Question

A nuclear reactor generating plant supplies 53.0 MW of useful power steadily for a year. Assuming that, in addition, twice this power is wasted in heat production, determine the mass (in kilograms) converted to energy in a year at this plant.

Answer 1

Multiply 159* 10^6 W by the number of seconds in a year.
That will give you the energy produced. Divide that by c^2 for the mass loss.

This is about thermodynamics and the Einstein E = m c^2 equation, not quantum physics.