Architecture and management of a geological repository - Andra

11 – Operational SafetyThese risks are the risk of explosion associated with the emission of gas from some waste packages(cf. Section 11.3), the risk of fire in underground installations (cf. Section 11.4) focusing on scenariosthat would involve disposal packages (cf. Section 11.5 and 11.6) and the risks involved with thetransfer of packages of radioactive material with the shaft drop scenario (cf. Section 11.7) and B wastedisposal packages falling in cells (cf. Section 11.8).This analysis is based on the current understanding of the main risks identified on the basis of currentknowledge of the installations. It may evolve with the expansion of installation definition studies.11.3 Study of the risk associated with the emission of explosive gases fromsome B waste packagesMost of the B waste packages (B2 and B5 in particular) emit gases between the time of their arrival atthe surface installations and their emplacement in the underground disposal cells. These gases arecaused by radiolysis which is associated with the effect of ionizing radiation (β, γ) emitted byradioactive materials on hydrogenated products present in the waste packages (organic materials,water in the conditioning matrix).These radiolysis gases are mainly hydrogen (more than 90% of the gaseous releases) and, to a lesserextent, methane 163 . The emission of these gases can cause an explosion if their concentration exceedstheir lowest explosive limit 164 .The purpose of this section is to ensure that the emission of these gases by B waste packages is notliable to entail a risk of explosion [107].11.3.1 Waste package characteristicsWaste packages affected by the release of explosive gases are mainly B2 and B5.1 type packages thatcontain organic matter.Table 11.3.1Emission rates of explosive gases released by some B waste packagesPrimarypackageB2.1B2.2Nature and contents ofthe primary packageMetal drum withbituminised sludgeEmission rates of explosive gases (H 2 , CH 4 )(l/drum/year)- 10 liters/drum/year (average value)- 57 litres/drum/year (maximum value corresponding to aminority of drums)B5.1 Container with hulls andend caps andtechnological waste(including organic waste)- 10 litres/drum/year (average value)- 500 litres/drum/year (maximum theoretic value forpacakages corresponding to a maximum value of organiccompounds and a maximum activity value)Average values of gas emitted from the packages have been used for simulations in the surfaceinstallations and in disposal cells as they correspond to several hundred or several thousand packages;on the other hand, the theoretical maximum values has been used for simulations relating to the phasein which a transfer cask loaded with a disposal package is transferred in the drift.The concentration of gases in a given installation has been estimated as a function of its ventilationcharacteristics. The per hour air renewal rate is taken as being equal to 2 in installations in surfacenuclear facilities. For the ventilation of B waste disposal cells, the data used are an airflow of 3 m 3 /swhich corresponds, for a 250m-long cell full of packages, to an air renewal rate per hour of 5.163 Radiolysis is also the cause of the release of very small amounts of carbon dioxide and carbon monoxide. These gases are diluted by theventilation of the installations as are those produced by vehicles with thermal engines.164 The lowest explosive limit is the minimum concentration of gas above which there is a risk of explosion in the presence of a source ofignition. The lowest explosive limit is 4 % for hydrogen and 5.3 % for methane.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM454/495