This paper describes the MULASSIS ID radiation analysis tool, which is based on the Geant4 Monte Carlo radiation transport toolkit. MULASSIS permits simulation of a wide range of particle physics in almost any shielding material, including treatment of ionization and nuclear interactions of energetic protons, electromagnetic cascades for electrons and photons, and low-energy neutron transport, with the tracking of all secondary particles that are produced by interactions. The implementation of Geant4 physics in a 1D (spherical or planar) shielding application simplifies the input requirements, and because of this, the tool can be operated through a series of web pages from the ESA SPENVIS web-site. Outputs of MULASSIS include particle fluence at boundary layers, total ionizing dose to layers, non-ionizing energy loss at boundary layers and pulse height energy deposition spectra.