[en] The present report deals with the conditions under which gas will migrate from a radioactive low-level waste repository located in a hard rock formation below the sea bottom. Chemical reactions resulting in gas (hydrogen) production will take place in the stored waste. Unless the gas produced can escape from the rock cavern an over-pressure that may bring about contamination of the groundwater regime may arize in the cavern. The purpose of the present study is to provide a rough estimate of the capability of the rock formation in consideration to convey the gas produced in the repository to the surface. The gas produced in the water filled repository will be subject to very fast gravitational segregation, and as a result a gas cushion will be created at the roof of the cavern. Three particular stages may be distinguished in the present flow problem, these are (i) the accumulation of the gas at the roof of the cavern, (ii) the escape of the gas into the fractures, once the critical thickness of the gas cushion has been reached, and (iii) the flow of gas, once the water in the fractures has been displaced. A first part of the study is devoted to analysing the transport properties of a single fracture. The governing flow equations and expressions for the flowrates of the gas and break-through times of the gas front to reach the upper boundary (sea bottom) of the rock formation are derived. The physical properties of a single fracture are used for the modelling of the fractured rock mass in consideration. A second part of the study is devoted to a number of numerical examples worked out to study the conditions of the gas flow for the projected low-level radioactive waste repository. The following parameters are studied: (i) the critical thickness of the gas cushion, (ii) break-through times of the gas, and (iii) flowrates of the gas. (Authors)