A nuclear reaction takes place when a nucleus is struck by another nucleus or particle.   If the original nucleus is transformed into another, this is called transmutation.   An example is shown on the right

Energy and momentum must be conserved in nuclear reactions.   A gGeneric reaction is shown on the right.

The reaction energy, or Q-value, is the sum of the initial masses less the sum of the final masses, multiplied by c2:

If Q is positive, the reaction is exothermic, and will occur no matter how small the initial kinetic energy is.

If Q is negative, there is a minimum initial kinetic energy that must be available before the reaction can take place.