Probabilistic safety assessment and human reliability analysis for the Wendelstein 7-X fusion device

Probabilistic Safety Assessment (PSA) allows an integrated identification of safety "bottlenecks" including a focus on specific equipment, human errors, failure modes in complex systems of fission, fusion and other devices. The objective of this paper is to demonstrate the application of probabilistic safety assessment for an increase of safety and reliability of thermonuclear fusion installations taking into account both equipment and operators' reliability. Analysis of possible failure of the cooling circuit in one of the most recently constructed fusion devices, namely Wendelstein 7-X (W7-X) stellarator, is used as an example. In the paper, PSA goals for fusion installations were proposed, namely calculation of unavailability of the systems and analysis of accidents leading to equipment damage. For safety analysis of the considered cooling circuit of W7-X, a more precise model, reflecting the effect of human errors was developed. On a system level for the fusion device, the PSA was performed as an alternative to the so-called RAMI approach which focused on the operational functions rather than on physical components. The calculated risk decrease and cost efficiency of risk decrease investments are an easy and clear indicator which supports decision making regarding risk decrease alternatives implementation.