  We present the design and implementation of a thermal model, developed in
COMSOL, aiming to probe the wafer-scale thermal response arising from realistic
rates and energies of cosmic rays at L2 impacting the detector wafer of Athena
X-IFU. The wafer thermal model is a four-layer 2D model, where 2 layers
represent the constituent materials (Si bulk and Si$_{3}$N$_{4}$ membrane), and
2 layers represent the Au metallization layer's phonon and electron
temperatures. We base the simulation geometry on the current specifications for
the X-IFU detector wafer, and simulate cosmic ray impacts using a simple power
injection into the Si bulk. We measure the temperature at the point of the
instrument's most central TES detector. By probing the response of the system
and pulse characteristics as a function of the thermal input energy and
location, we reconstruct cosmic ray pulses in Python. By utilizing this code,
along with the results of the GEANT4 simulations produced for X-IFU, we produce
realistic time-ordered data (TOD) of the temperature seen by the central TES,
which we use to simulate the degradation of the energy resolution of the
instrument in space-like conditions on this wafer. We find a degradation to the
energy resolution of 7 keV X-rays of $\approx$0.04 eV. By modifying wafer
parameters and comparing the simulated TOD, this study is a valuable tool for
probing design changes on the thermal background seen by the detectors.
