Page 241 - Managing Global Warming

200 Managing Global Warming

1 2 3
( 1 H, sometimes called protium; 1 H, deuterium (D, stable); and 1 H, tritium (T, unstable));
3
and other light elements such as helium (usually 2 He) and boron. The reactivity
(i.e., the probability that the collision of two fuel particles leads to a reaction)
of some of these reactions is shown in Fig. 5.1.
Due to it being the reaction with the highest reactivity at achievable temperatures,
most near-future fusion concepts are based on deuterium-tritium (D-T) fusion.
The reaction is:

2 3 4
D+T ¼ H+ H ! He 3:5MeVð Þ +n 14:1 MeVÞ
ð
1 1 2
4
This reaction results in a charged alpha particle ( 2 He) and a neutron. The alpha particle
can be constrained by magnetic fields and interacts strongly with other charged par-
ticles (ions and electrons) in the fusion plasma to heat it up. The neutron is not well
constrained and its energy is deposited in whatever material surrounds the plasma,
usually causing considerable damage to the material at an atomic level. For energy
production, it can be used to make more tritium, which has a short half-life of about
12years and so does not occur naturally in any abundance:
6 4 3
ð
ð
2
1
3 Li + n ! He 2:05MeVÞ + T2:75 MeVÞ
7 4 3
ð
1
2
3 Li + n ! He + T+ n 2:5MeVÞ
1E-20
1E-21
1E-22
Fusion reactivity <sn> (m 3 s –1 ) 1E-24
1E-23
1E-25
1E-26
1E-27
1E-28
1E-29 DT D-He3 DD p-B
1E-30
1 10 100 1000
Plasma temperature (keV)
Fig. 5.1 Fusion reactivities for common reactions. The fusion power is then given by
P fus ¼n D n T hσνiE fus . The deuterium-tritium (DT) reaction has a higher peak at a lower
temperature than the alternatives, making it the most achievable option. The fully aneutronic
hydrogen (proton)-Boron (p-B) reaction requires very high temperatures. See section 5.2 for
more information.
Data from Bosch H-S, Hale GM. Improved formulas for fusion cross-sections and thermal
reactivities, Nucl Fusion 1992; 32: 611–631.