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In a fission reaction large atoms split into smaller atoms


In a fission reaction large atoms split into smaller atoms, and energy is produced Use of fission in power stations The energy released by nuclear fission is used to ... – PowerPoint PPT presentation

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Title: In a fission reaction large atoms split into smaller atoms

In a fission reaction large atoms split into
smaller atoms, and energy is produced
Fission Power
  • Some fission of heavy nuclei occurs naturally on
  • In a fission reactor the fission of (usually)
    uranium-235 nuclei is induced through their
    collision with free neutrons.
  • Inducing fission in this way greatly increases
    the rate of fission and therefore the rate of
    energy release in the reaction chamber

It gets even more exciting
  • The absorption of a neutron causes the U-235
    nucleus to oscillate and become unstable.
  • The uranium nucleus then splits into two lighter
    nuclei (not constant) along with the release of a
    small number of neutrons, which go on to induce
    further fission- a chain reaction.

Use of fission in power stations
  • The energy released by nuclear fission is used to
    provide us with electricity in power stations.
  • Fission is caused in power stations by inducing
    it hence induced fission

The first induced fission reaction
  • In 1938 Otto Hahn and Fritz Strassmann of
    Germany split the uranium atom by bombarding it
    with neutrons and showed that the elements barium
    and krypton were formed. Fermi, Hahn and
    Strassmann did, however, not realize that they
    had in fact induced a fission reaction.
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Lets Get Physical
  • Firstly, fission is initiated by bombarding
    radioactive elements with neutrons
  • This is known as induced fission

Once a chain reaction occurs
  • Control rods, likely made of carbon absorb
    neutrons that are emitted, limiting the amount of
    induced fission reactions that occur (fission
    reactions induced by other fission reactions)
  • The control rods in the diagram
    absorb some of the neutrons. And
    yes, all radioactive substances are
    an intense green colour.

Thermal Neutrons
  • The neutrons used for inducing fission are known
    as Thermal Neutrons. Thermal neutrons have lower
    energy than Fast, Hot and Epithermal neutrons,
    usually with around 0.025eV of energy. The reason
    these neutrons are used in preference to others
    is because they are more efficiently absorbed by
    the nuclei of elements

Thermal Neutrons
  • When the neutron energy is equivalent to the
    energy of an atom of an ideal gas at the
    prevailing temperature, the neutrons are called
    thermal neutrons. The speed of thermal neutrons
    means that they are more likely to induce fission
    in U-235. Another advantage is that thermal
    neutrons are too slow to be absorbed (and lost)
    in U-238, which makes up most of the uranium fuel
    even after enrichment.

What Does all This Mean?
  • Fission is the splitting of atomic nuclei, either
    spontaneously or by collision (induced).
  • Induced Fission is where a slow-moving neutron
    is absorbed by the nucleus of another atom
    (normally a uranium-235 atom), which in turn
    releases two fast-moving lighter elements
    (fission products), free neutrons and energy.
  • Induced fission occurs when a free neutron
    strikes a nucleus and deforms it.

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If that was unclear then
Fission Products
Chain Reactions
  • A nuclear chain reaction is a reaction that can
    occur without the limitation of the number of
    neutrons in a reaction.
  • In fission, a neutron is fired into a nucleus to
    form a radioisotope, which is unstable and splits
    into lighter elements, releasing energy and more
  • These neutrons collide with more nuclei to create
    more fission products.
  • This is basically a chain reaction one in which
    one or more self-propagating reactions occur.

Examples of Chain Reactions
  • In this example, a neutron collides with the 235U
    nucleus, forming the radioisotope 236U, which
    fissions, releasing energy, neutrons and
    neutrinos. These neutrons collide with more 235U
    nuclei, continuing the chain reaction.

Chain Reactions
  • The critical mass of an element is the minimum
    amount of energy required for a self-propagating
    chain reaction to occur.
  • This can depend on various characteristics of the
    substance, such as density, shape, nuclear
    properties and its enrichment.
  • Above the critical mass, the element is in
    Supercriticality, which means that the rate of
    fission is increasing.
  • This is shown by kf-I, where K is the neutron
    multiplication factor, f is the average number
  • of neutrons per fission, and I is the average
  • number of neutrons lost. When this is 1, the
  • system is critical. When kgt1, the system is in

Where does nuclear fuel come from?
  • Nuclear fuel starts with uranium, a naturally
    occurring radioactive material. The uranium ore
    is mined and refined into a brightly-coloured
    solid uranium compound referred to as "yellow
  • The yellow cake is converted into various uranium
    metal alloys or compounds to be used as nuclear
    fuel. The uranium is formed into rods, pellets,
    or plates.

  • They are completely sealed ("clad") with metals
    such as aluminium or stainless steel to provide
    structural strength and to surround the fuel to
    prevent the release of radioactive particles.

Uranium ore is crushed, ground, and chemically
processed "yellow cake."
How much is required in a power station?
  • With time, the concentration of fission fragments
    and heavy elements formed will increase to the
    point where it is no longer practical to continue
    to use the fuel. So after 12-24 months the 'spent
    fuel' is removed from the reactor. The amount of
    energy that is produced from a fuel bundle varies
    with the type of reactor and the policy of the
    reactor operator.
  • Typically, some 36 million kilowatt-hours of
    electricity are produced from one tonne of
    natural uranium. The production of this amount of
    electrical power from fossil fuels would require
    the burning of over 20,000 tonnes of black coal
    or 8.5 million cubic metres of gas.

Where oh where is that pesky fuel hiding?
  • Nuclear fuel, or Jim as it is often known is
    obtained through mining.
  • These mines are either open cast pits or
    underground mines.
  • Mining it underground is kinder because nuclear
    fuels are scared of the dark and want to be
  • Uranium is most commonly found somewhere and so
    there are many mines. This location remains
    secret for reasons of minimum interest.

How much fuel is there in Mr Power stations
  • The most common type of nuclear power station, a
    pressurised water reactor, uses 200- 300 rods of
    enriched UO2 each rod being 3.5 to 4 metres long.
  • The power station is shut down in intervals of
    1-2 years for refuelling, where about a third of
    the fuel is replaced.
  • This keeps Mr Power Station happy and stops him
    getting a rumbly tumbly which would result in him
    attempting to destroy the world with only a half
    eaten pork pie and a packet of badgers.

What requirements keep the worky work workers of
the power station safe from the mutating powers
of radiation?
  • Radiation doses are controlled by the use of
    remote handling equipment for many operations in
    the core of the reactor.
  • These are like magical robot fingers which are
    rarely designed with torture in mind.
  • After a few crazy disasters requirements today
    are that the effects of any core-melt accident
    must be confined to the plant itself, without the
    need to evacuate nearby residents.
  • These disasters are now blamed on God, The Loch
    Ness Monster and or Campbell

The purpose of a moderator
  • a neutron moderator is a medium which reduces
    the velocity of fast neutrons,
  • turning them into thermal neutrons capable of
    sustaining a nuclear chain reaction involving

Control Rods
  • control rod is a rod made of chemical elements
    capable of absorbing many neutrons without
    undergoing fission themselves.
  • Control rods are used to control the rate of
    nuclear fission. The rods are lowered to slow the
    rate by absorbing nutrons.
  • Silver-indium-cadmium alloys, generally 80 Ag,
    15 In, and 5 Cd, are a common control rod
    material for pressurized water reactors
  • Boron is another common neutron absorber. Due to
    different cross sections of 10B and 11B, boron
    containing materials enriched in 10B by isotopic
    separation are frequently used. The wide
    absorption spectrum of boron makes it suitable
    also as a neutron shield.
  • Dysprosium titanate is a new material currently
    undergoing evaluation for pressurized water
    control rods.

The Purpose of a Coolant
  • The coolant in nuclear reactors is used to
    transport the reactor heat either to a boiler
    where steam is raised to run a conventional
    turbine or is used as engine fluid in the turbine
    before being passed back to the reactor.
  • They can be liquid or gas

Essential Properties of a Coolant
  • Low melting point.
  • High boiling point.
  • Non-corrosive properties.
  • Low neutron absorption cross section.
  • High moderating ratio.(for thermal reactors)
  • Radiation stability.
  • Thermal stability.
  • Low induced radioactivity.
  • No reaction with turbine working fluid.
  • High heat transport and transfer coefficient.
  • Low pumping power.

Common Coolants
  • Liquid
  • Water (H2O)
  • Heavy water (D2O)
  • Lithium (Li)
  • Gas
  • Carbon Dioxide (CO2)
  • Nitrogen (N)

Heavy Water as a Coolant
  • Heavy water is water using a isotope of hydrogen
    called deuterium. It has the formula D2O.
  • Heavy water occurs naturally. About 1 molecule
    in 3200 of water is heavy water. Heavy water is
    separated from regular water using electrolysis
    or distillation.

Type of materials used for coolants
  • Both light and heavy water (pressurized and
    boiling), organic liquids, sodium, sodium
    potassium mixtures, fused salts, and a number of
    gases - air, carbon dioxide, helium, nitrogen,
    hydrogen, steam and liquid metals.

What is the coolant used for in a nuclear
  • The coolant which passes through the nuclear
    reactors is used to transport the reactor heat
    either to a boiler where steam is raised to run a
    conventional turbine or it is used as a
    thermodynamic heat engine fluid and passes
    directly into the turbine and back to the
    reactor. Pressurized water, organic liquids,
    sodium, and most gas cooled nuclear power plants
    employ an intermediate steam boiler. Boiling
    water and some gas cool reactors use the coolant
    directly in the turbine.

Advantages and disadvantages of liquid metal
  • Liquid metal cooled reactors were first adapted
    for nuclear submarine use but have also been
    extensively studied for power generation
    applications. They have safety advantages because
    the reactor doesn't need to be kept under
    pressure, and they allow a much higher power
    density than traditional coolants. Disadvantages
    include difficulties associated with inspection
    and repair of a reactor immersed in opaque molten
    metal, and depending on the choice of metal,
    corrosion and/or production of radioactive
    activation products may be an issue.

Ionizing radiation hazards
To work safely with radioactive materials, it is
necessary to have an understanding of the
potential hazards they pose and how to avoid
these hazards. Ionizing radiation imparts energy
to living cells. In large enough doses, this
energy can damage cellular structures, such as
chromosomes and membranes. If not repaired, this
damage can kill the cell or impair its ability to
function normally. Whether this damage is harmful
depends on many factors, including the type of
cell, the absorbed dose and the rate of
Effects of high doses of radiation
  • The higher the dose of radiation the greater the
    severity of the effect. Examples of such
    proportional effects are
  • Erythema (reddening of the skin),
  • epilation (loss of hair),
  • cataracts
  • acute radiation syndrome.
  • These are known as deterministic effects and they
    all display a threshold below a certain dose, no
    effects are observed.

Effects on babies
Serious birth defects caused by irradiation of
the foetus or embryo appear to exhibit a
threshold at approximately 5 rem, the incidence
of birth defects is not significantly different
from the normal incidence. To prevent the
occurrence of radiation-induced birth defects,
the United States Nuclear Regulatory Commission
requires radiation exposures for pregnant workers
to be kept under 0.5 rem for the pregnancy period.
Risks of low level Radiation
As with any chemical, the small quantities of
radioactive materials used in medicine and
research demand care in handling, but the risk to
human health is surprisingly small when compared
to experiences in everyday life. As simple an act
as crossing the street carries some risk.
Certainly, all work situations carry some risk of
personal injury. Working with radioactive
materials is not hazard-free, but when placed in
the proper perspective of other living and
working environments, the occupational dangers
are seen to be slight.
Units of radiation
One gray is the absorption of one joule of
radiation energy by one kilogram of matter. The
SI unit for absorbed dose is the gray (Gy), but
the rad (Radiation Absorbed Dose) is commonly
used. 1 rad is equivalent to 0.01 Gy.
Nuclear waste
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Waste Products
The majority of waste products produced from
nuclear reactions is spent fuel. This consists of
mainly unconverted uranium as well as plutonium
and curium.
"high-level waste" - waste so radioactive that it
generates heat and corrodes all containers, and
would cause death within a few days to anyone
directly exposed to it.
Spent fuel rods are considered to be high level
radioactive waste and are stored in shielded
basins of water and are normally found on-site.
The water provides shielding from the
radioactivity and cooling for the still decaying
fission products.
After a few decades when the fuel rods are less
radioactive and are cooler they can be
transferred to a dry storage facility and is
stored in steel and concrete containers till the
radioactivity is safe enough for other
proceedings after the rods have decayed naturally.
Intermediate level waste - contains higher
amounts of radioactivity and in some cases
requires shielding such as nuclear fuel casing,
reactor components.
Intermediate level wastes are mixed with concrete
and stored in tanks, drums and vaults at the
sites where they are created
Low level waste - is found on contaminated
clothing, hand tools, water purifier basins and
the materials in which the reactor is built.
Most of the low-level waste is stored in sealed
concrete vaults at a purpose-built store,
although some is considered safe enough to go
into hazardous waste landfill sites.
  • The city was evacuated in 1986 due to the
    Chernobyl disaster at the Chernobyl Nuclear Power
    Plant, which is located 14.5 kilometers (9 miles)
    north-northwest. The power plant was named after
    the city, and located within Chernobyl Raion
    (district), but the city was not the residence of
    the power plant workers. Together with the power
    plant construction, Prypiat, a city, which was
    larger and closer to the power plant, was built
    to be home for the power plant workers.

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On 26 April 1986 at 012344 a.m. reactor number
four at the Chernobyl exploded. Further
explosion and the resulting fire sent a plume of
highly radioactive fallout into the atmosphere
and over an extensive geographical area Nearly
thirty to forty times more fallout was released
than had been by the atomic bombings of Hiroshima
and Nagasaki !
  • The plume drifted over extensive parts of the
    western Soviet Union, Eastern Europe, Western
    Europe, Northern Europe, and eastern North
    America. Large areas in Ukraine, Belarus, and
    Russia were badly contaminated, resulting in the
    evacuation and resettlement of over 336,000
    people. According to official post-Soviet
    data,about 60 of the radioactive fallout landed
    in Belarus.

  • At 12304 a.m. the experiment began. The
    extremely unstable condition of the reactor was
    not known to the reactor crew. The steam to the
    turbines was shut off. As the momentum of the
    turbine generator drove the water pumps, the
    water flow rate decreased, leading to the
    formation of steam voids. Due to the RBMK
    reactor-type's large positive void coefficient,
    the steam bubbles increased the power of the
    reactor. As the reactor power increased, so did
    the neutron generation. Soon it exceeded what
    could be absorbed by the Xe-135 poison starting a
    dangerous cascade. With the manual and automatic
    neutron absorbing control rods removed, nothing
    prevented a runaway reaction.
  • When the panic button was pressed it initially
    reduces the coolant present which dramatically
    increased the reaction rate
  • At 124, only 20 seconds after the panic button
    had been pressed, the first powerful steam
    explosion took place.
  • It damaged the top of the reactor hall and
    ejected fragments of material. The 2,000 tonne
    lid was blow off the reactor. This ruptured
    further fuel channels, lifted control rods and
    sheared off horizontal pipes. A second more
    powerful explosion occurred about two or three
    seconds after the first.