A primary line of evidence is the frequency and locations of earthquakes beneath the volcano, caused by upward flowing magma. (theconversation.com)
The slow upward flow of water insures that the short lived radioactive gases produced by fission die off before the reactor coolant reaches the above - ground portion of the plant (another bonus of gravity driven flow!). (smrllc.com)
Vents in the top of the chassis allow heat to escape, while a vented underside let cool air in, and the basic upward flow of heat provides all the airflow necessary. (pcmag.com)