Nuclear thermal propulsion (NTP) is an enabling technology for expanding spaceflight capability beyond cislunar space. Hydrogen propellant is required for best efficiency by mass, but long-term hydrogen propellant storage is difficult as it must be stored as cryogenic liquid. This is further complicated by the fact that hydrogen is a very good neutron attenuator and absorbs heat as a result of attenuating radiation, consequential due to the presence of a radiation source from NTP. Design of spacecraft using NTP will need to consider propellant heating from this radiation as well as from the space environment. Monte Carlo simulation is the typical method for evaluating such radiation attenuation in a material. This paper describes a simplified analytical model for approximating radiation heat deposition for design of spacecraft using NTP. The analytical model predicts similar amounts of radiation-induced heating as Monte Carlo methods, providing an alternate analysis method during conceptual design of spacecraft using NTP. Analysis of a selection of simplified spacecraft configurations is included to illustrate the method. It was found that the analytical method has good agreement with the existing state-of-the-art for radiation heating in spacecraft propellant.