Almost a century has past since physicists first began to suspect the atom might contain the phenomenal energy it does, and over half a century since Hiroshima awakened the whole world. Yet while the nuclear reaction is well understood and can be controlled it is still under exploited. Assuming the importance of nuclear power in the future, with fossil fuels disappearing and sustainable electricity generation in its infancy I intend to make a conclusion about the way forward for the nuclear industry. To do this I will research how nuclear power is generated before investigating the different ways in which it is harnessed. I will also investigate recent research into the improvement of nuclear facilities. The main factors I shall be looking at are the efficiency of power generation, safety and environmental issues, and economic viability.
The basis behind nuclear power stations is the nuclear fission reaction that takes place in the core. This reaction releases energy in the form of heat, which is then harnessed by various forms of reactor. From the reactor the process is much the same as for other forms of power station, with the accumulated heat energy being used to power turbines in order to produce electricity through electromagnetic induction.
(Ref. 1)
The basis of the reactions that take place in the core is the tendency of all matter to reach a more stable state. This is well illustrated in simple chemical reactions, where the instability of incomplete ‘outer shells’ of electrons is removed through the transfer of electrons from one atom to another. As more stable matter has, by definition, less potential energy the reaction must release energy to the surroundings. This can be seen through such examples as the reaction of carbon and oxygen, which releases large amounts of heat energy. Nuclear reactions are largely analogous to chemical reactions, but take place on a much smaller scale. They also release far greater quantities of potential energy. Again the reactions that occur are due to the inherent instability of the material involved. This instability is a result of the forces present within the nucleus. The strong nuclear force is the dominant force between all the nucleons, the particles that make up the nucleus. However there is also a significant force of electric repulsion between the positively charged protons present. Instability occurs in especially small or large nuclei, where the electric force between the protons is able to overcome the strong nuclear force holding the nucleus together. In large atoms, which are used in fission reactions, this is because there are a large number of protons, meaning the nuclear force has relatively less affect. Such nuclei can split into constituent parts with a lower total potential energy. If this happens the energy difference is released to the surroundings, mostly as heat. An example of the fission reactions that can take place is shown below. In the example the binding energy per nucleon decreases from around –7.5MeV to around –8.5MeV, with around 1MeV per nucleon being released to the surroundings.
235U92 + 1n0 → 144Ba56 + 90Kr36 + 21n0
It is very uncommon for the fission reaction described to take place naturally. Radioactive decay of the material is far more probable. Here again an analogy can be drawn with the example of a carbon-oxygen reaction. Just as it is necessary to heat the two chemicals to cause reaction, the nuclear reaction must also be artificially induced. By firing a neutron at an already unstable nucleus fission can be produced. The recipient nucleus absorbs the neutron, thus increasing it mass. Just as chemicals reach activation energy the nucleus becomes sufficiently unstable for a reaction to occur. Many such reactions can be induced because the fission reaction produces loose neutrons that can trigger more nuclei to become unstable. By controlling the number of neutrons that trigger reactions the process can be controlled.
In a nuclear bomb, such as the Hiroshima bomb, the fission reactions in an unstable substance are allowed to become ever more numerous, releasing huge amounts of uncontrolled energy. To allow the safe exploitation of the energy it is important to limit the number of reactions that occur. This is done via an arrangement of carbon, boron or cadmium rods that penetrate the core. The material in the rods is used to absorb neutrons released from previous reactions. By removing neutrons from the core the chain reaction can be slowed and controlled. The rods can be moved to speed up or slow down the reaction. The control of the reaction is the essential key to harnessing the power released.
Once the reaction is controlled the system is very much like any other power station. Heat is produced, at the nuclear reactor in our examples. This is because the transfer of heat is inevitable as a result of the law that entropy must increase. The second law of thermodynamics states that heat transfer results in higher entropy, as explained below:
The total entropy of a system must increase. A decrease in the energy of the high-energy region (the core) will result in a small decrease in the number of possible arrangements of that energy within it. The addition of that energy to the low energy region (the coolant) will cause far more ways of arranging the energy to be produced. As such a transfer of energy from the core to the coolant will occur in order to increase total entropy, given as being proportional to ln (W) where W is the number of arrangements. Hence heat is transferred to the coolant.
The gathered heat energy is then used to create steam by increasing the heat energy of a body of water to a point where it boils. The steam produced has a large amount of energy, which is transferred to kinetic energy in turbines by forcing the steam through them. The kinetic energy of the turbines is then finally transferred to electrical energy by the generator before being refined and sent out.
The central role of the reactor system is to take the heat energy generated in the core and use it to create steam for the turbines. The main difference in the main types of reactor is the coolant used. While all eventually heat water to produce steam some do so directly, while others use another medium to take heat from the core to the water. The systems I have chosen to analyse are as follows.
These systems use water as a coolant, either as a medium or to heat water directly from the core. ‘The boiling water reactor typically allows bulk boiling of the water in the reactor.’ (Ref. 2) Pressurised water reactors instead use pressurised water as a medium to collect heat and transfer it to a separate pool of water to create steam. Diagrams of both can be seen below. (Ref. 1)
The boiling water reactor simply works by continuously circulating an enclosed flow of water through the reactor. The heat energy from the core is transferred directly into the water ‘producing steam at a pressure of about 1000 pounds per square inch.’ (Ref. 2) The steam then rises though the reactor whereupon the pressurised nature of the atmosphere in the reactor forces it into the main steam pipes (see diagram). The pressurised steam then is forced through the turbines, whereupon much of the heat energy is transferred to kinetic energy. The kinetic energy of the turbines is then transferred to electrical energy by the generator. The cooled steam is then further cooled in the condenser. The condenser cools the water by using pipes fed with river water or seawater. Thus any remaining heat energy is fed to the outside. After being through the condenser the water is in liquid form. The now cool water is then pumped back into the reactor for another cycle.
The pressurised water reactor has more cooling cycles than the BWR. The water used directly to cool the core is separate from the water used to produce steam. Thus one cycle of water is used purely as a coolant. This is the same basic system as gas cooled and metal cooled reactors. The first cycle of pressurised water, at an original cool state, passes through the reactor and takes heat energy from the reaction. It remains in liquid form as a result of its pressurised state. It is then piped through another body of cool water, which in turn receives heat energy from the first, cooling the first cycle ready to be fed back through the reactor. The second body of water boils once it has enough energy. The resultant steam is forced through the turbines, transferring its energy to motion and causing the generator to spin and produce electricity. The second water cycle is then also cooled to return it to a liquid state. The diagram shows a cooling tower but it can also be done as said before with river water or seawater. The cooled water can then be pumped back for another cycle.
The main advantage of water-cooled systems is their low capital costs, especially BWR systems, and their low running costs. The materials used are generally far cheaper and more easily obtainable than in other systems. They are usually preferred as the low cost option, for example in less developed countries and for small-scale plants. The PWR system also reduces exposure to radiation by restricting radioactivity to the first cycle only. As such these plants have proliferated from the early stages of nuclear power generation to the present. PWR reactors are by far the most common reactors world-wide, although this is not true in Britain (Appendix A)
The two systems have different problems. The BWR system is problematic because its single cycle means that radioactivity is more easily spread throughout the whole system. Reducing this affect and implementing more safety measures as a result is costly and often inefficient. For example it is harder to prevent workers from coming into contact with a contaminated cycle. The PWR system, which uses water as an intermediate coolant is also inefficient because of the low thermal conductivity of water. As such more coolant per second must flow, meaning a greater pumping capacity is required, while energy is transferred less efficiently. It can also limit the speed of the reaction used, so lowering power output.
There is a third special form of water-cooled reactor, pioneered by Atomic Energy Canada Ltd. It uses heavy water (2 atoms deuterium, 1 atom oxygen) as a coolant in place of pressurised water. This heavy water does not have to be pressurised, eliminating the need for a special pressuriser. Heavy water also has another important role in these systems; it is a much more effective moderator than light water. A moderator slows down the neutrons released after each reaction. To understand the importance of this we must briefly return to nuclear theory.
Uranium found naturally is mostly composed of U-238. A very small percentage is composed of the much more fissile U-235. When a neutron is absorbed by U-238 it is unlikely to cause a fission reaction. It will usually be absorbed to create radioactive U-239, which will slowly decay. U-235 in contrast will nearly always split when hit by a neutron. The moderator increases the likelihood of neutrons hitting the few U-235 atoms by slowing them down. Slow neutrons are not easily absorbed into U-238, and so can travel on until they reach U-235, continuing the reaction.
All water has moderative properties, which benefit all water reactors, but those of heavy water are far superior. By using an excellent moderator the heavy water reactors can use uranium which has not been enriched with U-235 or the plutonium isotope Pu-239. This is of great importance for countries or areas that cannot process uranium independently. Most of these reactors are found in stations that serve a particular area. For example they are abundant in France where there is not a national power network. Instead a modular system is used with each community receiving power from a particular station. Such a system lacks stations of sufficient scale to process fuel, and these reactors are a convenient alternative.
Alongside water-cooled reactors gas-cooled systems are the most popular and widely used reactors in the world today. They are especially numerous in Britain where they account for the vast majority of combined generating capability (Appendix A). A typical gas-cooled system diagram is shown below (Ref. 1). It uses carbon dioxide as a coolant.
An example of gas-cooled reactors is the Dungeness complex on the south coast of England. Dungeness A, the older system, is a Magnox reactor, so named because the fuel is encased in a Magnesium alloy. Dungeness B, a more modern development, is an advanced gas cooled reactor (AGR) like the diagram above (Ref. 3). In both types of gas cooled reactor carbon dioxide or a similar gas is circulated around the reactor to cool the core. The heat energy it absorbs is then transferred by means of water pipes that just penetrate the reactor, but remain distant from the core. The subsequent transfer of energy from the gas to the water inside the pipes enables the production of steam. As with the other systems that steam is then used to turn the turbines, which reduces its temperature, before being re-condensed by a final cooling cycle using sea water or cooling towers
The main advantage of the gas-cooled systems over the water-cooled systems is the higher efficiency possible as a result of different operating possibilities. To begin with the fact that carbon dioxide is a gas at room temperature means that no energy is used in a state change, as is the case as water is changed to a gas. This means that a greater proportion of the reaction energy is used to create electricity. The structure of gas reactors also allows a greater and freer circulation of coolant, so heat is transferred more readily, again leading to greater output and efficiency.
The gas system also shares the advantage of a PWR system in that the radioactivity produced in the core remains within the first coolant cycle, helping to reduce the risk of leakage and lower the cost of containing the radioactivity.
However, despite its superiority to water systems, a gas system is still relatively inefficient. Carbon dioxide is still a poor conductor compared to other materials, especially metals. This, like with the PWR system, limits the speed of the reaction, and thus reduces the power output of the station. Gas-cooled systems are also more complex and expensive than water reactors, and lose the benefits of water’s natural property as a moderator.
Liquid metal reactors, or fast breeder reactors as they are also known, are very much in their infancy. They are being developed primarily by those nations that most rely on nuclear power, namely the USA, Russia and France. The principles behind the system offer great benefits in increased efficiency and productivity, possibly making nuclear fission a viable energy source rather than the secondary position it occupies today.
(Ref. 1)
The basic design of the liquid metal system is similar to the PWR and gas systems. A primary cooling circuit removes heat from the core. It then transfers the heat energy absorbed to a secondary circuit of water (the diagram above has simplified this process). The energy transferred to the water then causes steam to form, which is used to power turbines. However, by using liquid metal in the primary circuit instead of water or carbon dioxide the system properties are drastically altered.
Metals are far better conductors of heat energy than gases or water. Usually the metal used is liquid sodium, potassium or a mixture of the two. This greater thermal conductivity allows energy to be removed much faster from the core, providing a much greater power output than other systems. This makes economic sense as well as merely providing a more efficient and productive ‘machine’.
ΔQ/Δt = -λA × ΔT/Δx
(ΔQ/Δt – rate of energy flow, A – Cross sectional area, ΔT/Δx, temperature gradient, λ – thermal conductivity)
If comparing like reactors the cross sectional area will be the same, and the temperature gradient will also be identical if both coolants are at the same original temperature. Thus ratio of rate of energy flow in between the two systems is the same as the ratio of the thermal conductivity of each. This ratio is 132 : 0.023 or 5740 for liquid metal against water and slightly greater for liquid metal against carbon dioxide. By removing heat from the core faster by this ratio the metal-cooled reactor can run at higher temperatures and produce far greater power.
The reason why these reactors are also called fast reactors or fast breeder reactors is another of their benefits. They are designed to produce fuel within the reactor from previously unproductive elements of the input fuel. Instead of being slowed, fast neutrons are allowed to collide with the input fuel, causing uranium 235 to react, and changing uranium 238 to the more fissile isotope plutonium 239. The produced fuel then replaces spent fuel in the reaction so the reactor effectively uses a larger percentage of input fuel. Thus a greater proportion of the energy stored in the original fuel is converted. ‘Fast breeders can extract about 60 times the amount of energy from uranium that existing thermal reactors do’ (Ref. 4). Whereas coal power stations can have an efficiency of perhaps 20%, and conventional nuclear stations an efficiency of up 40%, a liquid metal cooled reactor can have an efficiency of over 60% and upwards.
The systems themselves are far more expensive to create and maintain than gas-cooled or water-cooled reactors, and so there must be sufficient demand for the costs to be worthwhile. ‘Fast breeders are expected to cost 20 per cent more to build than pressurised water reactors’ (Ref. 4). Because of their cost and complexity LMC reactors are most suitable for large-scale production not local or isolated use.
The coolant used in the LMC reactors also poses problems. Sodium and potassium are both highly reactive, especially and the enormous temperatures involved in the core make them potentially explosive. Thus part of the large maintenance cost is the result of preventing the metal coming into contact with normal air. A layer of inert gas must be pumped outside the path of the coolant to do this.
Obviously all power plants are different, with varying circumstances that determine required power output and with differing limits on resources. A power station created to serve a sparsely populated French department can not be compared to a station on the US power grid. However I believe that I can make a general recommendation for a station to be placed on the UK National Grid or similar. While the modern liquid-metal cooled reactor is not yet fully developed (indeed the UK has not devoted any substantial resources towards research in this area) I believe the possible benefits outweigh this cost. Water or gas cooled reactors have been proven to work and are capable of generating very significant amounts of electricity but it is revealing that nuclear power accounts for only a small percentage of the national requirement. Due to the complicated procedures that must be observed in these stations and because of the efficiency problems of current reactor systems most can not produce electricity all the time and at a quick enough rate to serve current demand. The LMCR systems that are being pioneered can at least partially solve these problems by increasing the efficiency of stations, both in terms of quicker energy transfer, enabling faster reactions and quicker electricity production, and in terms of more effectively using the fuel input. This would enable nuclear stations to fulfil more of the requirements for major electricity producers in the current market. In my opinion the future of nuclear power, especially in the UK where the government has proven unwilling to show much faith in the industry is to become a far more viable economic prospect rather than just a token secondary system and a means of producing military material. Bar a miraculous advance in the development of nuclear fusion power the only way I believe this can be done is to use the most advanced systems of the day, regardless of the original outlay. At present LMC reactors are just that. LMC reactors are the most realistic way forward, as Ian Fells of Newcastle University puts it, ‘I would put my money in the fast reactor rather than fusion because fast reactors have been proven’. This policy should still leave room for specialised reactors in specific conditions while using advanced technology to make large scale nuclear generation mainstream.
Before this can happen however the nuclear industry must convince the government and the public that it is a 100% safe technology, and must stress the advantages of nuclear power, namely the lack of carbon emissions and the exploitation of a more plentiful resource than coal, oil or gas. The Uranium Institute recently commented in Core Issues (Ref. 5) ‘our industry…must be seen to be beyond reproach. It must BE beyond reproach.’ It must also demonstrate that nuclear power is reliable. On a visit to Dungeness power station I discovered that the plant is often off-line, when maintenance or checks are being carried out. This ‘downtime’ must be reduced to levels acceptable in a power hungry nation. Particularly in the UK, but also in other ‘nuclear-able’ nations winning these arguments is equally important if not more so than improving current technology, although to some extent the two can go hand in hand. I believe that nuclear power must inevitably be the means of generation for much of the foreseeable future as fossil fuels diminish because so-called ‘sustainable’ sources have yet to be tapped in a viable manner. Moreover the will of humans to control nature is contradictory to depending on its will for our energy. Although now well understood there is much room for improvement in the nuclear power industry, and if plants have to be built on a serious scale and in numbers similar to the fossil fuel plants of today then efficiency and safety will almost certainly improve. Techniques are being developed to increase the rate of heat transfer to make plants more efficient, such as oscillating liquid technology, which improves the convection currents in the liquids. The future must include greater efficiency, greater economies of scale and ever stricter safety protocols for there to be a future at all.
Reactor Type No MWe No MWe
(in operation) (total)
------------------------------------------------- -------------- --------------
Pressurised light-water reactors (PWR) 249 218,886 289 255,714
Boiling light-water reactors (BWR) 94 77,585 98 82,165
Gas-cooled reactors, all types 35 11,699 35 11,699
Heavy-water reactors, all types 35 18,671 50 26,161
Graphite-moderated light-water reactors (LGR) 15 14,785 16 15,710
Liquid-metal-cooled fast-breeder reactors (LMFBR) 3 928 7 3,308
(Ref. 5)
British Nuclear Fuels plc Calder Hall 1 (Seascale, Cumbria) 50 GCR 100 5/56 10/56 UKAEA Calder Hall 2 (Seascale, Cumbria) 50 GCR 100 11/56 3/57 UKAEA Calder Hall 3 (Seascale, Cumbria) 50 GCR 100 3/58 4/59 UKAEA Calder Hall 4 (Seascale, Cumbria) 50 GCR 100 12/58 5/59 UKAEA Chapelcross 1 (Annan, Dumfriesshire) 50 GCR 100 11/58 3/59 UKAEA Chapelcross 1 (Annan, Dumfriesshire) 50 GCR 100 2/59 8/59 UKAEA Chapelcross 1 (Annan, Dumfriesshire) 50 GCR 100 7/59 12/59 UKAEA Chapelcross 1 (Annan, Dumfriesshire) 50 GCR 100 11/59 3/60 UKAEA Nuclear Electric plc Bradwell 1 (Bradwell, Essex) 123 GCR 100 8/61 8/62 TNPG Bradwell 2 (Bradwell, Essex) 123 GCR 100 4/62 12/62 TNPG Dungeness A1 (Lydd, Kent) 220 GCR 100 6/65 12/65 TNPG Dungeness A2 (Lydd, Kent) 220 GCR 100 9/65 12/65 TNPG Dungeness B1 (Lydd, Kent) 555 GCR 100 12/82 4/85 APC Dungeness B2 (Lydd, Kent) 555 GCR 100 12/85 12/85 APC Sizewell A1 (Sizewell,Suffolk) 210 GCR 100 6/65 3/66 EE/B&W/TW Sizewell A1 (Sizewell,Suffolk) 210 GCR 100 1/66 9/66 EE/B&W/TW Sizewell B (Sizewell,Suffolk) 1188 PWR 100 1/95 5/95 PPP Hinkley Point A1 (H.P., Somerset) 235 GCR 100 5/64 4/65 EE/B&W/TW Hinkley Point A2 (H.P., Somerset) 235 GCR 100 10/64 5/65 EE/B&W/TW Hinkley Point B1 (H.P., Somerset) 585 AGR 100 9/76 10/78 NPC Hinkley Point B2 (H.P., Somerset) 585 AGR 100 2/76 9/76 NPC Oldbury 1 (Oldbury, Avon) 217 GCR 100 8/67 12/67 TNPG Oldbury 2 (Oldbury, Avon) 217 GCR 100 12/67 9/68 TNPG Wylfa 1 (Anglesey, Wales) 475 GCR 100 12/69 11/71 EE/B&W/TW Wylfa 1 (Anglesey, Wales) 475 GCR 100 9/70 1/72 EE/B&W/TW Hartlepool 1 (Hartlepool, Cleveland) 575 AGR 100 6/83 8/83 NNC Hartlepool 2 (Hartlepool, Cleveland) 575 AGR 100 9/84 10/84 NNC Heysham A1 (Heysham, Lancashire) 550 AGR 100 4/83 7/83 NNC Heysham A2 (Heysham, Lancashire) 550 AGR 100 6/84 10/84 NNC Heysham B1 (Heysham, Lancashire) 625 AGR 100 6/88 7/88 NNC Heysham B2 (Heysham, Lancashire) 625 AGR 100 11/88 11/88 NNC Scottish Nuclear Limited Hunterston B1 (Ayrshire, Strathclyde) 575 AGR 100 1/76 6/76 TNPG Hunterston B2 (Ayrshire, Strathclyde) 575 AGR 100 3/77 3/77 TNPG Torness 1 (Dunbar, East Lothian) 625 AGR 100 9/87 5/88 NNC Torness 2 (Dunbar, East Lothian) 625 AGR 100 12/88 2/89 NNC
(Ref. 6)