1. The commonly expected goal for the nuclear fuel industry for nuclear reactors is to ensure that the fuels operate without leakage. A VVER-440 nuclear reactor has about 45,000 fuel elements, most of which operate during 4 reactor cycles under extreme physical and chemical conditions. The consequences of fuel failure influence the continuous power generation, in extreme cases may even require unplanned and immediate removal of the affected fuel assembly during the reactor cycle. Fuel failure does not preclude safe reactor operation, but the direct operational and economic impacts can be significant in some cases. One of the primary tasks is to continuously monitor the integrity of the fuel elements by measuring the isotope-selective radioactivity of the primary coolant. During the reactor campaigns, an advanced leakage model should be used to monitor the hermetical state of the reactor zone and to estimate the operational age and the leakage parameters of the defected fuel assembly.
2. Most of the models that are applied to predict the fuel defects are stationary-type, i.e. describe the leakage processes in equilibrium state of the reactor. However, the leakage events generally occur during transient states of the reactor. In the last 5 years the Nuclear Analytics Research Team in the Institute of Nuclear Technics in R+D cooperation with MVM Paks Nuclear Power Plant Ltd. developed a new type of time-dependent leakage model to identify defective fuel assemblies in VVER-type nuclear reactors. The model is able to mathematically describe the leakage process during a reactor cycle considering most of the reactor operational parameters. The model has an open type structure, which can be supplemented with additional modules that can influence the time dependence of leakage phenomena through various physical and chemical events.
3. The aim of the PhD research program is to further develop the system of leakage equations by mathematically modelling leakage phenomena that have not been considered so far, and by integrating them into the existing system as a new module: isotope-dependent numerical description of leakage functions, error prediction for calculated leakage parameters, spiking-events, inserting new decay-chains (¹³⁴Cs and ¹³⁷Cs isotopes, other fission products (¹⁰⁶Ru, ¹⁵⁴Eu, etc.)
4. The candidate must develop a theoretical mathematical description of the new modules and implement its numerical calculation modules and integrate them into the existing leakage expert system in MATLAB programming environment. The theoretical model must be validated with the results of operating databases and gamma spectrometry data series from VVER-440 nuclear reactors. Gamma spectrums and reactor operational data series from previous campaigns should be used for the calculations. The new calculation modules of the leakage model must also be adapted and extended to the VVER-1200 nuclear reactor. The yields of isotope production by fission and activation processes have to be determined by detailed burnup calculations applied MCNP software package. These calculations must also be performed in individual fuels of the reactor and the U and Pu isotopes released to the primary coolant, as well as for the surface contamination.

Expected results: The development of the model contributes to the safer, more economical and less radioactive waste operation of VVER-type nuclear reactors by describing predictable fuel failures. The final result of the PhD research program is the creation of an improved time-dependent model that takes into account significantly more nuclear physical and chemical factors, compared to the models known so far from the literature. This R+D project can also contribute to increasing the safety of nuclear energy production at the international level.