As little as three years ago you could go on to the OSTI website and freely download many studies on this topic produced by DOE and AEC laboratories. Unfortunately a few years ago the DOE started removing many of the reports from that public website. However, a good publicly available technical reference with respect to this topic is the book "Space Nuclear Power", by Buden and Angelo. Within the book, there is an entire chapter dedicated to state of the art shield designs as of the 1980's and not much has changed since then.
The preferred materials are typically intermittent layers of Lithium-Hydride (which protects against neutrons) and tungsten (to protect against x-rays and gamma rays). The shield (typically called a shadow shield) is placed between the reactor and the crew at just the right geometry to provide a radiation shadow for the crew compartment. If the crew compartment is placed a long ways away from the reactor, then the size of the shield can be minimized, because the required shadow angular limits is reduced and the effects of geometric attenuation also reduce the required thickness of the shield. However, as the distance between crew and shielding increases, then the structural mass of the craft increases. At some distance, there is a minimum spacecraft mass, but this distance is dependent on the reactor, shield and spacecraft design.
Although I cannot provide the resources for this right now, I can promise you that to net absorbed dose from cosmic radiation will dwarf that received due to the reactor. In fact for nuclear thermal propulsion and some nuclear electric propulsion designs, the use of nuclear propulsion reduces the time in space and reduces the expected radiation exposure compared to non-nuclear mission designs. It is ironic, but nuclear power reduces the radiation dose in most mission studies.
