Failure modes of passive decay heat removing safety systems of modern nuclear power plants

The purpose of this master’s thesis is to gain an understanding of passive safety systems’ role in modern nuclear reactors projects and to research the failure modes of passive decay heat removal safety systems which use phenomenon of natural circulation. Another purpose is to identify the main physical principles and phenomena which are used to establish passive safety tools in nuclear power plants. The work describes passive decay heat removal systems used in AES-2006 project and focuses on the behavior of SPOT PG system. The descriptions of the main large-scale research facilities of the passive safety systems of the AES-2006 power plant are also included. The work contains the calculations of the SPOT PG system, which was modeled with thermal-hydraulic system code TRACE. The dimensions of the calculation model are set according to the dimensions of the real SPOT PG system. In these calculations three parameters are investigated as a function of decay heat power: the pressure of the system, the natural circulation mass flow rate around the closed loop, and the level of liquid in the downcomer. The purpose of the calculations is to test the ability of the SPOT PG system to remove the decay heat from the primary side of the nuclear reactor in case of failure of one, two, or three loops out of four. The calculations show that three loops of the SPOT PG system have adequate capacity to provide the necessary level of safety. In conclusion, the work supports the view that passive systems could be widely spread in modern nuclear projects.

Finnish Maritime Personnel's Conceptions on Safety Management and Safety Culture

The purpose of this thesis is to explore Finnish maritime personnel’s conceptions of safety management and its relationship with the concept of safety culture. In addition, the aim is to evaluate the impact of the ISM Code on the prevailing safety culture in the Finnish shipping business. A total of 94 interviewees and seven Finnish shipping companies were involved in this study. Thematic interviews were applied as the main research method for the study. The results were analysed qualitatively. The results indicate that maritime safety culture can simultaneously demonstrate features of integration, differentiation and ambiguity. Basically, maritime personnel have a positive attitude towards safety management systems since they consider safety management beneficial and essential in general. However, the study also found considerable criticism among the interviewees. The interviewed maritime personnel did not criticise the ISM Code as such, yet they criticised the way the ISM Code has been applied in practise. In order to understand the multiple perspectives of safety culture more comprehensively, multiple theoretical perspectives and methodological approaches are needed. This study indicates that safety culture and the impacts of the ISM Code should not be unambiguously studied solely quantitative methods or qualitative methods. By examining safety culture from several methodological and theoretical perspectives, one may gain a more versatile and holistic overview of safety culture.