(c) doc b

8. Nuclear Fusion Reactions & formation of 'heavy elements'

Doc Brown's Chemistry KS4 science GCSE Physics Revision Notes

What is nuclear fusion? Why do nuclear fusion reactions release so much energy? Why is it difficult to sustain a nuclear fusion reaction? What is an artificial or man-made element? How are heavy elements like the trans-uranium elements made-synthesised? What is an ion particle accelerator? Can we build a nuclear fusion reactor power station? These revision notes on nuclear fusion and synthesising heavy elements should help with GCSE/IGCSE physics courses and A/AS level physics courses

RADIOACTIVITY and NUCLEAR PHYSICS INDEX

 

8a. Nuclear Fusion Reactions and the formation of 'heavy elements'

  • At the extremely high temperatures (107 oC = 10 million degrees!) in the 'heart' of stars the atomic nuclei have such enormous speeds and kinetic energies that on collision they can fuse together - the nuclear process of fusion.

  • The extremely high energy is needed to give the particles sufficiently high kinetic energy to overcome the natural and massive repulsion forces of the two positive nuclei involved e.g. two positive hydrogen nuclei (+) <==> (+).

  • The process by which a heavier atomic nucleus is made from two smaller atomic nuclei is called fusion and these changes also release enormous amounts of energy.

    • In a nuclear fusion process two smaller atomic nuclei may fuse into one larger nucleus or a larger nucleus that either of the starting nuclei plus a smaller nucleus.

    • Either way a heavier nucleus is created.

    • One advantage over nuclear fission is that little radioactive waste is produced, BUT, technically, nuclear fusion has proved technically very difficult to produce a continuous energy output, and we are a very long way from a nuclear fusion power station.

  • The lightest atom is hydrogen, this is converted to helium and gradually all the other elements up to uranium must have been formed in stars like the Sun.

    • We would like to be able to reproduce this fusion process e.g. converting hydrogen into helium as an energy resource to generate electricity.

    • Attempts are being made by nuclear scientists and engineers to build prototype nuclear fusion reactors BUT the task of maintaining nuclear fusion is proving extremely difficult.

    • You have to maintain an extremely high temperature and confine and control the plasma of hydrogen atoms fusing into helium atoms with powerful magnetic fields and this is proving technically very difficult, since you can't use a physical container.

    • So far, in a few experimental fusion reactors, fusion has only been created for a fraction of a second but cannot be controlled and sustained yet!

    • In fact its taking far more power to create the fusion than any energy released, not a good deal for the consumer at the moment!

  • Examples of fusion nuclear equations (get the balancing?) ....

  • (a) (c) doc b (initially a heavier isotope of hydrogen is formed and a positron)

  • (b)(c) doc b  + (c) doc b

  • (c)(c) doc b

  • (d)(c) doc b

  • (e)(c) doc b

  • (f) (c) doc b (the most abundant helium isotope found today)

  • (g)(c) doc b

  • (h) helium nuclei fuse to form lithium, beryllium etc.

  • (i) then from carbon to oxygen etc. (c) doc b

  • (j) and lots of alternative fusions like (c) doc b... etc.

  • (k) gradually building up elements with increasing atomic and mass numbers, and finally the massive isotope of uranium, (c) doc b the biggest 'naturally' occurring atom!

  • (a), (b) and (f) are believed to be the main initial energy releasing fusion nuclear reactions in the Sun, they happen quite nicely at 15000000oC!

  • On 'Earth' super-heavy' elements are being made in nuclear reactors by bombarding elements like uranium (atomic number 92) with lighter particles (described below).

Advanced Chemistry Page Index and Links


APPENDIX 'COLD FUSION'

Cold fusion is nuclear fusion at low temperatures e.g. room temperature (NOT millions of degrees!).

In 1989 two scientists called Stanley Pons and Martin Fleischmann reported in scientific research paper that using a simple electrolysis cell system they had caused hydrogen atoms to fuse at room temperature.

They reported that much more heat energy was evolved compared to the electrical energy passed into the cell.

However their paper had NOT been peer reviewed, that is, read and their work validated by other independent scientists.

Many scientists were sceptical about their work and since 1989, few scientists, if any?, have reliably reproduced their results.

Therefore, the scientific community does not officially accept that cold fusion is possible at the moment.

This is how science works, results must be reproducible in laboratories all around the world and so cold fusion theory is not accepted as a viable scientific concept.


8b. The production of Trans-Uranium Elements - very heavy elements!

  • Heavy atomic nuclei tend to be naturally unstable and for example, many long lived isotopes of uranium (U92) finally decay via a series of relatively short-lived radioisotopes to produce stable isotopes of lead (82Pb).

  • No element higher than uranium (92U) is found in nature except for traces of neptunium (93Np) and plutonium (94Pu) isotopes. These are found in uranium ores but are produced by neutron-uranium collisions rather than from the Earth's origin. The neutrons come from the spontaneous fission of the unstable uranium isotope 235U and gives rise to heavy element 'synthesis' sequence e.g.

    • 238U == + n ==> 239U == beta decay ==> 239Np == beta decay ==> 239Pu

  • Even heavier or 'trans-uranium' elements can be made by bombarding a heavy atomic nucleus with a smaller ionised atom particle, in an ion particle accelerator.

    • However many of the heaviest are only produced in minute quantities as little as a few hundred atoms in accelerator collisions.

    • In an accelerator the two atoms are ionised and accelerated in powerful electromagnetic fields to very high speeds eg close to speed of light, but in opposite directions and are then allowed to collide. The high kinetic energies are needed to overcome the repulsion of the two positive nuclei.

    • See examples 1. to 3. below.

  • The heavier elements are also made by neutron bombardment in a nuclear reactor.

    • Although most neutrons partake in nuclear fission reactions (see section 9.), in some cases this will create a bigger nucleus.

      •  e.g. Np and Pu 'natural' examples above and example 4. below.

  • So, from these two methods, a whole series of man-made or 'artificial' elements from atomic number 93 to 112 have been synthesised.

  • Where they are formed in nuclear reactors from neutron collision (e.g. plutonium), they can be chemically separated in quantities ranging from micrograms to kg in order study their physical and chemical properties.

  • Note again, the balancing of nuclear equations e.g.

(c) doc b

formation of einsteinium from uranium and nitrogen nuclei

(c) doc b

formation of californium from uranium and carbon nuclei

(c) doc b

formation of lawrencium from californium and boron nuclei

(c) doc b

formation of americium from plutonium and neutrons


RADIOACTIVITY and NUCLEAR PHYSICS INDEX

1. Atomic structure, fundamental particles and radioactivity

2. What is radioactivity? Why does it happen? What radiations are emitted?

3. Detection of radioactivity, measurement, dose units, ionising radiation sources, background radiation

4. The properties and dangers of alpha, beta & gamma radioactive emission

 5. The uses of radioactive Isotopes emitting alpha, beta or gamma radiation

6. Half–life of radioisotopes, how long does material remain radioactive? Uses of decay data & half–life values

7. Nucleus changes in radioactive decay? how to write nuclear equations? Production of Radioisotopes

 8. Nuclear fusion reactions and the formation of 'heavy elements'

 9. Nuclear Fission Reactions, nuclear power energy resources


Advanced Chemistry Page Index and Links

(c) doc b(c) doc bRADIOACTIVITY multiple choice QUIZZES and WORKSHEETS

Easier-Foundation Radioactivity Quiz

or Harder-Higher Radioactivity Quiz

 (c) doc b five word-fills on radioactivity * Q2 * Q3 * Q4 * Q5and ANSWERS!

crossword puzzle on radioactivity and ANSWERS!


ALPHABETICAL SITE INDEX for chemistry     

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