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The Science of Nuclear Power

Summary

Nuclear Power is produced when a nucleus absorbs a neutron and splits into two lighter nuclei. This releases enormous amounts of energy which in turn produces heat. In fact the Uranium, which is the most common element used to produce nuclear power today, has an energy content about 3 million times greater than that of fossil fuel. Consequently 1 gram of Uranium is equivalent to approximately 3 tonnes of coal. Nuclear reactors harness the heat which is produced from the energy released when the atom splits and convert it into electrical energy. Current Nuclear Power plants require the the use of the rare Uranium isotope U-235 and consequently only use one fiftith of the total energy content. Next generation reactors forecast to be available in 2020's will use all the energy in Uranium or the more abundant Thorium.

Nuclear reactors produce vast amounts of radioactive waste including large amounts of very long lived radioactive atoms. These radioactive particles are a product of the splitting of the atom. We are constantly exposed to low-level radioactivity from cosmic rays from outer space and naturally occurring radioactive isotopes which in general do not cause any harm. However at high levels of exposure there are numerous biological effects of radiation. These cause cell death, cancer induction and can cause genetic damage. The waste of nuclear reactors is highly radioactive and long lived, and as a consequence must be isolated from humans for around 100,000 years. The current concensus is that Nuclear Waste should be disposed in secure containers and placed deep underground. Future technology promises to turn the long lived radioactive particles into shorter lived atoms.

Physics of fission

Atoms consist of an electron cloud and a nucleus. The electrons each have the same mass and the same negative electric charge. The mass of an atom is given almost entirely by the nucleus which consists of protons and neutrons. Protons have a positive electric charge and neutrons have no electric charge. The chemical properties of atoms are governed by the number of electrons in the cloud. These match the number of protons in the nucleus and the electric charge of the electron and proton balance exactly.

One nucleus will spontaneously transform into a different nucleus if the final state nucleus is more stable and if the laws of physics allow the transformation. This process is usually accompanied by the release of ionizing radiation and is often called "radio-active decay". Nuclei that exhibit this behaviour are said to be "unstable" or "radioactive". Most of the matter found naturally on Earth is stable and does not undergo this transformation. Some examples of common radiactive isotopes found naturally that have this property are 40K (potassium-40) which is present in seawater and many salts, 14C (Carbon-14) and Uranium and Thorium.

Nuclear Fission energy is released when a very heavy atomic nucleus absorbs a neutron and splits into two lighter fragments. The energy release in this process is enormous. It is 10 million times greater than the energy released when one atom of carbon from a fossil fuel is burned.

As this process happens, heat is produced. The heat is converted into electricity via conventional steam and gas turbines like those used at Fossil-fuel based power plants. Harnessing this heat is an engineering problem.

There are 3 nuclear isotopes of importance to nuclear power that exhibit this behavior.

These are: 235U (Uranium-235) , 239Pu (Plutonium-239) and 233U (Uranium-233). Of the 3, only 235U is found naturally on Earth. Natural Uranium found on Earth consists of 99.3 % 238U and 0.7% 235U. The two other isotopes, 239Pu and 233U can be created from the far more abundant 238U and Thorium nuclei via advanced Nuclear techniques.

Some basic Nuclear Physics

Radioactive Waste from Nuclear Reactors

The generation of electricity in a Nuclear Power Plant is made by splitting uranium atoms. The uranium used as fuel in a nuclear plant is formed into ceramic pellets about the size of the tip of your little finger. The uranium atoms in these pellets are bombarded by atomic particles, they split (or fission) to release particles of their own. These particles, called neutrons, strike other uranium atoms, splitting them. When the atoms split, they also release heat. This heat is known as nuclear energy and is essentially responsible for creating electricity.

Other reactions also take place in the nuclear reactor such as neutron capture. Neutron capture is a term used for the scenario where a neutron comes close to a nucleus (in this case uranium) and the nucleus captures it and becomes a different nucleus. In this case when uranium-238 captures a neutron it becomes uranium-239. After uranium-239 emits a beta particle (electron) it becomes neptunium-239. Then, neptunium-239 emits a beta particle and becomes plutonium-239. The plutonium can also be used as nuclear fuel.

Certain changes take place in the ceramic fuel pellets during their time in the reactor of the nuclear power plant. The particles left over after the atom has split are radioactive. During the life of the fuel, these radioactive particles collect within the fuel pellets. The fuel remains in the reactor until trapped fission fragments begin to reduce the efficiency of the chain reaction. Some of the fission products are various isotopes of barium, strontium, caesium, and iodine. The spent fuel also contains plutonium and uranium that was not used up. The fission products and the left over plutonium and uranium remain within the spent fuel when it is removed from the reactor and are called high level waste as they are both thermally hot and very radioactive.

Half-Life of Radioactive Isotopes

An atom that is radioactive will decrease its radioactivity in time and eventually decay. How fast the radioactivity decreases depends on the half-life. The half-life is defined as the time it takes for half of the radioactivity to decay. Hence an isotope with a short half-life will decay quickly. The half-life is also inversely proportional to the intensity of racioactivity. Therefore the higher the intensity of radioactivity the shorter the half-life.

The approximate half-lives of some of the isotopes in the spent nuclear fuel are listed below:

Isotope Half-life
Strontium-90 28 years
Caesium-137 30 years
Plutonium-239 24,000 years
Caesium-135 2.3 million years
Iodine-129 15.7 million years

How Nuclear Reactors Work

When an atom undergoes fission it splits into smaller atoms, other particles and releases energy. Read about the physics of fission. It turns out that it is possible to harness the energy of this process on a large enough scale for it to be a viable way of producing energy. Read about how power plants work.

The fundamental point about nuclear energy is that the energy content of 1 gram of Uranium is equivalent to approximately 3 tonnes of coal. This means that we need to consume about 3 million times less material with Nuclear Power compared to using Coal or any other Fossil Fuel. This substantially reduces the volumes of fuel and waste of nuclear power compared to Fossil Fuels.

The Different Types of Nuclear Reactors

There are are a number of different types of Nuclear Reactors currently in operation throughout the world. Some of the most common types are described here.

Pressurized Water Reactors

Pressurized Water Reactors (PWR's) are by far the most common type of Nuclear Reactor deployed to date. Ordinary water is used as both neutron moderators and coolant. In a PWR the water used as moderator and primary coolant is separate to the water used to generate steam and to drive a turbine. In order to efficiently convert the heat produced by the Nuclear Reaction into electricity, the water that moderates the neutron and cools the fuel elements is contained at pressures 150 times greater than atmospheric pressure. You can find out more details of PWR reactors at the following link.

Pressurized Water Reactors.

Boiling Water Reactors

In a Boiling Water Reactor (BWR), ordinary light water is used as both a moderator and coolant, like the PWR. However unlike the PWR, in a Boiling Water Reactor there is no separate secondary steam cycle. The water from the reactor is converted into steam and used to directly drive the generator turbine. These are the second most commonly used types of reactors. Find out more about Boiling Water Reactors at the following link.

Boiling Water Reactors.

High Temperature Gas Cooled Reactors.

High Temperature gas cooled reactors operate at significantly higher temperatures than PWRs and use a gas as the primary coolant. The nuclear reaction is mostly moderated by carbon. These reactors can achieve significantly higher efficiencies than PWRs but the power output per reactor is limited by the less efficient cooling power of the gas. You can find out more about these types of reactors at the following link.

High Temperature, Gas Cooled pebble reactors

Heavy Water Reactors

Heavy Water reactors are similar to PWRs but use water enriched with the deuterium isotope of Hydrogen as the moderator and coolant. This type of water is called "heavy water" and makes up about 0.022 parts per million of water found on Earth. The advantage of using Heavy water as the moderator is that natural, unenriched Uranium can be used to drive the nuclear reactor. You can find out more about these reactor types at the following link.

CANDU Heavy Water Reactors

Energy Lifecycle of Nuclear Power

The performance of Nuclear Power can also be measured by calculating the total energy required to build and run a Nuclear Power plant and comparing it to the total energy it produces. The following set of calculations is also taken from the independently audited, Vattenfall Environmental Product Declaration for its 3090 MW Forsmark power plant. A more detailed description is here. Vattenfall have also made available the aggregated data set as a spreadsheet. You can download it from here.

The following table displays the source and the amount of energy required to produce 1 KW-Hr of electricity from the Forsmark power plant. The table includes the energy used in construction of the plant, mining the Uranium, enriching it, converting it to fuel, disposing the waste and decommissioning the plant. The Forsmark plant is assumed to run for 40 years. There is an additional 0.026 grams of Uranium consumed in generating this one KW-Hr of electricity. This 0.026 grams includes the Uranium used to generate power at Forsmark and the Uranium consumed by the French Nuclear Power plants that produced the electricity that enriched the Forsmark Fuel.

Energy Source Contribution by mass Conversion to Energy Energy Contribution
Coal 0.467 grams 0.00676 KW-Hr/gram 0.0031 KW-Hr
Crude Oil 0.32 grams 0.011 KW-Hr/gram 0.0035 KW-Hr
Lignite 0.234 grams 0.0038 KW-Hr/gram 0.00089 KW-Hr
Natural Gas 0.115 grams 0.015 KW-Hr/gram 0.00173 KW-Hr
Hydro-Electricity 0.00146 KW-Hr 1 0.00146 KW-Hr
Wood 0.041 grams 0.0042 KW-Hr/gram 0.00017
Total     0.0107 KW-Hr

So the Forsmark Plant produces 93 times more energy than it consumes. Or put another way, the non-nuclear energy investment required to generate electricity for 40 years is repaid in 5 months. Normalized to 1 GigaWatt electrical capacity, the energy required to construct and decommission the plant, which amounts to 4 Peta-Joules (PJ), which is repaid in 1.5 months. The energy required to dispose of the waste is also 4 PJ and repaid in 1.5 months. In total this is less than 0.8% of the all the electrical energy produced by the plant.

The calculations of the operating energy costs include the energy required to mine and mill the Uranium. In the case of the Forsmark power plant some of the Uranium is sourced from the Olympic Dam mine in South Australia. This mine has a rather low Uranium concentration (0.05% by weight). A detailed and audited environmental description of the Olympic Dam mine is available here. A succinct description of the energy inputs of the mine is here. These data show that the Olympic Dam mine supplies enough Uranium for the generation of 26 GigaWatt-years of electricity each year (including the Uranium needed to run the power plants for enrichment). The energy consumed by the the mine is equivalent to 22% of a GigaWatt-Year. The energy gain is over a factor of 100. The Olympic Dam mine energy cost includes the energy required for mining and smelting it's huge Copper production.

Another Uranium source for Forsmark is the Rossing Mine in Namibia. A description of the operations of the mine is available here. The Rossing mine produced 3037 tonnes of Uranium in 2004, which is sufficient for 15 GigaWatt-years of electricity with current reactors. The energy used to mine and mill this Uranium was about 3% of a GigaWatt-year. Thus the energy produced is about 500 times more than the energy required to operate the mine.

It is worth noting that the widely quoted paper by Jan Willem Storm van Leeuwen and Philip Smith (SLS), which gives a rather pessimistic assessment of the Energy Lifecycle of Nuclear Power, assumes a far larger energy cost to construct and decommission a Nuclear Power plant (240 Peta-Joules versus 8 Peta-Joules(PJ)). The difference is that Vattenfall actually measured their energy inputs whereas Willem Storm van Leeuwen and Smith employed various theoretical relationships between dollar costs and energy consumed. This paper also grossly over-estimates the energy cost of mining low-grade Ores and also that the efficiency of extraction of Uranium from reserves would fall dramatically at ore concentrations below 0.05%. Employing their calculations predicts that the energy cost of extracting the Olympic Dam mine's yearly production of 4600 tonnes of Uranium would require energy equivalent to almost 2 one-GigaWatt power plants running for a full year (2 GigaWat-years). You can follow this calculation here. This is larger than the entire electricity production of South Australia and an order of magnitude more than the measured energy inputs.

The Rossing mine has a lower Uranium concentration (0.03% vs 0.05% by weight) than Olympic Dam and the discrepancy is even larger in the case of Rossing. Here SLS predict Rossing should require 2.6 Giga-Watt-Years of energy for mining and milling. The total consumption of all forms of energy in the country of Namibia is equivalent to 1.5 GigaWatt-Years, much less than the prediction for the mine alone. Furthermore, yearly cost of supplying this energy is over 1 billion dollars, yet the value of the Uranium sold by Rossing was, until recently, less than 100 million dollars per year. Since Rossing reports it's yearly energy usage to be 0.03 GigaWatt-years, SLS overestimates the energy cost of the Rossing mine by a factor of 80.

Additionally SLS predict that the yield of of Uranium extracted from low grade Ores will fall to 0 at concentrations of 0.0002% (2 ppm). The Olympic Dam mine extracts gold at high efficiency at concentrations of 0.0005% (5 ppm (parts per million)) and there are many other gold mines which produce gold profitably at this concentration. Given the huge Uranium reserves present at 5 ppm, it unlikely we will ever need mines that operate lower than this.

The energy cost of the Olympic Dam mine is the sum of all the operations for mining Copper, Uranium, Gold and other minerals. It is therefore the upper limit of the energy cost of extracting the Uranium alone. The Rossing mine which only produces Uranium, is a better measure of the energy cost future Uranium mines.

It is interesting to see what effect using the correct energy cost of 8 PJ for the construction, decommissioning and waste disposal of a power plants and the measured energy consumption of the Rossing mine has within the methodology of Willem Storm van Leeuwen and Smith. If we assume that the energy cost of extraction scales inversely with concentration and employ the Rossing experience as a benchmark, ore concentrations as low as 0.001% (10 ppm) provide an energy gain of 16. This also (and very unrealistically) assumes no further progress in mining technology or efficiency improvements in Nuclear Power operations over the course of hundreds of years. As shown here there is an estimated 1 trillion tonnes of Uranium at concentrations of 10 ppm or higher within the Earth's crust. This provides a resource over a factor of 300 times greater than predicted by Willem Storm van Leeuwen and Smith to be recoverable. So once the correct energy cost for plant construction and mining operations are used, the work of Willem Storm van Leeuwen and Smith imply that resource exhastion will not be a problem for Nuclear Power for the foreseeable future.

Storm and Smith have released a rebuttal of this argument. You can find this rebuttal here. We have responded in detail to the questions raised by Storm and Smith. You can find our reponse here. Storm and Smith have issued a rebuttal to our response. It is here. Our answer to this is here.

Our additional investigations strengthen the conclusion that there is far more minable Uranium than predicted by Storm and Smith.

Advanced Nuclear Fission Technology

Please read the physics of fission before reading this section.

Uranium Fast Breeder Reactors

Natural Uranium consists of 0.7% 235U and 99.3% 238U. All commercial Power reactors used in the world today utilize the 235U component in natural Uranium as the primary means of maintaining a chain reaction. The most troublesome component of nuclear waste are the tran-Uranic elements that occur when 238U captures a neutron and transmutes to 239Pu. Further neutron captures on this element lead to a buildup of long-lived transuranic nuclei. However 239Pu is also fertile and undergoes fission like 235U. Advanced reactor designs exploit this to convert the 238U to 239Pu. If the reactor avoids the slowing down, "thermalization" of neutrons, there are sufficient excess neutrons that it is possible to convert more 238U to 239Pu than 235U is consumed. These reactors use the unmoderated "fast" neutrons directly produced via the fission process.

Thus these reactors "breed" 239Pu from 238U and so produce more fuel than they consume. The use of fast-breeder technology makes it possible to increase the efficiency of Uranium use by over a factor of 50. It is then possible to exploit the vast quantities of depleted Uranium stockpiled around the world to generate electricity. In addition the excess neutrons can be used to transmute the long-lived transuranic waste from current Nuclear Power reactors to ever-heavier isotopes until they eventually fission. Thus these reactors can be used to "burn" the most troublesome component of nuclear waste.

There have been a number of reactors that demonstrated that the Fast-Breeder concept works in principle at large scale. The superphenix and monju reactors in France and Japan have had serious technical issues and superphenix has been shut down. This type of reactor is more costly to construct and more difficult to operate than a conventional second-generation Power Reactor. Never-the-less these experiments have provided valuable data that will be used as input for the "Fourth Generation" nuclear reactors presently under research and development.

Thorium Breeder Reactors

Thorium is an element that is 3 times more abundant than Uranium on earth. It has a single stable isotope 232Th. In a nuclear reactor this isotope can capture a neutron and be converted to 233U. 233U undergoes fission like 235U and 239Pu. However when 233U fissions it releases more neutrons than either 235U or 239Pu. Consequently it is possible to to construct a breeder reactor that utilizes thermal neutrons to both generate energy and to breed 233U from Thorium given sufficient initial quantities of 233U mixed with 232Th.

A further advantage of Thorium breeders is that the amount of transuranic waste is vastly decreased compared to a Uranium or Plutonium based reactor.

The Indian Nuclear Power program has a long term goal of utilizing the Thorium cycle to obtain energy independence.

Accelerator Driven Fission.

Over the last 10 years the idea of using a high current, high energy accelerator to drive Nuclear Fission has emerged. These are the Accelerator Driven Systems or ADS. Unlike a Nuclear Reactor, an ADS drives Nuclear reactions that will stop if the proton beam from the accelerator stops. Thus there is no prospect of nuclear chain reactions proceeding out of control. ADS can be used to either generate energy or to transmute transuranic long-lived waste into much shorter lived radio-nuclei, or they can be used to do both. In addition these systems hold prospect of efficiently burning 238U and naturally occurring Thorium, which is 3 times more abundant than Uranium. Over the last 10 years the ADS has progressed from proof of principle experiments to one tenth scale test facilities.

More details of Accelerator driven Fission systems are found here

The Fourth Generation Reactors

In 2002 the Gen iV Internation Forum (GIF) nations (Argentina, Brazil, Canada, France, Japan, Korea, South Africa, Switzerland, Russia, United Kingdom and the United States of America) proposed a long term research and developement program to investigate 6 promising new reactor designs.

The six design concepts are:

These reactor concepts are designed to address the energy needs of the World into the far future (post 21st century).

  • They efficiently utilize Uranium (many can employ depleted Uranium or "spent" fuel from current reactors).
  • Destroy a large fraction of nuclear waste from current reactors via transmutation.
  • Generate Hydrogen for transportation and other non-electric energy needs.
  • Be inherently safe and easy to operate.
  • Provide inherent resistance to Nuclear Weapons proliferation.
  • Provide a clear cost advantage over other forms of energy generation.
  • Carry a financial risk no greater than other forms of energy generation.

These reactor concepts are at various levels of development. The first deployments of Generation IV reactors are not expected until 2015. Most will not be ready before 2025. However the long term potential of these projects is enormous. For example one Molten-Salt Reactor designed to consume one tonne of Uranium per year, could supply sufficient Hydrogen to supply 3 million passenger vehicles. The waste from the plant's year's operation would occupy half the volume of a typical domestic refrigerator. The radioactivity of the waste would diminish to background levels in about 500 years.

Radiation

Dose

The biological effects of heavy particle ionising radiation are measured with a quantity called Absorbed Dose. However, while two differing particles may have the same amount of energy, they may have very different responses from our living, biological cells, so a factor called Relative Biological Effectiveness (RBE) is also used. RBE is a measure of the amount of cell damage a given type of particle causes as compared to a dose of x-rays with the same energy. The combination of RBE with Absorbed dose is measured in units called Sieverts.

Estimated dose (Micro-Sieverts) Activity
5 sleeping next to your spouse for one year
10 a year of watching TV at an average rate
10 a year of wearing a luminous dial watch
10 a year of living in the USA from nuclear fuel and power plants
10 a day from background radiation (average, varys a lot throughout the world)
20 having a chest x-ray
65 flying from Melbourne to London, via Singapore
300 Yearly dose due to body's potassium-40
460 maximum possible offsite dose from Three Mile Island Accident
400 - 1000 Average annual dose from Medical sources
7,000 having a PET scan
8,000 having a chest CT (CAT) scan
50,000 off-site dose from accident at Chernobyl Nuclear Power Plant (estimates vary widely)
2,000,000 Typical single dose to Cancer region from Radiation Therapy
700,000 - 13,000,000 staff and firefighters at the Chernobyl Nuclear Power Plant during and immediately after the accident
65,000,000 Typical total dose to Cancer region from Radiation Therapy

Background, naturally occurring environmental sources of radiation include cosmic radiation (which occurs when subatomic particles from outside the solar system interact with the Earth's atmosphere and produce a shower of gamma rays, neutrons and leptons), terrestrial radiation (due to naturally occurring radionuclides such as uranium, radium and thorium which are present in rocks, soil and water), internal radionuclides (most common is potassium-40 which emits beta and gamma particles as it decays, and may be found as a fraction of the total amount of potassium in the body). The largest source of our dose from background radiation is from radon-220 and radon-222 gases, which are the airborne products of the decay of terrestrial uranium. Inhalation of radon gas contributes approximately two-thirds of the dose that is received from natural, background sources.

Biological Effects of Radiation.

Thus we can see that during our normal daily activities we are continually exposed to ionising radiation both from natural and human-made sources. The passage of ionising radiation through the body may produce adverse biological effects. The effects of concern are primarily, although not solely, due to damage to the genetic material inside the cell; the DNA or `double-helix' molecule that carries genetic information. The current scientific data suggests that there is no safe minimum, or threshold, for adverse radiation effects on the DNA of biological systems and that even small doses can produce consequences for the organism. The low-dose response of biological systems it believed to be linear - that is, smaller doses of radiation produce a proportionately smaller risk of adverse effects like a straight-line graph.

The biological effects fall into the following categories;

Cell Death or Apoptosis

- biological systems are composed of many individual tiny cells; each with its own DNA (only red blood cells don't have any DNA - they lose it during development). If there is catastrophic damage to vital cell function by radiation energy absorption the cell may cease to function and `die'.

Cancer Induction

- in this case the DNA is damaged or altered but the alteration is not lethal to the cell. If the normal regulator genes which control the rate at which cells divide and die are rendered malfunctioned the cell may become `immortal' and multiply at an abnormal rate producing an out-of-control growth of a line of abnormal cells. This is a cancer. Most tissues can produce cancers with enough radiation damage but rapidly dividing tissue lines, such as blood-forming `haemopoietic' lines which may produce leukemias, are particularly vulnerable. The risk for cancer production in the general population is estimated to be approximately 0.06 cases per million Micro-Sieverts of absorbed dose.

Genetic Damage to Future Generations

- this can arise because mutations, or changes in the pattern of bases in the DNA, can occur in the DNA that ends up in sperm or eggs and becomes a permanent feature of any resulting babies. Most often, radiation induced damage to such egg or sperm DNA is incompatible with the life of the fetus in utero but there is a finite chance of a live baby being born with defects. This is of particular concern because the damage to the genetic material can then be passed on to all future generations and become a permanent feature of the gene-pool; damaging many individuals. Of course, such mutations are also a natural part of life and the evolution of biological systems. The concern is that we do not want to increase the mutation rate above the natural background rate. The estimated risk of permanent damage to a second generation (grandchild) individual is 0.02 cases per million Micro-Sievert of exposure.

Dose-Response Tissue Reactions

- or `radiation burns'. These are the sorts of effects seen immediately after the bombing of Hiroshima; but lesser effects can occur with smaller doses. However, there does seem to be a `threshold' for this kind of effect with very small doses encountered in normal life (apart from sun-burn!) not producing detectable damage.

More information on the biological effects of radiation can be found at the International Commission on Radiological Protection web site.

A Study on Childhood Cancers in Great Britain

A study on the incidence of childhood cancer around nuclear power plants in Great Britain by the Health Protection Agency for the committee on Medical Aspects of Radiation in the environment concluded that there have been no increase in childhood cancers for children living less than 25km from a nuclear power plant. The report was published in 2005.

Download the report here.

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This page, its contents and style, are the responsibility of the authors and do not necessarily represent the views, policies or opinions of The University of Melbourne.