I would like to suggest reading the paper entitled "Why a Nuclear Reactor Cannot Explode like an Atom Bomb" in

https://science.fusion4freedom.com/why-a-nuclear-reactor-cannot-explode-like-an-atom-bomb/#:~:text=While%20a%20nuclear%20reactor%20can%20never%20explode%20like,set%20coal%20dust%20on%20fire%20causing%20an%20explosion

-The nuclear explosive bomb and the nuclear reactor operate on the same operating principle: chain reaction of the interactive fission U 235. The bombardment of uranium 235 by fast neutrons produces a fission by chain reaction accompanied by an enormous thermal energy in addition to beta and gamma radiation.

The most likely fission products are Sr 94 amu and Xe 140 amu plus two fast neutrons. The reaction is therefore a chain reaction and almost instantaneous. The annihilation of mass in this reaction is 0.215 amu transformed into 200 Mev of energy according to Einstein's formula E = dm.c ^ 2. One kilogram of U 235 can produce approximately 7.2 x 10 ^ 13 joules "1.72 x 10 ^ 13 calories", which is equivalent to the heat produced by the combustion of 1800 tons of coal or Mazot. This is the main advantage of nuclear power plants.

1-The speed of the reaction is what distinguishes the explosive reaction of atomic bombs from the normal combustion reaction of nuclear reactors. For an explosion to occur, the reaction rate or the release of thermal energy must occur very quickly causing a very large pressure difference upstream and downstream of the resulting shock wave, otherwise no explosion will occur.

2-The objective of the nuclear bomb is to release an energy of about 8 x 10 ^ 13 joules, equivalent to 20,000 tons of highly explosive TNT, in about 20 seconds, while the power capacity of the reactor of the nuclear power plant can release the thermal from zero (emergency stop) to full station power, say 1000 MW (10 ^ 9 Joules per second).

3-The reaction rate in nuclear reactors is kept at a low level by using natural uranium composed of 0.74% U 235, 0.006% U 234 and 0.993 U 238 or enriching uranium 235 at a concentration limited to 3-5%. In addition, the U fuel is distributed over THOUSANDS of fuel rods over the entire large volume of the reactor core which is the opposite geometry of the nuclear bomb. The rest of the reactor core is mainly inert U 238, mixed with graphite or any other suitable moderator, which absorbs most of the neutrons from the reaction causing the transformation of U 238 to Pu 239.

*It should also be mentioned that Pu 239 is much more expensive than natural uranium where here we have a unique case where the price of fuel is absolutely negative. The reactor also uses movable Cd or boron alloy control rods for penetrate vertically into the core and absorb any desired amount of neutrons.

- In order to increase safety, the reactor core is contained inside a thick steel vessel surrounded by a concrete container reinforced with steel rods 1 to 2 meters thick, in the shape of a 'hemisphere.

-Conversely, in modern atomic bombs, the reaction is reinforced by a concentration of U-235 or Pu 239 enriched or fertilized to more than 90% with a mass of about 20-30 Kg, contained in the correct geometry, at inside a relatively small container equipped with explosive sources emitting fast neutrons. In addition, the material and design of the walls reflect the neutrons that escape inside the reaction.

** Even in the event of accidents or occurrences such as the shutdown of the control rods, an earthquake hitting the reactor, a series of cooling water pump failures, all causing excessive heating, the nuclear reactor CORE of a nuclear power plant can crack, break down, melt, but never explodes.

- This is why the Chernobyl reactor did not explode and cannot become like a nuclear bomb. It's really impossible, but it just melted and collapsed due to the overheating and swelling of its core.

Ref

I. Abbas, Quora English and French sections, 2020, 2021.

Well, the explosion at Chernobyl was rather instant ( < 1sec) localized ( in terms of reactor active zone) release of ~ 10 tons TNT equivalent energy. That however happened because of combination of extreme radioactive Xe-135 'reactor poisoning' and well beyond designed reactor reactivity ( too many absorptive rods pulled out). Another contributing factor was reactor being too large so local criticality within it could be achieved and positive void coefficient (graphite used as moderator so removal of water into vapor only boost the reaction). The explosive energy release later confirmed by 1) Enormous explosive action 2) seeing instantly melted fuel elements thrown away long distance 3) Ration of radioactive Xenon isotopes in air confirming instant energy release.

Normally the chain reaction is slown by

1) over-criticality being rather low so it essentially rely on ~ 1% of so-called 'delayed' neutrons : neutrons released with delays up to seconds.

2) Moderation substantially extents time of neutron generation, so slows the chain reaction. However, its not essential: the are exist fast neutron reactors (breeders) not using moderator and some although low efficiency nuclear explosive device using moderation was also demonstrated (Uranium Hydride bomb).

3) Constant presence of many neutrons while bombs are timed pulsed neutron source triggered.

The peculiarity of Chernobyl situation was effect of 'saturated absorber' well known in lasers. E.g. some light absorber centers that are very efficient but present in small quantity so can be saturated will shorten avalanche generation very seriously e.g. turning nanosec pulses in ~ 10 picosecond, although it will be harder to achieve initial criticality.

The breeder reactors lacking moderation created slowdown may use some extra measures to maintain controlability e.g. operate in periodic pulsed mode defined by fast moving mechanical part.

Generally, it is very hard to make strong nuclear explosion ( even most efficient nuclear devices still waste most of the material as it is expelled too fast), e.g. you may see description of 'criticality incidents'.