# Difference between revisions of "Coupled multi-physics calculations"

Serpent 2 has been written with multi-physics applications in mind. The capability to model arbitrarily detailed density and temperature distributions means that most temperature and density distributions given by e.g. thermal hydraulics or CFD solvers can be brought into Serpent using one of the multi-physics interface file formats without loss of detail. There are dedicated routines for controlling the iteration between the neutronics solver (Serpent) and the solver(s) for the coupled fields in order to obtain the converged coupled solution.

## Multi-physics interface

The universal multi-physics interface can be used to easily bring in temperature and density solutions from any external solvers. With the multi-physics interface, the solution fields (temperature/density) can be overlaid on top of the base geometry model without having to make additional modifications (such as defining multiple materials with different temperatures) to the input.

See the detailed description: multi-physics interface.

## Internal solvers

Serpent is able to internally couple to the FINIX fuel behavior module that can be licensed separately from VTT.

## Power relaxation

Serpent relaxes the power distribution calculated in the iterations using the stochastic approximation based method[1], where the power distribution at iteration $n$ is calculated by

$P_{\mathrm{rel}}^{n} = P_{\mathrm{rel}}^{n-1} - \frac{s_{n}}{\sum_{i = 1}^{n} s_{n}} d \left(P_{\mathrm{rel}}^{n-1} - P^{n}\right),$

where $P^{n}$ is the unrelaxed power distribution tallied on iteration $n$, $P_{\mathrm{rel}}^{n-1}$ is the relaxed power distribution after the previous iteration, $s_{i}$ is the active neutron population simulated on iteration $i$ and $d$ is an underrelaxation factor that can be defined by the set relfactor option.