See our Submission to the Cap de la Hague expansion Planning Inquiry
While no one questions the dirty nature of oil and coal fuelled power stations, nuclear power is frequently referred to as "clean", and as having no impact on the greenhouse effect, or climate change. When we examine a fossil fuel powered station, we are aware of the general environmental effects of coal mining or oil extraction, as well as the emissions of carbon dioxide, particulates, sulphur etc, including uranium. Where are the comparisons with nuclear fuel? We hear conflicting stories about the effluent, hardly anything about the emissions, and nothing at all about the process of preparing the fuel that is "burnt" in a nuclear reactor.
When fossil fuel is burned in a power station it heats water in a steam generator to produce steam to a high temperature. The steam then passes through a turbine, rotating the turbine shaft to generate electricity. The steam is then cooled and liquified in a condenser, and returned to the steam generator to repeat the process. In general, power station efficiency is between 30 and 40%, although combined cycle gas turbines are around 50% efficient, and combined heat and power stations can typically achieve efficiencies of around 70%.
The main difference between a fossil fuelled power station and a nuclear one is that the burning of fossil fuel is replaced by the fission of uranium or plutonium as the source of heat to produce the steam.
When coal is burnt, we know that the reaction between carbon, the main constituent of coal, and the oxygen in the air produces carbon dioxide, which comprises the same elements but in an altered state. Nuclear reactions differ from chemical ones in that the elements and their isotopes can change as the result of the reaction, and new isotopes can be produced. The other main difference is that the amount of energy that is released during a nuclear reaction is in the order of a million times greater than the energy released from burning fossil fuels.
IonisationCertain radioactive emissions are capable of causing ionization in their passage through matter - i.e. they can knock electrons out of atoms or molecules or create ions either directly or indirectly, and thus damage cells and cause cancer. |
The new isotopes that are formed during nuclear reactions may be stable or unstable. If they are unstable they decay to form other new isotopes, emitting radiation or particles that can cause biological damage: radioactivity. If the new nucleus that forms from the decay of an isotope is itself unstable, a further radioactive decay, or even a sequence of them, occurs. The initial radioisotope is then said to give rise to a radioactive decay chain.
Because the particles and radiation emitted during decay can be hazardous, the biological hazard arising from a particular radioisotope does not cease until its decay chain has reached its end - that is, until a stable nucleus has been formed. Some radioisotopes are more hazardous than others because of the ease with which they are incorporated into the cell structure of the human body.
Half-lifeThe half life of a radioisotope is the time that it takes for half the atoms in the sample to decay. Although we cannot predict when a particular nucleus will decay, no matter how many atoms there are, the halving of the number always takes the same time. |
A decay chainUranium-238 has a half life of 4½ million years and emits alpha-particles. It decays to produce thorium-234 which has a half life of 24 days and produces beta-particles, to protactinium-234 with a half life of 7 hours emitting beta-particles, to uranium-234 with a half life of a quarter of a million years which decays by emitting alpha-particles... and so on until it finally creates the stable isotope lead-206. |
As well as man-made radionuclides, radioactive isotopes of all common elements occur naturally, for example carbon, potassium and hydrogen. Life on earth is adapted to a certain level of radiation, and has developed self-repair systems to deal with the damage that ionising radiation, free radicals etc cause. It has been theorised that a certain level of radiation is biologically tolerable, and keeps the body's defences on their toes. The difficulty for us arises in identifying what that basic level is. Because there is natural background radiation, and because it is irreducible, it does not necessarily follow that this sets an allowable scale for man-made radiation emissions, as radionuclides can be concentrated in the food chain, for example. It has also been theorised that man-made radionuclides may have a different biological effect, rather than just adding to the natural background radiation.
Radiation
Radiation is absorbed by the material through which it passes. During radioactive decay the alpha or beta particles, and the gamma rays that are given off can all penetrate matter, although alpha-particles can be stopped by a piece of paper or the human skin, whereas beta-particles require a few millimetres of metal to absorb them. Gamma-rays, on the other hand, are very penetrating and require lead shields or a metre of concrete to stop them. The dose of radiation received is the amount of energy absorbed per unit mass of matter. Exposure to ionizing radiation can be harmful as the radiation can cause cancers in the living population and genetic changes that may produce heritable defects in future generations. |
The effects of radiationThe biological effects of irradiation vary according to the type and energy of the radiation involved, and to the chemical and physical characteristics of the isotope. In addition to the absorbed dose and the quality of the radiation absorbed, biological changes can be influenced by a large number of other factors: some organs are more susceptible to damage than others, internally ingested radioactive materials may concentrate in certain organs, and some organs may be regarded as more vital than others. For example, a dose of ionizing radiation to the eye is three times more damaging than the same dose to the skin. The energy of alpha-particles is entirely absorbed within millimetres of the source, so if it comes into contact with body cells the ionizing radiation that is released is concentrated and the damage is greater. If an alpha-particle emitter enters the body - for example, the lungs - it will damage or kill cells. Although beta-particles are more penetrating, they have a range in air of a metre or so and are losing energy as they go, so the ionization they produce is spread out over a greater distance, and any biological damage they cause will be spread over several cells if it is absorbed by the body, and may therefore be more able to be repaired by the body's repair mechanisms. To a limited degree, it may be preferable for cells to be destroyed rather than damaged, as damaged cells may reproduce and give rise to mutations or cancer. X-rays and gamma-rays are forms of electromagnetic radiation and can pass through the human body, although a large dose can still be very dangerous. You can pick up a lump of plutonium with rubber kitchen gloves, but if you breathe even the tiniest fraction, i.e. one ten millionth of a gram, it will probably kill you. |
Uranium mining releases radon-222, a gaseous radioisotope that has a half life of 3.8 days and emits alpha-particles of densely ionizing radiation. Radon is formed in granite-type rocks from the radioactive decay chain of uranium-238 and naturally diffuses up through the rock to the atmosphere. However, as it has a short half-life it generally decays before reaching the surface. Underground mining for uranium releases the gas into the atmosphere and poses a severe risk for miners, estimated to increase their risk of lung cancer sixfold.
Uranium usually occurs naturally as the oxide uraninite, also known as pitchblende. Extraction involves crushing the rock and treating it chemically to separate out the uranium - this is usually undertaken at the mine and all processes use fossil fuels.
The extraction of useable uranium from ore is inefficient; only 10 kg of uranium oxides can be obtained from a tonne of ore, leaving 990 kg of rock contaminated with radioactivity to be disposed of. The radiation risk from the waste is minimized if it is buried, but cost cutting procedures mean it is often just dumped on the surface around the processing plant.
The uranium leaves the mine as an impure form of uranium trioxide, and is purified to uranium dioxide by high temperature heating in hydrogen gas. One type of reactor, in Canada, can use this material as fuel. The British Magnox reactor can use it in the form of uranium metal, which is produced by heating the uranium dioxide with hydrogen fluoride gas, and then with magnesium metal turnings. However, most of the world's nuclear reactors use uranium dioxide enriched in uranium-235 as fuel.
Enrichment is carried out using uranium hexafluoride by either gaseous diffusion or gas centrifugation. Gaseous diffusion is very energy intensive, and relatively inefficient. It has to be repeated hundreds of times before enough uranium-235 is extracted from a sample to enable it to be used in a reactor. Gas centrifugation is also energy intensive, but the percentage of uranium-235 produced is relatively greater and the process only needs repeating around half a dozen times. However, only tiny amounts can be extracted at a time, and the process needs to be replicated several hundred times, running simultaneously.
Once the uranium hexafluoride has been enriched to the required extent, it can be converted to enriched uranium dioxide by heating it with hydrogen and steam, with waste products of hydrogen fluoride gas and uranium hexafluoride. Fluorine is the most reactive of all the elements, reacting, often explosively, with almost all other elements. Both fluorine and hydrogen fluoride attack glass, and cause severe burns if they come in contact with skin. Hydrofluoric acid is highly dangerous as it cannot be washed off the skin, and quickly penetrates very deeply. These chemical hazards pose more of a risk than the radiological risks from processing the fuel, and also produce releases to the environment.
During routine operation of nuclear reactors, small amounts of radioisotopes are unavoidably released into the general environment. These are either fission products that have leaked from the fuel, or are radioisotopes formed by interactions with the coolant. Soluble fission products leaking into the coolant can be removed by passing the cooling water through a chemical clean-up plant. Some of the gaseous fission products carried in the coolant - notably krypton and xenon - are discharged into the atmosphere. The main hazard in this case is from krypton-85, because of its relatively long half-life (10.8 years). Tritium, which has a half-life of 12.3 years and decays by beta-particle emission, is difficult to contain and is emitted as a gas and also in water, where it can replace normal hydrogen molecules. Water molecules can also pick up a rare oxygen isotope which reacts in the reactor coolant water emitting alpha-particles to produce carbon-14, which has a half-life of 5,730 years and undergoes beta-decay. After reaction with the water, it is released as carbon dioxide gas or hydrocarbons.
In an attempt to compare the effects of emissions from fossil fuel combustion with nuclear reactors, it is important to note that unlike the uranium which is released during coal burning, particles of plutonium released from nuclear reactors are small enough to stay airborne, remain in the environment almost permanently, and may be fatal if breathed.
The core of a Pressurised Water Reactor contains about 110 tonnes of enriched uranium, which lasts for up to 6 years. The spent fuel is removed before all the fissile material has been used up, because some of the accumulating fission products begin to impede the chain reaction During this time the composition and radioactivity of the fuel has changed: it has absorbed neutrons, plutonium has been formed and has itself undergone reaction to create different plutonium isotopes. Each isotope will decay differently, with different radioactive products. From all these nuclear reactions within the reactor, the spent fuel is 100 million times more radioactive than fresh fuel. Fission products account for 76% of that radioactivity.
The spent fuel can be reprocessed to separate the uranium, plutonium and fission products from each other. The fission products then comprise the highly radioactive nuclear waste, which must be stored and ultimately disposed of. Radioactive wastes differ in the hazard they represent. They are classified loosely into low, intermediate and high level wastes, depending on the activity per unit mass or volume, and whether or not the decay produces so much heat that the wastes require cooling. The half-lives of the radioisotopes and the way in which they decay are unlikely to be factors in the classification.
After it has been removed from the reactor, spent fuel is stored under water on the reactor site for a year or more to allow radioactive fission products with short half-lives to decay. The spent fuel is then transported by rail to reprocessing plants at Cap de la Hague or Sellafield in steel flasks. Inevitably some fission products leak into the cooling-pond water, both at the reactor site and at the reprocessing plant, and the water is treated as low-level radioactive waste and discharged to sea.
At the reprocessing plant the fuel pins are shredded and dissolved in nitric acid. This process releases gaseous fission products such as krypton-85. Intermediate-level wastes arise from the fuel cladding and breakdown products of fission that are insoluble in nitric acid.
The fission products are then separated from the uranium and plutonium, and form the main high level waste which is then vitrified and stored again to allow further radioactive decay to take place. About one tonne of fission products is produced annually by a typical 1,000 MW PWR reactor.
The uranium is then separated from the plutonium. If uranium became scarce, the plutonium and uranium-235 could be used to make reactor fuel, but due to a world glut nearly all of this uranium and plutonium is simply stored. Security must be high because storing separated uranium presents a terrorist threat. The industry is keen to develop mixed oxide reactors (MOX) which can use reprocessed fuel, but these present their own problems.
PlutoniumPlutonium is an artificial radioisotope created in nuclear reactors. It accumulates in bones and is highly toxic, often described as one of the most hazardous substances known. Even after reprocessing, and "recycling" in a MOX plant, the amount of plutonium initially produced in the reactor does not reduce. The half life of plutonium-239 is 24,400 years: for every kilogram of plutonium created today, there will still be 500g around in 24,400 years. |
At each stage of the reprocessing, liquid and solid low-level radioactive wastes are produced. Although the high-level waste contains 97% of all the radioactive products, it only occupies a small fraction of the total volume of waste. Reprocessing 4m3 of spent PWR fuel produces a total volume of 642.5m3 of waste products, of which 600m3 are low-level, 40 m3 intermediate-level and 2.5m3 high level radioactive waste. The total volume of waste products created by reprocessing is 161 times greater than that of the spent fuel.
As a result of imperfections in the separation processes, liquid low-level waste discharges into the sea contain small quantities of isotopes of plutonium, which are all alpha-emitters. The strong currents in the sea at the discharge points disperse this radioactivity widely, however in the case of radioactivity dispersal is not analogous with the dilution of impact. When radioisotopes are released into the sea, they eventually become incorporated into the marine food chain or in sediments. They can then enter the human food chain when seafood is eaten, or when sediments are washed ashore by currents. At Sellafield in Cumbria it has been shown that sea spray can carry plutonium miles inland.
The factors on which risk assessment of these discharges is based include:
These factors depend on information supplied by COGEMA, which has been contradicted in various investigations undertaken by scientists commissioned by Friends of the Earth and Greenpeace. There are some indications that neither the modelling nor the monitoring may be detailed enough to allow an accurate evaluation of the risks to selected groups in the local population.
Inevitably there will be inaccuracies and inconsistencies in anything as complicated as predicting the health effects of industrial discharges, but two important questions have to be asked:
Any conclusions driven primarily by short term gain may prove to be enormously costly to future generations.
Much of the information contained in this article was drawn from the Open University textbook Nuclear Power, written by Malcolm Scott and David Johnson, and the Dictionary of Environmental Science and Technology, Andrew Porteous.
Animations by: Mike Johnson, Thoth Web Design
