The percent by mass of each element in a compound is known as

mass percent

Explanation:

Mass percent composition is also known percent by weight. It is abbreviated as w/w%. For a solution, mass percent equals the mass of an element in one mole of the compound divided by the molar mass of the compound, multiplied by 100%.

a boiling water reactor does not boil the water. a pressurized water reactor boils the water faster. a pressurized water reactor is heated directly by the core. a pressurized water reactor does not use fission.