CANDU Reactors are specifically designed such that they do not require enriched uranium, and can operate entirely on naturally-occurring uranium. A CANDU design is generally used by parties that do not desire uranium enrichment facilities, due to the cost of those facilities.
That said, a CANDU reactor CAN use enriched uranium, they are fully capable of supporting that fuel type.
CANDU reactors use natural uranium; the fissile isotope is 235U.
Most water moderated reactors use yellowcake powder: a uranium oxide enriched to 3% uranium-235. A few reactors use metallic uranium, sometimes enriched past 20%. Some experimental reactors use plutonium or mixed uranium & plutonium.
This is the CANDU reactor, developed in Canada. They have been successful and produce a large amount of power in Canada and other countries where they have been built. CANDU stands for CANadian DeUterium. The heavy water (D2O) is a better moderator than ordinary (light) water, and allows a reactor to be built that runs on unenriched uranium (u-238) as opposed to slightly enriched uranium (u-235, about 4%). The heavy water slows the fast neutrons down more, allowing better absorbtion, and the subsequent use of u-238.
This is the Candu type, which was uniquely developed in Canada to use heavy water moderator and natural uranium fuel
Uranium can be enriched to 99,9 %.But usually, for the use in nuclear reactors, the enrichment is under 20 %.
Uranium-235 in combination with Uranium-238, enriched from natural levels of about 0.7% U-235 to about 5% U-235. There are other configurations, but this is the most common.
Nuclear energy
Predominantly enriched uranium, but some reactors can use natural uranium.
Heavy water has the advantage of being a good moderator and of absorbing fewer neutrons than does light water, so that natural (unenriched) uranium can be used. Light water demands enriched uranium, around 4 to 5 percent U-235. So you can make a choice: use heavy water which is expensive to produce, or use light water and expensive enriched uranium. You can see the different approach between the US and Canada. In the US there was experience of enrichment from the WW2 Manhattan project, in Canada there was no such experience but they had cheap hydro power to use to produce heavy water, so developed the Candu type of reactor.
Uranium and plutonium can be used as nuclear fuels for nuclear reactors.
We might use californium as a neutron source in a nuclear reactor. Californium is a neutron emitter, and it can be used to "enhance" start-up abilities of a reactor where the fuel isn't as "good" as it might be in a core of, say, highly enriched uranium.
The use is to produce electricity from a nuclear reactor plant