[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.