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Radioactivity

10.6.5 Generation of Electricity from Nuclear Fission

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Nuclear Reactor

  1. A nuclear reactor produces tremendous amount of energy through nuclear fission.
  2. The energy liberated from the fusion of nuclear fuel heats the surrounding water.
  3. As a result, steam is generated to drive turbines, which in turn drive the electrical generators.
  4. The main components of a nuclear reactor:
    1. Graphite moderator
      Fast moving neutrons are slowed down by collisions with nuclei in the moderator so that they can cause further fissions. In some nuclear power plant, the moderator is water.
    2. Uranium rod (Fuel)
      Fission reactions occur in the uranium rod to produce nuclear energy. The uranium used is often 'enriched' by increasing the proportion of the isotope uranium-235 above the natural value of 0.7% to 3%.
    3. Control rod
      The rate of the fission reaction is controlled by inserting or withdrawing these rods. The nuclei in the rods absorb neutrons without undergoing any reaction. Sometimes the rod is made of cadmium.
    4. Coolant
      To take away the heat from the nuclear reactor. Substances with high specific heat capacity such as 'heavy' water and carbon dioxide are used.
    5. Thick concrete wall
      To prevent the escape of harmful radiations.
    6. Steam generator
      Water in the generator is heated and changed into steam. The steam then drives the turbines.
    7. Turbine
      To turn the dynamo in the electrical generator to produce electricity.

 

10.6.4 Nuclear Fusion

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  1. In nuclear fusion, two or more small and light nuclei come together to form a heavier nucleus.
  2. This process is accompanied by the release of a huge amount of energy.
  3. Below are two examples of fusion reactions:
  5. Fusion is much more difficult to achieve than fission because the hydrogen nuclei repel each other. Therefore, the nuclei must be heated to 108 K or more so that the nuclei will have enough of kinetic energy to overcome the electrical repulsion between the nuclei.
  6. Example
    1. The Sun get its energy from the fusion of hydrogen nuclei.
    2. A hydrogen bomb uses the principle of nuclear fusion for its design.

 

 

10.6.3 Chain Reaction

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  1. If neutrons from the fission of uranium-235 continue to split other nuclei causing further fission, a chain reaction has occurred.
  2. The number of nuclei which undergo fission multiplies rapidly.
  3. In order for a chain reaction to take place, a minimum of one neutron from each fission must trigger further fission.
  4. At the same time, the mass of fission material must exceed a certain minimal mass known as the critical mass. If the material is less than this value, too many neutrons escape without hitting nuclei, preventing a chain reaction from happening.
(Chain Reaction)

 

 

10.6.2 Nuclear Fission

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Nuclear Reaction

  1. In a nuclear reaction, the mass of the parent particles will become less (know as mass defect). The defected mass is then converted into energy called the nuclear energy.
  2. In short, nuclear energy is the energy released owing to the defect of mass in a nuclear reaction.
  3. There are 2 types of nuclear reaction
    1. nuclear fission
    2. nuclear fusion
  4. Nuclear fission is the process of splitting nucleus into 2 smaller nuclei whereas nuclear fusion is the process which 2 small nuclei combine to form a larger nucleus.

Nuclear Fission

  1. Nuclear fission is a process involving the splitting of a heavy nucleus into two nuclei of roughly equal mass and shooting out several neutrons at the same time.
  2. Nuclear fission seldom occurs spontaneously. Usually, it occurs when the heavy nucleus is bombarded by a neutron.
  3. Fission reaction resulting from neutron absorption is called induced fission. Nuclei that undergo fission without initial neutron absorption are undergoing spontaneous fission.
  4. Two typical examples of fission reactions:


Example 1
In a nuclear reaction, the mass difference in the reaction is 1.5 x 10-8kg. Find the heat released in this reaction. [Speed of light = 3.0 x 108 ms-1]

Answer:
Mass defect, m = 1.5 x 10-8kg

Heat released,
E = mc²
E = (1.5 x 10-8)(3 x 108)
E = 1.35 x 109 J


Example 2
A nuclear explosion released 8.2 x 1013 J of energy. What is the mass defect of uranium-235 in this reaction?
[Speed of light = 3.0 x 108 ms-1]

Answer

 

10.6.1 Nuclear Energy

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  1. In a radioactive decay, one element changes into another in a process called transmutation.
  2. The mass of the daughter particles are less than that of the parent particle. This difference in mass is called mass defect or mass loss.
  3. Mass defect is the missing mass in a nuclear reaction and the missing mass will turn into thermal energy and kinetic energy of the product particles.
  4. The mass lost is converted into energy.
  5. Einstein's formula can be used to calculate the amount of nuclear energy released from the defect of the mass in a radioactive decay or nuclear reaction.


    Einstein Formula

    E = mc²

    where 
    m = mass change, in kg
    c = speed of light, in m s-1
    E = energy changed, in J

 

10.5.1.4 Applications of Radioisotopes in Archeology

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  1. In archaeology radioisotope carbon-14 is used to study and estimate the age of ancient artifacts. This method is named as the radiocarbon dating. 
  2. Radiocarbon dating can be used to estimate the age of organic materials, such as wood and leather, up to about 58,000 to 62,000 years.




Example:
A piece of wood found in a cave of an archaeology site has a carbon-14 activity 25% of the activity from a live plant. Estimate the age of the wood. [Half-life of carbon-14 = 5730 years]

Answer:
100% → 50% → 25%

Carbon-14 take 2 half-life to decay from 100% to 25%, hence the age of the wood
= 2(5730)
= 11460 years

 

10.5.1.2 Uses of Radioisotope in Agriculture

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Pest Control


  1. Male insect is sterilised by exposing to radioactive radiation and then released back to the ecosystem.
  2. This can ensure that their reproducing effort do not generate new generation and hence reduces the population of the insect.

Tracer

  1. The metabolism of phosphorus by plants can be studied using phosphate fertilisers that contain phosphorus-32. 
  2. A small amount of phosphorus-32 is used in fertilisers. 
  3. The radiation produced by phosphorus-32decay is detected by a Geiger-Muller counter. 
  4. This method can trace the passage of phosphate ions in plants.
  5. Carbon-14 is used to study the passage of carbon during photosynthesis in plants.

Develop New Species of Plant

  1. Radioactive radiation is targeted to the seeds of plants and hence causes mutation to the genes.
  2. By chance, this may develop some superior agricultural products.

Preserve Food


  1. The gamma rays from cobalt-60 are used to kill bacteria in food to make fresh vegetables and fruits last longer without any change in quality, flavour and texture of food.
  2. Gamma rays are used to inhibit budding in potatoes.

 

10.5.1.1 Uses of Radioisotope in Medical

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Radiotherapy


Gamma rays of cobalt-60 can be used to destroy cancer cells in patients. This treatment is known as radiotherapy.

Tracer to Detect Blood Clots or Tumour

  1. A small amount of sodium-24 is injected into the patient's body. 
  2. Radioactive imaging is then used to detect accumulation of sodium-24 and therefore detect tumours and blood clots before they become dangerous.

Sterilising Medical Instrument


  1. Gamma ray emitted from radioactive cobalt-60 can kill germs such as bacteria and fungus. 
  2. Medical instruments such as surgical equipment, syringes and bandages can be sterilised by using gamma rays.

Heart Pacemaker


(Heart Pacemaker)
  1. A heart  pacemaker is a medical device which uses electrical pulses to contract the heart muscles, and hence to regulate the beating of the heart. 
  2. Plutonium-238 in a nuclear battery is used to produce small electric shocks in the heart pacemaker. 
  3. People with irregular heartbeats need to have a heart pacemaker implanted inside their chest.

Tracer to Investigate Activities of Thyroid Glands

  1. A radioactive tracer is a substance containing a radioisotope that is used for tracking purposes.
  2. In medicine, radioactive substances are introduced into the body of patients and the radiation emitted is detected by using a gamma camera. This diagnostic process is known as radioactive imaging.
  3. In medicine, radioactive iodine-123 or iodine-125 is introduced into patient's body.
  4. The rate of accumulation of radioactive iodine in thyroid glands is measured through radioactive imaging. This is used to evaluate the function of the thyroid gland.

 

10.5.1 Applications of Radioisotopes

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  1. There are two types of isotopes, namely
    1. the stable isotopes (non-radioactive)
    2. the non-stable isotopes (radioactive).
  2. Unstable isotopes go through radioactive decay and emit radiation and they are known as radioisotopes.
  3. Radioisotopes have many applications in the following field
    1. Industri
    2. Medical
    3. Agriculture
    4. Archeology