School of Engineering and Technology, Deakin University.
With the recent Fukushima disaster in our collective minds, the topic is both relevant to the study of thermodynamics, and to the role that engineers play in society. Although it has been contended that we shall eventually regard the disaster in Japan as a measure of the inherent safety of nuclear power (Higson, 2011), many sections of society believe otherwise.
Ukraine, still reeling from the devastation caused at the Chernobyl nuclear power plant
25 years ago is testament to these contrary beliefs. During a radioactive fire that burned for 10 days, 190 tons of toxic materials were expelled into the atmosphere. The wind blew 70% of the radioactive material into the neighbouring country of Belarus in which people continue to suffer medically, economically, environmentally and socially from the effects of the disaster to date. The estimated cost of the core meltdown is $200 billion.
Therefore the major issue surrounding nuclear energy is the safety concerns involved in not only the safe operation of the plant but the storage and transport of waste material. With no useful applications for uranium waste product it creates major environmental issues of contamination of food and drink in surrounding areas. Although not solving the issue entirely, the waste product of Thorium can be used in a secondary reactor, therefore minimising waste. Waste management can also be handled by using more efficient forms of enrichment such as the gaseous centrifuge method
2. The nuclear process
Uranium appears as the ninety-second element on the period table, and is a dense metal that although once thought to be rare, is relatively abundant on the earth’s surface. Uranium is found in the oxide form (U3O8), the world’s biggest such deposit of which is found at Olympic Dam in South Australia (Geoscience Australia, 2011). Uranium is generally found in one of three naturally occurring isotopes: U-238 occurring in 99.1% of ores; U-235; and U234, itself formed from the decay of the U-238 isotope. (Australian Uranium). The isotopes are essentially identical, except their atomic masses differ due to the number of protons and neutrons. For example, a U-235 atom has 92 protons and 143 neutrons for an atomic mass of 235 units. Because of the difference in masses between isotopes, it is possible to separate and categorise the different isotopes.
Australia contributes to around 25 percent of the world’s annual uranium production from the country’s three producing mines: Rio Tinto’s Ranger open-cut mine in the Northern Territory; BHP Billiton’s Olympic Dam underground polymetallic mine in South Australia; and Heathgate Resource’s Beverley in-situ leach mine in South Australia (Australian Uranium Association). Uranium One’s Honeymoon mine, another in-situ leach operation in South Australia, will also soon begin production as the
country’s fourth uranium mine (Uranium One, 2011).
Figure 2.1 – World uranium oxide production (ABARES, 2011).
The economic value of Australian uranium oxide production in 2011, estimated to be around 8,700 tonnes of which all of it is exported, accounts for revenue of around $960 million, the value of which is expected to triple by 2016 (Petchey, 2011 p. 167). This is dependent on several more mines that are expected to come on-line in the near future, such as BHP Billiton’s Yeelirri operation and Energy and Metals Australia’s Mulga
Rocks project, both in Western Australia.
Figure 2.2 – Australian uranium oxide production (ABARES, 2011).
2.3 Uranium enrichment
The efficiency of the uranium enrichment process directly contributes to the economical feasibility of a nuclear plant, as it accounts for around 67% of nuclear fuel costs (Pouris,
1986 p. 558). The aim of the uranium enrichment process is to alter the naturally occurring ratio of isotopes, which is 99.3% U-238 and 7% of U-235 so that it is more in favour of U-235. The percentage of U-235 isotope must be around 3% to 5% for the nuclear fission process to occur.
Pouris (1986 p. 559) details six technologies available that are used in the enrichment process: gaseous diffusion; gas centrifuge; separation nozzle system; laser enrichment process; chemical exchange process, and the advanced vortex tube technique. The gas centrifuge technique is the most common method used today (World Nuclear Association, 2011), with the gaseous diffusion process also commercially used. In both methods, uranium metal is converted to a uranium hexafluoride gas (UF6), and after the isotopes are separated, two streams are created. One is a UF6 with a high percentage of U-235, and the other is U-238 that has been depleted of its U-235 isotope. This depleted uranium, or DU, is the main nuclear waste produced, with estimates that there is a global stockpile of around 1.4 million tonnes (Falk & Bodman, 2006 p. 2).
Figure 2.3 – World uranium enrichment capacity and methods (Falk & Bodman, 2006).
2.4 Nuclear fission
The creation of energy during a nuclear reaction is a result of a process called nuclear fission. This is when the atom of a U-235 isotope is split as a result of a collision with a neutron; this results in two new atoms and two or three neutrons. These neutrons can then split other atoms, and once a critical mass is achieved, a chain reaction takes place (Joseph, 2004). This process results in an extraordinary release of energy in the vicinity
of 6.73 x 1010 kJ/kg (Cengal & Boles, 2007 p. 58), and at the same time emitting only
two to six grams of carbon equivalent per kilowatt hour, about the same as wind or solar energy (McDonald, 2002). ). To put this into perspective, the energy content of petrol is around 4.4 x 104 kJ/kg (Pulkrabek, 1997).
The energy released from the fission process is in the form of kinetic energy, gamma rays, and heat. This energy is then used to create steam to power a turbine and generator, and so the rest of the energy generating process is the same as for a conventional gas or coal fired power plant.
3. Thorium – an alternative to uranium
Thorium or thorium-232 is a radioactive metal that has been researched, developed and to a point successfully used for power production for many years (Makhijani, A., Boyd, M. 2011). India currently has 5 thorium reactors in operation and has just decommissioned one in 2010 after 50 years of service. Other countries to test Thorium include; Germany, United Kingdom, Netherlands, Canada and the United states of America, all of which have terminated their research and development (R&D) programs. Thorium itself is not actually fissionable but is used to breed uranium and other fissionable isotopes needed for a nuclear chain reaction to start and to take place. Scientist have estimated that there is more potential energy (exergy) in the world’s supply of thorium than that of Uranium, coal, natural gas and crude oil combined, but this has not been confirmed.
Thorium has a longer half-life than uranium and as such there is more present in the earth’s crust. There is approximately three times as much thorium as uranium in the world. It is also found in higher concentrations (2-10%) by weight for thorium, compared to (0.1-1%) by weight for uranium which makes it a lot easier to mine and refine. Thorium is also more efficient than uranium as the waste products from the initial reaction (intermediate materials) can be used as fuel in a second reactor. At the same operating output there would need to be 50 primary reactors to every secondary reactor, not ideal but a brilliant way to minimise waste. Radiation levels for an active thorium fuelled reaction are much lower than that for an active uranium fuelled reaction therefore, they can be used for much smaller independent units for towns, cities or some people have theorised individual houses could be fuelled by an independent reactor. An approach such as this to energy distribution would minimise transmission losses to close to zero.
Thorium doesn’t contain any fissile isotopes and therefore cannot start or sustain a nuclear chain reaction independently. Most applications use uranium or plutonium (uranium-235 or plutonium-239) to kick-start the reaction (usually 20% enriched uranium) and a small amount to continue and control the reaction. Thorium also requires energy intensive pre-treatment and refinement which increases cost and produces waste before it is even ready to be used. A 1997 International Scientific Convention on nuclear fuels concluded that the major reasons why thorium had not been more widely used for energy production was that the natural thorium ore contained no fissile isotopes and was not nearly as easy to weaponise as Uranium. A reason unrelated to power production but a global pressure never-the-less.
Thorium offers some great advantages in energy density, radiation and intermediate fission products but is inferior to uranium in the refinement and start-up phase. Thorium is more expensive and refinement and pre-treatment issues will need to be addressed before wide scale use begins.
Thorium generates 3.6 billion kWh of heat per ton (MacKay, D 2010), assuming a 40% efficient power station that is 1.44 billion kWh of electricity per ton. Assuming an energy density of coal of 24 MJ/kg (Fisher, J 2003) and a coal power station efficiency of 30% it can be found that coal produces 2.0kWh/kg which is equal to 2000 kWh per
ton. Uranium generates 24 million kWh of heat per ton , assuming 40% efficiency,
same as thorium, 9.6 million kWh of electricity per ton.
Figure 3.1 – Power outputs of each fuel source (power leaving the power station).
4. Energy analysis of power generation
The energy analysis was conducted on a nuclear powered water-steam Rankin cycle system. The analysis determined an overall efficiency of 23.76% which is common for a power plant in countries such as China and Russia although the world leader in nuclear power, Germany, has achieved efficiencies of 40% (Shilling, H., 2005). The figures for temperature and pressure, and the design of the power station were obtained from
Ishiyama, S., Muto, Y., Kato, Y., Nishio, S., Hayashi, T., Nomoto, Y. 2008.
Figure 4.1: Schematic diagram of the power generation system that was analysed.
5. Nuclear waste and the environment
The use of radioactive materials in a nuclear reactor is essentially to heat the water used in the heat engine of the nuclear power plant. The basic thermodynamic principles involved in the function of this heat engine are the same as that of any other power plant utilising steam to do work on the turbine, which in turn does work on the generator to produce electricity.
The advantage of a fission reaction from the nuclear fuel source is that far less fuel mass is required in order to generate the heat required to turn water into the steam used in the thermodynamic cycle to ultimately produce usable electricity. As an example, a typical family consisting of four people would consume approximately 4.5 ft3 per year of coal from a coal burning power plant (Ragheb, 2011) where as if they were using nuclear fuel, this amount would be as little as only 0.0021 ft3 per year. Clearly an enormous difference when you consider this saving multiplied by the millions of families occupying large cities.
Waste from a nuclear reactor in the form of spent nuclear fuel, verses used fossil fuels, has the distinct advantage in that it is able to be recycled and reused. According to the World Nuclear Association [WNA], typical nuclear waste fuel is comprised of approximately 94% of the non fissile uranium isotope U-238, but it also comprises 6% of fissionable products such as U-235 (almost 1%), plutonium (almost 1%) and other fissionable products for the remaining 4%. These remaining materials are highly radioactive and are able to be reprocessed to produce new nuclear fuel.
The difference in the amount of waste materials produced annually from spent nuclear fuel to the waste of carbon emitted from fossil fuels is staggering. In the years 2000 to
2009, 7.7 billion metric tonnes of carbon emissions was due to fossil fuels (co2.now.org). Compare this with the waste produced from its nuclear counterpart. The worldwide use of nuclear power stations only produce approximately 200,000 m3 of low and intermediate level radioactive waste, and about 10,000 m3 of high level waste. Then consider that components of this waste material will be able to be recycled
into further nuclear fuel capable materials (WNA).
All forms of toxic wastes need to be stored, transported, handled, used and disposed of safely to protect people and the environment. Nuclear materials and radioactive wastes need not be treated any differently. Annually, approximately 20 million consignments of radioactive materials are transported on railways, public roads and ships worldwide and although there have been accidents over the years; highly radioactive material has never escaped from its storage vessel (WNA). With approximately 430 nuclear power stations operating worldwide in as many as 32 different countries (WNA), the safe transport and handling of this material is crucial. The following table shows the main breakdown of nuclear fuel cycle materials used and their corresponding transportation.
Figure 5.1- (WNA, 2011).
With the different types of transport methods that are available to transport radioactive materials, it is important that the packages or containers used are designed in such a way that they will not be susceptible to the rigors of harsh environments or weather experienced during travel which could compromise the integrity of the transport container, ultimately risking safety to both people and the environment.
Although I could not find any direct documentation to support this, an idea that perhaps the storage facility for the nuclear waste could be on site at the nuclear reactor which would help to eliminate many safety issues surrounding the safe transportation and handling of the spent nuclear fuel? The same could be said for the facilities used to refine and manufacture the nuclear materials. A more in-depth analysis based solely on the production and transportation of nuclear materials and the distance they travel in order to be used could be done for further knowledge of this subject. Mining of uranium is only viable in a few areas (WNA) so clearly geographical location is one of the issues.
6. Nuclear safety
There is always a question about the reliability and safety on nuclear power. There are
435 nuclear power plants in 30 countries around the world. There are also thirty new nuclear power plants currently under construction (Nuclear, 2011). So why is it so important for nuclear power plants to be reliable and safe?
Although there has only been three major nuclear power plant incidents in the history of nuclear power history(Association, 2011). If there is a plant failure or melt down the possible outcome could create a major disaster affecting human/animal life and the surrounding environment for many years to come. In this section I will discuss some of the safety features currently available or in development. These safety devices are in place to try and prevent nuclear radiation either through the plants design or through a safety system. Note that this is not all the safety features of a nuclear power plant but just a select few.
6.1 Safety features
In Coy’s paper he writes about some of the safety features currently in use and in development. The HRT-PM Pebble Bed system which has been under development for decades and is the closest of the newest technologies to commercial electricity generation. A Pebble bed is uranium fuel encased in more than 300,000 tennis ball sized pebble. The fuel is graphite coated fuel seed, the radioactive fission products are absorbed in the pebbles coating. This keeps the operating temperature below the level where the pebbles begin to degrade and prevents them from getting hot enough to melt down even if the plant losses cooling for days. The drawback of this technology at the moment is that above 600 megawatts they loss there safety advantage over standard reactors because the peddles can get so hot that they cannot shed the heat fast enough and can go into melt down like ordinary fuel rods(Coy, 2011).
The Westinghouse AP1000 is a new style of reactor still based of the fuel rod system is currently under construction in China, India and possibly soon the U.S.A. This plant uses a passive safety system meaning that is requires no active intervention for it to work. The plant has a huge emergency water reservoir above the reactor that is held
back by valves. If the cooling system fails for some reason the values open and gravity takes over and the water pours down to cool the outside of the containment vessel. This process is repeated by when the water heats up and becomes steam it rises back up the vessel and cools to become water droplets again to fall back down and continuo the cooling process. This passive safety system contains enough water to cool the vessel for three days after which water will need to be pumped from an onsite lake. The first of these plants is meant to be online by in 2013(Coy, 2011).
The ESBWR boiling water reactor developed by GE Hitachi uses passive safety systems similar to the Westinghouse AP1000 and has a possible power output of 1500 MW. This was certified this year but as of yet no orders have been places for this reactor(Coy, 2011).
The mPower pressurized water developed by Babcock and Wilcox, Bechtel has a lower power output of 125 MW and is expect to be deployed in 2020. It can be installed as one or in modules within built passive safety systems and below ground containment storage this system is based off the latest nuclear submarine reactor(Coy, 2011)s.
Liquid fluoride thorium reactors are being developed by the U.S, France and India it expected to have power outputs of around 1000MW and the earliest deployment date of 2025. The benefit of thorium is that it produces almost no plutonium as a by product and a reactor would shut down safely even if there was a total loss of coolant(Coy,2011).
This is just a short list of some of the safety feature available and in development for nuclear power plants. There is a continuous efforts being made to continue to improve the safety of nuclear power and to avoid the terrible incident like Chernobyl, 3 Mile Island and Fukushima.
The majority of current nuclear plants attain their fuel sources from uranium. By enriching the uranium usually through the gaseous diffusion method the vapour UF6 is obtained so that nuclear fission can take place with the energy acquired used to create steam to power a turbine and generator. The basic thermodynamic principles involved in the function of this heat engine are the same as that of any other power plant utilising steam to do work.
Thorium has also been used as a fuel source for nuclear plants. With a higher abundance, concentration and potential energy compared to uranium it has the potential to replace uranium in as the preferred fuel source. Through our calculations it can be contented that thorium has a far greater power output then either coal or uranium with
3.6 billion kW.h of heat per ton. However thorium cannot start or sustain a nuclear chain reaction which inhibits its total overhaul of uranium. It also requires energy intensive pre-treatment and refinements which creates waste product before it is even ready to use.
The major drawback from nuclear power is its environmental impact. All forms of toxic wastes need to be stored, transported, handled, used and disposed of safely to protect people and the environment. A possible way to decrease the risks associated with toxic waste is to provide storage facilities on the site of the nuclear reactor. Measures have been taken to prevent the spread of nuclear radiation, whether it is through plant design or safety systems such as massive emergency water reserves in place for failure of cooling systems.
If the threat of nuclear radiation and contamination can be contained then it seems nuclear power will be the way of future power generation. This is further backed up with depleting fossil fuels and the media concern over coal fire power plant carbon emissions and the massive abundance of energy available from enriched uranium and thorium.
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