The Argonaut reactor is a water-moderated research reactor that was designed by the Argonne National Laboratory. There are currently few reactors of this type under operating conditions in the world, one of which, located at the Nuclear Engineering Institute in Brazil, has been in operation since 1965. This study analyzes a nonpostulated accident caused by the collision of the reactor coverer into the core during crane manipulation. This event causes the release of particulate and volatile fission products into the reactor hall. Thus it was necessary to evaluate the isotopic inventory and the fuel burnup for more than 50 years of operation. These data are used to calculate the source term and the instantaneous dose at 1 m from the reactor core.