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The Challenge of Fusion Power

Page history last edited by Andrew Alder 1 year, 7 months ago

A page of energy issues


I recently wrote an answer on Quora of which I'm quite proud. So I reproduce it here.


I have said similar things before, see below, but I think not quite so well. I particularly like the King Midas in reverse phrase (with thanks of course to The Hollies).


The question


What are the challenges to overcome before fusion power can be achieved?


My answer 

Most of them have to do with materials.


We need two or three materials, in theory it could be just one but that seems unlikely. There are three functions that need to be addressed.


  • We need a plasma facing material for the chamber lining or first wall. This material needs to withstand an enormous flux of very fast neutrons without contaminating the plasma or generating large quantities of nuclear waste.
  • We need a material for the breeding blanket which will generate significantly more than one Tritium nucleus for every neutron it absorbs, again without creating large quantities of nuclear waste.
  • And we need a material that when exposed to very fast neutrons will efficiently convert their energy into a form which can eventually generate electricity.


None of these objectives is negotiable. And so far, and despite decades of well-funded research, there is no known material which does any of these three things. But research is continuing. It's unwise to say "Impossible!" where technology is concerned. Who knows what ideas are lurking just around the corner of current thinking?


In particular, development of these materials is a key objective of the IFMIF and of ITER itself.


That third requirement might be seen as a challenge of itself. When nuclear fission takes place, the two fragments are propelled apart, and most of the energy released is in the speed of these fragments. These fragments are heavy and about equal in mass, so they pick up roughly the same amount of speed and energy, and they are electrically charged, so they would need a great deal of speed to cause any further nuclear reaction. They do not have that much speed, so instead they quickly give up this energy to their surroundings, without causing further nuclear reactions. The energy they lose can be used to heat a coolant, and eventually to raise steam.


Nuclear fusion is a bit more difficult. The initial result of D+T fusion (the fusion reaction that ITER and the latest Chinese tokamak both intend to use) is He-5, which quickly splits into He-4 and a neutron. Again, most of the energy released is in propelling these fragments apart.


But there the similarity ends. As momentum must be conserved, and one of the fragments is only one-quarter the mass of the other, this lighter fragment gains far more speed. And energy is proportional to the square of velocity. So this lighter fragment picks up most of the energy, about 80% of it in fact. And this lighter fragment (the neutron) has no charge, so it needs relatively little energy to react with any nucleus it hits.


And it has lots of energy. These are the fastest neutrons ever produced for peaceful purposes, and also the most penetrating and dangerous form of nuclear radiation yet produced for peaceful purposes.


They are King Midas in reverse. Everything they touch turns to nuclear waste.


If you can figure out a practical way to convert this energy into electricity, you will be very, very popular. There would still be material issues. But it would be a major breakthrough.


On the other hand, many other highly talented people have already spent their entire careers fruitlessly seeking such a breakthrough, so do not underestimate the challenge.


See also

some of my previous attempts at making some of the same points



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