Observations of natural and induced processes have shown that numerous types

of fusion reactions for which Q > 0 can be identified. The variables for different

*

reactions are the interacting nuclides , the reaction products which emerge, the Q-value of the reaction, and the dependence of the probability for the reaction to take place on the kinetic energy of the reactants. The fusion reaction most readily attainable under laboratory conditions and which is expected to be the first used for power generation purposes is the d-t reaction

d + t-^n + a + 17.6 MeV. (1.20)

Another most accessible fusion reaction involves deuterium nuclei as fuel:

p + t + 4.1 MeV n + h + 3.2 MeV

where h is chosen to represent the helium-3 nucleus (3He2+). This representation may appear somewhat unusual, but is seen to simplify notation in subsequent chapters. Equation (1.21) shows that d-d will fuse via two distinct reaction channels known to occur with almost equal probabilities at specific reaction conditions. Further, fuel deuterons may also fuse with two of the reaction products (tritons and helium-3) giving, in addition to the reactions of Eqs. (1.21) and (1.20),

d + h-^> p + a + 18.3 MeV . (1.22)

The above fusion reactions involve deuterons and the successively more massive light nuclides. Continuing along this pattern, a large number of reaction channels have been identified in specific cases of which d-6Li fusion is an example:

Appendix В displays the light-nuclide part of the Chart of the Nuclides.

1 Be + n + 3 A MeV

1 Li + p + 5.0 MeV p + a + t + 2.6 MeV 2a + 22.3 MeV h + a + n + 1.8 MeV

Here, each reaction channel possesses a unique probability of occurrence.

Fusion reactions involving the lightest nucleus, that is the proton, may occur according to the processes

■ h + a+ 4.0 MeV Г a+6Li + 2.1 MeV d + 2a + 0.6 MeV

p+uB -+3a + 8.7MeV (1.24c)

as well as others. Some reactions based on t and h are

t +1 —> 2n + a +11.3 MeV (1.25a)

h + h—>2p + a + 12.9 MeV (1.25b)

and

t + h —> n + p + a + 12.1 MeV . (1.25c)

Several features associated with fusion reactions need to be noted. First, the physical demonstration of a fusion reaction is not the only consideration determining its choice as a fuel in a fusion reactor. Other considerations include the difficulty of bringing about such reactions, the availability of fusion fuels, and the requirements for attaining a sufficient reaction rate density.

Another feature of the various fusion reactions listed above needs to be emphasized: in each case a different fraction of the reaction Q-value resides in the kinetic energy of the reaction products. Thus, a fusion reactor concept based on high-efficiency direct energy conversion of charged particles would appear particularly suitable for those reactions which are characterized by a high fraction of the Q-value residing in the kinetic energy of the charged particles. This is of particular interest because the neutrons appearing as fusion reaction products invariably induce radioactivity in the materials surrounding the fusion core.

Third, the fusion fuels are evidently the light nuclides displayed on the Chart of the Nuclides. In a subsequent chapter we will show that a subatomic short­lived particle called a muon and produced in special accelerators, may also play a role as a fusion reaction catalyst.

Most current fusion research and development activity is based on the expectation that the d-t reaction, Eq. (1.20), will be used for the first generation fusion reactors. While the world’s oceans as well as fresh water lakes and rivers contain an ample supply of deuterium with a particle density ratio of d/(p+d) ~ 1/6700, tritium is scarce; it is a radioactive beta emitter with a half life of 12.3
years, with the total steady state atmospheric and oceanic quantity of tritium produced by cosmic radiation estimated to be on the order of 50 kg. Since a 1000 MWt plant will bum about 250 g of tritium each operating day, a station inventory in excess of 10 kg will be required for every d-t based central-station fusion power plant so that other sources of tritium fuel are required.

The main source of tritium is expected to be its breeding by capture of the fusion neutron in lithium contained in a blanket surrounding the fusion core. The relevant reactions in 6Li and 7Li are

n+6Li—>t + a (1.26a)

and

п+7/л—> t + cc + n (1.26b)

with the latter possessing a high energy threshold Ethresh ~ 2.47 MeV. Lithium-6 and lithium-7 are naturally occurring stable isotopes existing with 7.5% and 92.5% abundance, respectively, and exist terrestrially in considerable quantity.

Additional sources of tritium may involve its extraction from the coolant and moderator of existing fission reactors, particularly heavy water reactors, where tritium is incidentally produced by neutron capture in deuterium via

n+2#—»3#. (1.27)

Of course, tritium could also be produced by placing lithium into control and shim rods of fission reactors.

Reaction (1.26b) is particularly interesting because the inelastically scattered neutron appearing at lower energy can continue to breed more tritium. Thus, in principle, it could be possible in such a system to produce more than 1 triton per neutron bom in the d-t reaction. Indeed, present concepts for d-t reactors generally assume a lithium-based blanket surrounding the fusion core that allows for tritium self-sufficiency. These and additional concepts will be discussed in subsequent chapters.