unique feature to aid safety is that fuel balls form the core's mechanism and can be replaced one by one as they age, however they are very expensive.
- SSTAR (Small, Sealed, Transportable, Autonomous Reactor) could be remotely shot down it necessary
- CAESAR (Clean And Environmentally Safe Advanced Reactor), which is still a concept, would use steam as a moderator
- Subcritical reactors are designed to be safer and more stable, but researchers are still fighting with economical and technical difficulties. An energy amplifier is an example for this which uses an energetic particle beam to stimulate a reaction which would produce enough energy to power the particle accelerator which would generate huge profits of energy
- Thorium based reactors. It is possible to create U-233 from Thorium-232, so this type is essentially a breeder reactor (a reactor that produces nuclear fuel)
- AHWR (Advanced Heavy Water Reactor) still under development
- KAMINI, which uses U-233 for fuel
There is a categorization, however, which is worth to be mentioned, namely the classification according to the "generation". All of the above mentioned reactors are either Generation II or III reactors, Generation I reactors having been retired some time ago. The real beauty of the reactor toPic, however lies in Generation IV and V+ reactors.
Generation IV reactors are a nuclear reactor designs, some only exist in theory, currently under developement. There is not much hope of seeing one of these reactors in public use until 2030. Research into these reactor types was officially started by the Generation IV International Forum (GIF) based on eight technology goals. The primary goals being to improve nuclear safety, improve proliferation resistance, minimize waste and natural resource utilization, and to decrease the cost to build and run such plants.
There are several designs here, namely the
- Gas cooled fast rectors feature a fast-neutron spectrum and closed fuel cycle for efficient conversion of fertile uranium. Several fuel forms are being considered for their potential to operate at very high temperatures and the tendency to retain fission products: composite ceramic fuel, advanced fuel particles, or ceramic clad elements of actinide compounds
- Lead cooled fast reactors that include a fast neutron spectrum, molten lead or lead-bismuth eutectic coolant, and a closed fuel cycle
- Molten salt reactor, where the primary coolant is molten salt. There have been many designs put forward for use of this type of reactor as a nuclear power plant and a few prototypes built
- The sodium-cooled fast reactor was created with the objective of producing a fast-spectrum, sodium-cooled reactor and a closed fuel cycle for efficient management of actinides and conversion of fertile uranium
- Supercritical water-reactor is a concept that uses supercritical water as the working fluid.
- The Very high temperature reactors are yet another concept which use a graphite-moderated nuclear reactor with a once-through uranium fuel cycle. This reactor design is planned to have an outlet temperature of 1,000°C
Generation V reactors are designs which are theoretically possible but research is not emphasized at present because of the Generation IV concepts. Though such reactors could be built with current or near term technology, they would stir little interest for reasons of economics, practicality, or safety. The current possibilities are:
- Liquid Core reactor, is a version where the fissile material is molten uranium and its cooling is solved by gas being pumped in through holes in the base of the containment vessel.
- Gas core reactor is a closed loop version of the nuclear light bulb rocket, where the fissile material is gaseous uranium-hexafluoride contained in a fused silica vessel. A working gas (such as hydrogen) would flow around this vessel and absorb the UV light produced by the reaction.
- Gas core EM reactor, which shows similarities with the Gas Core reactor, but a huge difference lies in the photovoltaic arrays which convert the ultraviolet light directly to electricity.
- FFR: Fission-fragment reactor
Generation IV, V reactors hold the key to the future of providing power for mankind in terms of the use of fissile reactions. In the future these will become safer, cheaper to build and more effective. As we have witnessed, there are plans for a "portable reactor" (some submarines and ships are nuclear-driven, but these would be very different). Who knows where we will end up? In 50 years time you might have your own pocket reactor in an organic implant, or even in the keys of a car.
What if there was a solution which would render all fission based reactors useless? What if pollution in current proportions would be put to an end by some sort of device which would sweep aside other power sources? What if you'd have the Sun working for you?
These questions remain unanswered for the time being but the solution holds immense possibilities. And the solution is the Fusion Reactor.
Controlled nuclear fusion could in principle be used in fusion power plants to produce power without the complex procedures of the handling of actinides, which are radioactive elements, but significant scientific and technical obstacles remain. Several fusion reactors have been built, but none of them has generated more thermal energy than electrical energy consumed so far. Despite continuous research dating back to the 1950s, no commercial fusion reactor is expected before 2050. The technical difficulties are immense and as we can see there is no hope to put these reactors to use for a long time. Nevertheless let's examine the concept and essence behind these advanced machines.
What is fusion?
Nuclear fusion is the opposing reaction of nuclear fission, where the nucleus of an atom is split. Fusion is the reaction where two lighter atomic nuclei collide and "fuse" themselves together forming a new nucleus slightly heavier than the sum of the original two's mass. One of the problems scientists are working on currently is the development of a magnetic field powerful enough to contain and compress the plasma. Plasma is gained by brisk heating of liquid hydrogen with a laser beam so that they overcome the forces which push them away and fuse together.
Fusion is encountered by us every day, for instance when we gaze at the night sky. All the stars radiate energy through fusion and become bright in our eyes. These energy radiations are reproducible at a staggering 105-107 K (an extremely high temperature). Fusion does not occur spontaneously on Earth because of the large forces of electrostatic repulsion between nuclei, but proper conditions were found upon detonating an H-Bomb. Immense heat was generated by the fission based bomb, so that the thermonuclear reaction could commence. In hot plasma, where the atoms are in an ionized state, violent clashes between nuclei produce fusion with energy released in accordance with the well-known energy-mass equation ΔE=Δmc2.The resulting fused nuclei absorb this as kinetic energy but lose it when hitting particles of the plasma this way maintaining a very high plasma temperature. In stars like the Sun, hydrogen nuclei fuse creating a chain reaction and forming helium nuclei.
During this reaction the amount of energy produced is:
ΔE=Δmc2=931[4*1.007825-4.002604] MeV =(appr.) 26,7 MeV
This cycle continues with the fusion of helium nuclei which form beryllium (1), then carbon (2) and finally oxygen (3).
There are numerous other mechanisms which are present in stars hotter than the Sun. Hans Bethe came up with the idea of stellar-nucleosynthesis during which the nuclei of heavier elements are built with the help of carbon as a catalyzer.
The real use of fusion on earth is that it would provide us with an endless supply of energy because deuterium can be extracted from water:
If we fuse a mixture of 1 kg of 21H and 31H, the result is 90000MeV energy, which is a huge amount, and if we consider the amount of available deuterium then the energy question is solved.
The current largest research operation, which involves more than 2000 physicists is the Joint European Torus (JET). In 1997, JET produced fusion power of over 10 MW sustained for over 0.5 seconds from 16.1 MW. ITER, their experimental reactor, which was announced in 2005, will be able to generate several times the input power for several minutes. JET has done huge research on magnetic field generators, and have finalized the Tokamak (an originally soviet design) which produces a toroidal magnetic field.
Hydrogen Fuel Cells
Hydrogen cells exist nowadays, but the energy needed to operate it is generated by traditional methods so if fusion power would become an utilizable energy source, then pollution could be further decreased with the help of these fuel cells. All internal combustion engine driven appliances could be refitted with these fuel cells reducing the quantity of emitted gases. Furthermore, not only vehicles, but factories could also be powered with fuel cells diminishing the need to actually burn anything.
Hydrogen is a versatile energy source and can be used to satisfy all needs of power. They are an ideal choice because they use the chemical potential for energy in hydrogen and convert it to electricity and the byproducts are only pure water and heat, which can be used for other useful purposes.
The list of advantages doesn't stop at only anti-pollution and economic reasons, there are efficiency arguments as well. These fuel cells can operate with two to three times the efficiency of traditional combustion technologies, these include even power plants. While a traditional power plant generates electricity at 33 to 35% efficiency, the efficiency of fuel cell systems range up to 60%. This 60% can even increase with the gainful usage of byproducts. In addition to these fuel cells operate quickly and have fewer parts which reduce the possibility of break-downs.
The structure of a hydrogen fuel cell is fairly simple in comparison to other solutions used nowadays. It mainly consists of an electrolyte positioned between two electrodes, an anode and a cathode. There are bipolar plates on each side of the cell and they collect the current and help the distribution of gases. In a Polymer Electrolyte Membrane (PEM) fuel cell, which is considered one of the most promising designs, hydrogen gas flows through channels to the anode, where a catalyst causes the hydrogen molecules to separate into protons and electrons. Only one proton can pass at a time through the membrane. While the protons do their journey, a flow of electrons follow an external circuit. This flow can be used to power different appliances, for example a motor. When these electrons return from their destination, they form water with oxygen and hydrogen protons (oxygen protons are gained from oxygen gas which flows into the cathode). This reaction is exothermic and the generated heat can be used for various purposes.
The power produced by a fuel cell depends on numerous factors, including the fuel cell type, size, temperature at which it operates, and pressure at which gases are supplied. One fuel cell generates less than 1V, an incredibly small amount of power. To increase the amount of electricity generated, fuel cells are combined in series to form a stack. These stacks (by saying "fuel cell" we mainly refer to the entire stack) may contain only a few or even hundreds of individual cells. This form makes fuel cells ideal for a wide variety of applications, from laptop computers (50-100 Watts) to entire homes (1-5kW), vehicles (50-125 kW), and central power generation (1-200 MW or more).