Piontkovskyi Y. Unfolding of energy distribution in VVER-1000 reactor core based on the signals from self-powered neutron in-core monitors

This thesis is dedicated to solving an actual scientific-technical task: enhancement of an operational safety of nuclear fuel at VVER-1000 nuclear power reactor by means of lowering the uncertainties of power distribution parameters within the reactor core based on the signals from self-powered neutron detectors, being installed under the in-core instrumentation system (ICIS). The systematization and analysis were performed of systems and detectors that are being used at NPPs worldwide to unfolding the power distribution in a reactor core. Their operational features, main advantages and drawbacks are considered. Major attention was paid to self-powered rhodium neutron detectors that are currently installed at Ukrainian NPPs under ICIS. Based on the results of analysis conducted, the development of numerical models and means was accomplished to enhance a precision of the transition function from in-core self-powered neutron detector current to the linear heat rate of a fuel assembly. The local sensitivity of rhodium in-core self-powered neutron detectors to neutron flux in VVER-1000 reactor core was computed. Also a contribution into rhodium detector signals from neighboring fuel assemblies as well as a contribution from fuel rods of a fuel assembly with installed rhodium detector and neighboring fuel rods are determined. It was shown that the numerical model of the core developed for unfolding of the linear heat rate does allow to take into account greater amount of fuel pins for computation of the transition function contrary to other methods that are being used at Ukrainian NPPs nowadays. The calculations were done to determine the influence of reactor core parameters (coolant temperature, boric acid concentration and positions of neutron adsorption elements of control rods) at signal parameters formation from the rhodium self-powered neutron detector. The task for accounting of rhodium detectors burnup was solved for linear heat rate unfolding, taking into account non-uniform rhodium material burnup across the radius of rhodium detector emitter, self-screening of rhodium material internal layers and parameters of the neutron field, in which the rhodium detectors operate. It is presented that for linear heat rate precision enhancement it is necessary to compute the individual burnup functions for each rhodium detector depending on its location in the reactor core volume. The validation checkout of calculational models was accomplished. All the models developed are recognized as acceptable ones for their application in solving all scientific and technical tasks under this research. The reactor core model of the research reactor VVR-M at Institute for Nuclear Research of the National Academy of Sciences of Ukraine was developed and presented. This model is dedicated to investigate reactor core neutron field parameters and to determine the most optimal conditions for irradiation of the test assembly with the rhodium self-powered neutron detectors of domestic production. The in-core optimal location to install and operate the test assembly with the rhodium self-powered neutron detectors is suggested and justified. The output signal from this assembly was calculated and compared with experimental values.