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How can we harness energy inside an atom?
Is nuclear power the answer to our sustainable energy needs?
The nucleus is an energetic bundle of particles being held together by a short-range strong force that counteracts the electrostatic repulsion of protons.
In the nucleus, the work done in holding the nucleus together is called binding energy.
Large nuclei tend to be less stable than small nuclei. They require more binding energy overall to hold them together. The nuclei we find in nature tend to have relatively long half-lives for radioactive decay, but they can break into smaller pieces if we upset the delicate equilibrium of forces that hold them together.
In 1934 Enrico Fermi had experimented in Rome with bombarding uranium nuclei with neutrons, and thought he had produced elements with lower atomic numbers as a result. However, the results seemed inconclusive, and in Berlin in 1938 the German Physicist Otto Hahn (1879-1968) carried out a similar experiment with his assistant Fritz Strassman (1902-1980) in which he believed he had generated barium. Hahn was not sure about his results, and consulted his colleague Lise Meitner (1878-1968).
Meitner was Jewish and had to flee the Nazi annexation of Austria for Sweden, from where she and her cousin Otto Frisch (1904-1979) corresponded by mail with Hahn. Meitner soon realised that Hahn has indeed 'split' the atom, and she calculated the process by which this could have occurred. Between them, Hahn and Meitner had discovered nuclear fission, the process by which large nuclei can be split into smaller ones using neutrons.
In nuclear fission, a neutron is fired like a bullet into the uranium nuclei. If the neutron is absorbed by the nucleus, the additional kinetic energy it introduces to the nucleus upsets the equilibrium of forces and the nucleus begins to 'wobble' or distort - Meitner compared it to the way that biological cells stretch and then split into two.
After fission of uranium, the original neutron is released, along with two further neutrons. The two smaller nuclei remaining - the daughter nuclei - are more stable as they require much less binding energy per nucleon. The excess energy is released as heat from the reaction.
Each uranium fission releases around 3 x 10-11 J of energy. This may not seem like much, but we can calculate how much energy would be released from even a relatively small quantity of uranium metal.
1 mole of any substance contains the same number of atoms. This number is the Avogadro constant:
NA = 6 x 1023 atoms
1 mole of any substance is the mass of the substance in grams equal to the relative atomic mass.
Atomic mass of uranium-238 = 238u
1 mole of U-238 has a mass of 238 g. This is about the size of a grapefruit.
So 238 g of uranium consists of 6 x 1023 atoms.
If all the atoms undergo fission, the energy released would be
E = 6 x 1023 x 3 x 10-11 = 2 x 1013 J
That is, around 2000 GJ of energy would be released! If all this energy was released instantaneously...
For this reason, research on nuclear fission was a priority for both sides in the Second World War. Ultimately, the USA was the first and only power to 'weaponise' fission in the form of two atomic bombs, which were dropped on the Japanese cities of Hiroshima and Nagasaki on August 6th, 1945.
The table below summarises some of the challenges, and solutions, in producing sustainable, controlled nuclear fission.
Activity: use the available information and the internet to complete the questions in the boxes on the sheet 'Nuclear-Fission'.
Activity: Understanding nuclear reactors
ATL
Information literacy skills: Make connections between various sources of information.
Critical-thinking skills: Analyse complex systems and projects into their constituent parts and synthesise them to create new understanding.
Look at the 'Schematic diagram of a nuclear reactor'. Research to find out what the different components of the design are for, and complete the table as follows:
In the column labelled 'Scientific problem' outline the problem that the component addresses.
In the column labelled 'Technological solution' describe the way the component solves the problem.
The first row has been completed for you.
Now, in your own words, explain, how the design of a nuclear reactor sustains a critical reaction, and how it prevents a supercritical reaction.
Assessment opportunities
In this activity you have practiced using skills that are assessed using Criterion A: Knowing and understanding.
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