Model checking verifies the correctness of nuclear power plant safety systems
Model checking as a process.
The object of Jussi Lahtinen’s dissertation was to find a more formal and mathematical approach to system verification and to develop model checking practices that are suitable for the nuclear industry. The traditional system verification methods, such as testing and simulation, do not have enough coverage to address the increasing digitalisation of safety automation systems.
‘Nuclear power plants have large safety systems in place that encompass multiple safety functions, such as the emergency diesel generator control system. In one of the techniques now developed, the system is divided into modules, and an algorithm is used for pinpointing the subset of modules that verifies the correctness of the system,’ says Lahtinen, explaining the practical implementation of the study.
At the architecture level, hardware failures in the microcircuits used for computing a number of functions, for example, are of pronounced significance. An individual software failure, on the other hand, is not all that safety-critical, given that the plant has a number of independent software-based safety systems in place.
‘Model checking is a highly effective method for finding latent design errors that may also be strange or unusual. Unlike testing or simulation, model checking is capable of achieving complete sequential coverage with respect to the requirement being examined,’ Lahtinen adds.
In the concluding phase of the dissertation, a method to support the structure-based testing of the function block diagrams used in the design of software-based systems was created. The method generates tests based on the structure of the function block diagram that describes the functioning of the safety automation system.
‘According to the feedback received from Fortum, the process of model checking as such may reveal errors that are easily ignored in testing. The comprehensive checking is also capable of analysing events over a very short time span,’ Lahtinen adds.
The results of the dissertation work have already been used in practical assignments for the Radiation and Nuclear Safety Authority concerning the Olkiluoto 3 project, for Fortum concerning the automation renewal of the Loviisa nuclear power plant, and for Fennovoima concerning the functional architecture of the foreseen Hanhikivi nuclear power plant. Further study on the integration of model checking with probabilistic risk analysis is already underway.
The dissertation work related to the study was supervised at the Aalto University Department of Computer Science by Professor Keijo Heljanko, and it was carried out at the VTT Technical Research Centre of Finland. A substantial part of the study was funded by the Finnish nuclear power plant safety research programme SAFIR. Model checking has been studied on a constant basis since 2007, and it is a very demanding method computationally.
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Doctoral dissertation: Model Checking Large Nuclear Power Plant Safety System Design