Within the CO$_2$ deposit near the south pole of Mars, according to this article 3 subunits have been found with one having a top CO$_2$ layer of about 300 meters thick covered by a water ice layer with a thickness of up to 20 meters (Figure 4).
The Russian OK-900A nuclear fission reactor has been used to power icebreakers. It has a fuel load of about 151 kg and a power production of about 171 megawatts with a total energy production of over 2000 GWh !
Dropping a similar reactor on the water ice layer covering the CO$_2$ subunit would sublimate the water ice beneath the reactor and it would sink down to the CO$_2$ layer, leaving a hole above it.
Looking at the CO$_2$ phase diagram above, the CO$_2$ solid heated by the reactor would turn into gas, leaving the shaft above, and the reactor would sink relatively fast to the bottom of the deposit, probably within days.
And what would happen next ?
Would not the lifting, cooling gas in the shaft deposit on the inner surface near the opening, eventually closing the shaft ?
Sitting at the bottom of the deposit the reactor would sublimate more and more solid CO$_2$, creating a big room around itself, with ever increasing pressure within. Would not that eventually lead to a huge explosion of pressurized gas ?
But if the shaft would stay open, will the initially very cold gas circulate enough to prevent the reactor from overheating ?