Thermal energy is the term used to describe the energy emitted by an object which atoms are moving at a certain rate due to the temperature, and or are rising its temperature, so thermal energy is closely related to the temperature of the objects, in this case the nuclear reactor core would be the one that has the most thermal energy since it has the highest temperature.
I don't no if this helps but the body heat from your hand causes the liquid to boil, which in turn makes the liquid evaporate, turning it to gas. The expanding gas pushes the liquid upwards and when you release your hand, equilibrium is re-established.