Emission of volatiles for the long-period Comet C/2006 W3 Christensen was observed before perihelion from 3.7 AU through perihelion passage at 3.13 AU till 5.0 AU after perihelion. Coma is found to be strongly enriched in molecules more volatile than water. At heliocentric distances 3.13–3.7 AU activity of the comet was mostly driven by CO emission (Bockelée-Morvan, D., et al. [2010]. Astron. Astrophys. 518, L149; Ootsubo, T., et al. [2012]. Astrophys. J. 752, id15). However, emission of water was observed as far as at 5 AU. We attempted to determine, what composition and structure of the nucleus are needed to reproduce the observed strong emission of CO molecules. For this purpose we simulated evolution of a model nucleus orbiting like Comet C/2006 W3. We fitted simulated profiles of fluxes of molecules to the described in literature fluxes derived from observations (Bockelée-Morvan, D., et al. [2010]. Astron. Astrophys. 518, L149; Ootsubo, T., et al. [2012]. Astrophys. J. 752, id15; de Val-Borro, M., et al. [2014]. Astron. Astrophys. 564, A124). The agreement was possible only using model taking into account sublimation from ejected grains. We have found, that the nucleus should be very porous, composed of moderately fine grains, about 10 μm in diameter, and have tensile strength about 10 kPa, like estimated for Comet 9P/Tempel 1.