Bilodid I. Scientific and technical bases of decrease of superior conservatism in the safety analysis of nuclear installations

The thesis focuses on safety justification of VVER-440, VVER-1000 and RBMK-1000 spent fuel management systems, considering changes in the multiplication properties of nuclear fuel during operation without reduction in the level of nuclear safety. The scientific and technical basis for burnup credit during safety analysis of VVER and RBMK fuel management systems has been developed and implemented to decrease excessive conservatism associated with the approach when all fuel is regarded as fresh. This allows more realistic assessment of the multiplication properties of irradiated fuel and, hence, allows the fuel management system to be filled to the extent possible and the number of absorbers required to keep the system subcritical to be decreased. Which reduces the cost of handling of fuel assembly and amount of radioactive waste from nuclear facility. As a result of the implementation of the tasks set in the thesis, for the first time a complete and integrated scientific and technical work was performed: from the analysis of multilplication properties of spent fuel of various types to the development of methodological approaches and the implementation of the fuel burnup credit in the nuclear safety analysis of spent fuel management systems. The developed approaches, models and recommendations are acceptable for all storages of VVER-1000 and RBMK-1000 reactors in Ukraine. This allows me to conclude that the results of the thesis can be applied to all spent fuel management systems of these types of reactors, and the developed approaches and recommendations can be the basis for developing a justification for taking into account the fuel burnup of any reactors. The scientific significance of the thesis consists in obtaining the following results personally by the author: 1. For the first time, scientific and technical principles for taking into account the fuel burnup in the nuclear safety analysis of spent fuel management systems for VVER and RBMK reactors in Ukraine have been developed. 2. The developed approach allows the use of average fuel operating parameters in a reactor without simulating the irradiation of each fuel assembly. 3. For the first time, the effect of irradiation and process characteristics of VVER and RBMK nuclear fuel on its isotopic composition and multiplication properties during storage depending on the burnup has been comprehensively studied. 4. For the first time, the conservative, in terms of multiplication properties of fuel management systems, isotopic composition of VVER-1000 and RBMK-1000 spent fuel, has been defined and can be used for safety justification. 5. For the first time, the conservative spatial distribution of burnup in VVER 1000 and RBMK-1000 spent nuclear fuel has been determined and justified to ensure conservatism of criticality assessments of spent fuel management systems required by Ukraine's regulatory documents. 6. For the first time, the applicability of the chosen list of isotopes and burnup distribution for their credit in the criticality calculations has been justified. Based on the research carried out, a burnup credit in nuclear safety justification has been developed and implemented for containers of the dry spent fuel storage facility at the Zaporizhzhya NPP (ventilated storage casks) for VVER-1000 fuel and for wet spent fuel storage facility (ISF-1) at the Chornobyl NPP for RBMK-1000 fuel. Keywords: fuel burnup, criticality, storage, isotopic composition, safety analysis.