[Annushka]

Recently I watched Chernobyl program and I don't understand how they making uranium hot in reactor. What needs to happen to neutrons hitting atoms??

Is it this cycle, so hot water will do it? If so what will do it for a first time, how they turning reactor on?

Anyone knows??

[Babcock_Hall]

I know only the very basics of nuclear chemistry.  When you say "hot," do you mean as in high temperature, or as in more radioactive.  The word "hot" could mean more than one thing.

[Borek]

how they turning reactor on?

https://en.wikipedia.org/wiki/Startup_neutron_source

[Enthalpy]

I'm not sure I understand the question properly.

The reaction of uranium produces heat. This reaction is called fission. It heats water to obtain work by a turbine, then electricity by an alternator.

Nearly any heavy atom would make heat by splitting ("fission"). Uranium and plutonium are special because they split easily by absorbing neutrons, and their fission emits enough neutrons, so their fission can be a chain reaction. Making enough neutrons would be difficult by other means: this would consume more energy than uranium fission produces.

Uranium fission needs no initial heat. The only condition for a "chain reaction" is that more neutrons are produced than lost. This condition is not easy.

[pcm81]

"HOT" in nuclear jargon usually means radioactive. Used uranium fuel is "hot" because it contains fission products, which are radioactive.
Nuclear reactor startup usually involves withdrawal of several control rods from the core. The control rods contain boron-10 and eat up neutrons, but do not have chain reaction themselves; hence stopping chain reaction, so pulling them out enables chain reaction to increase.

If fuel rods are actually hot (temperature) then chances are they are also "hot" (radioactive), since the source of heat in the fuel is the fission chain reaction that produces radioactive fission products.