What is the difference between a thermal reactor and a nuclear reactor?

Question

What is the difference between a thermal reactor and a nuclear reactor?

Answer 1

The nuclear industry lingo gets a bit confusing to the uninitiated.  Stated briefly, a thermal nuclear reactor is a nuclear reactor that operates using what are referred to as "thermal neutrons."  

This is a fancy way of saying that the neutrons released from a previous fission of the uranium-235 fuel have to be "slowed down" by collisions with other atoms (their nucleai) so that they are going slow enough to fall into the next uranium nucleus needed to continue the chain reaction.

When the neutrons have collided enough (bounced around in the fuel) to be really slow, they are essentially in "thermal equilibrium" with their surroundings.  It just happens that uranium-235 has a higher probability of being caused to fission by such a slow neutron, just like a golf ball is more likely to fall into the cup if it is going slow rather than fast.

Thus, we have the "thermal" reactor, which is pretty much the opposite of a "fast" reactor, which uses faster neutrons and different isotopes for the fuel, isotopes that are more fissionable if more energetic neutrons are around, such as those needed to cause uranium-238 to fission.  Since the neutrons have not slowed down, they are going "fast" and the next fission also occurs "fast."  It is generally true that fast reactors are harder to control simply because things happen faster.

So, the answer to your question is that the thermal reactor is a subset of nuclear reactors.  Almost all the reactors in the world are thermal reactors, whether they are called pressurized water reactors (PWR) or boiling water reactors (BWR).  They depend on the water in the reactor core to slow down the neutrons quickly (before they can escape from the reactor core), billiard ball style, since the energy/momentum transfer is greatest when the colliding particles are about the same weight.

While the slowing down event needs to be fast, the final product of slow neutrons is the goal and, also, the name of the reactor type, at least as explained in terms of being "thermal" or not much more energetic (speedy) than the hydrogen in the water molecules.

Answer 2

The heat source for the steam turbines are other than rods of high grade uranium.