Page 244 - Managing Global Warming

Nuclear fusion: What of the future? 203

pressure rises in the components to calculate where to install rupture disks, expansion
volumes, and condensation tanks to ensure the release would be safely contained with
no risk of tritium leakage beyond a primary containment level—and in case this also
failed, there is also secondary containment in line with conventional nuclear defense-
in-depth principles [7].
Fusion plasmas, while containing a lot of energy, are intrinsically unstable and
require active control. In any case of loss of control, the plasma may disrupt, possibly
causing damage to the reactor, but it cannot sustain a runaway nuclear reaction. There
is some residual nuclear decay heat in the materials following the collapse of the
plasma (which also affects the maintenance of the plant), but this is dispersed across
a large volume of material and even a failure of plasma control followed by a complete
loss of coolant does not lead to melting of plant materials [8].
The neutrons produced by the D-T reaction cause activation of the materials
surrounding the plasma to form low- and intermediate-level waste. It is possible to
design materials (for example, so-called low-activation steels), which limit the quan-
tity of long-lived radionuclides formed and are intended to permit recycling of most
structural elements of a fusion reactor within hundreds of years, a far more tractable
problem than the tens of thousands of years required for the storage of fissile waste [9].
However, this does require adequate detritiation and separation of materials extracted
from the reactor and the full feasibility of these processes is not yet clear. The greatest
production of long-lived nuclear material in common concepts is carbon-14, produced
by neutron radiation of nitrogen, found in water (used as a coolant) and as a common
14
alloying element in steel. Although C occurs naturally in low levels (produced in
the atmosphere by high-energy cosmic rays), it is readily taken up by biological
organisms and can be incorporated into DNA, where its decay can be damaging
(the half-life of 14 C is 5730years). Therefore, the lifetime production of 14 Cbya
fusion power plant must be limited, leading to stringent manufacturing limits on
structural alloys.
While a fusion reactor does not intrinsically use fissile material in its operation, the
neutrons available from the reaction would allow the creation of nuclear isotopes if the
right elements were incorporated into the blanket. This would not be a straightforward
operation and would be immediately obvious to inspectors.

5.2 Fusion concepts

In order to generate significant fusion power, the reaction plasma needs high fuel
densities and high reactivity:

h
P fus ¼ n D n T σνiE fus
where P fus is the fusion power; n D , n T are the densities of deuterium and tritium fuel;
hσνi is the reactivity at the plasma temperature; and E fus is the energy released by the
fusion reaction. Fusion devices therefore aim at achieving high plasma densities and
temperatures. The two main routes are magnetic confinement (for example, tokamaks
and stellarators) and inertial confinement.