Answer:
The answer is
X: Converts mass into energy
Y: Is controlled by neutron absorption
Step-by-step explanation:
To maintain a sustained controlled nuclear reaction (nuclear fission), for every 2 or 3 neutrons released, only one must be allowed to strike another uranium nucleus. If this ratio is less than one then the reaction will die out; if it is greater than one it will grow uncontrolled (an atomic explosion). A neutron absorbing element must be present to control the amount of free neutrons in the reaction space. Therefore, nuclear fission is controlled by neutron absorption.
Nuclear fission and fusion both converts mass to energy.
In both reactions total mass of material decreases but the energy comes out from atoms.