Final answer:
The mass of fuel in both nuclear reactors and conventional coal power plants decreases when energy is produced, due to mass-energy equivalence. However, the mass decrease in coal power plants is too small to be observed in practice, unlike in nuclear reactors where it is more substantial and measurable.
Step-by-step explanation:
According to Einstein's theory of relativity, mass can be converted into energy and vice versa as described by the famous equation E=mc2. In a nuclear reactor, this conversion occurs at a measurable scale due to the process of nuclear fission, where the mass decrease is substantial enough to observe. This is because a significant amount of energy (in the form of heat and electricity) is produced from a relatively small amount of nuclear fuel.
Similarly, in a conventional power plant where coal and oxygen are used, there is a conversion of mass to energy as well. However, due to the conservation of mass-energy, the loss of mass that corresponds to the energy produced would also occur here. Yet, this change in mass is much smaller compared to that in nuclear processes and cannot be easily observed in practice because the energy per mass unit released during chemical processes such as the combustion of coal is much less than that released during nuclear reactions.
Therefore, while the mass of the combined coal and oxygen also decreases after being converted into energy, this is not practically observable due to the incredibly small scale of the mass change. The mass loss in burning coal is minuscule and beyond the capability of current measurement tools to detect compared to the measurable change in nuclear reactions.