# Coupled transient tutorial with MSCS

This tutorial describes how to use Serpent 2.1.29 to solve a simple reactivity insertion transient for a 3x3 rod unit cell with a central AIC control rod. We will solve the coupled neutronics-fuel temperature problem for an accidental fast removal and return of the control rod. We will not solve heat conduction in the fuel, but will use an adiabatic approximation instead: all heat is modelled as remaining at the place of deposition.

The XY and XZ cuts of the geometry we want to simulate are shown below:

The details of the transient we want to simulate follow: In the beginning of the transient, the system (made critical by soluble boron) is operating at a power level of 1.0 W with the delayed neutron precursors at their stable concentrations. The control rod is removed from the system with an instantaneous upward velocity of 100 000 cm/s, which is maintained until the control rod is completely removed from the geometry. After this, the control rod is inserted back with 5% of the removal speed until the initial position is reached again.

In order to simulate the transient, we will need to first generate the source distributions for neutrons and delayed neutron precursors in the beginning of the transient. As we want to start the transient from the critical state (starting from subcritcal or supercritical states is not currently supported), we will need to create a critical model of our system.

Let's start with the following input:

Colors in the input correspond to:

- Comments
- Control words
- Name definitions
- Name references

**Initial input for 3x3 rod 3D unit cell**

% --- Link XS-libraries etc. set acefile "./" % --- Neutron population and criticality cycles: set pop 10000 50 50 % --- Total power for normalization (1 W): set power 1 % --- Use reflective boundary condition in XY, black in Z set bc 2 2 1 % --- Pin definitions include "./include/pins" % --- Geometry definition include "./include/geometry" % --- Move control rod surface to starting position trans s sCR 0 0 -3.54 % --- Material definitions include "./include/materials" % --- Add geometry plots plot 2 500 1000 plot 3 500 500 % --- Add mesh plots mesh 3 500 500 mesh 2 500 1000 mesh 10 3 500 500 mesh 10 2 500 1000