Розподіл зазорів між тепловиділяючими збірками в активній зоні ВВЕР-1000

• Дата: 2018
• Автори: Abdullayev, A.M., Zhukov, A.I., Maryokhin, S.V., Riabchykov, S.D.
• Формат: Стаття
• Мова: Ukrainian
• Опубліковано: State Scientific and Technical Center for Nuclear and Radiation Safety 2018
• Онлайн доступ: https://nuclear-journal.com/index.php/journal/article/view/117
• Назва журналу: Nuclear and Radiation Safety

The safety of fuel loading of VVER reactors is justified by calculations of the neutronic characteristics of the forthcoming campaign. These calculations are based on the design parameters of fuel assemblies (FA) — fuel enrichment, materials, design features, etc. However, during operation, some parameters change in an uncontrolled manner. In particular, FA can deform — bend or twist, this leads to the appearance of increased gaps between the fuel assemblies. These regions filled with a moderator lead to an increase in comparison with the calculations for the generation of thermal neutrons and, as a result, to a surge in the power of the fuel rods surrounding these regions. Safety requirements limit the power of fuel rods. Therefore, design capacities are increased by means of the so-called engineering margin factor to account for random outbursts. The deviation of the size of the water gaps between the fuel assemblies from the design ones should be known to calculate this coefficient, for example, the size distribution function of the gaps. This information is most often obtained by modeling the mechanical state of fuel assemblies in the reactor core. Other approaches are based on experimental data. Measurements in the core during the campaign are not possible. Therefore, the geometric parameters of the fuel assembly after the discharging from the core are measured. The presented paper uses the data of such measurements obtained after the 24th fuel campaign of ZNPP unit 4. It is assumed that the fuel assemblies tend to retain the form they have in the “free” state, and mechanical interaction with neighboring fuel assemblies leads to a certain equilibrium state that can be easily analyzed. In contrast to similar calculations, the elastic energy functional of interacting fuel assemblies is proposed whose minimum gives the required size distribution function of the gaps. 24 and 25 campaigns were modelled; the role of inter-sector FA shuffling was studied. The distribution of the gaps between the fuel assemblies in VVER-1000 core is calculated based on the measured deformations of the fuel assemblies discharged from the core and the elastic characteristics of the fuel assemblies. It was demonstrated that 95 % of gaps in the cores both with FA-A and FA-WR do not exceed 7.6 mm. The results can be used to calculate the engineering margin factor in determining the peaking factors of energy release.