Tome Architecture and management of a geological repository - Andra

4 - General architecture of the repository in a granite medium4.4.1 Human protectionRepository-related activities call for the definition of safety functions that are specific to nuclearinstallations. Furthermore, protective measures for operators against the usual occupational risks ofsurface and underground construction work must also be taken into account.4.4.1.1 Definition of the safety functions relating to repository operationThe functions of nuclear safety are as follows:• Confining radioactivityThe radioactive materials have to be confined to prevent their dissemination. In particular, gaseousradionuclides likely to be released by certain packages will be limited as much as possible, andcontrolled ([20]). Checking the absence of package surface contamination also participates inradioactivity confinement.Meeting this safety function limits the risks of radionuclide inhalation and ingestion by workers andthe general public in the immediate vicinity of the repository installations.• Protecting humans from radiationThis function consists of protecting operators and the general public from radiation emitted bypackages.This entails inserting fixed or removable shields, keeping operators at a safe distance from the sourcesand managing their exposure time.• Controlling the criticality riskThis function consists of avoiding a criticality accident 10 , whose consequences could notablyundermine the radiation confinement and protection functions [22].This objective entails preventing the reactivity by controlling the fissile materials, packages geometry,their distribution in the repository installations and, possibly by installing neutron-absorbing materials(neutron poison) between them 11 .• Removing the residual thermal outputThis function consists of limiting the temperature levels inside the installations by dissipating thethermal output emitted by certain packages.• Removing radiolysis gasesThe explosive gases primarily arising from a radiolysis phenomenon 12 specific to certain packageshave to be removed. Ventilating the installations participates in achieving this objective during therepository operating phase.4.4.1.2 Human protection objectivesThe public and personnel must be protected from the radiological risks associated with the industrialactivities.101112A criticality accident is an uncontrolled nuclear chain reaction (fission) between neutrons and fissile materials (uranium-239, plutonium-239 and 241).The role of these types of materials capable of capturing neutrons, is to limit the nuclear reaction.The radiolysis phenomenon is linked to the effect of ionising rays (β, γ) emitted by the radioactive materials on hydrogenated productspresent in certain B waste disposal packages (organic matter, matrix water, concrete container water). It mainly takes the form of therelease of hydrogen, and also methane to a lesser extent.Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY84/228