Abstract
Fuel assembly bowing is a known phenomenon observed in many PWR reactors all over the world. The phenomenon is relevant to safety as it can lead to increased water gaps between assemblies which results in higher pin peaking factors. The goal of the present study is to assess the effect of assembly bowing not only for stead-state nominal conditions but also during a transient. The selected transient is the loss of one reactor coolant pump as it can be limiting especially regarding the Departure from Nucleate Boiling (DNB) safety criterion. This study focuses on an extreme case where the bowing is simulated in the core hot assembly by keeping the water gap constant over the whole core active length. The resulting cross-sections and form functions obtained from a 2d infinite lattice model are used in the nodal diffusion code DYN3D applying its pin-by-pin reconstruction method. For the transient simulation, DYN3D is coupled with the thermal-hydraulics subchannel code CTF on the SALOME platform. Several modelling options are compared: nominal geometry for neutronics and thermal-hydraulics (TH); mixed: neutronics with increased water gap, TH with nominal geometry; and increased water gap for both neutronics and TH. The results confirm that the increased water gap should be considered in both models in order to reduce the conservatism.
Highlights
Fuel assembly bowing is a known phenomenon identified in many PWR reactors all over the world (e.g. Germany, France)
The phenomenon is relevant to safety as it can lead to increased water gaps between assemblies which results in higher pin peaking factors in the neighbouring rods due to improved moderation
The selected transient is the loss of one reactor coolant pump (RCP) as it can be limiting especially regarding the Departure from Nucleate Boiling (DNB) safety criterion
Summary
Fuel assembly bowing is a known phenomenon identified in many PWR reactors all over the world (e.g. Germany, France). The phenomenon is relevant to safety as it can lead to increased water gaps between assemblies which results in higher pin peaking factors in the neighbouring rods due to improved moderation. It is generally assumed that the high peaking factors influence on safety parameters is (partially) compensated by the improved cooling thanks to the larger gaps. The goal of the present study is to assess the effect of assembly bowing for stead-state nominal conditions and during a transient. This study focuses on an extreme case where the bowing is simulated in a fresh MOX assembly by keeping the a constant additional 15mm to the nominal water gap over the whole core active length
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