Reliability Modelling of Systems with Context Dependent Functions Using the Example of the 11 Tesla Dipole Quench Protection System at CERN

Subject of this thesis is the reliability analysis of the Quench Protection System (QPS) of the 11 Tesla dipole magnet of CERN’s Large Hadron Collider (LHC). The objective is to identify weaknesses of the system and to assess its reliability under different operational conditions. The RIRE methodology, which was developed at CERN, is applied to the 11 Tesla dipole QPS. In the framework of RIRE, the functional structure of the QPS, its failure modes and their effects on the magnet and LHC operation are defined. Building on these results, the RMCF methodology was developed in this thesis. RMCF defines a systematic approach for the reliability modelling of systems that have to fulfil their functions in case of an undesired event, as can be the case for protection system. By applying RMCF in the second part of the work, the most critical failure mode of the QPS, which defines the insufficiently energizing of the quench heaters, is quantitatively analysed. The results show that the estimated system reliability meets the defined reliability requirement assuming that the QPS is maintained in periodic tests and is also tested after each quench in the 11 Tesla dipole magnets. This result is obtained independently of the number of quenches in the magnets per year. The variation of the frequency of the periodic tests shows that the system reliability can be significantly improved by performing frequent tests. Furthermore, the extension of the Quench Heater Discharge power Supplies (DQHDS) by a redundant triggering demonstrated to have a negligible improvement of the system reliability. Last, the operation with an unavailable quench heater poses increased risk and thus should be kept as low as possible. By combining the RIRE and RMCF methodologies, a procedure was developed for the extensive analysis of complex systems whose functions depend on an undesired event. The approach was successfully applied to the QPS of the 11 Tesla dipole magnet of CERN. The results of the analysis show that the concept of the quench heaters is acceptable and fulfils the defined reliability requirement. Based on the results, the design request for the redundant triggering in the DQHDS will not be implemented.