Nuclear Power Versus Renewable Energy

One of the biggest problems that we face in today’s modern power hungry times is the decision on a single source of power. As we continue to consume as a society we are very quickly burning through our supplies of fossil fuels to the point at which in a couple of decades we will have all but exhausted existing supplies.

It is for this reason that decisions need to be made on what source will generate future generations electricity. The future of energy production will lie with the criteria set out by fossil fuel energy production and how well they fit these criteria. Any new fuel source will have to be readily available and provide a constant supply of energy; it will need to be cheap and safe to produce; the same or fewer emissions than that of fossil fuels, although with today’s current eco standards the later will probably need to be true.

One particular source of energy seems the logical choice for the main replacement for fossil fuels and that is nuclear power, but how do other more renewable sources of energy compare to nuclear power? In 2002 it was reported by an MIT study that “nuclear power supplied 17% of the world’s electricity consumption” [1] and if this is compared to renewable sources such as wind power, geothermal and biomass generators which accounts for just 19% as calculated in 2008 [2]. The aim of this paper is to compare and contrast the viability of different energy sources for the future.

What is nuclear power?

Nuclear power in its current state is the controlled fission of radioactive material for the generation of electricity.

Nuclear power uses Uranium primarily as a fissible material, the first safety issues come from obtaining a source of uranium that is viable to use for energy generation. Uranium occurs naturally in a compound ore form, comprised of 99.3% U238 and 0.7% U235 [3]. Uranium 238 is not a suitable isotope for fission, infact in its natural ore it prevents the uranium from sustaining any sort of nuclear reaction. This can be explained by the energy of absorbed neutrons in the nuclear material, U235 will undergo fission at all energies of neutrons, but U238 will not. To overcome this problem, uranium ore needs to be enriched with a higher percentage of U235 (usually so that U235 comprises 3% to 5% of the ore [3]). The enrichment process introduces safety issues that are not present in other forms of energy production, the process produces uranium hexafluoride that is a highly toxic chemical that is difficult to store due to its corrosive nature. The main waste product is depleted uranium that the radioactive waste from extracting U235 from natural Uranium ore. There has been much controversy over Depleted uranium’s use as a weapon and its difficulty in being stored due to its radioactive nature, current estimates put global stockpiles of depleted uranium at 1.5 million tonnes [3].

The generation of power from radioactive sources makes use of a compound nuclear reaction. Neutrons are absorbed by enriched uranium, usually a uranium ore composed of mainly U235, which forms an excited compound nucleus. A compound nucleus will be inherently unstable and result in the fission of the excited compound nucleus and emission of two new nuclei and neutrons. Figure 1 illustrates a possible nuclear reaction inside a nuclear reactor:

Figure 1- Example of nuclear reaction. n+U235 → U*236 → Xe139 +Sr95 + 2n

As can be seen from figure 1 the example fission reactions require a single neutron to induce fission of U235 but the final products of the reaction include 2 neutrons which allows the reaction to become self perpetuating, this neutron feeding process is known as a chain reaction. This chain reaction will only occur in enriched uranium.

Nuclear power is generated using two different types of reactor that both have their merits and faults with regard to safety:

The fast reactor is a reactor based on an equal mix of Uranium 235 and 238 that can sustain the chain reaction required to generate energy. Fast neutrons, hence the name, sustain the reaction, in the case of the fast reactor neutrons interact with U238 whicch forms a nucleaus that does not under go fission but forms plutonium 239 instead. Although Pu 239 is able to undergo fission, it does not do so at the speed required to sustain the chain reaction and infact most of it becomes unused waste in the form of spent fuel. The fast reactor is designed to navigate around this by using the fast neutrons to produce more fissile Pu239 than spent Pu239 by not using a moderator to slow neutrons down this reduces the problem of producing lots of unused fuel as waste. Because this type of reactor does not use a moderator to control neutrons and due to the highly fissile material in the reactor core, it does require an efficient cooling system which does introduce certain safety issues. Fast reactors require a liquid sodium coolant, chosen due to its highly conductive nature, to remove heat from the core. The use of sodium though has its associated risks because its highly reactive and becomes corrosive when reacting with air which is obviously a problem when surrounding a radioactive core.

Thermal reactors use enriched uranium to produce energy instead of an equal mix and the enrichment produces the waste products as discussed earlier. Unlike fast reactors which do not use moderators thermal reactors, and the ones in question use a moderator and coolant. In the case of Pressurised water reactors and boiling water reactors they both use water as a coolant and moderator due to waters inherent property that it is a good neutron absorber. The water based thermal reactors are one of the most widely used nuclear reactors due to the availability and safety of the coolant and moderator. There are contamination risks with the boiling water reactor, as water is pumped through the reactor and boils it will pick up fragments of radioactive contaminants which could be released into the atmosphere via the cooling towers.

For note the world’s largest nuclear power plant uses 7 different boiling water reactors to generate electricity and is capable of generating a total of 8,212MW and a typical fossil fuel turbine is capable of generating 1000MW so a typical fossil fuel plant can be compared to outputting a similar amount of energy to that of a nuclear power plant [3b].

Safety of Nuclear Power

With the explosion of the Chernobyl nuclear power plant and the recent meltdown risks of the Fukushima reactors safety concerns are always inherent when discussing nuclear power.

Meltdowns are often a buzz word when associated with nuclear power plants and is often the biggest safety concern when considering nuclear power, but what exactly is a nuclear meltdown? A nuclear meltdown is not as serious as the word suggests or as the world media have suggested. A meltdown will occur when there is more heat inside the reactor core than that being removed by any coolant mechanism that are put in place. If this does happen then the nuclear fuel will literally melt causing it to melt through the reactor. This will result in damage to the reactor core and a possibility of the fuel escaping the reactor, but nuclear reactors are contained within a containment structure that is designed to prevent any radioactive material from contaminating the atmosphere and allowing coolant to be pumped into the structure. This only becomes an issue when the containment structure is damaged as was the case during the Fukushima earthquakes, with the exception of natural disasters modern nuclear power plants are designed with the up most safety in mind and have many different precautions in place to prevent and exposure and risk to the general public.

Control rods are one of the best designed safety features within a nuclear reactor, they are designed to avoid any rapid increase in reactor core temperatures. When fuel temperatures rise to a temperature at which coolant will be contaminated and need to be ejected, the control rod safety mechanism kicks in. They are designed to slow the neutron flux with in the reactor, much like the moderator does, and hence slow down the rate of energy generation so that coolant can carry heat away without the risk of contamination. Systems are in place so that this precaution is activated immediately when any reactor threat is detected. This safety measure reduces the risk of any radioactive material being ejected via cooling systems.

In fact studies suggest that exposure to radioactive sources due to nuclear power stations and nuclear fuel plants are less than 0.1% of human annual exposure to radiation, this can be seen from figure 1. So although there is a high chance of radioactive contamination the safety mechanism that are in place prevent this risks from occurring and keeping exposure down to a minimum.

Figure 2- Figures showing annual sources of radiation exposure [4]

With this in mind although there are danger risks when considering nuclear fuel as a replacement for fossil fuels, the number of design features installed to prevent this is more than adequate to ensure the safety of the technology.

Proliferation concerns

One concern with Nuclear power is not the safety risks of the power station or the waste they produce but instead that enrichment facilities can produce Uranium that is enriched for use in nuclear weapons. This is much a concern in the developing world where governments are not as stable and it would not take long for a fuel enrichment facility to start producing weapons grade uranium.

Renewable energy sources

So far we have looked at some of the pros and cons of nuclear power and it seems like a viable alternative to fossil fuels, but technological advances have allowed renewable energy sources to become a viable option for industrial production of energy. Renewable sources of energy can’t produce energy on the scales that nuclear power plants can but they do not suffer from the contamination complications that nuclear power does.

Solar power

Solar power exploits nature’s biggest source of energy the sun, which outputs a massive 3.84×10^26W [5] of which we receive 1.74×10^17 W or 1.366kW/m^2 [6]. This is a huge amount of energy to exploit and an obvious choice for renewable energy because the sun is expected to remain in the sky for the next four and a half billion years. This seemingly perfect candidate for energy production seems relatively unused as solar power accounts for just 0.9% [7] of worlds energy production. One of the main issues with solar technology is the photovoltaic cell, this being the standard solar panel technology, it has a relatively low efficiency and as a result the amount of energy outputted does not represent the full potential from the energy put in, also with the sun being an intermittent energy source, the only way photovoltaic cells can produce energy at nights is with the use of large banks of batteries, which are expensive and an environmental hazard.

There are seemingly new technologies creeping into the solar market, much simpler than the photovoltaic cell and much more efficient. One such technology is the solar tower. The solar tower uses a large field of heliostats which are mirrors that track the sun to focus the suns light on a single point which is a central tower. The central tower uses the heat supplied to drive a steam turbine capable of generating between 10 and 100 MW (dependant on the number of mirrors), for example the PS10 solar power plant in Spain uses 624 heliostats to generate 11MW and is still being constructed[8]. This may seem like a small amount of energy but a larger area solar power tower could provide much more energy, by increasing the area of the plant, by increasing the number of heliostats. One such project is currently under way in the Mojave desert which covers 4000 acres and aims to generate enough electricity to power the city of San Francisco [9]. Unlike Photovoltaic cells solar power towers have the capability of producing energy at night. When the suns energy is focused on the central tower some of the heat which is used to drive the turbines is also used to heat salts to molten temperatures in excess of 200 ‘C, molten salts can keep their temperature very well if insulated properly, and when required can be pumped through the turbine to generate steam to generate electricity [10].

Given enough time Solar power could become a replacement for fossil fuel energy production, but in the meantime it suffers from high initial costs, a large carbon footprint and a lack of backing. Currently Spain appears to be the pioneers of this technology and have already constructed numerous solar power towers including PS10 as discussed earlier.

Wind power

One by-product of the suns energy that has yet to be mentioned is that it supplies energy into the atmosphere causing temperature and pressure differences, this produces a bulk movement of air molecules resulting in the wind. The wind is a much more exploited energy source compared to solar power, with wind power producing 2.5% of the world’s electricity[11].

Wind power generates electricity in a very similar manner to a conventional power generator, but instead of using water as a fluid to turn a turbine, the wind is the fluid that turns the turbine. Although there is a significant amount of power available in the wind not all of it can be extracted and a theoretical limit has been calculated for the amount of energy that can be [12]. This limit means that 59.3% of energy in the wind can be used for energy generation. In reality much less than this is extracted because wind is not a uniform medium and as such suffers from intermittency issues similar to solar power, in that wind is not a constantly accessible resource.

Hydroelectric Power

One already established form of renewable energy is Hydroelectricity, the generation of energy using the conversion of gravitational potential energy of falling water through a turbine. Its currently the 4th largest form of power generation behind coal, gas and nuclear power producing 20% of the world’s current electricity demands [13]. Interestingly though Hydroelectricity has been exploited in the 1st world, with almost all possible sites being developed for hydroelectricity, its only in the developing world where hydroelectricity could be utilized to improve renewable energy generation, this would explain why it is the 4th largest energy provider [13].

Unlike wind power and solar power, hydroelectric power does not suffer from intermittency issues. The idea behind a hydroelectric dam is that it uses the water at a higher gravitational potential to fall through a turbine generating electricity. When there is a smaller demand for electricity the flow rate can be reduced and the reservoir can be used to store excess water, this provides the dam with water on demand, which is a unique feature of hydroelectric power.

Three gorges dam in china is a fantastic example of a 21st century hydroelectric plant, for reference it is capable of producing 22500MW [14] this is greater than 100 times the amount of energy produced by a solar power tower, and 5 times more energy than the largest nuclear power stations, so obviously hydroelectric dams are useful and utilizable replacement for fossil fuels.

Hydroelectricity does have its environmental faults, and is probably the most hazardous of the renewable power sources. In order to construct a hydroelectric plan a large enough volume of water is required with a big enough drop for potential energy conversion to warrant building a hydroelectric plant. To do this a river must be dammed which upriver will flood a large area of land, this causes huge amounts of destruction to natural habitats, downriver there is a chance that drinking water is contaminated which has obvious human impacts.

Conclusion on energy types

We have discussed the different aspects of nuclear power and many different types of renewable energy sources. It is my opinion that despite certain renewable already being established they have still not reached their potential to replace fossil fuels, especially with certain intermittent issues and as a result are not ready to replace fossil fuels. On the other hand, while renewable sources develop into a more viable energy source nuclear power is ready to replace fossil fuels in the interim.

If we compare energy production it would appear as if hydroelectric dams are the best option, but as discussed all sites in the developed world have been exploited and for comparison nuclear power stations are still being built as many possible sites still exist. Despite its safety issues and the non-renewable nature of nuclear fuel, nuclear power is capable of producing a steady supply of power that will meet demands upon request. In reality there is no one replacement for fossil fuels, as a society we have created a huge dependence on them and it is going to require a mixture of different power sources in order to develop a secure energy future and as a result our future probably lies with renewable energies and nuclear energies working in conjunction with each other.

References

1. Deutch J.M et al, Future of Nuclear Power, An interdisciplinary MIT study, 2009, page 1.

2. El-Asjry M. et al, Renewable energy policy network for the 21st century, REN21, 2010, page 9.

3. Falk J. & Bodman R.,Uranium Enrichment, Energy Science, November 2006, page 2.

3b. Power and technology, Full statistics of turbine energy generation, http://www.power-technology.com/projects/kashiwazaki/, accessed 16 April 2011.

4. Nuclear safety, The institution of engineering and technology, 2006, page 3.

5. Zeilik M. et al, Introductory Astronomy and Astrophysics, Saunders college publications, 1992, page G-11.

6. Wilson R. C., and Mordvinov A. V., Secular total solar irradiance trend during cycles 21-23, Geophys. Res. Lett, 2003, page 1199.

7. Wirman C., Electrical power annual: generation, US energy information administration, 2009, page 3.

8. Garcia-Sobrinos G., Tower of Power, Civil engineering, 2009, page 42-49.

9. Zook D., County supervisor concerned by Brightsource Mega Solar Project Impacts calls for a full review, Best Syndication News, 2011.

10. Barth D., Development of a High Temperature, Long-Shafted, Molten-Salt Pump for Power Tower Applications, Journal of Solar Energy Engineering, 2002

11. Ragheb M., Wind Power Systems:Harvesting the Wind, College of Engineering University of Illinois, 2011.

12. Gijs A.M. van Kuik, The Lanchester-Betz-Joukowsky Limit, Wind Energy Journals, 2007, 10:289-291

13. Hydropower and the World’s Energy Future, International energy agency publication, 2000, update 2009.

14. Fabian A., Taming the Yangtze, IET publications, 2009.