Radiological safety considerations in Accelerator Driven Systems (ADS) and Radioactive Ion Beam (RIB) facilities

With the advent of technologically advanced and complex facilities like accelerator driven subcritical or transmuting systems and radioactive ion beam facilities, the practitioners of radiation protection face new and unconventional challenges requiring satisfactory solutions. A high energy particle accelerator producing large number of neutrons by spallation reactions when coupled with a subcritical nuclear reactor to maintain the fissioning process without achieving criticality is known as the accelerator driven subcritical system which can also be used to transmute long lived nuclear waste containing transuranic elements. Radioactive Ion Beam (RIB) facilities aim at accelerating ions of unstable (radioactive) elements for studies of exotic nuclei which will open new frontiers in nuclear physics, nuclear astrophysics, material science and biology. Both the facilities are based on high energy, high intensity particle accelerators producing large fluence of high energy neutrons, gammas and muons. They initiate hadronic and electromagnetic cascades generating neutron- proton and electron-gamma showers making the radiation environment highly complex spatially and directionally around these facilities. Technological and theoretical developments necessary for a more precise radiation protection practices to cope with such directional, dynamic, pulsed and a mixture of different types of radiation fields are non-trivial in nature and are different from conventional radiological safety requirements. The major radiation safety considerations for these facilities can be broadly classified in two parts: 1. To protect the public from radiation hazards (for which we have to take into consideration skyshine, release of toxic gases in the environment, soil and groundwater activation) 2. To maintain hazards within limits for radiation workers (for this we have to consider bulk shielding, streaming of radiation through ducts and penetrations, induced activity in target, air, cooling water, walls and accelerator structures). Theoretical results obtained so far indicate that the very high activation of the structural materials in some components will require a detailed study and comparison of the behavior of different materials, namely their remaining activity several years after beam shutdown. The very high dose rates assessed in the computational studies performed also impose stringent requirements in the radiological protection and radiation safety issues associated with the operation and maintenance of these facilities, Presently, several new radioactive ion beam (RIB) facilities are planned, which typically will have three orders of magnitude larger radioactive inventory than already existing facilities. These radioactivities mostly are produced by ion beam irradiation with many kW of power. In the direct production method, protons of about one GeV hit different thick targets and produce a broad range of neutron- and proton-rich nuclei by fragmentation or spallation. In the indirect approach, secondary particles like fast neutrons, thermal neutrons or Bremsstrahlung photons are generated and then an intense source of neutron-rich fission fragments is obtained by fissioning of uranium or thorium targets. The radioactive ion production target is hot and shows out gassing, but at the same time ionized species are extracted. To safely confine and control the radioactive inventory of this open source special efforts are required. Considering the complex radiological safety issues, it appears essential to carry out probabilistic safety analysis (PSA) of radiological hazards concerning the ADS and RIB facilities based on hazards and operability (HAZOP) analysis and Failure Mode Effect and Criticality Analysis (FMECA). Fault Tree and Event tree methods can be used to depict system failure logic, identify different initiating events and analyse the safety systems required under such circumstance. (author)