Why uranium dioxide is used in nuclear reactors?

[Karagoz]

In Wikipedia I read that Uranium-235 is a nuclear is fuel in fission reactors, also that Uranium-235 is split (fission) and energy is revealed.

But in some images showing how nuclear reactors work, they show Uranium dioxide as nuclear fuel elements.

In Wikipedia, it's written that the oxides are used because the oxide melting point is much higher than that of the metal and because it cannot burn, being already in the oxidized state.

So they use Uranium dioxide instead of Uranium-235 to prevent the fuel elements being burned and melted?

Uranium dioxides do split and cause chain reactions?

How does the chain reaction works with Uranium dioxides?

 

[hilbert2]

The rate of the chain reaction depends on the number of uranium.235 nuclei per unit volume, the total volume of the fuel (sub/supercritical?) and on the capability of any other nuclei in the material to capture neutrons. I'm not sure how well the oxygen nuclei in $UO_2$ can stop neutrons but it's possible that they have little effect on the fission reaction.

 

[Nugatory]

So they use Uranium dioxide instead of Uranium-235 to prevent the fuel elements being burned and melted?

They use uranium dioxide instead of metallic uranium (a mixture of U-235 and U-238) to prevent the fuel elements from melting and burning.

However, a molecule of uranium dioxide is just a uranium atom and two oxygen atom bonded together, and it's still the uranium atom that fissions - the oxygen atoms are just along for the ride. The uranium atom in the uranium dioxide atom molecule can be either U-235 or U-238, and as long as enough of them are U-235 we have a satisfactory reactor fuel.