  We consider the processes $d +d \to n +{^3He}$, $d +{^3He} \to p +{^4He}$, $d
+{^3H} \to n +{^4He}$, ${^3He} +{^3He}\to p+p +{^4He}$, ${^3H} +{^3He}\to d
+{^4He}$, with particular attention for applications in fusion reactors. After
a model independent parametrization of the spin structure of the matrix
elements for these processes at thermal colliding energies, in terms of partial
amplitudes, we study polarization phenomena in the framework of a formalism of
helicity amplitudes. The strong angular dependence of the final nuclei and of
the polarization observables on the polarizations of the fuel components can be
helpful in the design of the reactor shielding, blanket arrangement etc..We
analyze also the angular dependence of the neutron polarization for the
processes $\vec d +\vec d \to n +{^3He}$ and $\vec d +\vec {^3H} \to n
+{^4He}$.
