Simulation of inter-wrapper flow

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SESAME Newsletter #1 (March 2016) 

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The inter-wrapper flow plays different roles in nominal and in decay heat removal situations.

In nominal conditions, a flow recirculation occurs in the inter-wrapper, due to the radial pressure profile caused by the above core structure, and the by-pass flow feeding the inter-wrapper region. The knowledge of the inter-wrapper flow permit to predict the hexagonal tubes temperature, and then to calculate the mechanical equilibrium of the core.

In decay heat removal situation at low flow rate, a significant fraction of the residual power can be extracted through the hexagonal tube to the inter-wrapper region by several ways:

  • natural convection in the gap: cold sodium penetrates downward in the peripheral region of the core, moves to the center and goes out by buoyancy. This is especially important when a direct heat exchanger immersed in the hot pool is operating.
  • radial conduction: in the peripheral region, and in the case of isolated subassemblies such as internal storage subassemblies, the neighboring subassemblies are cold, and form a heat sink for the heated ones.

To take into account these phenomena, an application named TrioMC2 has been developed. TrioMC2 couples a rough CFD model of the inter-wrapper region (calculated with TrioCFD) and a sub-channel description of the core (with TrioMC).

An implicit thermal coupling is performed through the hexagonal tubes, to avoid time step stability limitations. The algorithm is the following:

  • Knowing the inter-wrapper sodium temperature and heat exchange coefficient of the previous time step, the sub-channel code transmit to the CFD code 1/ an explicit flux across the hexagonal tube, and 2/ the variation of this flux according to a variation of the inter-wrapper sodium temperature.
  • TrioCFD solves his time step, and gives back to TrioMC the updated temperature and heat exchange coefficient.
  • Finally, the sodium temperature inside the subassemblies are updated according to the real inter-wrapper sodium temperature.

The inter-wrapper gap has only one mesh between two hexagonal tubes. Navier-Stokes equations are solved in the gap with:

  • Boussinesq hypothesis to take into account buoyancy effects
  • pressure drop correlations, to take into account regular and singular pressure losses
  • heat transfer correlations to calculate the heat fluxes exchanged through the hexagonal tube
TrioMC2 simulation of Plandtl with direct heat exchanger
TrioMC2 simulation of Plandtl with direct heat exchanger

An example of calculation is shown on the figure at right. This is a TrioMC2 modelisation of the japanese Plandtl loop, with low flow rate and a DHX operating.
This coupled approach allows to perform best estimate computations of a complete full scale reactor core in both nominal condition for design studies and accidental transient for safety studies thanks to a highly parallel architecture.