  We present radiative-transfer modelling of the dusty spiral Pinwheel Nebula
observed around the Wolf-Rayet/OB-star binary WR104. The models are based on
the three-dimensional radiative-transfer code TORUS, modified to include an
adaptive mesh that allows us to adequately resolve both the inner spiral turns
(sub-AU scales) and the outer regions of the nebula (distances of 10^4 AU from
the central source). The spiral model provides a good fit to both the spectral
energy distribution and Keck aperture masking interferometry, reproducing both
the maximum entropy recovered images and the visibility curves. We deduce a
dust creation rate of 8+-1 x 10^{-7} solar masses per year, corresponding to
approximately 2% by mass of the carbon produced by the Wolf-Rayet star.
Simultaneous modelling of the imaging and spectral data enables us to constrain
both the opening-angle of the wind-wind collision interface and the dust grain
size. We conclude that the dust grains in the inner part of the Pinwheel nebula
are small (~100A), in agreement with theoretical predictions, although we
cannot rule out the presence of larger grains (~1 micron) further from the
central binary. The opening angle of the wind-wind collision interface appears
to be about 40 degrees, in broad agreement with the wind parameters estimated
for the central binary. We discuss the success and deficiencies of the model,
and the likely benefits of applying similar techniques to the more the more
complex nebulae observed around other WR/O star binaries.
