“Why is geothermal energy an alternative source of nuclear energy?”
There is some problem with the doubt you asked. It is correct to say “Geothermal energy and Nuclear energy are alternative sources of energy”
Sun is the ultimate source of energy for earth. The energy of fuels, wind energy, water energy, food – all these are derived from sun. But Geothermal energy and nuclear energy are not derived from solar energy.
GeoThermal Energy Links for Reference
See a diagram of Geothermal plant
The centre of the Earth is around 6000 degrees Celsius – easily hot enough to melt rock. Even a few kilometres down, the temperature can be over 250 degrees Celsius if the Earth’s crust is thin. In general, the temperature rises one degree Celsius for every 30 – 50 metres you go down, but this does vary depending on location
In volcanic areas, molten rock can be very close to the surface. Sometimes we can use that heat.
Geothermal energy has been used for thousands of years in some countries for cooking and heating.
The name “geothermal” comes from two Greek words: “geo” means “Earth” and “thermal” means “heat”.