8. Conditions needed to realise a fusion reactor
Before we manage to realise a fusion reactor, we have to reach some fundamental
objectives in the following sequence:
- Breakeven. This is when the energy generated by fusion
equals the energy put in to maintain the plasma at thermonuclear temperatures.
- Ignition. This means that the fusion reaction
is self-sustaining due to the action of the helium nuclei produced.
- Technological feasibility. This is reached
when the net output of the whole plant is positive.
conditions
for a fusion reactor
So, in the future fusion reactor, the reaction has to be self-sustaining.
In other words, we suppose that the alpha particles trapped in the plasma
volume release their energy to the plasma so that it remains heated after
initial heating by external means. In the meantime, the neutrons transport
their energy to the reactor blanket, generating tritium and transforming
the energy into heat, which can then be used to produce electric energy.
The energy produced by fusion reactions is expressed as the kinetic energy
(heat) of the reaction products:
- Neutrons, which carry about 80% of the energy produced, leave
the plasma without any significant interaction. They are absorbed by
the lithium blanket placed around the reactor core and are used to regenerate
tritium. The lithium blanket has to be thick enough (about 1 cm) to absorb
the fusion neutrons (14-MeV neutrons), which are then used to heat a
liquid and generate electric energy through a heat exchanger.
- The heavier helium nuclei remain trapped in the plasma to which
they transfer their energy, thereby obtaining a self-sustaining reaction
without needing any further external heating.
fusion reactor schematic principle
This is the outline of the future thermonuclear reactor, in which the
energy released in the reaction (energy per unit time) will be proportional
to the density of the reacting nuclei, to the probability that the reaction
will take place and to the temperature of the plasma.
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