According to this site, the center of the Earth with its 6000° Celsius seems to be rather chilly when compared to the center of a nuclear explosion
Temperatures of a nuclear explosion reach those in the interior of the sun, about 100,000,000° Celsius, and produce a brilliant fireball.
That's 6 orders of magnitude more.
If you think of it, during an underground nuclear explosion, rock immediately around the bomb is vaporized.
The energy of the nuclear explosion is released in one microsecond. In the following few microseconds, the test hardware and surrounding rock are vaporized, with temperatures of several million degrees and pressures of several million atmospheres. Within milliseconds, a bubble of high-pressure gas and steam is formed. The heat and expanding shock wave cause the surrounding rock to vaporize, or be melted further away, creating a melt cavity. The shock-induced motion and high internal pressure cause this cavity to expand outwards, which continues over several tenths of a second until the pressure has fallen sufficiently, to a level roughly comparable with the weight of the rock above, and can no longer grow.
I would say that your robot will turn to plasma anyway, then the blast will disperse it around.