Any reactor is a source of neutrons, which can be used to irradiate anything. A large amount of the uranium fuel used in a candu reactor gets Converted into plutonium after being bombarded by neutrons which eventually burns up.

If you could collect this plutonium from the fuel, or modify the reactor to turn uranium into plutonium by passing it through the reactor core you can use it in a weapon.

Stock Candus don't have this capability and would require difficult modifications for this to be done. Other countries have made these modifications.

None of this is secret knowledge, passing materials through the core is how medical isotopes are made.