  We consider the production of $^3$He and $^2$H by $^4$He photodisintegration
initiated by non-thermal energy releases during early cosmic epochs. We find
that this process cannot be the predominant source of primordial $^2$H since it
would result in anomalously high $^3$He/D ratios in conflict with standard
chemical evolution assumptions. We apply this fact to constrain topological
defect models of highest energy cosmic ray (HECR) production. Such models have
been proposed as possible sources of ultrahigh energy particles and gamma-rays
with energies above $10^{20}$eV. The constraints on these models derived from
$^4$He-photodisintegration are compared to corresponding limits from spectral
distortions of the cosmic microwave background radiation (CMBR) and from the
observed diffuse gamma-ray background. It is shown that for reasonable primary
particle injection spectra superconducting cosmic strings, unlike ordinary
strings or annihilating monopoles, cannot produce the HECR flux at the present
epoch without violating at least the $^4$He-photodisintegration bound. The
constraint from the diffuse gamma-ray background rules out the dominant
production of HECR by the decay of Grand Unification particles in models with
cosmological evolution assuming standard fragmentation functions. Constraints
on massive black hole induced photodisintegration are also discussed.
