Evaluation and criticality assessment of radiological source terms to be used for fire risk studies at accelerator facilities.

The European Organisation for Nuclear Research (CERN) is one of the largest scientific laboratories worldwide. Nowadays, mainly focused on high energy particle physics, it provides the scientific community with a unique range of particle accelerator facilities that are used by over 600 institutes and universities around the world. In such particle accelerator facilities, high energy particles end up almost inevitably impinging onto the surrounding material, inducing nuclear reactions. This often results in activation, which is the artificial induction of radioactivity in otherwise non-radioactive materials. Particles ejected in radioactive decays are part of the so-called ionising radiation, and their interaction with living biological tissue can have harmful and eventually lethal results. The CERN Radiation Protection group ensures that the personnel of the laboratory, the public and the environment are protected from potentially harmful effects of ionizing radiation linked to the organization's activities. CERN is also a unique laboratory with respect to challenges related to fire protection. The complexity of the facilities and their radiological hazards often require dedicated studies to successfully prevent, mitigate and face potential accidental fires. To carry out these studies, the Occupational Health & Safety and Environmental Protection Unit of CERN launched the FIRIA project. Its aim is to develop an integrated approach to quantitatively assess potential discharges of radioactive substances induced by a fire accident. The accurate determination of the inventory of radionuclides released from activated materials as a consequence of fire is of the utmost importance in order to estimate the potential radiological consequences derived from such an event. This has revealed the need to evaluate the contribution of the thermally promoted out-diffusion of radionuclides. We refer as out-diffused to those radionuclides initially placed in the matrix of a solid, which due to thermally promoted diffusion reach the surface of the object that contains them and manage to escape from it, subsequently being released to the environment. The work presented here aims to meet the need for an accurate assessment of this phenomenon by designing, implementing and benchmarking a simulation software to realistically estimate the contribution of radioisotope out-diffusion for a wide range of possible fire scenarios.