The Terrapower Natrium is fast neutron spectrum sodium cooled reactor. It breeds and burns plutonium and should yield high fuel burnup. I haven't completed reading the Preliminary Safety Analysis Report (PSAR) at
https://www.nrc.gov/docs/ML2408/ML24088A065.pdf. It is a very interesting design. Mass production of these plants could yield interesting economics. I haven't read the referenced article yet, but they broke ground June 10th in Wyoming for the “secondary” plant and awaiting NRC approval.