  We present a study of the effects of clumping on the emergent spectral energy
distribution (SED) from dusty supernova (SN) shells illuminated by a diffuse
radiation source distributed throughout the medium. (...) The fully 3D
radiation transport problem is solved using a Monte Carlo code, MOCASSIN, and
we present a set of models aimed at investigating the sensitivity of the SEDs
to various clumping parameters. We find that, contrary to the predictions of
analytical prescriptions, the combination of an optical and IR observational
data set is sufficient to constrain dust masses even in the case where
optically thick clumps are present. Using both smoothly varying and clumped
grain density distributions, we obtain new estimates for the mass of dust
condensed by the Type II SN 1987A by fitting the optical and infrared
spectrophotometric data of Wooden et al. (1993) at two epochs (day 615 and day
775). (...) From our numerical models we derive dust masses for SN 1987A that
are comparable to previous analytic clumped graphite grain mass estimates, and
at least two orders of magnitude below the 0.1-0.3 Msol that have been
predicted to condense as dust grains in primordial core collapse supernova
ejecta. This low condensation efficiency for SN 1987A is in contrast to the
case of SN 2003gd, for which a dust condensation efficiency as large as 0.12
has recently been estimated. (Abridged)
