CFD analysis of Fuel Rod Bundles using a FV Unstructured Code

Researcher: Stefano Rolfo

Supervisor(s): Prof. D. Laurence
Sponsor: EPSRC (KNOO Project)
Start Date: September 2006 End Date: March 2010
Keywords: LES, Hybrid RANS/LES, Heat Transfer, Fuel Rod Bundles, Sodium Fast Reactor

Overall Research Aim

Rod bundles are a typical constitutive element of many nuclear reactor designs. They are composed of a set of rod elements, containing the fuel pellets, arranged in a variety of configurations. These pins are cooled by a fluid flowing parallel to the rods. Here different types of fuel bundle are considered and analysed using different CFD techniques.

Research Progress

Fuel rod bundle arranged into a triangular array

A fuel rod bundle arranged in a triangular array is investigated using well-resolved LES. When the pitch to diameter ratio (P/D) is below 1.1, the flow is characterized by an energetic and almost periodic cross-flow pulsation in the gap between adjacent sub-channels. The result is a meandering flow as shown by the temperature contours of Fig 1. Turbulence quantities are consequently quite different from those observed in pipes and channels and familiar steady state CFD techniques are not able to capture this phenomenon.
Fig 1: 3D Temperature field. P/D = 1.06, Re=6000


Fig 2: Comparison of the Hybrid results against DNS.
The weak point of well-resolved LES is that, for a wall-bounded flow, the resolution necessary tends to be close to the one required by DNS. This makes LES still very time consuming and affordable only a low Reynolds numbers. A way to overcome the problem is to hybridize LES with RANS in order to relax the mesh constrain imposed by the wall. In this work the hybrid RANS/LES proposed by Uribe et al is further developed with the addition of heat transfer modelling.

Some new modifications of the blending function, which control the merging between the two velocity fields, are also implemented in order to perform a greater proportion of RANS in presence of extremely stretched or skewed cells.

Sodium Fast Reactor (SFR).

Fast reactors with a liquid metal coolant have recently received renewed interest due to their efficient usage of the primary uranium resources; they are one of the proposals the Generation IV reactors. The rod bundle is characterized by the presence of a wire spacer in order to avoid collisions between fuel pins. The helical wires create a global swirling motion, promoting mixing between sub-channels. The figure opposite shows an anti clockwise rotation of the maximum stream-wise velocity (left). A very strong and complex secondary motion is evident (right).
Fig 3: Streamwise velocity (left) and secondary motion (right).

Last Modification: r14 - 2010-04-13 - 15:53:04 - JuanUribe

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Topic revision: r14 - 2010-04-13 - 15:53:04 - JuanUribe

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