Compared to conventional two-dimensional/one-dimensional DORT synthesis fluence methodology, the Westinghouse-developed three-dimensional parallel discrete ordinates transport code RAPTOR-M3G can provide best-estimate neutron fluence at extended beltline regions that are far away from the core midplane, which removes the over conservatism in the synthesis fluence determination methodology at these locations. Therefore, RAPTOR-M3G has been used on a four-loop Westinghouse pressurized water reactor plant to calculate the neutron fluence in support of its License Amendment Request for measurement uncertainty recapture power uprate. However, the U.S. Nuclear Regulatory Commission (USNRC) does not consider RAPTOR-M3G an approved fluence determination methodology per Regulatory Guide 1.190. A comparative study has been done between the RAPTOR-M3G code and the TORT three-dimensional single processor discrete ordinates transport code in the DOORS 3.2 code package to justify the use of RAPTOR-M3G. TORT and two-dimensional discrete ordinates transport code DORT have been approved by the USNRC for reactor vessel fluence calculation per Regulatory Guide 1.190. Being a single processor discrete ordinates code limited by current hardware and compilation configuration, TORT has not been used for commercial reactor vessel fluence production calculation because it imposes a great challenge to the computer memory that can be allocated to a single processor. A transport TORT model with the geometry meshes that meets all the requirements of Regulatory Guide 1.190 would not run on a single processor because of the memory limitation. In order to work around this, the TORT model has been broken into three segments axially: upper reactor environment (URE), middle reactor environment (MRE), and lower reactor environment (LRE). The three segments are bootstrapped to provide a flux map for the whole reactor vessel. Details and results of this comparative study are described in this paper. A founding between using theta-weighted and directional theta-weighted differencing schemes has also been reported in this paper.