  The formation and growth of helium bubbles due to self-irradiation in
plutonium has been modelled by a discrete kinetic equations for the number
densities of bubbles having $k$ atoms. Analysis of these equations shows that
the bubble size distribution function can be approximated by a composite of:
(i) the solution of partial differential equations describing the continuum
limit of the theory but corrected to take into account the effects of
discreteness, and (ii) a local expansion about the advancing leading edge of
the distribution function in size space. Both approximations contribute to the
memory term in a close integrodifferential equation for the monomer
concentration of single helium atoms.
  The present boundary layer theory for discrete equations is compared to the
numerical solution of the full kinetic model and to previous approximation of
Schaldach and Wolfer involving a truncated system of moment equations.
