Because uranium-235 isn't found naturally in large quantities, engineers increase its concentration in nuclear fuel so that a chain reaction is more likely to occur. what is the name of this process?

Nuclear Fusion. 

Having low atomic nuclei is the reason why uranium-235 is not commonly found in large nuclear reactions. 

So, scientist increases its concentration by fusing the lower atomic nuclei with high atomic nuclei in order to produce a heavier nucleus which will increase the release of energy.