Численное исследование температурного поля в активной зоне РУ БРЕСТ-ОД-300 при частичной блокировке проходного сечения на входе теплоносителя

Abstract

A reactor core design with shroudless fuel assemblies (FAs) has been adopted in the BREST-OD-300 reactor’s basic design. The choice of shroudless FAs is dictated by the fact that with this reactor core design, there are no separate isolated fuel rod cooling channels shaped by FA shrouds. Therefore, emergencies involving partial blocking of the core flow cross section at the core inlet that are considered for the newly designed lead-cooled reactor, will have less severe consequences for the fuel rods than they may be in the case of using shrouded FAs. Thus, with the shrouded FA’s flow cross section being fully blocked, the heat produced by the fuel rods can only be removed from them to the coolant flowing in the gap between the shrouds (both due to natural convection of coolant inside the FA and due to thermal conductivity throughout the fuel rod bundle in the affected FA, predominantly in the radial direction). Obviously, if the core power is not reduced to a significant extent, overheating of fuel rods and loss of their cladding tightness will be unavoidable. A hypothetical situation involving partial blocking of the BREST-OD-300 reactor plant’s core flow cross section was investigated by carrying out 3D calculations using a porous body model. The case of the FA flow cross section being fully blocked at the level corresponding to the first spacer grid from the bottom is considered. A conclusion can be drawn from the obtained study results that the growth of fuel rod cladding temperature that will take place when the flow cross section of even seven central FAs is blocked will not cause them to fail immediately (the cladding temperature will in this case make 790 °C), but in all likelihood, it will only result in their service life to become shorter. To obtain better accuracy of the reactor core analysis performed according to the porous body model, a more detailed reactor core model representing individual FAs and gaps between them should be used. Nonetheless, the results obtained from the performed numerical analysis show that the reactor core with shroudless fuel assemblies has a significant advantage over the design with shrouded fuel assemblies.