Assessment of the Operating Temperature of Absorber Assembly Structural Components and Confirmation of Their Cooling Reliability in VVER-440 Reactor Core

Abstract

The specific energy release in the structural materials of the absorber assembly (control part of the accident control assembly – ACA, also known as the shim assembly) has been calculated. It depends on the power of the fixed fuel assemblies (FAs) in adjacent cells. The value of the energy release is 17.5 W/cm³ on the most loaded section of the boron absorber insert for an average Kq=1.28 over sectors. The total energy release in the structural materials of the absorber assembly, in the coolant, and in the connecting bar material is 229 kW for fully inserted controls and 64 kW for controls lifted up by 154.8 cm from the core bottom. The surface temperature distribution in the absorber insert along the absorber assembly height is conservatively calculated based on the total energy release in the absorber insert material and the amount and rate of coolant flow through it. At a coolant temperature around the absorber insert corresponding to the maximum coolant heating in adjacent fixed FAs (46.6 °C), and in the absence of axial heat exchange, the maximum surface temperature of the absorber insert for fully inserted controls will be 312.7 °C (outer surface), and for controls lifted from the core bottom to 154.8 cm – 317.1 °C (inner surface), giving a margin to saturation of 14.3 °C and 9.9 °C, respectively, at a coolant saturation temperature of 327 °C. In the most conservative case considered in this paper, the maximum surface temperature of the absorber insert is lower than the coolant's saturation temperature. This indicates the absence of bulk and surface boiling of the coolant under operation of the most energy-loaded component, i.e., the absorber insert of the absorber assembly, meaning that the structural components of the absorber assembly will be reliably cooled in the VVER-440 reactor core.