In a nuclear reactor, the true fuel is a fissionable isotope such as U235 or Pu239. The U235 is a naturally occurring substance in that uranium is found all over the world but the particular isotope of U235 is effectively only found in trace amounts of natural uranium. In order for a sustained nuclear chain reaction to occur in a nuclear reactor, the U235 fraction in natural uranium has to be enriched from less than 1% up to around 4 or 5% for commercial nuclear reactors to operate. This enrichment process is done at places like URENCO just east of Eunice, NM (right next to the Waste Control Specialists facility in Texas). The isotope U235 is special in the sense that when it absorbs a neutron, it has a high probability of splitting through fission to give off a large amount of heat and 2 or 3 additional fast neutrons.
Just having the uranium enriched in the U235 isotope is not sufficient to run a commercial reactor, the neutrons given off in fission travel at much too fast a speed to sustain the chain reaction in a modern reactor core. You could say that the neutrons have to spend more time near a nucleus to be absorbed and this only happens when they are traveling slower. So in order for the neutrons to have a reasonable probability of even being absorbed by the fissile isotopes, they have to be slowed down by a considerable amount. This is done by bouncing them around many times, over and over off of hydrogen. The hydrogen is present due to the use of water running through the nuclear reactor as the means to remove the heat from the fission reactions taking place. The neutrons given off from fission bounce into the hydrogen in the water in the same way the cue ball on a pool table bounces into other pool balls and so slows down. The more times the cue ball hits other balls, the slower it tends to move on average.
The chain reaction cannot take place unless the neutrons given off in fission are absorbed by other fissionable isotopes so by having the right balance of enriched uranium in U235 and water content as a coolant and moderator (a moderator is the substance used to slow down the fission neutrons), a sustained chain reaction can then take place in a power reactor. The heat generated by the fission is transferred to the water which then is allowed to generate steam to turn a turbine and make electricity.
The rate at which power can increase in a nuclear reactor is dependent on the rate at which neutrons are produced in the process. Most of the fission neutrons are generated almost instantly but some are actually created through radioactive decay some time after the fission. These delayed neutrons end up making the average generation rate of neutrons rather slow. Modern power reactors are operated in such a way that these delayed neutrons are required to be present in order for the reactor to sustain a chain reaction making the reactor behave fairly slow in terms of changes in power levels.
The actual control of the reactor is then carried out by the use of what are called control rods. These are generally long pieces of metal with neutron poisons concentrated in them. The neutron poison is a material with a very high probability of absorbing neutrons with no resultant fission reaction taking place. These control rods have to first be removed from the reactor core to enable it to sustain a chain reaction.
Even without the controls in place, the physical design of a nuclear reactor prevents it from undergoing a nuclear explosion as found in modern weapons. The heat generated from a nuclear reactor can create a chemical based explosion but again, nothing that could generate a nuclear yield. In the case of Chernobyl, the heat from the reactor effectively flashed the water in the core into steam which was the source of the initial explosion. This was followed by a very hot graphite fire as the Russians used a graphite moderator instead of water as opposed to US designs. The Fukushima explosions were similarly chemical in origin due to hydrogen accumulation caused by the fuel overheating.
So based on reactor design and operation, it is not credible to have a nuclear explosion (and that feature is intentional).