Tome Architecture and management of a geological repository - Andra

4 - General architecture of the repository in a granite mediumThe risk reduction measures during the operating phase means essentially radiological protectionshields and measurements monitoring the absence of contamination and should limit human exposurebelow the limits set by Andra in its objectives for protecting humans from radiation [23] : 5 mSv perannum for personnel working in the nuclear zone, 0.25 mSv per annum for the public at large.Humans must also be protected from other potential nuisances generated by the repositoryconstruction, operation and closure activities [24], such as:- inherent physical risks (falling, crushing, electrical hazard, etc.) throughout the various repositoryactivities and particularly during construction:- fire risk;- explosion risk by radiolysis gases (hydrogen, etc.) generated by certain B waste packages;- risks inherent to the working environment (noise, dust, toxic gases emitted by heavy machinery,heat released by the exothermic packages, etc.).Environmental protection requirements (as a reminder: underground and surface water [25] air [26],landscape, vicinity, soil, fauna and flora) also need to be taken into consideration throughout therepository activities, including during the post-operation monitoring phase. This monitoring shouldalso include parameters likely to be involved in long-term safety functions.4.4.2 Radiological risks during operationThe operation of nuclear installations induces waste package-related radiological risks. Designmeasures can reduce these risks to levels below those set in the objectives and ensure operation of theinstallations is safe.4.4.2.1 Risk of external exposureWaste packages are sources of external exposure (through β, γ rays and neutrons), from the time oftheir reception in the surface installations until they are disposed of in the underground installations.The transport casks containing the primary packages delivered to the repository site have aradiological protection function, and their structure is designed for the radiological characteristicsspecific to the waste conveyed. Once they have been removed from their packaging, the primarypackages are handled and conditioned inside cells made inaccessible to the operators working on thembehind radiation shields (walls, viewing ports).Management of external exposure risk during transfer operations and until the packages are emplacedin the disposal cells, is controlled by placing radiation shields 13 between the radioactive sources andthe operators to attenuate the radiation flows. This is achieved by the disposal package transfer casks,the operating plugs of the C and CU disposal boreholes and the B waste tunnel gates. Protecting ordistancing the control unit of the machinery used to transfer or insert the packages into the disposalcells will also contribute to the operator dose reduction, therefore meeting the safety objective set byAndra.A preliminary estimate of doses received at the various workstations has been made for the repositoryproject in the clay medium [27], which has very similar operating processes.13The material used for these shields depends on the type of radiation emitted by the radioactive source:- for gamma rays, the materials used are heavy materials such as steel, concrete and leaded glass;- for neutron rays they are specific materials (with boron or cadmium, etc.) or hydrogenated materials;- alpha and beta rays do not need any particular shielding because they are stopped by the package envelope.Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY85/228