Abstract
The aim of the numerical study was the detection of possible vortices in the upper part of the core of a Pre-Konvoi Pressurized Water Reactor (PWR) which could lead to temperature cycling. In addition, the practical application of this Computational Fluid Dynamic (CFD) simulation exists in the full 3D analysis of the coolant flow behavior in the reactor pressure vessel of a nuclear PWR. It also helps to improve the design of future reactor types. Therefore, a CFD simulation of the flow conditions was carried out based on a complex 3D model. The geometry of the model includes the entire Reactor Pressure Vessel (RPV) plus all relevant internals. The core is modelled using the porous body approach, the different pressure losses along and transverse to the main flow direction were considered. The spacer-grid levels were taken into account to the extent that in these areas no cross-flow is possible. The calculation was carried out for nominal operating conditions, i.e., for full load operation. Furthermore, a prototypical End of Cycle (EOC) power distribution was assumed. For this, a power distribution was applied as obtained from a stationary full-core calculation with the 3D neutron kinetics code DYN3D. In order to be able to adequately reproduce flow vortexes, the calculation was performed transiently with suitable Detached Eddy Simulations (DES) turbulence models. The calculation showed fluctuating transverse flow in the upper part of the core, starting at the 8th spacer grid but also revealed that no large dominant vortices exists in this region. It seems that the core acts as a rectifier attenuating large-scale vortices. The analyses included several spacer grid levels in the core and showed that in some areas of the core cross-section an upward increasingly directed transversal flow to the outlet nozzle occurs. In other areas of the core cross-section, on the other hand, there is nearly any cross-flow. However, the following limitations of the model apply: In the model all fuel elements are treated identical and cross flows due to different axial pressure losses for different FA types cannot be displayed. The complex structure of the FAs (eg. flow vanes in spacer grids) could also influence the formation of large-scale vortices. Also, the possible influence of two-phase flows was not considered.
Highlights
Since 2005, events have occurred involving the increased operational oxidation of M5 fuel rod claddings at several German pressurized water reactors (PWRs)
The increased oxide layer was even found in the region of the fuel rod plenum, where no appreciable power is transferred from the fuel rod to the coolant
These might be due to the fact that the upper core area could be affected by increased cross-flow, as it is expected that the flow starts “orientation” towards the reactor pressure vessel (RPV) outlet even below the uppermost spacer grids (SGs)
Summary
Since 2005, events have occurred involving the increased operational oxidation of M5 fuel rod claddings at several German pressurized water reactors (PWRs). Depending on the height position, different thermal conditions may occur in the form of temperature fluctuations These might be due to the fact that the upper core area could be affected by increased cross-flow, as it is expected that the flow starts “orientation” towards the reactor pressure vessel (RPV) outlet even below the uppermost SG. Due to different coolant temperatures in neighboring fuel assemblies (FAs), the turbulent flow could lead to a cyclic temperature load on the cladding material and the initial oxide layer, especially in the upper core region. According to this hypothesis, it is assumed that the oxide layer ruptures due to the load from alternating temperatures, creating pathways where oxidizing species reach the metallic cladding rod surface. The oxide layer loses its protective effect and thicker oxide layers can develop
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