  The experimental bound on lifetime of nu_3, the neutrino mass eigenstate with
the smallest nu_e component, is much weaker than those of nu_1 and nu_2 by many
orders of magnitude to which the astrophysical constraints apply. We argue that
the future reactor neutrino oscillation experiments with medium-baseline (~ 50
km), such as JUNO or RENO-50, has the best chance of placing the most stringent
constraint on nu_3 lifetime among all neutrino experiments which utilize the
artificial source neutrinos. Assuming decay into invisible states, we show by a
detailed chi^2 analysis that the nu_3 lifetime divided by its mass, tau_3/m_3,
can be constrained to be tau_3/m_3 > 7.5 (5.5) x 10^{-11} s/eV at 95% (99%)
C.L. by 100 kt.years exposure by JUNO. It may be further improved to the level
comparable to the atmospheric neutrino bound by its longer run. We also discuss
to what extent nu_3 decay affects mass-ordering determination and precision
measurements of the mixing parameters.
