S280 Science Matters
Almost all of the mass of an atom resides in a highly compact nucleus consisting of protons and neutrons (nucleons). The neutron is neutral, but the proton carries a single positive charge.
The number of protons in a nucleus is the atomic number (Z). Each chemical element has a characteristic atomic number, which distinguishes its nuclei from those of all other chemical elements.
The proton and the neutron have similar masses. The mass number of a nucleus is the total number of nucleons it contains.
The different combinations of mass numbers and atomic numbers which are found in nuclei of the same element are called isotopes. Nuclear reactions involve the transformation of isotopes.
When nuclei are formed from their constituent nucleons, there is a decrease in mass. This mass decrease is converted into an amount of energy known as the binding energy which is released to the surroundings when the nucleus is formed in this wasy; conversely, this energy would have to be supplied in order to break the nucleus up into its constituent protons and neutrons.
The binding energy per nucleon increases with A up to about A = 60, and thereafter decreases slowly.
Nuclei that are stable (that is, do not undergo radioactive decay) have a neutron-to-proton ratio close to one at very low values of Z, but this ratio increases steadily to about 1.54 in the Z = 80-90 region.
The important modes of radioactive decay are α-decay, β-decay and positron emission, in which the respective emissions are α-particles (42He), electrons (β-particles) and positrons, and the nucleus is transformed into another element. Radiation in the form of γ-rays is usually emitted as well.
There are large differences in the amount of material needed tos top these different emissions: α-particles require the least and γ-rays the most.
The product of the radioactive decay of an isotope may itself be radioactive, and so on; that is, it might be part of a radioactive decay chain.
Radioisotopes decay more quickly if they have short half-lives; as the decay of any radioisotope proceeds, its rate of decay (the activity) decreases. This rate of decay cannot be changed by changing the physical conditions.
(Note: Nn/N0 = ½n)
The different neutron interactions with nuclei
|Scattering||Neutron loses kinetic energy and the tafet nucleus gains kinetic energy; the average energy loss of a neutron decreases as the mass of the target nucleus increases|
|radiative capture||Neutron absorbed by the target nucleus and only γ-rays are emitted|
|charged particle emissions||Neutron absorbed by the target nucleus and a charged particle is emitted together withe some γ-rays|
|fission||Neutron absorbed by the target nucleus to form a new nucleus, which splits into two different fission products; neutrons are emitted together with some γ-rays|
Neutron-induced fission leads to the splitting of a nucleus of high mass number such as 235U, into two fission products and, usually, 2 or 3 neutrons. The fission products are radioactive and generally undergo β-decay.
In nuclear fission, about 50 million times more energy is released per atom than in the burning of carbon
When the fission of a nucleus can be induced by neutrons of any energy, the nucleus is said to be fissile. Fissile nuclei include 235U, 239Pu and 233U, and are used as fuel in nuclear reactors.
Fission is a particular outcome of the absorption of a neutron by a nucleus. Other possible outcomes of neutron absorption are radiative capture and charged particle emission.
Instead of being absorbed by a nucleus, a neutron may simply be scattered by it. The lower the mass of the scattering nucleus, the larger is the average kinetic energy loss experienced by a neutron on scattering.
The chance that a neutron will cause fission in 235U is especially large for thermal neutrons, whose energies are low (less than 1 eV).
The chance that a neutron will undergo a radiative capture reaction with 238U is greatest at neutron energies between 10 eV and 1 keV, an energy range called the resonance absorption region.
Fast neutrons (with energies of around 1 MeV) in natural uranium are far more likely to be scattered by 238U nuclei than to induce fission in 235U. This is mainly because the 238U nuclei are much more abundant then 235U in natural uranium. Consequently, in order to induce fission in uranium with fast neutrons, it is very important that the uranium should be enriched in 235U above the natural abundance of 0.72%.
A self-sustaining chain reaction can be established, in which the fission rate (the number of fissions per second) is constant. This is called a critical system
A subcritical system is one in which the number of fissions per second is decreasing; a supercritical system is one in which the number of fissions per second is increasing.
Critical systems can be made based on neutrons with high (around 1 MeV) and low (below 1eV) energy. Reactors that operate using high-energy neutrons (fast neutrons) are called fast reactors; thsoe that use low-energy neutrons (thermal neutrons) are called thermal reactors.
In thermal rectors, scatterinf of the neutrons produced in fission reactions by low-mass moderator nuclei slows the neutrons down before they cause fission.
The chain reaction can be controlled by changing the amount of neutron-absorbing naterial in the reactor core.
If the only available fissile isotope were naturally occurring 235U, which comprises just 0.72% of uranium, the potential of nuclear fission as an energy source would be very limited.
In a nuclear reactor, the more common isotop 238U (99.28% of uranium) can undergo neutron absorption followed by two β-decays to form the fissile 239Pu. Similarly, the even more common 232Th will yield the fissile 233U. These processes could, in principle, bring about a several hundredfold increase in the supply of fissile material.
When the reactor in which such reactions occur produces more new fissile material than it consumes, the process is called breeding; if it produces less, the process is called conversion.
A reactor core consists of rods or pins of nuclear fuel (that is, containing a fissile isoptope), encapsulated in cladding to contain the fission products. A fuel element consists of a number of fuel pins, and a complete reactor core contains several hundred fuel elements.
The heat produced by fission is taken away by a coolant. The coolant is in contact with the cladding, and flows through the fuel elements. The higher the temperature of the stream entering the turbine, the greater is the efficiency of electricity generation, so the coolant in a water-cooled reactor is therefore pressurized to increase its boiling temperature.
In thermal reactors, the fuel elements are surrounded by a moderator, the function of which is to slow down neutrons to thermal energies, where the probability of fission in the fissile isotope is highest.
The power produced in the reactor is regulated using control rods, and the chain eaction is stopped using shutdown rods.
Because of the heat produced by fission-product decay, the cooling of a reactor core must continue even after teh chain reaction has been halted.
The complete core is surrounded by a concrete biological shield, to minimize the radiation exposure to the operating staff.
AGRs have only been built in the UK and were designed to overcome the limitations of the earlier Magnox reactors. AGRs use an enriched (2%) ceramic UO2 fuel, clad in stainless steel. The moderator is graphite, and the coolant is CO2. AGR reactors incorporate a prestressed concrete pressure vessel, which also serves as a biological shield. They have an efficiency of about 40%, which is comparable to that of a fossil-fuelled power station.
The fuel in PWRs is UO2 containing enriched (3%) uranium, clad in a zirconium alloy. Both the moderator and teh coolant are light water. The core is comparatively small, and the power density is nearly 100 kW l-1, which is much higher than for an AGR. They have a thick steel pressure vessel, and the reactor and steam generators are housed in a special containment building. PWRs are the dominant reactors world wide.
The RBMK reactor is unique to the former USSR, although most of its features are to be found in other reactors. It uses enriched (2%) UO2 fuel, clad in a zirconium alloy. The coolant is light water, which boils in the core, and the steam produced drives the turbines directly. The coolant is pressurized using pressure tubes, which form the cooling channels. The moderator is graphite.
Assessments of world uranium resources suggest that, if it confines itself to the fission of 235U in thermal reactors, then at present rates of consumption, the nuclear industry will only be able to supply energy for a few decades.
Theenergy supplied by nuclear power could be increased som 60-fold by using breeder reactors in which 239Pu is made from 238U and by an even larger factor if 239Pu breeding were supplemented by 233U breeding from 232Th.
Fast-breeder reactors have no moderator. A typical fuel consists of 20-30% 239PuO2 mixed with 238UO2. In the absence of a moderator, the core is very small and the power density very high, so liquid metal coolants (usually sodium) are used.
The core is surrounded by a breeding blanket of 238UO2, where most of the new 239PuO2 is formed. The rate at which an electricity generating programme basesd on breeder reactors can grow is greater the shorter the doubling time of the reactors.
Biological systems can be damaged by α-particles, β-particles, X-rays, γ-rays and neutrons, because these kinds of radiation cause ionization within cells.
Such ionization may break bonds in DNA or other biologically significant materials; this is a direct effect of radiation. Secondary damage occurs when ionization subsequently leads to the formation and diffusion of radicals and their reaction products.
The larger the amount of energy absorbed in a radiation doese, the greater will be the ensuing biological damage. A short time between doeses also leads to greater biological damage. The extent of the damage is dependent on the type of radiation.
α-particles tranfer their energy within a short distance. So do the charged particles that are produced by the interaction of neutrons with nuclei. These types of radiation have a high linear energy transfer (LET) and are densely ionizing, the biological damage being concentrated in a few cells or a single cell. The repair mechanisms in the cells cannot alwyas then keep pace with the damage caused.
β-particles and the electrons produced by X-rays and γ-rays, have a longer range than α-particles in biological materials. They have a lower LET than α-particles, and are lightly ionizing. The damage they cause in individual cells can be more readily repaired than α-particle damage.
One measure of radiation dose is the energy that the tissue absobs from teh radiation. It is expressed in grays (joules per kilogram of tissue) and is known as the absorbed dose
Densely ionizing radiations like α-particles are more effective in killing cells than lightly ionizing radiations like β-particles and γ-rays; the relative biological effectiveness. RBE, quantifies these differences, which depend on cell type and the biological effect observed. In estimating the effects of radiation on humnas, the different types of radiation are also assigned a quality factor.
A second measure of radiation dose, the dose equivalent, recognizes this varying biological effectiveness of the different types of radiation. It is the product of the absorbed dose and the quality factor, and is expressed in sieverts.
In order to take into account the varying sensitivity of body organs to radiation, the effective dose equivalent (measured in sieverts) is used. In practice, this quantity is often simply referred to as dose, which should be distinguished from the absorbed dose, measured in grays.
High instantaneous radiation exposures of over 100 mSv have non-stochastic effects. Low exposures have stochastic effects that are expressed as probabilities, notably those of contracting fatal cancers.
Data on such effects have been obtained mainly from studies of excess cancers among survivors of atomic bomb explosions, and patients who have received high doses during radiation treatment.
Risks of stochastic effects at low exposures can be obtained by extrapolating the data referred to in point 2 above to low radiation levels using either a linear or quadratic model.
There is some evidence that low levels of radiation may be beneficial.
The most recent ICRP recommendations for radiation dose limits in excess of natural background are, for teh public, 1 mSv yr-1 averaged over five years and, for radiation workers, 10 mSv yr-1 averaged over five years. The increased risk of a fatal cancer to a worker receiving a 20 mSv dose is estimated to be about 1 in 1 000.
One proposed explanation of some of the leukaemia clusters that occur in the UK is that the immunity of isolated communities to a leukaemia virus is lost when a new town or factory brings an influx of new people.
One leukaemia cluster occurs near a Sellafield reprocessing plant, but no association with parental radiation exposure has been established.
The production of delayed chromosomal abnormalities in stem cells irradiated by α-particles may produce an explantion for a link between childhood leukaemia and parental radiation exposure, but much more work is needed to establish this relationship.
There are five basic stages in the nuclear fuel cycle: uranium mining and extraction, fuel preparation, fuel consumption in the reactor, fuel reporcessing and waste storage/disposal.
In principle, reprocessing, which separates the uranium, plutionium and fission products in spent fuel from each other, can be omitted, and the spent fuel treated as nuclear waste.
The main hazards arising from uranium extraction come from the radioactive daughter products of uranium, the most important of which is the gaseous isotope radon-222.
The main risk from radon during mining is to those working underground, and this risk can be reduced by adequate ventilation. The environmental risk from uranium mining waste can be minimized by burying it.
The production of nuclear fule from yellowcake begins with conversion to pure uranium trioxide, UO3.
If enriched uranium fuel is needed, UO3 is turned into uranium hexafluoride, UF6, which is a gas above 50 deg C.
Enrichement of UF6 then occurs by gaseous diffusion or gas centrifugation. The latter is much the more efficient, having a much higher degree of enrichment at each stage.
The enriched UF6 is converted into enriched UO2 for fuel by heating it with hydrogen and steam.
The principal waste product from the enrichment process is uranium hexafluoride in which the uranium is depleted in 235U. The radiological risks from uranium enrichment and fuel fabrication are very small, and are outweighed by the chemical risks.
During normal reactor operation there are releases of radioisotopes such as 3H, 85Kr and 14C, which result in radiation exposure to the public.
Reactor workers are exposed to these releases, and also to radiation from the reactor andn its fuel.
The current estimate of the collective dose leading to one excess cancer death is 20manSv. This implies that over the lifetime of a PWR, with a staff of 400 and a collective dose of 45 man SV over 30 years, there will be about two excess cancer deaths among the workers. The risk to the public is very much smaller than this.
Spent PWR fuel consists of about 95% 238U, 0.9% 235U, 2.5% fission products and 1.6% plutionium. The spent fuel is about 100 million times more radioactive than fresh fuel; this is mainly due to the fission products.
The handling of spent fuel involves the management of nuclear waste, which, for the purposes of disposal, is divided into three categories: low, intermediate and high level. High-level waste requires cooling facilities.
Spent uranium metal fuel has been reprocessed at Sellafield for many years. From 1994, oxide fuels from British and foreign reactors have been reporcessed in a new plant, known as THORP.
One year's operation of a 1 000 MW PWR produces about 4 m3 of spent fuel, which, on reporcessing, yields 2.5 m3 of high-level waste, 40 m3 of intermediate-level waste and 600 m3 of low-level waste.
Current marine discharges at Sellafield are much lower than in the 1970s, and the estimated risks to the general public are small. However, 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.
There are two basic strategies for dealing with nuclear waste: firstly, sorage of spent fuel followed by later disposal; secondly, storage and then reprocessing of spent fuel, followed by storage of the highly active fission-product waste and its subsequent disposal.
The currently favoured method of disposal is burial in geological formations deep underground.
High-level wastes should be allowed to decay for hundreds or thousands of years in a secure location before they can be allowed to enter the environment. For the initial decades, they must be cooled because of the heat produced by fission-product decay.
Waste disposal of deep underground might be mobilized by groundwater, which can move through interconnected voids in the rock.
The principal criterion for a high-level waste depository site is that it has a low hydraulic gradient; rocks such as clays or granites, which often have low hydraulic conductivity, fulfil this criterion. Groundwater flow velocities in these rock types are low. Evaporites, the very existence of which (since they are water soluble) demonstrates that they have not been penetrated by water, may also be suitable.
It may be difficult to establish the suitability of a site because the distribution of fissures, which are partly responsible for groundwter flow, may be uneven and unpredictable.
Proposals for high-level waste disposal currently centre on the placement of vitrified wste in depositories at depths of between 300m and 1 000m.
In such depositories there would be multiple barriers to prevent the radioisotopes from entering the environment, including encapsulation of the vitrified waste in metal containers and the use of a surrounding material that restricts transport of radioisotopes.
There are no firm proposals at present to build such a depository in the UK, following opposition to test programmes in the 1970s.
Nearly two thousand million years ago, there were natural reactors at Oklo, in Gabon. Study of the distribution of fission products and the idotopes in their decay chains has shown that very few of them have migrated far from their site of formation, particularly the plutonium isotopes.
Since the migration of radioisotopes is site specific, it is not possible to draw any general conclusions from the Oklo phenomenon.
The fact that there is now always strong local opposition to nuclear waste depositories of any kind has meant that the choice of sites has become primarily a political issue. As a consequence, sites cannot be chosen only on the basis of their geological suitability.
There are strong differences of opinion on whether high-level waste disposal can ever be undertaken with acceptable safety. The main factors giving rise to these doubts centre on whether it is possible to make predictions of radioisotope transport and geological stability over the very long time-scales involved.
The case for nuclear fuel reprocessing being part of wste management is weakened if there is not a serious commitment to a fast=breeder reactor programme.
Both economic and radiological arguments favour the delaying of reprocessing in countries where no failities exist. However, in the UK, the large capital investments that have already been made in reprocessing and vitrification plant must be taken into account.
The principle threat to the public from an accident in a nuclear reactor is the threat of the release of the fission product radioactivity in the core into the environment.
The Windscale accident of 1957 occurred because the reactor normally operated at only 150 deg C, a temperature at which strain energy produced by irradiation built up in the graphite moderator.
Routing release of the strain energy was achieved by the increase in temperature which occurred when the fan-driven air-cooling was stopped. On one occasion this was not fully effective. The energy released caused an excessive rise in temperature, and a subsequent fire in the fuel and graphite; the fire was eventually put out with water.
Sustantial amounts of volatile radioisotopes were released, which were estimated to have been responsible for 32 additional cancer deaths.
The 1979 accident at Three Mile Island began with rising coolant temperature and pressure caused by the failure of a pump for feedign water to the steam generator.
The resultant rise in temperature and pressure automatically opened a pressure relief valve in the primary coolant circuit and shut down the reactor.
The pressure relief valve failed to close when the coolant pressure decreased, and water was continuously lost from the primary cooling circuit. This loss of water became more damaging when the operators misread the situation and turned off the automatically triggered emergency core cooling system. The level of teh coolant eventually fell below the top of the fuel rods.
Heat from fission product decay caused the dwindling coolant to boil and the fuel to partially melt. The zirconium alloy cladding reacted with steam, producing hydrogen gas.
Coolant containing fission products flooded the basement of the containment building, but very little radioactivity was released to the environment.
In the version of the RBMK reactor used at Chernobyl there was a positive void coefficient at low pwer, which tended to increase the power produced when the coolant boiled.
In the 1986 accident at Chernobyl, zenon poisoning led the operators to run the reactor at low power with nearly all the control rods withdrawn from the core. To do this, they blocked automatic emergency shutdown mechanisms.
The positive void coefficient then led to an uncontrollable power surge, which blew the cap off the reactor; sections of the core disintegrated and melted, the graphite caught fire, and a radioactive cloud was dispersed over much of Europe.
It is difficult to estimate the excess cancer deaths that the accident will cause. World-wide figures varying from tens of thousands to hundreds of thousands have been proposed by different organizations.
In the UK, some restrictions on the consumption and movement of sheep affected by the uptake of radioactivity were still in force ten years after the accident.
Because nuclear accidents are so infrequent, the risk that one will occur cannot be estimated from experience or statistics.
Risk estimates can be made by specifying all conceivable sequences of events which culminate in accidents, and then calculating the risks of the accidents as multiples of the estimated probabilities of the sequential contributing events. The probabilities of at least some of the contributing events will be derivable from experience and statistics.
The best-known attempt to do this is the Rasmussen Report, which assessed the risks associated with 100 commercial nuclear power plants operating in the USA.
The Rasmussen Report concluded that the risks of death from the 100 nuclear power plants were comparable with those from meteorites, adn hence very low. The risk estimates in the report were assigned uncertainties of a factor of ten, up or down.
Since the 1970s, there has been a trend towards diminished state control of energy production in the UK. The energy industries have therefore been judged more in terms of cost and short-term returns on investment, rather than by other criteria such as security of supply and balanced diversity, which were favourable to nuclear power, and supported by central planners.
The seasonal and daily variations in demand for electricity require a flexible means of supply. There must be some power stations that can work continuously to provide the base load. Demands above the base load are managed by starting up or shutting down other generators when necessary.
The financial success of any commercial project depends on accurately estimating the capital cost of the project, its running costs and the demand for the product.
Factors outside the control of the project management which can change these estimates significantly include changes in interest rates, in safety requirements or simply in demand.
Finally, in very complex, long-term projects, particularly those for which there is little previous experience, estimating development and construction costs can be very difficult. The estimates are more often than not in error, and costs are invariably underestimated.
Discounted cash flow analysis is a technique that an organization can use when trying to predict and compare the profitability of future long-term projects. As used here, it works with figures that have been assumed to have an inflation rate of zero.
The positive and negative cash flows over the lifetime of the project are converted to their positive and negative present values, the values they would have at the start of the project. To make these calculations a discount rate has to be estimated; this is an interest rate that might be earned by alternative investment.
If the eum of the present values, the net present value, is positive, the project is deemed to be more profitable than the alternative investments.
Another way of using discounted cash flow analysis to compare project profitabilities is to compare internal rates of return. The internal rate of return is the discount rate that yields a net present value of zero.
In justifying the PWR reactor at Sizewell, the CEGB used the method of net effective cost. This subtracts the savings that arise from not using older, less efficient plant, from the cost of new plant.
In the UK, a substantial proportion of the research and development costs of nuclear power were borne by the government through the UKAEA. These 'hidden costs' of nuclear power were therefore covered by a government subsidy.
Between 1975 and 1987, a high proportion of government investment in energy research was devoted to fast-breeder reactors rather than to energy efficiency, renewable energy sources or nuclear fusion, although that proportion declined substantially with time. The UK fast breeder programme was terminated in 1994.
The chief differences between the costs of nuclear and fossil-fuelled power stations are the costs of (i) construction, (ii) fuel, and (iii) decommissioning.
For fossil-fuelled stations, the largest cost component is due to the fuel, whereas for nuclear power stations the capital cost component is the dominant one. As a consequence of these differences, comparisons of estimated costs for the two types of power station are sensitive to the economic assumptions made.
The costs of decommissioning andn waste storage are influenced by (i) the actual cost, (ii) when it takes place, and (iii) what discount rate is assumed if the cost is to be met by investment. The absence of any experience of the decommissioning of large reactors, combined with the long delay before it will be completel, make these costs very hard to estimate.
Nuclear weapons are of two kinds; in a fission weapon, a subcritical mass of a fissile isotope centred on a neutron source is made critical by explosive compression; in a fusion weapon, a fission weapon is used to heat and compress a mixture of tritium and deuterium. A fission weapon is therefore an essential feature of both types of weapon.
The fissile isotopes used in fission weapons consist of about 10 kg of 235U or 239Pu with isotopic purities of over 90%. An enrichment plant is needed to make the 235U; a nuclear reactor and a reporcessing plant are needed to make the 239Pu.
It is not necessary to have a nuclear power programme to make weapons-grade plutionium; a single small reactor is sufficient.
Plutionium that is over 90%239Pu is produced only at low fuel burn-up. Higher burn-up yields a higher proportion of heavier isotopes such as 240Pu, which reduces the effectiveness of the weapon as a result of spontaneous fission.
There can only be a connection between a civil nuclear power programme and nuclear weapons if the civil programme contains enrichment or reprocessing facilities.
Nuclear wepons proliferation is supposedly controlled and prevented by the Non-proliferation Treaty. This gives th IAEA the power to inspect nuclear installations.
The case of Iraq suggests that without the full cooperation of signatories, it is very difficult, under normal circumstances, to make the Treaty fully effective.
Proper international control and inspection of both reactors and reporcessing facilities is one way of weakening possible links between nuclear power and nuclear weapons.
Of the fusion reactions that have been considered as power sources, that between deuterium and tritium produces most energy.
Because the positively charged nuclei repel one another, controlled fusion reactions must be conducted at very high temperatures while the nuclei are contained and squeezed together, for example by intense magnetic fields.
After more than 50 years of research, controlled fusion has been achieved for very brief periods of time, but it has not yet been harnessed to produce electricity. Those in the field still speak of a fusion reactor for producing electrical power being decades away.
The UK is extremely well endowed with renewable energy resources.
The developments necessary to use renewable energy sources for electricity production have already been made for wind, tidal, solar, hydro and biofuels, although improvements are possible. Others, like the use of waves and geothermal energy, require differing degrees of development.
Because some of the renewable energy sources are unpredictable, and others are predictable but not continuous, back-up electricity supplies are needed if electricity is to be available on demand.
The cost of producing electricity from renewable energy sources varies widely, but none currently produce enlectricity as cheaply as is possible using fossil fuels.
At present, the total contribution that the renewable energy sources make to UK electricity production is less than 3%, although the potential contribution is much larger than this.
Between 1960 and 1994, UK energy consumption increased by 30%, while world energy consumption trebled. In the UK there was a big shift away from coal to the use of natural gas and oil, the latter being important for transportation.
The UK is very well endowed with fossil fuels compared to most Western European countries, and is self-sufficient in energy terms.
Fuel substitution occurs in electricity production to meet changes in fuel supplies. The substitutions may be short term - to cover strikes - or long term, to adjust to changing fuel costs.
The government uses the Non-Fossil Fuel Obligation to encourage the use of non-fossil fuels for electricity production. This results in a premium being paid for electricity from such sources. The obligation will not apply to nuclear power after 1998.
The alternative, renewable, sources provided less than 3% of the UK's electricity in 1994, but the supply from some sources, notably onshore wind power and biomass, are increasing quite quickly.
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