That's true. We had two reactor options during the cold war that were choices to extend our Uranium supply. One was the Fast Breeder reactor and the other was the liquid thorium reactor. My dad worked on the Fast breeder which would use liquid sodium as a coolant to allow the reactor to operate at much higher temperatures and high neutron flux so as to burn actinides more efficiently and convert uranium into plutonium to be either burned or taken out and used for weapons. Although, it's silly, American commercial fast breeders wouldn't really be used for plutonium production even if they got off the ground because U.S. regulations wouldn't allow anyone to access the fuel very easily. Unlike Chernobyl, which was a weapons/energy plant, designed for easy access to it's fuel. Hence the lack of containment which all American plants are required to have.
Realistic breeder fuel cycles also use a mix of actinides that would be unattractive for bomb makers: you're going to start with fuel that's been through an LWR once or twice, so contamination with Pu240 and Pu241/Am241 will be high to begin with and will just get worse the more cycles you spend in a fast spectrum.
Now, hypothetically, a country like Iran could develop a fast reactor that's optimized for producing plutonium for military purposes, but practically this would be much more difficult than developing a heavy water reactor like the U.S. used at Savannah River.
The isotope which is most problematic in commercial fuel cycles would actually be Np237. It's easy to separate Np chemically from other elements, and Np237 is longer lived than other isotopes of Np, so it can be prepared in a very pure form. Np237 has a low spontaneous fission cross section and a critical mass close to pure U235 -- it would be an attractive material for primitive gun-type bombs.
Note that Np237 is produced from U235 by the chain of absorbing two neutrons, making U237, and then beta decay to Np237. A plutonium-fueled or thorium-fueled reactor isn't going to make as much of it as our current reactors do.
