Efficiency of a thermal reactor I was told that the efficiency of any thermal reactor increases if it is operated at higher temperature- in this case, nuclear reactor was referred to. But I cannot seem to understand why...
 A: Presumably this is in the context of using reactors to generate electricity. Any power generator works by transferring heat from a hot source to a cold sink, and using the heat flow to do work. This work then generates electricity.
The archetypal heat engine is the Carnot engine, and its efficiency at converting heat to work is given by:
$$ \eta = 1 - \frac{T_c}{T_h} $$
where $T_h$ is the temperature of the hot heat source and $T_c$ is the temperature of the cold heat sink (both temperatures need to be in Kelvin). So the efficiency is increased by increasing $T_h$ or decreasing $T_c$. In practive the cold sink is the environment, and there's little we can do to change that. So the only practical way to make a heat engine more efficient is to increase $T_h$ i.e. run it hotter.
The Carnot engine is an ideal case that doesn't exist in the real world, but the behaviour of real electricity generators is basically similar and the same principle applies. That's why you increase the effiency of your power station if you run the reactor hotter.
