Institute for Solid-State Nuclear Physics
Institut für Festkörper-Kernphysik
Human economy effectuates large-scale use and refactoring of landscape and natural habitats. The higher the efficiency of the economy the lower the impact on the environment. The highest impact comes from agriculture, followed by the energy industry. They both experienced a remarkable efficiency gain in the past 200 years. However, production and use of energy for electricity, transportation, heating/cooling, industrial, and commercial purposes have varying degrees of impact on our environment, and in recent years, the relationship between these energy sources and their impact on the environment has taken center stage. Determining the net cost-benefit of the various energy sources, and in particular their impact on the environment, is a very complex puzzle. One part of that puzzle is how the production and use of the energy sources affects the earth's climates and those that live in those climates.
In assessing the effect on the environment of any particular source of energy one needs to consider the entire process of extracting, converting and housing the particular energy source.
Most sources of energy we use on Earth are produced by the Sun’s energy, whether it is oil, natural gas, coal, wood, hydropower, wind, photovoltaics and solarthermal. Nuclear, geothermal, and tide are not based on the Sun’s energy. The following is a very brief summary of which types of energy sources provide us with the most amount of energy for the least amount of negative impact on the environment, i.e. which one has the highest efficiency. The various energy sources, examined below, are categorized as fossil fuels, so-called "renewable" energies and nuclear energy.
All fossil fuels (oil, natural gas, coal) when converted to electricity, heat, or gasoline, produce the so called “greenhouse gases”, which is mainly carbon dioxide (CO2). It is a wide-spread assumption that the accumulation of greenhouse gases in the atmosphere may contribute to global warming and climate change. Extracting these fuels, through mining or other methods, produces more greenhouse gases in addition to other pollutants and damage to the landscape. Finally, building facilities to convert fossil fuels to electricity or other forms of energy also produces greenhouse gases and other pollutants. So, greenhouse gases and other pollutants are emitted when fossil fuels are extracted, converted, and when the facilities for these activities are built. It is estimated that by the end of 2009 the emission of greenhouse gases is about 50 billion tons of carbon dioxide equivalent .
Coal extraction and consumption not has the largest CO2 footprint but also when burned, coal emits Radon and Uranium, and in some cases Thorium into the atmosphere. These radioactive materials do exist naturally in the soil that contains the coal.
The mining of fossil fuels, especially coal, takes the overwhelming share of mining activities, diminishing mineral mining (including uranium) with ensuing destruction of natural habitats.
Fossile fuels were built up by the Sun over millions of years and are now being consumed within hundreds to thousands of years, so the consumption speed is a factor of 10,000 faster than production. That's why fossil fuels can not be considered as a sustainable energy source on a long-term scale.
Energy or resources consumed by a power plant are never renewable but this term has been established for the use of energy from naturally running power sources like the Sun, Earth's internal heat (geothermal) or energy from orbs constellations (tide). This energy source must not be consumed faster than its (re)generation at any time. "Renewables" means essentially photovoltaics, concentrated solar power CSP (both driven directly by the Sun's radiation), wind and hydro energy (driven indirectly by the Sun).
Energy must be converted to electricity or mechanical energy in order to be useful. It is a physical fact that this conversion is more efficient with higher energy density. The energy produced by the Sun in one second is enough to last us on Earth for about one million years based on world energy consumption in 2010. This energy, however, is radiated almost uniformly over the entire surface of the Sun, so most of the energy is lost into space. Only 0.00000005% of the Sun's energy reach the Earth, resulting in a constant irradiation of 1,400 Watt per square meter (W/m?) at the upper atmosphere. During the night the solar radiation is zero, so the average value is half of it. This is further reduced by another factor of 2 when the light travels through the atmosphere, and by another factor of 2 in areas with a latitude of middle Europe or the USA. At these latitudes the usable average Sun energy is only 200 W/m?.
This energy density is very low for technologies that convert it to electricity. However, it becomes useful when natural processes concentrate the energy. This happens when the Sun drives water on mountains where it flows down in rivers, or when the Sun produces regions of high atmospheric pressure resulting in wind. Therefore, from the "renewables", hydro and wind power have the highest ERoEIs (see above). Hydroenergy has the additional advantage that day/night and other fluctuations are essentially balanced, so it can provide a stable and continuous energy supply. Wind energy is extremely unstable so that huge storage facilities are needed. This undermines the advantage of a natural concentration of the solar energy in wind.
Photovoltaics is the attempt to convert the Sun's radiation energy into electrical energy without concentration. The missing concentration makes this technology extremely ineffective and brings it to the bottom of the ERoEI list. A large-scale use of photovoltaics is only possible with Silicon-based technology. For other technologies like CIS there are not even sufficient materials on Earth to use it on a large scale. The efficiency of installed photovoltaics including shading effects and conversion losses is 10%, so only 20 W/m? is the actual electricity output, in southern countries up to 40 W/m?.
Concentrated solar power (CSP) is a technical solution where the power density is increased before converted into electricity in order to gain higher efficiency. Since there is a non-linear dependence on the solar irradiation these plants can only be operated in desert contries with a useful electricity output. The ERoEI can be remarkably high but this is also reduced by needed day/night storage facilities and the requirement of long-distance power transportation.
Biomass is another application. Here, biological processes are used to concentrate the solar power. Since the combined efficiency for photosynthesis which stores the energy in the plants and the electricity generation is barely 1% the area consumption is the largest of all "renewable" energies. Even though no storage is needed the ERoEI is in the same region as photovoltaics.
The most important character of an energy source is not the total amount of energy it contains but its energy density, that is, the energy/unit volume, or energy/unit mass, or energy/unit area. Low energy density has a twofold disadvantage. It consumes large areas (which maximizes the footprint and impact on the environment) and it produces the electricity very inefficiently. The main difficulty with renewable energies is that they are of very low energy density, except for hydropower. To demonstrate this point, let us consider the Bluewater Power Station in Perth, Australia. This power station uses coal to produce about 460 MW of electricity. It consumes about 3.2 million tons of coal every year. The footprint of the Bluewater power station is 145 hectares or 1.5 million m?, which means that this power station has power density of about 300 W/m? compared to 20 W/m? for photovoltaics.
The production of an appreciable amount energy using renewable sources is hampered by the low energy density and fluctuations during the day and the seasons and due to weather conditions. Overcoming these hurdles leads to some environmental problems. The low density requires the use of hundreds or even thousand of solar panels or wind turbines occupying huge amount of land. Solar panels need to be kept clean and rotated in order to follow the Sun to collect the maximum amount of the Sunlight and that requires the use of motors that consume part of the energy generated. Rare earth metals are used in manufacturing the solar panels and electric generators for the wind turbines. Rare earth metals are available only from a small number of countries (mainly China) and that will lead to a monopoly that is not unlike the current oil monopoly. Solving the fluctuation problem requires the use of storage devices like electric batteries. Once again, very large number of batteries needs to be used for any reasonable scale solar or wind turbine stations. Eventually those large numbers of solar panels, motors, wind turbines, and batteries need to be decommissioned and buried somewhere. Currently, the manufacturing of solar panels, motors, wind turbines, and batteries uses energy produced by fossil fuels. Finally, manufacturing hundreds or thousands of solar panels, motors, wind turbines, and batteries makes the ratio “energy returned/energy invested” (ERoEI) quite low compared to fossil fuels.
It is unfortunate that all “renewable energy” sources like solar, wind, and tide are low energy density sources and cannot replace the much higher energy density of fossil fuels. Industry and heavily populated areas demand very large and steady supply of energy. Such demand can only be met by high-density energy sources. It is unfortunate because renewable energy sources cause less emissions than fossil fuels. Renewable sources like solar and wind energy are not completely clean and they do have negative impact on the environment due to the large area consumption. However, they might be useful in niche applications. Actually, hydropower is the only "renewable" power source which is roughly as economic as fossil energy sources.
If we stop using fossil fuels in the future, we need to replace them by another high energy density source. Furthermore, oil is a valuable source for petrochemicals or other materials and it would be wise not to burn it for energy.
Any piece of matter is equivalent to energy according to Albert Einstein’s famous formula E = mc2 where m is the mass to be converted, E is the energy produced, and c is the speed of light. Theoretically, the current worlds primary energy demand can be met by converting about 6,000 kg of matter to energy every year. Unfortunately, so far, no simple straightforward method of converting matter completely to energy has been found.
Current technology allows only the conversion of a small fraction of the mass into energy. For conventional fuels the mass change is tiny after the chemical energy is released. For instance, black coal has a mass change of 0.00000003% after burned. This gives an idea of the fraction of the energy density we are actually using and how much is left as "waste". Contrary, nuclear fission of uranium results in a mass change of 0.1%. This still seems to be still low but it is 3 million times higher than the energy density of black coal. However, this is the maximum burn-up possible with nuclear reactors. Current reactor types burn barely 1% of the mined uranium, so only 0.001% of the mass is converted to energy, therefore the actual ratio to black coal is 30,000. For nuclear fusion the converted mass is already a few percent but the technology to use it is not yet available. To further demonstrate this ratio let us assume a power station of 1,000 MW electrical output. A nuclear power station of this size needs about 100 tons of Uranium every year while a coal fired power station need about 2 million tons every year.
Generation IV fast reactors comes close to the theoretical limit of 3 million times higher energy density than coal while current nuclear technology has "only" a factor of 20,000 higher energy density. The higher energy density of nuclear fuel means an extremely low impact on the environment. Compared with coal and similar fuels this means that
the land consumption by mining for those resources is reduced by this factor,
the fuel transportation effort and costs which are quite high for fossil fuels are reduced by this factor,
the area consumption by the power plants is much smaller related to the energy output, and
the "death toll" which is fairly high for the coal industry is remarkably reduced.
Since the total amount of nuclear fuel needed for the same electricity output is so small, enrichment as well as recycling of nuclear fuel, even though often discussed as "pollutant", is still extremely small compared with the impact of fossil fuels.
It is often claimed that "nuclear waste" is the main problem of nuclear energy but what are the actual numbers? The nuclear energy is released when the heavy nucleus is split into lighter nuclei. These nuclei are called “fission products” and they can not be used for further energy production. Some of these nuclei are radioactive for a long time and must be treated as waste. Another part of the radioactive waste is coming from heavy nuclei that have been activated by capturing neutrons in the reactor. These so-called actinides can be used in Generation IV reactors since they still carry 99% of the nuclear energy that has not been released. The DFR is an advanced and more economical concept.
A 1,000 MW power plant can supply 1 million people with electricity (including embodied energy, not only the electricity on the personal bill). From the 100 tons natural uranium per year needed 1 ton has been burned in current reactors and therefore ends up in fission products. Per person this is 1 gram per year. For a lifelong nuclear electricity supply this is only 80 grams waste per person. Most of these fission products decay in a few ten to 100 years and only 10% require long-term storage of about 100,000 years, so the actual long-lived fission product waste (LLFP) is only 8 grams per person-life. Since these LLFPs decay slowly they have a very low radiotoxicity.
There is no common concept how to treat this waste. Some countries store the used fuel elements as they come from reactors in unused mines, others separate the fission products (80 grams per person-life from which 8 grams are long-lived) and encapsulate them in molten glass (coquilles) for final storage. No final storage facility is in use yet but even from the intermediate storage facilities the chance of any of this waste escaping into the environment is extremely small. It has not happened in all the decades since the beginning of commercial reactor operations in the 1950s. Many radioactive isotopes are useful in medicine and industry. Unfortunately, there are currently very few processing facilities and there are no plans to build more.
During operation a nuclear power plant does not directly emit greenhouse gases into the atmosphere. Some emissions from services around a nuclear power plant are negligible but for the construction of the plant a lot of concrete is needed which has a considerable CO2 balance. There is no doubt that the total greenhouse gas emissions are negligible compared with fossil fuel based plants but how does it compare with the "renewable" energies?
Taking into account the total lifecyle, a typical wind park like in Dornstedt, Germany, with a power output of 17 GWh per year and a lifetime of 20 years requires 7,200 tons of concrete. A typical reactor like the AP-1000 needs 20 times as much concrete but produces 450 times as much electricity per year. Moreover, the lifetime is at least 40 years (probably much larger), so the concrete consumption related to the electricity output is a factor of 40 smaller.
Again, the higher energy density is the dominating factor. For non-fossil power plants, concrete is the main greenhouse gas driver and it can be concluded that even wind power has a factor 40 larger carbon footprint than nuclear power. Similar conclusions can be drawn for other resources and other energy sources like solar energy.
The main advantages of the DFR are its compactness, low complexity and cost structure wich is reflected in the superb ERoEI. These advantages are achieved by a high power density, profound simplifications made possible by the utilization of basic physical control loops for reactor regulation, and abandonment of a fuel cycle industry. The net ERoEI gain is a factor of 10-20 to contemporary water cooled reactors and another factor of 2 to SFR's
The complete burnup of the fissionable material diminishes the fuel costs of the DFR to insignificance. This means that for the same uranium or thorium mining effort one obtains 100 times the energy. However, the minimg costs are already a small fraction in the cost structure of nuclear power plants. As long as the high grade ore mines last thermal reactors are in better competition to the generation IV breeder reactors.
The reduced size and effort to build and operate the DFR compared with the energy output further reduces the environmental impact. Only a few plants are necessary to provide electricity for an entire country and the plants are almost invisible. The uranium and thorium mining is completely negligible, in fact, stockpiled actinide resources in countries that have already used nuclear power for decades are sufficient for several centuries of operation. Additionally, the high neutron excess of the DFR can be used for transmutation. This means that even the long-lived fission products can be at least partially deactivated which reduces the radioactive waste problem remarkably.
In vehicles, there is currently no alternative to chemical fuels. Electric cars do not provide a solution for long-distance transportation. The main reasons are the low efficiency, the low energy density, and short lifetime of batteries, not to mention logistic problems emerging from long recharging times. Contrary to what is promoted by some companies and politicians, there is no solution of these problems coming into view. Moreover, batteries need large amounts of lithium which has a high environmental impact when extracted.
The high temperature of the DFR makes it possible to replace fossil fuels used for vehicles with synthetic ones for costs at the low end of fossil fuels. Moreover, these fuels can be produced totally CO2 neutral and do not consume any further resources. Not even the vehicles have to be replaced which is a very economic and environmental-friendly solution. Furthermore, the fuels do not need to be shipped around the world since they can be produced locally. This makes big oil spill catastrophes part of the history.
 The Sun's total power is 400 trillion TW (Terawatt). However, this power is emitted over the Sun's surface of 6,000,000 trillion m?, so that the radiation density is 70 MW/m?. Since the Earth is 150 million km away this number is reduced by factor of 50,000 which results in an 1400 W/m? at the upper atmosphere, the so-called solar constant. Summed over the Earth' surface this is a power of 200,000 TW, or 15,000 times the world's energy demand.
 Einsteins formula describes the maximum energy density E/m which is simply c? = 90,000 TJ/kg = 25 TWh/kg.
 The electricity output of current nuclear power stations is a bit less efficient since they work at lower temperatures as coal-fired stations, so the ratio of the "electricity densities" is about 20,000.