Direct-drive, plastic shells imploded on the OMEGA laser system [T. R. Boehly, Opt. Commun. 133, 495 (1997)] with a 1 ns square pulse are simulated using the multidimensional hydrodynamic code DRACO in yield degradation in "thin" shells is primarily caused by shell breakup during the acceleration phase due to short-wavelength (ℓ>50, where ℓ is the Legendre mode number) perturbation growth, whereas "thick" shell performance is influenced primarily by long and intermediate modes (ℓ≤50). Simulation yields, temporal history of neutron production, areal densities, and x-ray images of the core compare well with experimental observations. In particular, the thin-shell neutron production history falls off less steeply than one-dimensional predictions due to shell-breakup-induced undercompression and delayed stagnation. Thicker, more-stable shells show burn truncation due to instability-induced mass flow into the colder bubbles. Estimates of small-scale mix indicate that turbulent mixing does not influence primary neutron yields.