SESAME Newsletter #1 (March 2016)
SIMULATION OF INTER-WRAPPER FLOW
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
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.