Secondary Flows in PWR Hot Legs, CFD Brings Some Light to Better Understand Hot Leg Flows
NURETH-14 - 2011 September 25-30

Presented at:
2011 September 25-30
Toronto, Canada
Session Title:
B3-1 Core Thermalhydraulics and Subchannel Analysis

Serge Bellet (EDF - Septen)
Sofiane Benhamadouche (EDF R&D)


Secondary flows play an important role in several locations of the primary circuit of a PWR. A full program for a better understanding of these kinds of flows with CFD computations has been launched at eDF. Since the beginning of 2009, eDF started new activities around CFD simulations of swirling and secondary flows. A progressive approach (increasingly close to the industrial needs) is adopted with different items relative to bibliography study, academic to semi-industrial test cases, simplified upper plenum and hot leg, mock-up and reactor simulations. The present paper focuses on a recent work, based on a pure CFD approach, dealing with the understanding of the complex structure of secondary flows which are observed in the hot leg. Different simulations have been performed to find the origin of the secondary flows in the upper plenum and also to understand their evolution along the hot leg. 

A first simulation (empty plenum) enlighten on the origin of the two “main” secondary structures widely observed in other numerical simulations and experiments. Two counter rotating vortices are created due to a geometrical effect (asymmetric location of the hot leg). The axis that separates these two structures is rather vertical at the inlet of the hot leg and turns clockwise while going through the leg. The rotation is not that important. A second simulation (with the guide tubes) enlighten on the role of the RCCA guide tubes located as an obstruction of flow in front of the hot leg nozzle inlet. It is shown that secondary motions are created along the tubes due to an upward/downward flow and these structures can have a non negligible trace at the hot leg inlet. However, in particular far from the leg inlet, the global structure consists in two counter rotating vortices similar to the one observed on an empty plenum but with a different orientation (the axis that separates the two structures is shifted by around 20° compared to the configuration without guide tubes) and a similar rotation. Concerning the temperature, the global distribution of the temperature is similar in both cases, in particular at the beginning of the hot leg. While going further in the hot leg, the influence of the secondary motions is clearly observe, what makes the temperature distribution less homogenous and less smooth.

One of the most important conclusions is also that Reynolds Stress Models supported by an adequate mesh refinement can capture complex secondary flows, which is not the case of standard approaches such as the ones which use a standard k-ε model for example.

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