Uranium dioxide is a black semiconducting solid. It can be made by heating uranyl nitrate to form .
This is then converted by heating with hydrogen to form UO. It can be made from enriched uranium hexafluoride by reacting with ammonia to form a solid called ammonium diuranate, . This is then heated (calcined) to form  and UO which is then converted by heating with hydrogen or ammonia to form UO.
The UO is mixed with an organic binder and pressed into pellets, these pellets are then fired at a much higher temperature (in H/Ar) to sinter the solid. The aim is to form a dense solid which has few pores.
The thermal conductivity of uranium dioxide is very low compared with that of zirconium metal, and it goes down as the temperature goes up.
Corrosion of uranium dioxide in water is controlled by similar electrochemical processes to the galvanic corrosion of a metal surface.
While exposed to the neutron flux during normal operation in the core environment a small percentage of the  in the fuel absorbs excess neutrons and is transmuted into .  rapidly decays into  which in turn rapidly decays into . The small percentage of  has a higher neutron cross section than . As the  accumulates the chain reaction shifts from pure  at initiation of the fuel use to a ratio of about 70%  and 30%  at the end of the 18 to 24 month fuel exposure period.