# 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.

## 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.