Spent nuclear fuel (SNF) assemblies, after a sufficient period of cooling, can be transferred into dry casks for interim storage. In the United States, this method of storage is practiced before the canisters can be permanently moved to a geological repository. After a cooling period of about three years, the decay heat of SNF assemblies is sufficiently low to allow their storage in a dry cask with passive cooling. The SNF assemblies are placed inside a stainless steel canister which is welded shut to prevent leaks. The canister is then placed inside a thick (~ 70 cm) concrete cask which is maintained on a large concrete pad on the property of the nuclear power plant. Dry cask storage was first used in 1986 and has become increasingly prolific as the SNF inventory increases due to the lack of a permanent repository. Ongoing reactor operation increases the commercial SNF inventory by over 2000 tonnes/year [1], indicating that the use of dry cask storage will only increase as time goes on. When a permanent geological repository is made available, only the stainless steel canister must be moved to the repository, not the entire concrete cask. Safety considerations require that the radiation dose rates at the surface of the canister be well characterized including the contributions of decay photons, neutron capture photons, and neutrons. The neutron multiplication factor of the canister must also be well known to ensure that the configuration of SNF assemblies remains subcritical. This study simulated a loaded dry cask canister using Monte Carlo radiation transport code, MCNP6 [2]. Radiation dose rate contributions on the surface of the canister and the neutron multiplication factor, keff, were calculated for a canister containing 32 pressurized water reactor (PWR) SNF assemblies with all interstitial space filled with the following materials: Portland concrete, sand, borosilicate glass, light water, and a reference case with no fill material.