Inter-assembly gap treatment

[fpepe-polito]

This sub-issue focuses on the treatment of the inter-assembly gap.
Until now, three models are adopted in DASSH to treat this region:

1. flowing gap coolant: models axial advection and inter-subchannel conduction;
2. stagnant fluid: only models conduction in the horizontal plane;
3. duct-average model: the inter-assembly gap coolant temperature equals the average of the adjacent duct wall temperatures.

The following sub-tasks can be addressed:

• implementation of a fourth model which treats inter-subchannel mass and momentum exchanges;
• comparison of the four available methods.