Nuclear Energy – The Facts Behind The Fuss
School of Physics & Astronomy
The University of Manchester
Sizewell B is a nuclear power station on the Suffolk coast. It is the UK’s only Pressurised Water Reactor. Its construction began in 1988 and it started generating electricity in 1995.It is scheduled to be decommissioned in 2035.
Total power supplied to the national grid is 1198 MW.
There are approximately 520 full time EDF Energy employees plus over 250 full time contract partners.
The thermal efficiency of the reactor is 33% (3000MWth).
The reactor fuel is uranium dioxide (UO2), which is formed into a ceramic fuel pellet that is compatible with the water coolant. The fuel pellets are stacked into a Zircaloy clad fuel rod.
Many fuel rods are placed in a square lattice to construct a fuel assembly (see below). 200 17×17 fuel assemblies with 300 pellets per rod are generally needed to fuel the entire reactor core
The reactor core is housed in a reactor pressure vessel that is composed of steel about 25cm thick.
3 GW is generated by 17,340,000 pellets
The Pellets are about 1 cm in length producing a linear power of about 200 W/cm ~ 20 kW/m
The Pellet’s mass is about 5g producing a specific power of about 40 W/g.
If the Fuel Pellet remains in the reactor for three years then the total energy produced by one pellet is given by
3 x 365.25 x 24 x 3600 x 200 W or Js^-1 ~ 20 GJ
Total energy per unit mass = 20 GJ/5 g = 4 GJg^-1 = 4000 GJkg^-1 = 4000 TJ^t-1
= 46,000 MWdt-1 (MW per day per tonne) = 1.1 Billion kWht^-1
Now compare this with the energy released by burning gas (0.4 MWdt^-1) and eating Chocolate (0.2 MWdt^-1) t = tonne
% Nuclear energy used to produce electricity in different countries:
~ 20% USA
~ 80% France
~ 32% Switzerland
~ 30% Japan – pre Fukushima
~ 16% Russia
~ 5% Mexico
~ 2.5% Brazil
– ~14% Worldwide
A simple electric generator that school students learn about consists of a coil of wire rotating inside a magnetic field.
As the coil rotates it cuts the magnetic field lines between the N and S poles generating an alternating voltage. You get maximum voltage when the movement of the coil, the magnetic field lines and the conductor cutting the field lines are all perpendicular to each other.
Large electrical generators do not look like this. Electromagnets rotate and the coils don’t. There are also many coils arranged in different directions to allow a reasonable steady voltage.
In all generators mechanical energy is converted to electrical energy.
About 86% of all worldwide electrical generation uses steam turbines. For this to happen you need a source of energy to convert water into steam. This could be coal, gas or nuclear fission.
Nuclear Fission reactors use uranium as the fuel. The use of uranium as an oxide dates back many years and was used to colour glass.
Discovered as a metal in pitchblende in 1789 by M. Klaproth
Martin Heinrich Klaproth (1 December 1743 – 1 January 1817) was a German chemist who discovered uranium (1789), zirconium (1789), and cerium (1803).
Uranium is named after the planet discovered in 1781 by Klaprot
Radioactive properties discovered by Henri Becquerel in 1896
Antoine Henri Becquerel (15 December 1852 – 25 August 1908) was a French physicist, Nobel laureate, and the discoverer of radioactivity along with Marie Skłodowska-Curie and Pierre Curie, for which all three won the 1903 Nobel Prize in Physics.
Becquerel’s discovery of spontaneous radioactivity is a famous example of serendipity, of how chance favours the prepared mind. He thought that phosphorescent materials, such as some uranium salts, might emit penetrating X-ray-like radiation when illuminated by bright sunlight. His first experiments appeared to show this. By May 1896, after other experiments involving non-phosphorescent uranium salts, he arrived at the correct explanation, namely that the penetrating radiation causing photographic plates to become fogged came from the uranium itself, without any need for excitation by an external energy source.
The image above is of Becquerel’s photographic plate which has been fogged by exposure to radiation from a uranium salt. The shadow of a metal Maltese Cross placed between the plate and the uranium salt is clearly visible.
Discovery of the Neutron
Proposed by Chadwick in 1932
The alpha-particles from the radioactive source hit the beryllium nuclei and transformed them into carbon nuclei, leaving one free neutron. When this neutron hit the hydrogen nuclei in the wax it could knock a proton free
In 1930 Bothe and Becker had observed highly penetrating uncharged radiation from the bombardment of beryllium. At first this radiation was thought to be gamma radiation.
In 1931 F. Joliet and I. Curie measured fast protons emerging from paraffin bombarded with these particles. This was not in itself inconsistent with the assumed gamma ray nature of the new radiation, but detailed quantitative analysis of the data became increasingly difficult to reconcile with such a hypothesis.
n 1932, James Chadwick performed a series of experiments at the University of Cambridge, showing that the gamma ray hypothesis was untenable. He suggested that the new radiation consisted of uncharged particles of approximately the mass of the proton, and he performed a series of experiments verifying his suggestion. These uncharged particles were called neutrons, apparently from the Latin root for neutral and the Greek ending -on (by imitation of electron and proton).
Sir James Chadwick CH FRS (20 October 1891 – 24 July 1974) was an English physicist who was awarded the 1935 Nobel Prize in physics for his discovery of the neutron in 1932.
Enrico Fermi bombarded uranium with neutrons in 1934. He discovered that slow neutrons were more easily captured than fast ones, and developed the Fermi age equation to describe this. After bombarding thorium and uranium with slow neutrons, he concluded that he had created new elements; although he was awarded the Nobel Prize for this discovery, the new elements were subsequently revealed to be fission products.
Enrico Fermi (29 September 1901 – 28 November 1954) was an Italian physicist.
Nuclear fission of heavy elements was discovered on December 17, 1938 by Otto Hahn and his assistant Fritz Strassmann, and explained theoretically in January 1939 by Lise Meitner and her nephew Otto Robert Frisch. Frisch named the process by analogy with biological fission of living cells. It is an exothermic reaction which can release large amounts of energy both as electromagnetic radiation and as kinetic energy of the fragments (heating the bulk material where fission takes place). In order for fission to produce energy, the total binding energy of the resulting elements must be less negative (higher energy) than that of the starting element.
Bombardment of uranium by neutrons causes the nucleus to split into two main fragments
A neutron is absorbed by a uranium-235 nucleus, turning it briefly into an excited uranium-236 nucleus, with the excitation energy provided by the kinetic energy of the neutron plus the forces that bind the neutron. The uranium-236, in turn, splits into fast-moving lighter elements (fission products) and releases three free neutrons. At the same time, one or more “prompt gamma rays” (not shown) are produced, as well.
Mass decrease due to this reaction is appreciable as mass has been converted into fission energy as shown by Einstein’s famous equation E = mc^2
Crucially other neutrons are emitted during fission reactions to produce further fission reactions.
1. A uranium-235 atom absorbs a neutron and fissions into two new atoms (fission fragments), releasing three new neutrons and some binding energy. 2. One of those neutrons is absorbed by an atom of uranium-238 and does not continue the reaction. Another neutron is simply lost and does not collide with anything, also not continuing the reaction. However, one neutron does collide with an atom of uranium-235, which then fissions and releases two neutrons and some binding energy. 3. Both of those neutrons collide with uranium-235 atoms, each of which fissions and releases between one and three neutrons, which can then continue the reaction.
Energy has to be expended to release a nucleon and this is known as the Binding Energy
Nuclei can be fissioned apart or fused together depending on their position on the binding energy per nucleon curve.
One nuclear fuel pellet undergoes 6 x E12 fissions per second
Each cubic micron has 10 fissions per second
The total energy released per fission is about 200 MeV or 32 pJ
Kinetic Energy from fission fragments 165 ± 5
Prompt γ−ray energy 7 ± 1
Kinetic Energy of fission neutron 5 ± 0.5
β rays from fission products 7 ± 1
α rays from fission products 6 ± 1
Neutrinos from fission products 10 ± 1
The Role Of Neutrons
The Neutron is uncharged so it can approach a nucleus at low energies without coulomb repulsion.
Cross section of interaction is greater at low energies. Cross section is used to express the likelihood of interaction between particles. When particles in a beam are thrown against a foil made of a certain substance, the cross section s is a hypothetical area measure around the target particles of the substance (usually its atoms) that represents a surface. If a particle of the beam crosses this surface, there will be some kind of interaction.
About 200 MeV of energy is released per fission which is 45 million times greater per atom of fuel than in a chemical reaction
About 2.5 neutrons are released per fission and the release of new neutrons gives the possibility of a chain reaction.
Stable or Non-Stable Reactions
A reproduction constant k = no. of neutrons in one generation/no of neutrons in previous generation
If k is greater than 1 then the reaction is supercritical as found in nuclear weapons
If k is less than 1 then the reaction is subcritical and the reaction dies out
If k = 1 the reaction is critical and a stable chain reaction occurs
Controlled Nuclear Fission
Fuel was originally uranium metal but now there are many variations
The moderators are carbon, water and deuterium oxide. A neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235.
Cladding (stainless steel or ceramic) contains the fuel pellet and prevents release of radioactive fission products. It has a ribbed surface to improve heat transfer.
Coolant, either a gas or liquid circulated through the core of the reactor for heat extraction
Control Rods usually boron or cadmium with large capture cross section to maintain k=1. They are used in nuclear reactors to control the fission rate of uranium and plutonium.
The shield is usually steel and concrete and used for radiation protection and to provide protection for the pressure vessel against impact from large vehicles and attacks involving moderate amounts of explosives.
The shielding is made up of concrete reinforces with steel pipes.
Two thirds of all nuclear reactors in the world are pressurised water reactors.
Boiling water reactors are 22% more efficient but there is a greater chance of contamination.
So that’s the theory but how does it happen in the real world?
Nuclear Fuel Cycle
Only one uranium isotope can be used.
Uranium is considered as a non-renewable energy source as we have a finite quantity of it.
28% of Earth’s supply is in Australia where there are currently 3 working mines.
It is milled at the mine and leached with sulphuric acid to separate out the uranium. It is recovered from solution and dried as U3O8 – yellowcake
Approximately 200 tonnes are required for 1000 MWe reactor for one year.
Conversion and Enrichment
Most Nuclear Reactors need enriched uranium to operate. Notable exceptions are Magnox and Candu
235U is increased from 0.7% to 3 – 4% during enrichment.
Uranium needs to be in the form of a gas before it can be enriched
U3O8 is converted into UF6 – uranium hexafluoride
Centrifuges can be used to separate the two streams
The Tricastin enrichment plant in France (background) and the four nuclear reactors (foreground) that provide over 3000 MWe power for it.
AP1000 Fuel Rod Assembly
AGR Fuel Assembly
Magnox Fuel Assembly
The fins improve heat transfer
Magnox Fuel Rods
First Commercial Reactor
The first commercial reactor in the UK was at Sellafield. Sellafield is a nuclear reprocessing site, close to the village of Seascale on the coast of the Irish Sea in Cumbria, England. Windscale was built first followed by Calder Hall. Both are now being decommissioned.
The above left picture is of Sellafield in the past and the above right picture is of Sellafield today.
First Fleet of Reactors
Magnox is a now obsolete type of nuclear power reactor which was designed in the United Kingdom, and was exported to other countries, both as a power plant, and, when operated accordingly, as a producer of plutonium for nuclear weapons. The name magnox comes from the alloy used to clad the fuel rods inside the reactor.
Wylfa Nuclear Generating Facility contains the world’s last operating Magnox reactor.
Berkeley, Gloucestershire, first grid connection 1962, shut down 1989
Bradwell, Essex, first grid connection 1962, shut down 2002
Calder Hall, Cumbria – first grid connection 1956, shut down 2003
Chapelcross, Dumfriesshire, first grid connection 1959, shut down 2003
Hunterston, West Kilbride, first grid connection 1964, shut down 1990
Hinkley Point, Somerset, first grid connection 1965, shut down 1999
Trawsfynydd,Gwynedd, first grid connection 1965, shut down 1991
Dungeness A, Kent, first grid connection 1965, shut down 2006
Sizewell A, Suffolk, first grid connection 1966, shut down 2006
Oldbury, Gloucestershire, first grid connection 1967, decommissioning due 2008 but extended, Reactor 2 closed 30.6.11, Reactor 1, 29.2.12
Wylfa, Anglesey, first grid connection 1971, Reactor 2 closed 25.4.12 but reactor 1 can operate until about December 2015
Second Fleet of Reactors
An advanced gas-cooled reactor (AGR) is a type of nuclear reactor. These are the second generation of British gas-cooled reactors, using graphite as the neutron moderator and carbon dioxide as coolant. The AGR was developed from the Magnox reactor, operating at a higher gas temperature for improved thermal efficiency, requiring stainless steel fuel cladding to withstand the higher temperature. Because the stainless steel fuel cladding has a higher neutron capture cross section than Magnox fuel cans, enriched uranium fuel is needed, with the benefit of higher “burn ups” of 18,000 MWt-days per tonne of fuel, requiring less frequent refuelling. The first prototype AGR became operational in 1962 but the first commercial AGR did not come on line until 1976.
All AGR power stations are configured with two reactors in a single building. Each reactor has a design thermal power output of 1,500 MWt driving a 660 MWe turbine-alternator set. The various AGR stations produce outputs in the range 555 MWe to 670 MWe though some run at lower than design output due to operational restrictions.
Dungeness, in Kent, 2 units Connected in 1983 and 1985. Decommissioning was due to start 2008
Heysham in Lancashire, 4 units connected in 1983, 1984 and 1988. Decommissioning due 2014 – 2023
Hunterston in Ayrshire, 2 units connected in 1976 and 1977. Decommissioning was due to start 2011
Hartlepool in Durham, 2 units connected in 1983 and 1984. Decommissioning was due to start 2014
Hinkley Point in Somerset, 2 units connected in 1976. Decommissioning was due to start 2011
Torness in East Lothian, 2 units connected in 1988 and 1989. Decommissioning due 2023
Dounreay Fast Reactor, 14 MW, connected in 1962 but shut down in 1977
Dounreay Prototype Fast Reactor, 250 MW, connected in 1975 but shut down in 1994
Sizewell B in Suffolk, 1188 MW, connected in 1995 and decommissioning due 2035
Storage of Spent Fuel
The spent fuel is highly radioactive and very (thermally) hot. Initially the waste is stored in ponds at the reactor site. The water cools the rods and acts as shielding. The rods can also be stored in dry stores with air cooling.
Dungeness Storage Pond, Kent and THORP Storage Pond, Sellafield
Only about 4% of U is burnt up and the 235U content is reduced to less than 1%
Some Plutonium remains from the fission reactions
Reprocessing separates the Uranium and Plutonium from the waste products by chopping up the fuel rods and dissolving them in acid
Uranium can re-enriched
Plutonium can blended with Uranium to produce MOX fuel
Waste in the UK in categorised by the activity:
Low Level Waste (LLW) below 4 GBq/te of αlpha or 12 GBq/te of βeta/gamma activity;
Intermediate Level Waste (ILW): Wastes with radioactivity levels exceeding the upper boundaries for LLW, but which do not need heating to be taken into account in the design of storage or disposal facilities;
High Level Waste (HLW): Waste in which the temperature may rise significantly as a result of their radioactivity, so this factor has to be taken into account in designing storage or disposal facilities.
New LLW Container
LLW Repository near Drigg
Magnox swarf, is the debris or waste resulting from the Magnox reactor
Nuclear Power Generation and radioactive waste
80 year lifetime use of electricity for 1 person generates this much high level waste
Storage of Canisters
Calcine as a melt is poured into stainless steel cylindrical containers (“cylinders”) in a batch process. When cooled, the fluid solidifies (“vitrifies”) into the glass. Such glass, after being formed, is highly resistant to water. The glass canister (type CSD-V) is a stainless steel cylinder measuring 1.34 meters in height and 0.43 meter in diameter. Each canister holds 150 litres (400 kg) of solidified glass containing 14% fission products, corresponding to the treatment of approximately 1.7 metric ton of used fuel.
After filling a cylinder, a seal is welded onto the cylinder. The cylinder is then washed. After being inspected for external contamination, the steel cylinder is stored, usually in an underground repository. In this form, the waste products are expected to be immobilized for a long period of time
The picture above right shows loading silos with canisters containing vitrified HLW.
Challenges of Radioactive Waste
The table below shows that high level waste is the most dangerous but requires a smaller space for disposal than low and intermediate level waste.
The multiple barrier concept is a series of engineered and natural barriers working together to contain and isolate used nuclear fuel from the biosphere. Each of these barriers provides a unique and stand-alone level of protection – if any of the barriers deteriorate, the next one will come into play.
Barrier 1 is the used nuclear fuel pellet itself. Fuel pellets are made from uranium dioxide powder, baked in a furnace to produce a hard, high-density ceramic. Ceramics are extremely durable; they do not readily dissolve in water, and their resistance to wear and high temperatures make them one of the most durable engineered materials.
Barrier 2 is the fuel element and the fuel bundle. The function of each element is to contain and isolate the fuel pellets.
Barrier 3 is the used nuclear fuel container. Used nuclear fuel bundles will be placed into large, very durable containers designed to hold 360 fuel bundles each. The inner vessel is made from 100-millimetre-thick steel, which provides the mechanical strength to withstand the pressures of the overlying rock and future glacial loading. The outermost layer of the container is corrosion-resistant copper. The container prevents water from reaching the used nuclear fuel bundles, preventing radio-nuclides in the fuel from escaping into the underground environment. The container is engineered to remain intact for at least 100,000 years, and expected to last much longer, keeping the used nuclear fuel completely isolated from the surroundings.
Copper is picked because it is known to be durable under deep rock conditions.
Barrier 4 is bentonite clay, backfill and sealants. After the used nuclear fuel containers are placed in the vault, all open spaces in each underground chamber will be filled with engineered materials designed to minimize any seeping of water through the repository. Once the vault is completely filled it will be sealed with bulkheads of special, high-performance concrete.
Barrier 5 is the Geosphere. The vault will need to be be approximately 500 metres underground – the exact depth will depend on the site. It will be excavated within a suitable sedimentary or crystalline rock formation.
The geosphere forms a natural barrier of rock, which will protect the repository from disruptive natural events and human intrusion. It will also help maintain favourable conditions for long-term containment and isolation of the used nuclear fuel, as well as limit movement of any radionuclides if other barriers fail.
Engineered Geological Disposal Facility
A deep geological repository is a nuclear waste repository excavated deep within a stable geologic environment (typically between 300to 700 m). It entails a combination of waste form, waste package, engineered seals and geology that is suited to provide a high level of long-term isolation and containment without future maintenance.
The UK Government is committed to implementing geological disposal for the safe and secure management of higher activity radioactive waste over the long term and favours an approach for selecting a site that is based on working in partnership with communities.
After its closure, a geological repository is a passive system whose natural and engineered barriers work together to isolate and contain radioactivity until it has decayed or has insignificant hazard potential in terms of being able to re-enter the environment in harmful concentrations.
A repository for nuclear fuel in Forsmark will be located in Söderviken, close to the Forsmark Nuclear Power Plant. Here, at a depth of approximately 500 metres in bedrock that is 1.9 billion years old, plans are underway for a final repository for some 12,000 tonnes of spent nuclear fuel.
How Deep ??
Deep borehole disposal is the concept of disposing of high-level radioactive waste from nuclear reactors in extremely deep boreholes (Deep geological repository). Deep borehole disposal seeks to place the waste as much as five kilometres beneath the surface of the Earth and relies primarily on the thickness of the natural geological barrier to safely isolate the waste from the biosphere for a very long period of time so that it should not pose a threat to man and the environment. The concept was originally developed in the 1970s, but recently a proposal for a first experimental borehole has been proposed by a consortium headed by Sandia National Laboratories.
Constructing the Borehole
Drill the first stage of the borehole
Insert the casing.
Pour in the cement basement.
Drill the next stage of the borehole.
Insert the casing.
Pour in the cement basement
Drill the next stage of the borehole
Placement of Canisters
Insert the casing, insert the canisters, pour in the grout and allow it to set
Separation of Canisters
Insert Bentonite clay, insert another stack of canisters, repeat until the bottom km of the borehole is filled.
Sealing the Borehole
Pour in some backfill (crushed granite), insert heater and seal the borehole, pour in more backfill and seal the borehole again and fill the rest of the borehole with backfill.
Storage cannot be relied upon in the long-term to provide the necessary permanent isolation of the wastes from man’s environment, and future generations should not have to bear the burden of managing wastes produced today. Hopefully we can come up with a permanent method of isolating waste before the geological barriers fail.
Uranium vs Thorium
One of the teachers on the course asked about the use of Thorium in nuclear reactors.
On the face of it Thorium looks a better nuclear fuel than Uranium and it can’t be used to make weapons.
One reason why thorium reactors have not made more progress in the past is that nuclear fuel breeding traditionally has been a very slow and capital-intensive process. For this reason, the DBI reactor is designed to be started-up using conventional nuclear fuels, with low enough capital and operating costs that it can compete with other conventional nuclear power plants and pay for its costs in the first few years, even before the bred Uranium-233 is available.
Another and perhaps the major reason why thorium use for energy production has not made more progress over the past decades is that thorium is not nearly as easy to weaponise. A 1997 international scientific symposium on nuclear fuel cycles concluded that the principal reason thorium had not been used more widely to date is that the ore contains no fissile isotope.
Do you think you could run a nuclear power plant?