  Experimental observation of nucleon instability is one of the missing key
components required for the explanation of baryon asymmetry of the universe.
Proton decays with the modes and rates predicted by(B-L)-conserving schemes of
Grand Unification are not observed experimentally. There are reasons to believe
that (B-L) might not be conserved in nature, thus leading to the nucleon decay
into lepton+(X) and to phenomena such as Majorana masses of neutrinos,
neutrinoless double-beta decays, and most spectacularly to the transitions of
neutron to anti-neutron. The energy scale where (B-L) violation takes place
cannot be predicted by theory and therefore has to be explored by experiments.
Different experimental approaches to searching for (B-L)-violating transition
of neutron to antineutron are discussed in this paper. Most powerful search for
neutron to antineutron transitions can be performed in a new reactor-based
experiment at HFIR reactor (ORNL) where sensitivity can be >1,000 times higher
than in the previous experiments.
