As an alternative energy carrier alongside electricity, hydrogen is considered one of the key factors in the impending required changes in the energy system transition. Due to the decrease in natural gas extraction in the Netherlands, the reuse of the natural gas network which eventually will become available for hydrogen transmission has emerged as a topic, recently. Although it seems to be possible to reuse the existing natural gas transmission network for the transmission of hydrogen, a quantitative risk analysis is still lacking and there is uncertainty about the safety of hydrogen transmission. This report aims to quantify the risk of the reuse of an existing natural gas pipeline for 100% hydrogen transmission and compare it with the natural gas case. A risk analysis entails the quantification of both the probability of failure and the adverse consequences. This research investigated the current methodology to determine the probability of failure for hydrogen transmission and proposed a new calculation method using a Bayesian network and historical natural gas data to derive new failure frequencies. The consequence of failure for both hydrogen and natural gas will be modelled using the SAFETI-NL; the influence of the gas different characteristics on the lethality will be quantified and risk values are produced, assuming the probability of failure to be similar for both gases when transported through natural gas pipelines. This research illustrated the promising application of a Bayesian network is a promising method to derive failure frequencies while taking into the significant influence of the pipeline characteristics. However, from the results, it emerges that further research is required with more redundant data. Also, future research should investigate the application of natural gas data since some failure scenarios are not entirely independent of the transmission gas, which could result in less favourable failure frequencies. Furthermore, calculations in SAFETI-NL have shown that hydrogen release is associated with a shorter duration and lower release power than natural gas, resulting in less significant consequences in the vicinity of the pipelines. Also, the low weight and high diffusive coefficient of hydrogen cause the consequences to decrease much faster with distance. As a result, there is concluded that hydrogen transmission is accompanied by lower risk values than natural gas, assuming the failure frequencies to be similar when both gases transported through natural gas pipelines. Although, numerous input parameters affect the risk values determined, of which at least the probability of ignition for hydrogen release has to be investigated in future research.