The surface source write/read capability in the 3-D neutronics code MCNP has been implemented in the CAD based DAG-MCNP. We performed neutronics calculations for a detailed solid model of an ITER first wall/shield module to assess the accuracy of the results obtained using the surface source for toroidal fusion systems. To further understand the sensitivity of the results to the size of the surface source and boundary conditions, we performed calculations for a simplified 3-D ITER model. The results show that use of the surface source approach is accurate provided that the surface source and associated reflective boundaries are extended beyond the component of interest by at least 10 cm and the surface source is generated/placed as close as possible to the front surface of that component.