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dan964

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Section 5.4: Applications of Nuclear Physics
1. Nuclear Fission Reactors
  • The role of fission reactors is to provide a controlled release of energy, using a nuclear reaction.
  • The heat energy generated by a fission reaction is used to heat water to produce steam which turns turbines, and generators, responsible for the generation of electricity.
  • The main components to a fission reactor are the fuel and control rods, the coolant, moderators and radiation shielding.
Reactor Core
  • Nuclear fission occurs in the reactor core which consists of the fuel rods, control rods and the moderator.
  • A typically fuel rod is enriched uranium-235, which is used to provide the material and mass for fission.
  • A typical control rod is cadmium or boron which absorbs the excess neutrons to prevent the fission. They are also easily moveable in order to control the rate of the fission reaction, when they are lowered the reaction slows, when pulled out, the reaction rate increases.
  • Most reactors have two sets, one of which is used as regulating rods for routine control, and one as safety rods in the case of emergency shutdown.
  • A moderator is used in order to slow down the neutrons to an appropriate energy to enable them to undergo fission (uranium-235 requires slow neutrons for fission). Common moderators include deuterium oxide (heavy water), graphite, beryllium or ordinary water. Heavy water is often expensive to produce, so graphite is more economically viable despite being less efficient.
Heat Exchanger and Electricity Production
  • The coolant is used to remove the heat generated in the reaction. This is usually air, helium, ‘heavy’ water, molten sodium or molten sodium chloride.
  • The primary coolant carries the heat to a heat exchanger to heat water to form steam. This steam is then used to drive a turbine to generate electricity (using a generator). It is constantly circulated to carry more heat.
  • The secondary coolant is used to cool the steam after it has been used to drive the turbine, and it is recirculated back into the heat exchanger.
Radiation Shielding
  • There are two layers of radiation shielding in the reactor.
  • The first layer is designed to protect the walls of the reactor by reflecting neutrons back into the reactor core.
  • The other layer is a biological shield and consists of very thick concrete which is used to protect the personnel and also prevents radiation from leaving the reactor centre.

Practicals and Research

1. The Manhattan Project
Development of the Project
  • The Manhattan Project was a project designed for the production of atomic bombs during the second-half of World War 2. It started in 1939, after Einstein wrote a letter to the US president Franklin D. Roosevelt suggesting the production of atomic bombs following the German discovery of nuclear fission in 1938.
  • In 1940, it was confirmed that uranium-235 undergoes fission with slow neutrons. This required a method of separation (gaseous diffusion then electromagnetic separation) to isolate it from the more common uranium-238.
  • In 1940, the first transuranic element neptunium-237 was developed in a nuclear reactor. In the middle of 1941, plutonium-239 was developed as a suitable material for use in the atomic bomb.
  • In December 1942, Enrico Fermi created the first chain reaction in a reactor.
  • In 1945, the first uranium-235 bomb which consisted of the barrel containing one piece of uranium-235 that could be fired into another piece of uranium-235 to form the supercritical mass was dropped on Hiroshima.
  • The first plutonium-239 bomb which was a sphere of plutonium-239 surrounded by explosives, which was dropped on Nagasaki.
Impact on Society
The impact on society during the 1940s was overall significant and positive:
  • The project was the largest and most expensive project of World War 2 (it costed about US $2 billion dollars)
  • The project resulted in the abrupt termination of World War 2, and despite the loss of many Japanese civilians, it indirectly prevented any more loss of life.
The impact on society during the 1940s to the 1980s was overly significant, although the negative consequences of the project were still felt in the hostility between the USSR and the USA:
  • Its development led to the Cold War, a standoff between the USSR (now Russia) and the USA, dividing the world into two blocs, the Western Bloc (USA, Western Europe and allies) and the Eastern Bloc (USSR, Eastern Europe and allies).
  • A positive consequence of the project was that each was in fear of the other launching a weapon, and this prevented active warfare between the two sides.
The Manhattan Project was significant as it led to advances in nuclear technology including:
  • the production of electricity using nuclear reactors
  • producing transuranic elements allowing the number of known elements to be increased
  • the production of isotopes used in medicine used for diagnosis and treatment, and in agriculture used for soil monitoring and in industry for monitoring such as the inspection of metal welds for defects
 
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